fractal behavior and detectibility limits of geophysical surveys the detectibility limits of a large scale geophysical measuring network such as arrays of seismometers for seismic hazard assessment arrays of air guns and vibrators in seismic profiling and station spacing in gravity magnetic magnetotelluric and other geophysical surveys for exploration of oil minerals and groundwater depends on the fractal dimension of the network and the anomaly lovejoy et al 1986 korvin 1992 the geophysical anomaly resulting from the fractal nature of sources turcotte 1992 such as length of fault okubo and aki 1987 robertson et al 1995 velocity inhomogeneities sato 1988 nonrandom distribution of density thorarinsson and magnusson 1990 and susceptibility pilkington and todoeschuck 1993 pilkington et al 1994 maus and dimri 1994 1995 1996 reflectivity todoeschuck et al 1990 dimri 1992 etc cannot be accurately measured unless its fractal dimension does not exceed the difference of the 2 d euclidean and fractal dimension of the network

using fractal crustal magnetization models in magnetic interpretation1 evidence from borehole susceptibility logs and the spectral analysis of aeromagnetic data suggests that the three dimensional distribution of magnetization within the crust can be described as fractal this property can be exploited in magnetic interpretation methods which explicitly require statistical information on the spatial variation of magnetization specifically we address the problem of magnetic source depth estimation through downward continuation and gridding aeromagnetic survey data using the method of kriging when magnetic data are continued downwards the depth at which the power spectrum flattens out the white depth can be taken to be an estimate of the top of the source distribution this procedure assumes that individual sources are uncorrelated with each other taking into account the correlation of the magnetization using a fractal description leads to a reduction in this depth estimate gridding of randomly distributed magnetic measurements using kriging requires an estimate of the covariance of the data compared with the assumption of a white uncorrelated magnetization distribution using fractal covariances for kriging produces gridded estimates which more closely reflect the statistics of the underlying magnetization process and produce maps with a justifiable degree of smoothness

spreading and convective dissolution of carbon dioxide in vertically confined horizontal aquifers injection of carbon dioxide co2 into saline aquifers is a promising tool for reducing anthropogenic co2 emissions at reservoir conditions the injected co2 is buoyant relative to the ambient groundwater the buoyant plume of co2 rises toward the top of the aquifer and spreads laterally as a gravity current presenting the risk of leakage into shallower formations via a fracture or fault in contrast the mixture that forms as the co2 dissolves into the ambient water is denser than the water and sinks driving a convective process that enhances co2dissolution and promotes stable long term storage motivated by this problem we study convective dissolution from a buoyant gravity current as it spreads along the top of a vertically confined horizontal aquifer we conduct laboratory experiments with analog fluids water and a mixture of methanol and ethylene glycol and compare the experimental results with simple theoretical models since the aquifer has a finite thickness dissolved buoyant fluid accumulates along the bottom of the aquifer and this mixture spreads laterally as a dense gravity current when dissolved buoyant fluid accumulates slowly our experiments show that the spreading of the buoyant current is characterized by a parabola like advance and retreat of its leading edge when dissolved buoyant fluid accumulates quickly the retreat of the leading edge slows as further dissolution is controlled by the slumping of the dense gravity current we show that simple theoretical models predict this behavior in both limits where the accumulation of dissolved buoyant fluid is either negligible or dominant finally we apply one of these models to a plume of co2 in a saline aquifer we show that the accumulation of dissolved co2 in the water can increase the maximum extent of the co2 plume by several fold and the lifetime of the co2 plume by several orders of magnitude

origin and timing of dauphine twins in quartz cement in fractured sandstones from diagenetic environments insight from fluid inclusions abstract electron backscattered diffraction techniques ebsd show that dauphine twins in quartz are widespread in many tectonometamorphic environments our study documents that under diagenetic temperatures

filling box flows in porous media we report upon a theoretical and experimental investigation of a porous medium filling box flow by specifically examining the details of the laminar descending plume and its outflow in a control volume having an impermeable bottom boundary and sidewalls the plume outflow initially comprises a pair of oppositely directed gravity currents the gravity currents propagate horizontally until they reach the lateral sidewalls at y pm l the flow then becomes of filling box type with a vertically ascending first front separating discharged plume fluid below from ambient fluid above the flow details are described analytically by first deriving a new similarity solution for darcy plumes with mathit pe o 1 where mathit pe is the peclet number from the similarity solution so obtained we then derive expressions for the plume volume flux and mean reduced gravity as functions of the vertical distance from the source regarding the plume outflow a similarity solution adopted from huppert woods j fluid mech vol 292 1995 pp 55 69 describes the height and front speed of the gravity currents whereas a semi implicit finite difference scheme is used to predict the first front elevation versus time and horizontal distance as with high reynolds number filling box flows that studied here is an example of a coupled problem the gravity current source conditions are prescribed by the plume volume flux and mean reduced gravity conversely discharged plume fluid may be re entrained into the plume be it soon or long after reaching the bottom impermeable boundary to corroborate our model predictions analogue laboratory experiments are performed with fresh water and salt water as the working fluids our experiments consider as independent variables the porous medium bead diameter and the plume source volume flux and reduced gravity predictions for the gravity current front position and height compare favourably against analogue measured data good agreement is likewise noted when considering either the mean elevation or the profile of the first front results from this study may be adopted in modelling geological plumes for example our equations can be used to predict the time required for discharged plume fluid to return to the point of injection in the case of aquifers closed on the sides and below by impermeable boundaries

the effects of quartz dauphine twinning on strain localization in a mid crustal shear zone abstract we use microstructural electron backscatter diffraction ebsd and crystal vorticity axis cva analyses to evaluate the effects of quartz dauphine twinning on strain localization we compare a granodiorite mylonite and a tonalite mylonite from the grebe shear zone a mid crustal transpressional shear zone in fiordland new zealand ebsd analysis reveals identical quartz grain boundary migration dynamic recrystallization microstructures in both samples with abundant dauphine twinning only in the granodiorite mylonite bulk cva patterns further distinguish the untwinned and dauphine twinned samples indicating a transition from simple shear to pure shear dominated transpression respectively the quartz cva pattern from the dauphine twinned sample indicates that twinned and untwinned grains record a transition from simple shear to pure shear dominated deformation respectively pole figures of the dauphine twinned sample reveal high temperature slip systems in the untwinned and dauphine twinned grains we conclude that dauphine twinning formed early in transpression and rendered the twinned grains less deformable relative to the untwinned grains over time the effectiveness of dauphine twinning in localizing strain is therefore strongly influenced by its timing relative to the evolving shear zone kinematic deformation geometry

dispersal of buoyancy driven flow in porous media with inclined baffles here we explore the effect of a series of low permeability tilted baffles on the ascent of a buoyant fluid injected into a porous rock from a linear well motivated by several industrial processes particularly co2 sequestration we first consider both theoretically and experimentally the dynamics associated with flow past an individual inclined baffle if the incident flux is sufficiently large then a pool of injectate grows beneath the baffle and spills over both ends partitioning the flux but otherwise the injectate only flows over the updip end of the baffle leaving a stagnant zone under the downdip part of the baffle in a multi layered system the flow may then be described using nonlinear recurrence relations based on the flow past an individual baffle using this approach in the particular case in which there is a regular distribution of baffles we show that on a scale greater than individual baffles the flow adjusts to a plume of constant width rising at an angle to the vertical which depends on the geometry of the baffles this constant width plume may be described by an effective directional permeability at angles to the vertical where we find where is the effective permeability of the regions between baffles within the boundaries of this plume the majority of pore space is bypassed indeed the plume scale effective porosity is largely associated with the pools of injectate which collect beneath each baffle we show that since the constant width plume has such a large lateral extent the total pore space occupied by the plume per unit height is larger than for a homogeneous formation furthermore these pools lead to a large surface area between injectate and formation water which enhances the reaction of the injectate with the formation water however we also show that in steady state it may be hard to determine this plume scale effective porosity since the ratio of the effective darcy speed and the effective interstitial speed of the plume only relates to the porosity of the updip part of these pools in which the injectate is flowing

on the use of seismic data to monitor the injection of co2 into a layered aquifer abstract one method to monitor the migration of co2 through a confined layered aquifer is to use time lapse seismic reflection data we examine how such information may be combined with a flow model for a co2 plume spreading through a water saturated aquifer to provide constraints on the migration of a plume of injected co2 we focus on the estimation of the partitioning of the injected flux between the different layers in a multi layer geological formation and on estimation of the permeability of each layer since there is a minimum depth of the injected co2 which is detectable seismically we show that even in a one layer formation the model predicts two different possible permeabilities for the formation corresponding to a fast pressure driven flow and a slower buoyancy controlled flow these two estimates of the permeability are sensitive to changes in the seismic detection threshold if there is additional data on the bottom hole well pressure we can distinguish between the solutions in a multi layer system supplied from a single well the non uniqueness in the inversion process leads to uncertainty in both the partitioning of the flux between the layers and the permeability of the different layers in the formation the model constraints lead to a relatively small range of possible distributions of the flux but if there is a lack of information about the relative pressure in the different layers near the well there is a much larger range of possible values for the permeability of each layer we develop some simple bounds on the problem by considering the case in which the near well pressure difference between the layers is related to either the vertical hydrostatic or the vertical co2 static pressure difference we examine the implications of the technique for interpreting the historic data set relating to co2 injection at the sleipner field north sea in which there are nine distinct layers in the formation this leads to estimates for the possible flux partitioning between the layers and also some constraints on the possible range of effective permeability in each layer estimates of the partitioning of the flux can inform estimates of the efficiency of co2 injection in the different layers however the uncertainty in the estimate of the effective permeability leads to uncertainty in estimates of the propagation speed of the flow in each layer and hence assessments of the hazards of leakage and of the rate of capillary trapping or dissolution of co2 following injection

multiple steady states in exchange flows across faults and the dissolution of co2 this is the author accepted manuscript the final version is available from cup via http journals cambridge org download php file 2fflm 2fflm769 2fs0022112015001007a pdf code 58f6f56d3a1302266ca4bcf617ba24df

time lapse walkaway vertical seismic profile monitoring for co2 injection at the sacroc enhanced oil recovery field a case study abstractgeologic carbon storage involves large scale injections of carbon dioxide into underground geologic formations changes in reservoir properties resulting from co2 injection and migration can be characterized using monitoring methods with time lapse seismic data to achieve economical monitoring vertical seismic profile vsp data are often acquired to survey the local injection area we investigated the capability of walkaway vsp monitoring for co2 injection into an enhanced oil recovery field at sacroc west texas vsp data sets were acquired in 2008 and 2009 and co2 injection took place after the first data acquisition because the receivers were located above the injection zone only reflection data contain the information from the reservoir qualitative comparison between reverse time migration images at different times revealed vertical shifts of the reflectors center indicating the presence of velocity changes we examined two methods to quantify the changes in velocity standard full wa

tracking mechanical dauphine twin evolution with applied stress in axial compression experiments on a low grade metamorphic quartzite abstract the stress dependent evolution of mechanical dauphine twinning has been investigated in axial compression experiments on a low grade metamorphic quartzite applying both time of flight neutron diffraction and electron backscatter diffraction the data of the experimentally stressed quartzite samples were compared with those of the naturally deformed starting material to monitor dauphine twinning in relation to different experimental stress states this comparison shows that in the experimental conditions of 500 c temperature and 300 mpa confining pressure dauphine twinning initiates below 145 mpa differential stress and saturates between 250 mpa and 460 mpa differential stress a single grain orientation analysis sgoa has been developed based on the distinction of quartz grains free of dauphine twin boundaries dtbs and containing dauphine twin boundaries comparing pole figures and inverse pole figures of dtb free grains of the starting material with those of the experimentally stressed samples shows a significantly different orientation distribution of the positive 10 1 1 r and the negative 01 1 1 z rhombs in dtb containing grains the sgoa allows to distinguish between host and twin domains using dtb free grains the sgoa furthermore reveals a particular pattern with one of the r rhomb maxima parallel to the axial compressive stress direction and a girdle with two r rhomb submaxima perpendicular to it we believe that this relationship between the axial compressive stress direction and the rhomb orientation distribution shows the potential of the sgoa in the reconstruction of the paleostress state in naturally stressed quartz bearing rocks

co2 percolation experiment through chlorite zeolite rich sandstone pretty hill formation otway basin australia underground co2 sequestration is highly recommended as an effective means of significantly decreasing co2 concentration in the atmosphere mineral storage is the more secure technology but requires the presence of high concentration of divalent cations in the pore fluid results from co2 percolation experiments through chlorite zeolite rich sandstone samples from the pretty hill formation otway basin australia are presented the dissolution of the laumontite 7 wt and chamosite 7 wt are the potential sources of calcium iron and magnesium required for carbonate precipitation the percolation experiment was setup to reproduce at laboratory scale the in situ temperature and pressure conditions t 95 degrees c and p 10 mpa the fluid injected at constant flow rate is a rock equilibrated brine subsequently enriched in co2 up to partial pressure of 6 mpa we observe feldspars laumontite and chamosite dissolution kaolinite and silica precipitation and a noticeable sink of co2 in the sample which is attributed to the precipitation of both amorphous carbon due to the reduction of co2 and fe rich carbonate permeability decreases of about one order of magnitude due to the localization of the kaolinite precipitation in the main flow paths while porosity increases the high reactivity of this sandstone makes this reservoir a valuable target for co2 mineralization but the associated permeability decrease may limit the injection rate and the spreading of the co2 in the reservoir

self similar dynamics of liquid injected into partially saturated aquifers the injection of liquid from a central well into a partially saturated aquifer of finite thickness is described using similarity solutions the solutions illustrate that injection leads to a growing zone around the source in which the fluid fills the whole depth of the aquifer beyond this zone the current adjusts to the depth in the far field as the fluid slumps under gravity the radial extent of the zone in which the aquifer is fully flooded depends on the ratio of the buoyancy driven flow speed to the pressure driven flow speed associated with the injection new laboratory experiments using a model porous medium support the model predictions in the case of an initially unsaturated layer the analysis is then developed to allow for a fully saturated aquifer containing fluid of lower density than the injectate and a further class of similarity solutions is developed again these are shown to be consistent with new laboratory experiments in concluding we briefly consider how the results may be combined to explore the self similar dynamics of a relatively dense fluid injected into an aquifer which is partially saturated with fluid of smaller density

a study of caprock hydromechanical changes associated with co2 injection into a brine formation a numerical study of hydromechanical changes during a deep underground injection of supercritical co2 in a hypothetical brine aquifer caprock system is conducted the injection process is simulated using a newly developed computer model for multi phase analysis of co2 and brine water flow coupled with heat transfer and rock deformations in this modeling co2 is injected at a constant rate over a 10 year period at a depth of 1 300 1 500 m the injection zone is overlain by a 100 m thick caprock located at 1 200 1 300 m which in one of the studied cases is intersected by a vertical fault the hydraulic mechanical as well as hydromechanical responses caused by the injection are studied this includes the spread of the co2 plume effective stress changes ground surface uplift stress induced permeability changes and mechanical failure analysis the analysis shows that most hydromechanical changes are induced in the lower part of the caprock near its contact with the injection zone whereas the sealing mechanism of the upper part may remain intact despite an injection pressure close to the lithostratic stress value

gravity currents with residual trapping motivated by geological carbon dioxide co 2 storage we present a vertical equilibrium sharp interface model for the migration of immiscible gravity currents with constant residual trapping in a two dimensional confined aquifer the residual acts as a loss term that reduces the current volume continuously in the limit of a horizontal aquifer the interface shape is self similar at early and at late times the spreading of the current and the decay of its volume are governed by power laws at early times the exponent of the scaling law is independent of the residual but at late times it decreases with increasing loss owing to the self similar nature of the current the volume does not become zero and the current continues to spread in the hyperbolic limit the leading edge of the current is given by a rarefaction and the trailing edge by a shock in the presence of residual trapping the current volume is reduced to zero in finite time expressions for the up dip migration distance and the final migration time are obtained comparison with numerical results shows that the hyperbolic limit is a good approximation for currents with large mobility ratios even far from the hyperbolic limit in gently sloping aquifers the current evolution is divided into an initial near parabolic stage with power law decrease of volume and a later near hyperbolic stage characterized by a rapid decay of the plume volume our results suggest that the efficient residual trapping in dipping aquifers may allow co 2 storage in aquifers lacking structural closure if co 2 is injected far enough from the outcrop of the aquifer

buoyant currents arrested by convective dissolution 1 when carbon dioxide co2 dissolves into water the density of water increases this seemingly insubstantial phenomenon has profound implications for geologic carbon sequestration here we show by means of laboratory experiments with analog fluids that the up slope migration of a buoyant current of co2 is arrested by the convective dissolution that ensues from a fingering instability at the moving co2 groundwater interface we consider the effectiveness of convective dissolution as a large scale trapping mechanism in sloping aquifers and we show that a small amount of slope is beneficial compared to the horizontal case we study the development and coarsening of the fingering instability along the migrating current and predict the maximum migration distance of the current with a simple sharp interface model we show that convective dissolution exerts a powerful control on co2 plume dynamics and as a result on the potential of geologic carbon sequestration

on the role of caprock and fracture zones in dispersing gas plumes in the subsurface 1 waste stored in a geological disposal facility can generate gas and depending on the geological environment this gas may migrate into the rock mass here we develop a simplified physical model to describe the initial stages of the dispersal of a gas plume as it rises from such a geological disposal facility located at a depth of many hundreds of meters below the surface typically the plume becomes confined below a low permeability layer and then spreads laterally until reaching a fault or fracture zone when it may continue rising upward since the gas is soluble in the groundwater the gas may partially dissolve as it displaces the groundwater in addition in the generic geology considered herein since the source of gas gradually wanes with time the spreading plume tends to thin out leading to some residual trapping of the gas behind the plume we show that depending on the distance of the fault or fracture zone from the geological disposal facility different fractions of the source gas may be diverted farther upward into the formation rather than being trapped in the original layer this can have an important impact on the subsequent pattern of dispersal of the gas by the groundwater flows which may be key information in any safety assessment

draining viscous gravity currents in a vertical fracture we consider the flow of instantaneous releases of a finite volume of viscous fluid in a narrow vertical fracture or hele shaw cell when there is a still narrower vertical crack in the horizontal base of the cell the predominant motion is over the horizontal surface but fluid also drains through the crack progressively diminishing the volume of the current in the fracture when the crack is shallow on the scale of the current it saturates immediately with the draining fluid in this case we obtain an exact analytical solution for the motion when the crack is deeper and does not saturate immediately we calculate numerically the motion of the fluid in both the fracture and the crack in each case the current advances to a finite run out length and then retreats we describe both phases of the motion and characterize the run out length in terms of the controlling parameters

modeling leakage pathways in subsurface formations fluid drainage through multiple fractures in porous media insights from hele shaw cell experiments abstract arresting the recent observed warming of the earth s climate is a challenge requiring the reduction of anthropogenic emissions of carbon dioxide one option for reducing emissions into the atmosphere is to capture and sequester the released carbon dioxide in geological formations however potential geological storage first requires a risk assessment of carbon dioxide escaping to overlying layers and back to the atmosphere through leakage pathways in the formation this in turn requires an understanding of fluid flow through the leakage pathways in this study the effect of leakage pathways on the flow of a gravity current was investigated using an analogue system a hele shaw cell fluid was introduced through the top edge of the cell and flowed out through one or more holes in its impermeable base the height of the accumulated fluid above the base of the cell at various points along its length and the outflow rate of fluid through the holes was measured this measurement was conducted with varying conditions of the location number and strength of source as well as the location and number of holes at steady state the fluid motion was in accordance with darcy s law for horizontal flow in a long thin current in another set of experiments in which inflow was stopped and the fluid was allowed to drain out of a single open hole the outflow rate was in accordance with darcy s law for one dimensional flow in the vertical direction until the fluid height above the hole fell below a certain limit this threshold height was found to be 1 3 cm which was similar in magnitude to the length of the hole a time series of photographs tracked the flow of colored dye the photographs demonstrated that at steady state the fluid traveled for some distance beyond the hole before draining through it this implies that contaminants may be transported in a formation even beyond an outlet before finally draining out through it the photographs also documented the shape of the interface between the newly introduced dye and the ambient fluid in the cell these shapes were found to be different in a cell placed horizontally versus one placed on an incline suggesting that the geometry and flow conditions in a formation affecting the transport of an injected fluid these findings may be used as a basis for predicting behavior of a fluid injected into a fractured formation in the subsurface

mechanical twinning in quartz shock experiments impact pseudotachylites and fault breccias abstract increasing use of diffraction methods to study preferred orientation of minerals has established that quartz in deformed rocks not only displays characteristic c axis orientation patterns but that there is also generally a distinct difference in the orientation of positive and negative rhombs in the trigonal quartz crystal structure positive and negative rhombs are structurally different and particularly negative rhombs e g 01 1 1 are much stiffer than positive rhombs e g 10 1 1 here we focus on the role of mechanical dauphine twinning under stress as a cause of this difference and illustrate with ebsd measurements ubiquitous twinning in quartz bearing rocks subjected to high stresses characteristic twinning is observed in experimentally shocked sandstones and stishovite bearing quartzites from the vredefort meteorite impact site in south africa similar twinning is documented for quartz associated with pseudotachylites from the santa rosa mylonite zone in southern california whereas quartz in underlying ductile mylonites are more or less twin free it suggests that twinning was produced by local seismic stresses that caused fracture and frictional melting on fault surfaces quartz bearing breccias from the safod san andreas fault observatory at depth drilling project also show evidence of twinning and suggest high seismic stresses in the currently creeping segment of the san andreas fault at parkfield from these observations it appears that dauphine twin microstructures can be diagnostic of high local and transient stresses

error estimation and optimization of gravity surveys1 abstract gravity data are often acquired over long periods of time using different instruments and various survey techniques resulting in data sets of non uniform accuracy as station locations are inhomogeneously distributed gravity values are interpolated on to a regular grid to allow further processing such as computing horizontal or vertical gradients some interpolation techniques can estimate the interpolation error although estimation of the error due to interpolation is of importance it is more useful to estimate the maximum gravity anomaly that may have gone undetected by a survey this is equivalent to the determination of the maximum mass whose gravity anomaly will be undetected at any station location given the data accuracy at each station assuming that the maximum density contrast present in the survey area is known or can be reasonably assumed from a knowledge of the geology the proposed procedure is as follows at every grid node the maximum mass whose gravity anomaly does not disturb any of the surrounding observed gravity values by more than their accuracies is determined a finite vertical cylinder is used as the mass model in the computations the resulting map gives the maximum detection error and as such it is a worst case scenario moreover the map can be used to optimize future gravity surveys new stations should be located at or near map maxima the technique is applied to a set of gravity observations obtained from different surveys made over a period of more than 40 years in the abitibi greenstone belt in eastern canada

bouguer density determination by fractal analysis density values for the bouguer reduction of two gravity data sets from iceland are determined using a new method based on minimization of the roughness of the bouguer anomaly surface the fractal dimension of the surface is used as a gauge of the roughness the analysis shows the size of topographic features supported by crust without isostatic compensation to be 25 to 30 km in southwest iceland and 9 to 10 km inside the active rifting zone the densities selected for these areas are 2490 and 2730kg m3 respectively

buoyant dispersal of co2 during geological storage carbon capture and storage is currently the only technology that may allow significant reductions in co2 emissions from large point sources seismic images of geological co2 storage show the rise of co2 is influenced by horizontal shales the buoyant co2 spreads beneath impermeable barriers until a gap allows its upward migration the large number and small scale of these barriers makes the prediction of the co2 migration path and hence the magnitude of co2 trapping very challenging we show that steady buoyancy dominated flows in complex geometries can be modeled as a cascade of flux partitioning events this approach allows the analysis of two dimensional plume dispersal from a horizontal injection well we show that the plume spreads laterally with height y above the source according to y h 1 2 l where l is the width of the shales and h is their vertical separation the fluid volume below successive shale layers and therefore the magnitude of trapped co2 increase as y h 5 4 above the source so that every additional layer of barriers traps more co2 than the one below upscaling small scale flow barriers by reducing the vertical permeability common in numerical simulations of co2 storage does not capture the dispersion and trapping of the co2 plume by the flow barriers

dynamic recrystallization of quartz under greenschist conditions abstract the sem electron channelling technique has been used to investigate the dynamic recrystallization of three quartz grains representative of optical textures observed in quartzite deformed under greenschist facies conditions the degree of recrystallization has been determined at 10 20 50 and 100 respectively we demonstrate that dynamic recrystallization is a sequential process involving subgrain grain rotation and grainboundary migration it initiates via subgrain polygonization on a single crystal slip system and continues via subgrain and neoblast rotation on several slip systems in the early stages of recrystallization

spatial and temporal evolution of injected co2 at the sleipner field north sea time lapse three dimensional 3d seismic surveys have imaged an accumulation of injected co 2 adjacent to the sleipner field in the north sea basin the changing pattern of reflectivity suggests that co 2 accumulates within a series of interbedded sandstones and mudstones beneath a thick caprock of mudstone nine reflective horizons within the reservoir have been mapped on six surveys acquired between 1999 and 2008 these horizons have roughly elliptical planforms with eccentricities ranging between two and four in the top half of the reservoir horizon areas grow linearly with time in the bottom half horizon areas initially grow linearly for about eight years and then progressively shrink the central portions of deeper reflective horizons dim with time amplitude analysis of horizons above within and below the reservoir show that this dimming is not solely caused by acoustic attenuation instead it is partly attributable to co 2 migration and or co 2 dissemination which reduce the impedance contrast between sandstone and mudstone layers growth characteristics and permeability constraints suggest that each horizon grows by lateral spreading of a gravity current this model is corroborated by the temporal pattern of horizon velocity pushdown beneath the reservoir horizon shrinkage may occur if the distal edge of a co 2 filled layer penetrates the overlying mudstone if the buoyant plume draws co 2 upward or if the effective permeability of deeper mudstone layers increases once interstitial brine has been expelled topographic control is evident at later times and produces elliptical planforms especially toward the top of the reservoir our results show that quantitative mapping and analysis of time lapse seismic surveys yield fluid dynamical insights which are testable shedding light on the general problem of co 2 sequestration

an inverse method for estimating thickness and volume with time of a thin co2 filled layer at the sleipner field north sea migration of co 2 through storage reservoirs can be monitored using time lapse seismic re ection surveys at the sleipner field injected co 2 is distributed throughout nine layers within the reservoir these layers are too thin to be seismically resolvable by direct measurement of the separation between re ections from the top and bottom of each layer here we develop and apply an inverse method for measuring thick ness changes of the shallowest layer our approach combines di erences in traveltime down to a speci c re ection together with amplitude measurements to determine layer thicknesses from time lapse surveys a series of synthetic forward models were used to test the robustness of our inverse approach and to quantify uncertainties in the absence of ambient noise this approach can unambiguously resolve layer thickness if a realistic ambient noise distribution is included layer thicknesses of 1 6 m are accurately retrieved with an uncertainty of 0 5 m we used this approach to generate a thickness map of the shallowest layer the delity of this result was tested using measurements of layer thickness determined from the 2010 broadband seismic survey the calculated volume of co 2 within the shallowest layer increases at a rate that is quadratic in time despite an approximately constant injection rate into the base of the reser voir this result is consistent with a diminished growth rate of the areal extent of underlying layers finally the relationship between caprock topography and layer thickness is explored and potential migration pathways that charge this layer are identi ed

controls on the dissolution of co2 plumes in structural traps in deep saline aquifers 1 sequestration of carbon dioxide in deep saline aquifers has been proposed and investigated as a viable solution to help mitigate carbon emissions from fossil fuels much research has been directed at understanding the transitions of supercritical co2 from being a mobile fluid phase to being trapped by capillarity or dissolved in groundwater such transitions lead to a reduction in mobility of co2 and hence in the risk of leakage to the surface following injection buoyant plumes of co2 migrate updip towards structural traps in the geological strata however some of this co2 may be capillary trapped in pore spaces or dissolved in groundwater en route since co2 saturated groundwater only has a small co2 concentration the dissolution of any large structurally trapped plumes of co2 may be controlled by the availability of unsaturated groundwater in an aquifer of finite vertical extent this may be rate limited by a combination of i the background hydrological flow coupled with ii the slow lateral exchange of relatively dense co2saturated groundwater with unsaturated groundwater in an inclined aquifer this may be controlled by the slow along aquifer component of gravity structurally trapped plumes of co2 may therefore persist for many thousands of years and since they are potentially highly mobile may represent an important contribution to the long term risks associated with co2 sequestration at particular sites

modelling carbon dioxide accumulation at sleipner implications for underground carbon storage an analytical solution to the equations describing the flow of a buoyant fluid released into a porous medium below a horizontal impermeable boundary is used to model the growth of co2 accumulations beneath thin mudstone beds in the utsira sand reservoir at sleipner in the north sea here supercritical co2 has been injected at a rate of 1 mt yr since 1996 and imaged by time lapse seismic data in 1999 2001 and 2002 the co2 rises as a narrow plume and is partially trapped by a number of thin mudstones before reaching the caprock to the reservoir the radii of the individual layers of trapped co2 increase as the square root of time since initiation as predicted by the modelling for constant input flux however apparent negative initiation times for horizons low in the reservoir suggests that net input fluxes for these layers have decreased with time most probably as the spreading layers have increased their leakage rates accumulation of co2 in the layers higher in the reservoir was initiated up to 3 yr after injection started modelling of the thickness profiles across three of the higher layers suggests that their net input fluxes have increased with time the observation that the central thicknesses of the deeper layers have remained approximately constant or have slightly decreased since first imaged in 1999 is consistent with the model predictions that the central thickness is directly proportional to net input flux however estimates of the permeability of the reservoir from the rate of increase of the radii of the co2 accumulations are an order of magnitude less than measured permeabilities on the reservoir sandstone permeabilities estimated from the modelling of layer thickness changes scatter in the same range these discrepancies may arise from 1 approximations in the model not being valid 2 the measured permeabilities not being representative of the permeability for two phase flow on the scale of the reservoir or considered less likely 3 that much less co2 is being stored in the imaged co2 accumulations than estimated from the seismic reflection profiles the most probable cause of the discrepancy is that the relative permeability for the co2 phase is significantly reduced at lower co2 saturations

depth estimation from the scaling power spectrum of potential fields summary depth estimation from potential field power spectra requires a realistic assumption of the statistical properties of the source distributions density and susceptibility distributions in the earth s crust exhibit a long range dependence which is adequately described by scaling random fields with a spectral density proportional to some power of the wavenumber the theoretical power spectrum for a half space model of scaling sources explains the shape of observed power spectra of real potential field data very well minimizing the misfit between the model and the observed power spectrum yields an estimate for the depth to the top of the sources after demonstrating this approach on synthetic magnetic data we reinterpret power spectra of gravity and aeromagnetic data from utah hawaii and saskatchewan finding depth values that differ significantly from earlier interpretations all three power spectra are best explained by source distributions starting at surface level even the power spectrum from an aeromagnetic survey of a sedimentary basin with virtually non magnetic basin fill in the latter case a priori information on the intensity and the scaling exponent of the field caused by the basement had to be included to obtain an approximate estimate of the basin depth in general potential field power spectra are dominated by scaling properties of their source distributions and contain only limited depth information

the effect of a fissure on storage in a porous medium we consider the two dimensional buoyancy driven flow of a fluid injected into a saturated semi infinite porous medium bounded by a horizontal barrier in which a single line sink representing a fissure some distance from the point of injection allows leakage of buoyant fluid our studies are motivated by the geological sequestration of carbon dioxide co2 and the possibility that fissures in the cap rock may compromise the safe long term storage of co2 a theoretical model is presented that accounts for leakage through the fissure using two parameters which characterize leakage driven both by the hydrostatic pressure within the overriding fluid and by the buoyancy of the fluid within the fissure we determine numerical solutions for the evolution of both the gravity current within the porous medium and the volume of fluid that has escaped through the fissure as a function of time a quantity of considerable practical interest is the efficiency of storage which we define as the amount of fluid remaining in the porous medium relative to the amount injected this efficiency scales like t 1 2 at late times indicating that the efficiency of storage ultimately tends to zero we confirm the results of our model by comparison with an analogue laboratory experiment and discuss the implications of our two dimensional model of leakage from a fissure for the geological sequestration of co2

converging gravity currents over a permeable substrate we study the propagation of viscous gravity currents along a thin permeable substrate where slow vertical drainage is allowed from the boundary in particular we report the effect of this vertical fluid drainage on the second kind self similar solutions for the shape of the fluid fluid interface in three contexts i viscous axisymmetric gravity currents converging towards the centre of a cylindrical container ii viscous gravity currents moving towards the origin in a horizontal hele shaw channel with a power law varying gap thickness in the horizontal direction and iii viscous gravity currents propagating towards the origin of a porous medium with horizontal permeability and porosity gradients in power law forms for each of these cases with vertical leakage we identify a regime diagram that characterizes whether the front reaches the origin or not in particular when the front does not reach the origin we calculate the final location of the front we have also conducted laboratory experiments with a cylindrical lock gate to generate a converging viscous gravity current where vertical fluid drainage is allowed from various perforated horizontal substrates the time dependent position of the propagating front is captured from the experiments and the front position is found to agree well with the theoretical and numerical predictions when surface tension effects can be neglected

modelling carbon dioxide sequestration in layered strata motivated by the geological sequestration of carbon dioxide co2 we study the propagation of gravity currents in a porous medium bounded by a thin layer of much lower permeability we formulate a model for drainage assuming that the fluid remains simply connected throughout using this model we examine the propagation of both two dimensional and axisymmetric currents numerically we find that for the fixed flux situation solutions approach a steady state which is described analytically the approach to this final solution depends on both the permeability contrast and thickness of the thin layer and in many cases the current first overshoots before relaxing back to its ultimate steady state finally we examine propagation along multiple thin lower permeability layers as a reduced order model of the plume of co2 currently being injected at sleipner in the north sea

gaussian scaling noise model of seismic reflection sequences evidence from well logs a simple model for reflection series is desirable for two reasons first to have a means of generating model reflection sequences on which to test geophysical data processing techniques specifically methods of predictive deconvolution and second to suggest a way of parameterizing real reflection sequences for classification and further geologic study in this note we wish to discuss a model suggested by hosken 1980 our discussion is founded upon the powerful ideas of self similarity and scaling developed by mandelbrot 1983 and on the paucity of typical lengths in geology typical lengths are common in physics problems for example the length of an organ pipe governs the wavelengths of the notes played when taking photographs geologists include some manmade object to give scale to the photo otherwise it is very hard to tell if say folds are of centimeter size or form whole mountains

some implications of cold co2 injection into deep saline aquifers when co 2 is injected down a well the temperature at the bottom of the well depends on surface conditions heat exchange with the wall of the well and pressure work within the well typically the temperature of the co 2 at the bottom of the well is lower than the local geothermal temperature as this relatively cold co 2 flows into the porous matrix local thermal equilibrium manifests a thermal front behind which the porous matrix and co 2 adjust to the cold injection temperature as the temperature of the injected co 2 increases across the thermal front the co 2 becomes less viscous and less dense in relatively high permeability rock as the flow spreads from the well it becomes buoyancy driven and so at the thermal front the flow adjusts from a deep slow flow to a relatively shallow fast flow the increased depth in the near source cold region has two significant implications first it increases the near source storage potential as more rock is flooded with co 2 but it may also enhance the leakage into the seal rock which occurs in regions where the current is sufficiently deep for the pressure to exceed the capillary entry pressure

using image warping for time lapse image domain wavefield tomography abstracttime lapse seismic data are widely used for monitoring subsurface changes a quantitative assessment of how reservoir properties have changed allows for better interpretation of fluid substitution and fluid migration during processes such as oil and gas production and carbon sequestration full waveform inversion fwi has been proposed as a way to retrieve quantitative estimates of subsurface properties through seismic waveform fitting however for some monitoring systems the offset range versus depth of interest is not large enough to provide information about the low wavenumber component of the velocity model we evaluated an image domain wavefield tomography idwt method using the local warping between baseline and monitor images as the cost function this cost function is sensitive to volumetric velocity anomalies and it is capable of handling large velocity changes with very limited acquisition apertures where traditional fwi fails we described the theory and workflow of our method la

streamline based simulation of carbon dioxide storage in a north sea aquifer 1 we model carbon dioxide co2 storage into a deep north sea aquifer using streamline based simulation we assume incompressible flow of liquid like co2 and aqueous phases we simulate dissolution of co2 and a rate limited precipitation reaction advective transport and reactions are solved along streamlines while dispersion and flow due to gravity segregation of the phases are solved on the underlying grid geological storage is modeled on a one million cell model the distribution of co2 after injection is dominated by advective transport due to multiphase flow and co2 moves preferentially through high permeability channels without reaction the regional groundwater flow causes the co2 to continue to migrate until it reaches residual saturation where it continues slowly to dissolve precipitation leads to a decrease in porosity and permeability while co2 is stored in the solid phase the storage efficiency is low around 2 because of aquifer heterogeneity

fractal analysis of three dimensional spatial distributions of earthquakes with a percolation interpretation although many studies have shown that faults and fractures are self similar over a large range of scales none have tested the fault structure for self similarity in three dimensions in this study earthquake hypocentral locations in central and southern california were used to illuminate three dimensional 3 d fault structures for which we measured the fractal capacity dimension d0 3 d hypocentral distributions from the joshua tree big bear and upland aftershock sequences as well as background seismicity at parkfield were found to be fractal where d0 3 d increased with increasing event density asymptotically approaching a stable value the joshua tree data set stabilized at d0 3 d 1 92 0 02 the parkfield data set asymptotically approached d0 3 d 1 82 and the big bear data set approached d0 3 d 2 01 as a test of the effects of location errors upon the measured value of d0 3 d the upland aftershock data were located with both the southern california hadley and kanamori 1977 h k velocity model and the more accurate hauksson and jones 1991 h j velocity model events located with the h k model asymptotically approached d0 3 d 2 07 and events located with the h j model approached d0 3 d 1 79 suggesting that improved hypocentral locations may decrease the measured fractal dimension one interpretation of our results of d0 3 d 2 0 for all of the hypocentral data is that earthquakes only occur on the percolation backbone of a fault network i e the active part of the network that accommodates finite strain deformation sahimi et al 1993 we show that a percolation model that allows for healing of previously broken bonds is consistent with this interpretation

buoyancy driven flow from a waning source through a porous leaky aquifer we develop a series of models to describe the migration of a buoyant fluid through a layered permeable rock following release from a localized waning source in particular if the fluid is injected into a high permeability layer bounded above by a layer of lower permeability a plume migrates along the interface with some draining into the low permeability layer if the current is sufficiently deep to overcome the capillary entry pressure we show the motion of the fluid is controlled by a number of key factors with the dominant dimensionless numbers being the time scale for the source to decay compared to the time scale for draining through the low permeability layer the residual saturation of the gas and water in the formation as that phase is displaced by the other phase and the capillary entry pressure as measured by the critical depth of the current required for draining as compared to the initial depth of the current simplified analytical models are presented to illustrate some of the key controls on and transitions in the flow and the models are used to explore leakage and trapping prior to flow reaching a fault zone

similarity solutions for fluid injection into confined aquifers fluid injection into the deep subsurface such as injection of carbon dioxide co 2 into deep saline aquifers often involves two fluid flow in confined geological formations similarity solutions may be derived for these problems by assuming that a sharp interface separates the two fluids by imposing a suitable no flow condition along both the top and bottom boundaries and by including an explicit solution for the pressure distribution in both fluids when the injected fluid is less dense and less viscous than the resident fluid as is the case for co 2 injection into a resident brine gravity override produces a fluid flow system that is captured well by the similarity solutions the similarity solutions may be extended to include slight miscibility between the two fluids as well as compressibility in both of the fluid phases the solutions provide the location of the interface between the two fluids as well as drying fronts that develop within the injected fluid applications to cases of supercritical co 2 injection into deep saline aquifers demonstrate the utility of the solutions and comparisons to solutions from full numerical simulations show the ability to predict the system behaviour

dauphine twinning as evidence for an impact origin of preferred orientation in quartzite an example from vredefort south africa large meteorite impacts cause great changes to geologic features on earth ranging from cratering and ejecta blankets to phase transformations in minerals in this report we describe the effect of a shock wave on the crystallographic orientation of quartz crystals in quartzite from the vredefort impact site in south africa preferred orientation of bulk quartzite samples was measured by time of flight neutron diffraction with a random distribution of c axes a weak but distinct difference between the orientation distribution of positive and negative rhombs was observed as illustrated with pole figures and inverse pole figures results for the natural vredefort sample are compared with deformation

benchmarking of vertically integrated co2 flow simulations at the sleipner field north sea abstract numerical modeling plays an essential role in both identifying and assessing sub surface reservoirs that might be suitable for future carbon capture and storage projects accuracy of flow simulations is tested by benchmarking against historic observations from on going co2 injection sites at the sleipner project located in the north sea a suite of time lapse seismic reflection surveys enables the three dimensional distribution of co2 at the top of the reservoir to be determined as a function of time previous attempts have used darcy flow simulators to model co2 migration throughout this layer given the volume of injection with time and the location of the injection point due primarily to computational limitations preventing adequate exploration of model parameter space these simulations usually fail to match the observed distribution of co2 as a function of space and time to circumvent these limitations we develop a vertically integrated fluid flow simulator that is based upon the theory of topographically controlled porous gravity currents this computationally efficient scheme can be used to invert for the spatial distribution of reservoir permeability required to minimize differences between the observed and calculated co2 distributions when a uniform reservoir permeability is assumed inverse modeling is unable to adequately match the migration of co2 at the top of the reservoir if however the width and permeability of a mapped channel deposit are allowed to independently vary a satisfactory match between the observed and calculated co2 distributions is obtained finally the ability of this algorithm to forecast the flow of co2 at the top of the reservoir is assessed by dividing the complete set of seismic reflection surveys into training and validation subsets we find that the spatial pattern of permeability required to match the training subset can successfully predict co2 migration for the validation subset this ability suggests that it might be feasible to forecast migration patterns into the future with a degree of confidence nevertheless our analysis highlights the difficulty in estimating reservoir parameters away from the region swept by co2 without additional observational constraints

potential field power spectrum inversion for scaling geology we propose a method for inverting the power spectrum of gravity and magnetic data the method is demonstrated on aeromagnetic and bore well data from the german continental deep drilling project ktb density and susceptibility distributions in the earth s crust exhibit scaling behavior with power spectra proportional to f where f is the wavenumber and is the scaling exponent of the source distribution we model the sources of the potential field by a random function with scaling properties defined on a half space with its top at a specified depth beneath the observation plane comparing the theoretical power spectrum for this model with the power spectrum of the measured data we obtain the best values for the depth to source and the scaling exponent as a global minimum of the misfit function despite the simplicity of the model it offers a new understanding of the factors influencing the shape of the potential field power spectrum in particular the low wavenumber part of the power spectrum can be dominated by the scaling properties of the source distribution and not by the depth to some kind of basement the scaling exponent of the field varies with the type of surface geology the question of whether the scaling exponent can actually be used to identify different types of geology gives an interesting new aspect to power spectrum inversion

on the slow draining of a gravity current moving through a layered permeable medium we examine the gravitational dispersal of dense fluid through a horizontal permeable layer which is separated from a second underlying layer by a narrow band of much lower permeability we derive a series of analytical solutions which describe the propagation of the fluid through the upper layer and the draining of the fluid into the underlying region the model predicts that the current initially spreads according to a self similar solution however as the drainage becomes established the spreading slows and in fact the fluid only spreads a finite distance before it has fully drained into the underlying layer we examine the sensitivity of the results to the initial conditions through numerical solution of the governing equations we find that for sources of sufficiently large initial aspect ratio defined as the ratio of height to length the solution converges rapidly to the initially self similar regime for longer and shallower initial source conditions this convergence does not occur but we derive estimates for the run out length of the current which compare favourably with our numerical data we also present some preliminary laboratory experiments which support the model

co 2 breakthrough pressure and permeability for unsaturated low permeability sandstone of the ordos basin abstract with rising threats from greenhouse gases capture and injection of co 2 into suitable underground formations is being considered as a method to reduce anthropogenic emissions of co 2 to the atmosphere as the injected co 2 will remain in storage for hundreds of years the safety of co 2 geologic sequestration is a major concern the low permeability sandstone of the ordos basin in china is regarded as both caprock and reservoir rock so understanding the breakthrough pressure and permeability of the rock is necessary because part of the pore volume experiences a non wetting phase during the co 2 injection and migration process the rock may be in an unsaturated condition and if accidental leakage occurs co 2 will migrate up into the unsaturated zone in this study breakthrough experiments were performed at various degrees of water saturation with five core samples of low permeability sandstone obtained from the ordos basin the experiments were conducted at 40 c and pressures of 8 mpa to simulate the geological conditions for co 2 sequestration the results indicate that the degree of water saturation and the pore structure are the main factors affecting the rock breakthrough pressure and permeability since the influence of calcite dissolution and clay mineral swelling during the saturation process is excluded increasing the average pore radius or most probable pore radius leads to a reduction in the breakthrough pressure and an increase by several orders of magnitude in scco 2 effective permeability in addition the breakthrough pressure rises and the scco 2 effective permeability decreases when the water saturation increases however when the average pore radius is greater than 0 151 m the degree of water saturation will has a little effect on the breakthrough pressure on this foundation if the most probable pore radius of the core sample reaches 1 760 m the breakthrough pressure will not be impacted by the increasing water saturation we establish correlations between 1 the breakthrough pressure and average pore radius or most probable pore radius 2 the breakthrough pressure and scco 2 effective permeability 3 the breakthrough pressure and water saturation and 4 the scco 2 effective permeability and water saturation this study provides practical information for further studies of co 2 sequestration as well as the caprock evaluation

linking microseismic event observations with geomechanical models to minimise the risks of storing co2 in geological formations abstract for carbon capture and storage ccs in geological formations to be scientifically viable we must be able to model and monitor the effects of geomechanical deformation on the integrity of the caprock excess deformation may open fractures providing pathways for co2 leakage from the reservoir an acceptable geomechanical model must provide a good match with field observations microseismic activity is a direct manifestation of mechanical deformation so it can be used to constrain geomechanical models the aim of this paper is to develop the concept of using observations of microseismic activity to help ground truth geomechanical models microseismic monitoring has been ongoing at the weyburn co2 storage and monitoring project since 2003 we begin this paper by presenting these microseismic observations less than 100 events have been recorded documenting a low rate of seismicity most of the events are located close to nearby producing wells rather than the injection well a pattern that is difficult to interpret within the conventional framework for injection induced seismicity many events are located in the overburden without geomechanical simulation it is difficult to assess what these observations mean for the integrity of the storage formation to address these uncertainties we generate numerical geomechanical models to simulate the changes in stress induced by co2 injection and use these models to predict the generation of microseismic events and seismic anisotropy the initial geomechanical model that we generate using material properties based on laboratory core measurements does not provide a good match with either event locations or s wave splitting measurements made on the microseismic events we find that an alternative model whose reservoir is an order of magnitude softer than lab core sample measurements provides a much better match with observation as it leads shear stresses to increase above the production wells promoting microseismicity in these areas and generates changes in effective horizontal stresses that match well with s wave splitting observations this agreement between geophysical observations and a softer than lab measurements reservoir model highlights the difficulties encountered in upscaling lab scale results there is a strong need to link geomechanical models with observable manifestations of deformation in the field such as induced seismicity for calibration only then can we accurately assess the risks of leakage generated by mechanical deformation

geological carbon storage storage of the carbon dioxide that is produced by burning fossil fuels is one way to avoid the damaging consequences of climate change a range of observations suggests that geological carbon storage is much less risky than unabated carbon emissions to the atmosphere

capillary entry pressure and the leakage of gravity currents through a sloping layered permeable rock we examine the motion of a buoyant fluid injected into a water saturated porous rock as it spreads along a thin inclined low permeability barrier we account for leakage of the fluid across the barrier once the current is sufficiently deep so that the pressure exceeds the capillary threshold we show that at some distance from the source the pressure decreases below this threshold and all the remaining flux spreads laterally along the barrier we examine the controls on the partitioning of the flow between the draining flux and the laterally spreading flux and also the controls on the lateral extent of the draining region for the case of an instantaneous release and a maintained release of fluid we consider the implications of our work for the dispersal of co 2 plumes which may be sequestered in deep saline aquifers

status of co2 storage in deep saline aquifers with emphasis on modeling approaches and practical simulations carbon capture and storage ccs is the only viable technology to mitigate carbon emissions while allowing continued large scale use of fossil fuels the storage part of ccs involves injection of carbon dioxide captured from large stationary sources into deep geological formations deep saline aquifers have the largest identified storage potential with estimated storage capacity sufficient to store emissions from large stationary sources for at least a century this makes ccs a potentially important bridging technology in the transition to carbon free energy sources injection of co2 into deep saline aquifers leads to a multicomponent multiphase flow system in which geomechanics geochemistry and nonisothermal effects may be important while the general system can be highly complex and involve many coupled nonlinear partial differential equations the underlying physics can sometimes lead to important simplifications for example the large density difference between injected co2 and brine may lead to relatively fast buoyant segregation making an assumption of vertical equilibrium reasonable such simplifying assumptions lead to a range of simplified governing equations whose solutions have provided significant practical insights into system behavior including improved estimates of storage capacity easy to compute estimates of co2 spatial migration and pressure response and quantitative estimates of leakage risk when these modeling studies are coupled with observations from well characterized injection operations understanding of the overall system behavior is enhanced significantly this improved understanding shows that while economic and policy challenges remain co2 storage in deep saline aquifers appears to be a viable technology and can contribute substantially to climate change solutions

gas invasion into water saturated unconsolidated porous media implications for gas hydrate reservoirs abstract gas transport through unconsolidated porous media has been a recent focus in studies of hydrate reservoir dynamics and is important in numerous other geophysical contexts we conduct experiments to examine the controls on gas invasion through sieved sands that are suspended within the water column as air is injected slowly from below we monitor the thickness of the free gas column that accumulates beneath the sand to gauge the difference between the gas and liquid pressure or overpressure just prior to gas invading and passing through the sand layer comparing the threshold overpressures for initial gas invasion using different particle sizes our experiments demonstrate the control of capillary forces when the effective stress supported by particle contacts is sufficiently large at lower effective stresses smaller sediment overburdens we observe gas invasion before the capillary controlled threshold overpressure is reached these results demonstrate a transition from capillary invasion at high effective stresses to fracture dominated invasion at low effective stresses moreover video documentation reveals evidence for transitions from capillary to fracture dominated behavior within single invasion episodes as the rising gas nears the sediment surface after the initial invasion episode in each experiment we infer that observed reductions in the overpressure needed for subsequent invasions are caused by bubble break up and retention within the sediment column these experiments highlight the importance of effective stress and pore size in determining the mode of gas passage through unconsolidated sediments we discuss the implications of our findings for the dynamics of gas hydrate reservoirs and other geophysical systems

the effect of drainage on the capillary retention of co2 in a layered permeable rock buoyant plumes of co 2 spreading through water saturated permeable rock bounded by layers of lower permeability tend to spread laterally as they advance they may gradually leak through fractures or discontinuities in the lower permeability boundary leading to a gradual waning of the original plume and dispersal of co 2 higher in the formation with a finite release of co 2 the trailing edge of the plume recedes with time and capillary forces tend to trap a fraction of this co 2 within the pore space this also leads to a gradual waning of the plume with time and limits the mass of co 2 which can leak through the boundary and rise higher into the formation we explore the balance between these two effects and calculate some of the controls on the fraction of the co 2 plume which becomes trapped within the original layer of rock

the intrusion of fluid mechanics into geology during the last few years a number of fluid dynamicists often in collaboration with geologists have been investigating problems suggested by geology and creating the new field of geological fluid mechanics the research has uncovered many new fluid mechanical phenomena and the studies have ranged widely over theoretical contributions laboratory experiments and confirmatory field observations this paper presents an overview of part of the field with a strong emphasis on the novel fluid mechanical concepts the presentation includes discussion of some of the effects of cooling and crystallization and the resulting convective motions the flow of gravity currents at both high and low reynolds number and the erosion of solid retaining walls by hot turbulent flows

on the flow of buoyant fluid injected into a confined inclined aquifer we study the dispersal of a plume of incompressible buoyant fluid injected into a confined sloping aquifer which has an outflow at a single fault which may be up dip up slope or down dip down slope from the point of injection we develop a long time asymptotic solution for the motion of the injected fluid we show that for the case in which the outflow fault is up dip from the point of injection there is a critical injection rate above which the injected fluid floods the full depth of the aquifer and we show that for the case in which the outflow fault is down dip from the point of injection there is a critical injection rate below which all injected fluid initially flows up dip our analysis leads to expressions for the lateral extent of the injected fluid as a function of time and we consider the implications of the model for the dispersal of supercritical carbon dioxide injected into deep saline aquifers the work also indicates that the geometry of the system may have a significant effect on i the total volume of carbon dioxide which it is possible to sequester in a faulted aquifer and ii the interpretation of the dispersed position of any injected tracers

fractal geometry in the san andreas fault system it has been noted that the spatial distribution of earthquakes and the mode of strain release in the san andreas fault system is related to the complexity of fault geometry because of their rough appearance over many length scales faults can be regarded as fractal surfaces direct estimates of fractal dimension d of portions of the san andreas fault system between the northern gabilan range and the salton sea including the postulated extent of the great 1857 fort tejon earthquake are obtained from measured fault lengths analogous to the lengths of coastlines as discussed by mandelbrot regions characterized by complicated fault geometry are associated with larger values of d based on fault traces mapped at a scale of 1 750 000 d is 1 3 for this reach of the fault defined as a 30 km wide band about a main fault trace for that part near parkfield which could be associated with the nucleation of the 1857 earthquake d is 1 1 at this same scale d is 1 4 for the san andreas and related faults near san bernardino where the 1857 rupture stopped compared to 1 2 for the san andreas san juan fault segments near the point of arrest of the 1966 parkfield earthquake at finer map scales 1 24 000 and 1 62 500 critical lengths of 500 m and 1 km are identified which might relate to the extent of off san andreas fault offsets the critical lengths also suggest that fault geometry is not self similar if this fractal geometry persists through the seismic cycle it may be possible to use a quantitative measure of complexity to explain the occurrence of great and characteristic earthquakes along a given reach of fault

fluid drainage from the edge of a porous reservoir we report theoretical and experimental studies to describe buoyancy driven fluid drainage from a porous medium for configurations where the fluid drains from an edge we first study homogeneous porous systems to investigate the influence of heterogeneities we consider the case where the permeability varies transverse to the flow direction exemplified by a v shaped hele shaw cell finally we analyse a model where both the permeability and the porosity vary transverse to the flow direction in each case a self similar solution for the shape of these gravity currents is found and a power law behaviour in time is derived for the mass remaining in the system laboratory experiments are conducted in homogeneous and v shaped hele shaw cells and the measured profile shapes and the mass remaining in the cells agree well with our model predictions our study provides new insights into drainage processes such as may occur in a variety of natural and industrial activities including the geological storage of carbon dioxide

a geochemical clogging model with carbonate precipitation rates under hydrothermal conditions abstract a step wise numerical calculation method was developed to provide predictions of when and where carbonate deposits might be found through reservoirs during co 2 sequestration flow experiments through porous media using a supersaturated carbonate fluid were also performed in order to observe flow rates in order to evaluate precipitation rates and permeability change in the formation calculated flow rates based on the proposed geochemical clogging model were compared with the experimentally observed data both high and low temperature cases were studied to understand how hydrothermal conditions can affect precipitation rates of carbonate according to chemical kinetics growth rates of minerals are generally proportional to the saturation index s i that depends on temperature thus a supersaturated fluid has the advantage of improving the filtration and the amount of c fixation however when the ratio of filtration coefficient to pore fluid velocity u increases the permeability around the injection point tends to be significantly reduced by carbonate accumulation and thus this might result in insufficient injection of co 2 therefore it is essential to understand how to control both and u so that the precipitation of carbonate can be located as far away from the inlet as possible

dynamic recrystallization near the brittle plastic transition in naturally and experimentally deformed quartz aggregates the electron backscattering diffraction technique ebsd was used to analyze bulging recrystallization microstructures from naturally and experimentally deformed quartz aggregates both of which are characterized by porphyroclasts with finely serrated grain boundaries and grain boundary bulges set in a matrix of very fine recrystallized grains for the tonale mylonites we investigated a temperature range of 300 380 c 0 25 gpa confining pressure a flow stress range of 0 1 0 2 gpa and a strain rate of 10 13 s 1 were estimated experimental samples of black hills quartzite were analyzed which had been deformed in axial compression at 700 c 1 2 1 5 gpa confining pressure a flow stress of 0 3 0 4 gpa a strain rate of 10 6 s 1 and to 44 to 73 axial shortening using orientation imaging we investigated the dynamic recrystallization microstructures and discuss which processes may contribute to their development our results suggest that several deformation processes are important for the dismantling of the porphyroclasts and the formation of recrystallized grains grain boundary bulges are not only formed by local grain boundary migration but they also display a lattice misorientation indicative of subgrain rotation dynamic recrystallization affects especially the rims of host porphyroclasts with a hard orientation i e with an orientation unsuitable for easy basal slip in addition dauphine twins within porphyroclasts are preferred sites for recrystallization we interpret large misorientation angles in the experimental samples which increase with increasing strain as formed by the activity of fluid assisted grain boundary sliding

leakage from gravity currents in a porous medium part 2 a line sink we consider the propagation of a buoyancy driven gravity current in a porous medium bounded by a horizontal impermeable boundary the current is fed by a constant flux injected at a point and leaks through a line sink at a distance from the injection point this is an idealized model of how a fault in a cap rock might compromise the geological sequestration of carbon dioxide the temporal evolution of the efficiency of storage defined as the instantaneous ratio of the rate at which fluid is stored without leaking to the rate at which it is injected is of particular interest we show that the efficiency of storage decays like t 2 5 for times t that are long compared with the time taken for the current to reach the fault this algebraic decay is in contrast to the case of leakage through a circular sink neufeld et al j fluid mech vol 2010 where the efficiency of storage decays more slowly like 1 ln t the implications of the predicted decay in the efficiency of storage are discussed in the context of geological sequestration of carbon dioxide using parameter values typical of the demonstration project at sleipner norway we show that the efficiency of storage should remain greater than 90 on a time scale of millennia provided that there are no significant faults in the cap rock within about 12 km of the injection site

topographic controls on gravity currents in porous media we present a theoretical and experimental study of the propagation of gravity currents in porous media with variations in the topography over which they flow motivated in part by the sequestration of carbon dioxide in saline aquifers we consider cases where the height of the topography slopes upwards in the direction of the flow and is proportional to the nth power of the horizontal distance from a line or point source of a constant volumetric flux in two dimensional cases with n 1 2 the current evolves from a self similar form at early times when the effects of variations in topography are negligible towards a late time regime that has an approximately horizontal upper surface and whose evolution is dictated entirely by the geometry of the topography for n 0 result in a transition from an early time similarity solution towards a topographically controlled regime that has an approximately horizontal free surface we also analyse the evolution over topography that can vary with different curvatures and topographic exponents between the two horizontal dimensions finding that the flow transitions towards a horizontally topped regime at a rate which depends strongly on the ratio of the curvatures along the principle axes finally we apply our mathematical solutions to the geophysical setting at the sleipner field concluding that topographic influence is unlikely to explain the observed non axisymmetric flow

gas water rock interactions in frio formation following co2 injection implications for the storage of greenhouse gases in sedimentary basins to investigate the potential for the geologic storage of co2 in saline sedimentary aquifers 1600 t of co2 were injected at 1500 m depth into a 24 m thick sandstone section of the frio formation a regional brine and oil reservoir in the u s gulf coast fluid samples obtained from the injection and observation wells before co2 injection showed a na cacl type brine with 93 000 mg l total dissolved solids tds at near saturation with ch4 at reservoir conditions following co2 breakthrough samples showed sharp drops in ph 6 5 5 7 pronounced increases in alkalinity 100 3000 mg l as hco3 and fe 30 1100 mg l and significant shifts in the isotopic compositions of h2o dissolved inorganic carbon dic and ch4 geochemical modeling indicates that brine ph would have dropped lower but for the buffering by dissolution of carbonate and iron oxyhydroxides this rapid dissolution of carbonate and other minerals could ultimately create pathways in the rock seals or well cements for co2 and brine leakage dissolution of minerals especially iron oxyhydroxides could mobilize toxic trace metals and where residual oil or suitable organics are present the injected co2 could also mobilize toxic organic compounds environmental impacts could be major if large brine volumes with mobilized toxic metals and organics migrated into potable groundwater the d 18 o values for brine and co2 samples indicate that supercritical co2 comprises 50 of pore fluid volume 6 mo after the end of injection postinjection sampling coupled with geochemical modeling indicates that the brine gradually will return to its preinjection composition

curie temperature depth estimation using a self similar magnetization model summary the earth s crust is magnetized down to the curie temperature depth at about 10 to 50 km this limited depth extent of the crustal magnetization is discernible in the power spectra of magnetic maps of south africa and central asia at short wavelengths the power increases as rapidly towards longer wavelengths as expected for a self similar magnetized crust with unlimited depth extent above wavelengths of about 100 km the power starts increasing less rapidly indicating the absence of deep seated sources to quantify this effect we derive the theoretical power spectrum due to a slab carved out of a self similar magnetization distribution this model power spectrum matches the power spectra of south africa and central asia for a self similarity parameter of 4 and curie temperature depths of 15 to 20 km

the effect of dauphine twinning on plastic strain in quartz we present an electron backscatter diffraction analysis of five quartz porphyroclasts in a greenschist facies t 300 400 c granitoid protomylonite from the arolla unit of the nw alps mechanical dauphine twinning developed pervasively during the incipient stage of deformation within two porphyroclasts oriented with a negative rhomb plane z almost orthogonal to the compression direction z twin orientation twinning was driven by the anisotropy in the elastic compliance of quartz and resulted in the alignment of the poles of the planes of the more compliant positive rhomb r nearly parallel to the compression direction r twin orientation in contrast we report the lack of twinning in two porphyroclasts already oriented with one of the r planes orthogonal to the compression direction one twinned porphyroclast has been investigated with more detail it shows the localization of much of the plastic strain into discrete r twins as a consequence of the higher amount of elastic strain energy stored by r twins in comparison to z twins the presence of dauphine twins induced a switch in the dominant active slip systems during plastic deformation from basal regions without twinning to and pervasively twinned regions dynamic recrystallization is localized along an r twin and occurred dominantly by progressive subgrain rotation with a local component of bulging recrystallization part of the recrystallized grains underwent rigid body rotation approximately about the bulk vorticity axis which accounts for the development of large misorientation angles the recrystallized grain size piezometer for quartz yields differential stress of 100 mpa the comparison of this palaeostress estimate with literature data suggests that mechanical dauphine twinning could have a potential use as palaeopiezometer in quartz bearing rocks

buoyancy driven flow in a confined aquifer with a vertical gradient of permeability we examine the injection of fluid of one viscosity and density into a horizontal permeable aquifer initially saturated with a second fluid of different viscosity and density the novel feature of the analysis is that we allow the permeability to vary vertically across the aquifer this leads to recognition that the interface may evolve as either a rarefaction wave that spreads at a rate proportional to t a shock like front of fixed length or a mixture of shock like regions and rarefaction wave type regions the classical solutions in which there is no viscosity ratio between the fluids and in which the formation has constant permeability lead to an interface that spreads laterally at a rate proportional to t 1 2 however these solutions are unstable to cross layer variations in the permeability owing to the vertical shear which develops in the flow causing the structure of the interface to evolve to the rarefaction wave or shock like structure in the case that the viscosities of the two fluids are different it is possible that the solution involves a mixture of shock like and rarefaction type structures as a function of the distance above the lower boundary using the theory of characteristics we develop a regime diagram to delineate the different situations we consider the implications of such heterogeneity for the prediction of front locations during text co 2 sequestration if we neglect the permeability fluctuations the model always predicts rarefaction type solutions while even modest changes in the permeability across a layer can introduce shocks this difference may be very significant since it leads to the text co 2 plume occupying a greater fraction of the pore space between the injector and the leading edge of the text co 2 front in a layer of the same mean permeability this has important implications for estimates of the fraction of the pore space that the text co 2 may access

localizing co2 at sleipner seismic images versus p wave velocities from waveform inversion abstractthe presence of injected co2 in the utsira sand at the sleipner site norway is associated with a high negative p wave velocity anomaly that is a low postinjection velocity and a strong seismic response time lapse seismic imaging of co2 injection at sleipner is thus a viable monitoring tool of the injected co2 the work flow usually involves conventional seismic processing including stacking and results in seismic images multiple reflections interference effects such as tuning and the velocity pushdown effect due to co2 injection render these seismic images ambiguous in terms of the localization and the quantification of the co2 in the utsira sand nonetheless seismic images often form the basis for analyses that aim to quantify the injected co2 we employed elastic 2d full waveform inversion to invert prestack seismic sleipner data from preinjection 1994 and postinjection 1999 and compared the resulting postinjection p wave velocity model with the corresponding seismic image we fou

chrysotile dissolution rates implications for carbon sequestration abstract serpentine minerals e g chrysotile are a potentially important medium for sequestration of co2 via carbonation reactions the goals of this study are to report a steady state far from equilibrium chrysotile dissolution rate law and to better define what role serpentine dissolution kinetics will have in constraining rates of carbon sequestration via serpentine carbonation the steady state dissolution rate of chrysotile in 0 1 m nacl solutions was measured at 22 c and ph ranging from 2 to 8 dissolution experiments were performed in a continuously stirred flow through reactor with the input solutions pre equilibrated with atmospheric co2 both mg and si steady state fluxes from the chrysotile surface and the overall chrysotile flux were regressed and the following empirical relationships were obtained f mg 0 22 ph 10 02 f si 0 19 ph 10 37 f chrysotile 0 21 ph 10 57 where fmg fsi and fchrysotile are the log10 mg si and molar chrysotile fluxes in mol m2 s respectively element fluxes were used in reaction path calculations to constrain the rate of co2 sequestration in two geological environments that have been proposed as potential sinks for anthropogenic co2 carbon sequestration in chrysotile tailings at 10 c is approximately an order of magnitude faster than carbon sequestration in a serpentinite hosted aquifer at 60 c on a per kilogram of water basis a serpentinite hosted aquifer however provides a larger sequestration capacity the chrysotile dissolution rate law determined in this study has important implications for constraining potential rates of sequestration in serpentinite hosted aquifers and under accelerated sequestration scenarios in mine tailings

the influence of capillary effects on the drainage of a viscous gravity current into a deep porous medium the drainage of a viscous gravity current into a deep porous medium driven by both the gravitational and capillary forces is considered in two steps we first study the one dimensional case where a layer of fluid drains vertically into an infinitely deep porous medium we determine a transition from the capillary driven regime to the gravity driven regime as time proceeds second we solve the coupled spreading and drainage problem there are no self similar solutions of the problem for the entire time period so asymptotic analyses are developed for the height depth and front location in both the early time and the late time periods in addition we present numerical results of the governing partial differential equations which agree well with the self similar solutions in the appropriate asymptotic limits

fitting straight lines and planes with an application to radiometric dating conventional practice in geochronology is to fit a straight line or isochron to data consisting of two isotopic ratios by a method e g that of 1 2 or perhaps the more modern version of titterington and haliday 1979 that takes into account that fact that both ratios are measured with error in this paper we use matrix algebra to lay out a general method for fitting linear relations between any number of variables all subject to errors with known variances and covariances and the well known newton raphson method to do the optimization this leads to a good computational algorithm which may also be used e g to check whether coefficients in several linear relations are the same in many fields of science one needs to fit linear relations so our method is of wide utility its use is in no way restricted to radiometric dating

leakage from inclined porous reservoirs we investigate the effect of localized leakage on the storage of buoyant fluids in inclined porous reservoirs with application to the geological storage of co 2 we find that once the current has propagated some distance beyond the point of leakage its profile becomes steady in time save for the nose which advances at a constant speed crucially this steady state implies that the efficiency of storage defined as the instantaneous proportion of the injected fluid that does not leak tends to a finite value this contrasts with previous studies of localized leakage in horizontal reservoirs which found that the efficiency of storage tends to zero at late times we analyse the steady state efficiency and the time scales of evolution for a leakage point located either upslope or downslope of the injection point using analytical and numerical methods these findings are verified by model laboratory experiments finally we consider the implications of our results for the geological storage of co 2 under sloping cap rocks compromised by a fracture or fissure

heave of a surficial rock layer due to pressures generated by injected fluids 1 this paper examines the problem of heave developed in a surface or caprock layer of a storage formation due to the injection of pressurized fluids at depth the problem is of interest to the deep geologic disposal of hazardous wastes and the geologic sequestration of carbon dioxide by injection the paper develops a mathematical model that assumes elastic behavior of the storage medium while the surface rock layer is modeled as a thin plate elastic solutions are developed for the axisymmetric flexure deflections of the surface layer during internal pressurization over a flat circular region the model provides a convenient tool for the preliminary estimation of the influence of injection pressures on the integrity of surface rock layers