critically stressed fractures analysis of the shaikan field kurdistan region of iraq the shaikan field is a large producing oil field in the kurdistan region of iraq it consists of multiple fractured reservoirs consisting of limestones calcareous sandstones and mudstones the surrounding tectonic terrane is situated in the seismically active zagros taurus orogenic zone where present day stresses are high the regional stresses are found to impose conditions that satisfy failure along reservoir bound fractures suggesting that a significant proportion of fractures are likely to be critically stressed the in situ maximum principal stress magnitudes are estimated using three methods namely the tensile and compressive strengths of reservoir rock and leak off test lot data stress field orientations are determined from wellbore image log data which are used to interpret wellbore breakouts and the associated induced tensile fractures reservoir pressure has declined since production started and poroelastic responses have been assessed and used to estimate the present day stress state and the criticality of those fractures that are most likely to fail or slip although a conventional approach has been used the present authors argue that a new approach to stress response with changing pore pressure should be taken unlike the previous theory of criticality in which a reduction in pore pressure is considered to lead to a stabilization of the fracture network the present study suggests that a system may remain critically stressed regardless of pressure decline thematic collection this article is part of the the geology of fractured reservoirs collection available at https www lyellcollection org cc the geology of fractured reservoirs

post processing removal of correlated errors in grace data 1 gravity fields produced by the gravity recovery and climate experiment grace satellite mission require smoothing to reduce the effects of errors present in short wavelength components as the smoothing radius decreases these errors manifest themselves in maps of surface mass variability as long linear features generally oriented north to south i e stripes the presence of stripes implies correlations in the gravity field coefficients here we examine the spectral signature of these correlated errors and present a method to remove them finally we apply the filter to a model of surface mass variability to show that the filter has relatively little degradation of the underlying geophysical signals we seek to recover

present day stress orientations adjacent to active strike slip faults california and sumatra present day stress directions interpreted from well bore breakouts adjacent to two crastal scale active strike slip faults the san andreas fault in california and the great sumatran fault in sumatra indicate that the maximum horizontal stress direction sh is oriented at a high angle 70 90 to both faults the regionally defined stress fields spanning these faults show that adjacent young or actively growing folds have formed orthogonal to sh and are therefore in the thrust fault orientation these observations indicate a decoupling of the strike slip and compressional components of the deformation within these broadly transpressive zones borehole breakouts in 118 wells in western california indicate a regionally consistent stress pattern with sh generally oriented ne sw nearly perpendicular 80 90 to the strike of the san andreas fault the orientation of sh nearly perpendicular to the san andreas fault implies low shear stress on the fault and is consistent with geological interpretations of the coast and transverse ranges indicating active compressional deformation fault plane solutions for recent dip slip style earthquakes principal stress directions determined from inversion of earthquake focal mechanisms and induced hydraulic fracture orientations a stress trajectory map for western california is computed using an iterative statistical algorithm in which observed directional data such as breakout directions are used to obtain a model regional stress field analysis of well bore breakouts in 25 wells within the central and southern oil districts of sumatra indicates that the regionally defined sh adjacent to the active great sumatran strike slip fault is oriented at a high angle 70 80 to the fault this orientation of sh is consistent regionally with geologic stress indicators and focal mechanisms of dip slip earthquakes preliminary analysis of the stress field in the vicinity of the philippine and alpine faults suggests sh is also oriented at a high angle to these active strike slip faults similarly the minimum horizontal stress sh is oriented at a high angle to the the kane and dead sea transforms the observation of sh and sh in the vicinity of active strike slip faults being oriented nearly perpendicular and parallel to the faults suggests that large crustal scale strike slip faults may in general be inherently weak surfaces

gas hydrate dissociation in sediments pressure temperature evolution 1 hydrate bearing sediments may destabilize spontaneously as part of geological processes unavoidably during petroleum drilling production operations or intentionally as part of gas extraction from the hydrate itself in all cases high pore fluid pressure generation is anticipated during hydrate dissociation a comprehensive formulation is derived for the prediction of fluid pressure evolution in hydrate bearing sediments subjected to thermal stimulation without mass transfer the formulation considers pressure and temperature dependent volume changes in all phases effective stress controlled sediment compressibility capillarity and the relative solubilities of fluids salient implications are explored through parametric studies the model properly reproduces experimental data including the pt evolution along the phase boundary during dissociation and the effect of capillarity pore fluid pressure generation is proportional to the initial hydrate fraction and the sediment bulk stiffness is inversely proportional to the initial gas fraction and gas solubility and is limited by changes in effective stress that cause the failure of the sediment when the sediment stiffness is high the generated pore pressure reflects thermal and pressure changes in water hydrate and mineral densities comparative analyses for co2 and ch4 highlight the role of gas solubility in excess pore fluid pressure generation dissociation in small pores experiences melting point depression due to changes in water activity and lower pore fluid pressure generation due to the higher gas pressure in small gas bubbles capillarity effects may be disregarded in silts and sands when hydrates are present in nodules and lenses and when the sediment experiences hydraulic fracture

generalization of gas hydrate distribution and saturation in marine sediments by scaling of thermodynamic and transport processes gas hydrates dominated by methane naturally occur in deep marine sediment along continental margins these compounds form in pore space between the seafloor and a sub bottom depth where appropriate stability conditions prevail however the amount and distribution of gas hydrate within this zone and free gas below can vary significantly at different locations to understand this variability we develop a one dimensional numerical model that simulates the accumulation of gas hydrates in marine sediments due to upward and downward fluxes of methane over time the model contains rigorous thermodynamic and component mass balance equations that are solved using expressions for fluid flow in compacting sediments the effect of salinity on gas hydrate distribution is also included the simulations delineate basic modes of gas hydrate distribution in marine sediment including systems with no gas hydrate gas hydrate without underlying free gas and gas hydrate with underlying free gas below the gas hydrate stability zone for various methane sources the results are scaled using combinations of dimensionless variables particularly the peclet number and damkohler number such that the dependence of average hydrate saturation on numerous parameters can be summa rized using two contour maps one for a biogenic source and one for upward flux from a deeper source simulations also predict that for systems at steady state large differences in parameters like seafloor depth seafloor temperature and geothermal gradient cause only small differences in average hydrate saturation when examined with scaled variables although important caveats exist our model presents a unified picture of hydrate accumulations that can be used to understand well characterized gas hydrate systems or to predict steady state average hydrate saturation and distribution at locations for which seismic or core data are not available

the impact of lithologic heterogeneity and focused fluid flow upon gas hydrate distribution in marine sediments gas hydrate and free gas accumulation in heterogeneous marine sediment is simulated using a two dimensional 2 d numerical model that accounts for mass transfer over geological timescales the model extends a previously documented one dimensional 1 d model such that lateral variations in permeability k become important various simulations quantitatively demonstrate how focused fluid flow through high permeability zones affects local hydrate accumulation and saturation simulations that approximate a vertical fracture network isolated in a lower permeability shale kfracture kshale show that focused fluid flow through the gas hydrate stability zone ghsz produces higher saturations of gas hydrate 25 70 and free gas 30 60 within the fracture network compared to surrounding shale simulations with a dipping high permeability sand layer also result in elevated saturations of gas hydrate 60 and free gas 40 within the sand because of focused fluid flow through the ghsz increased fluid flux a deep methane source or both together increase the effect of flow focusing upon hydrate and free gas distribution and enhance hydrate and free gas concentrations along the high permeability zones permeability anisotropy with a vertical to horizontal permeability ratio on the order of 10 2 enhances transport of methane charged fluid to high permeability conduits as a result gas hydrate concentrations are enhanced within these high permeability zones the dip angle of these high permeability structures affects hydrate distribution because the vertical component of fluid flux dominates focusing effects hydrate and free gas saturations can be characterized by a local peclet number localized vertical focused and advective flux relative to diffusion relative to the methane solubility gradient somewhat analogous to such characterization in 1 d systems even in lithologically complex systems local hydrate and free gas saturations might be characterized by basic parameters local flux and diffusivity

dual porosity fractured miocene syn rift dolomite reservoir in the issaran field gulf of suez egypt a case history of the zonal isolation of highly fractured water carrier bed

inference of mantle viscosity from grace and relative sea level data summary gravity recovery and climate experiment grace satellite observations of secular changes in gravity near hudson bay and geological measurements of relative sea level rsl changes over the last 10 000 yr in the same region are used in a monte carlo inversion to infer mantle viscosity structure the grace secular change in gravity shows a significant positive anomaly over a broad region 3000 km near hudson bay with a maximum of 2 5 gal yr 1 slightly west of hudson bay the pattern of this anomaly is remarkably consistent with that predicted for postglacial rebound using the ice 5g deglaciation history strongly suggesting a postglacial rebound origin for the gravity change we find that the grace and rsl data are insensitive to mantle viscosity below 1800 km depth a conclusion similar to that from previous studies that used only rsl data for a mantle with homogeneous viscosity the grace and rsl data require a viscosity between 1 4 1021 and 2 3 1021 pa s an inversion for two mantle viscosity layers separated at a depth of 670 km shows an ensemble of viscosity structures compatible with the data while the lowest misfit occurs for upper and lower mantle viscosities of 5 3 1020 and 2 3 1021 pa s respectively a weaker upper mantle may be compensated by a stronger lower mantle such that there exist other models that also provide a reasonable fit to the data we find that the grace and rsl data used in this study cannot resolve more than two layers in the upper 1800 km of the mantle

dynamics of gas hydrate case of the congo continental slope a numerical model is developed to study the temperature effect on stability and phase transformation of gas hydrate the model uses a mathematical formulation based on the enthalpy form of the conservation law of energy the use of the enthalpy form instead of the temperature form as often done in the literature has made the problem numerically simpler the model is then applied to describe the effect of sea bottom temperature variations on the stability of gas hydrate occurrences and on the seafloor reflectivity in sediments of the congo continental slope indeed a migrating seafloor reflectivity front is observed on 3d seimic images from two surveys performed 6 months apart and interpreted as a migrating gas hydrate stability zone seawater temperature variations were recorded over a 230 day period at the seafloor the amplitude of these variations could explain a 75 m shift in water depth of the upper limit of the gas hydrate stability zone however the response of the sediment to a perturbation applied at the seafloor is not immediate because of the effects of thermal diffusion and of latent heat of hydrate dissociation thermal modelling shows that the depth of penetration of these perturbations is around 2 m for the saturated sediment and around 6 m for hydrate bearing sediments hydrate fraction of 0 1 the switch between gas and gas hydrate generated by these temperature fluctuations concern only the first meter of sediment these appear too small to explain the migration of the seafloor reflectivity considering that the wavelength of the seismic signal is significantly larger around 10 m however for temperature fluctuations with larger wavelength the switch between gas and gas hydrate will be more important and consequently could explain the reflectivity contrast as a possible migrating gas or gas hydrate front

systematic stress variations in the southern san joaquin valley and along the white wolf fault implications for the rupture mechanics of the 1952 ms 7 8 kern county earthquake and contemporary seismicity the southern san joaquin valley ssjv stress state is characterized by systematic variations in the regional principal stress directions and relative magnitudes as well as local and possibly temporal variations that appear to be the result of the 1952 ms 7 8 kern county earthquake the regional maximum horizontal principal stress shmax orientation in the san joaquin valley systematically rotates from ne sw compression along the western margins of the valley to n s compression in the ssjv this n s shmax stress direction in the ssjv is consistent with active development of a e w trending structural fabric of fold axes and shallow thrust and the reverse dip slip motion observed during the kern county earthquake on the south southeast dipping white wolf fault contemporary seismicity 30 40 years after the mainshock clusters in two separate areas along the white wolf fault the southwest region where the 1952 earthquake nucleated and the northeast region near where the earthquake rupture terminated earthquakes in the southwest and northeast regions show a diversity of focal mechanisms that include strike slip reverse oblique and to a lesser extent normal slip inversion of earthquake focal mechanisms for in situ stress in the southwest region indicates a strike slip reverse stress regime with s1 oriented approximately perpendicular to the ruptured fault plane implying low frictional strength in the nucleation zone of the 1952 earthquake inversion of focal plane mechanisms in the northeast region indicates a strike slip stress regime with s2 nearly perpendicular to secondary 1952 rupture planes also implying low frictional strength these results indicate near complete stress drops for fault planes associated with the 1952 earthquake and some of the contemporary earthquakes implying fault surfaces which are frictionally weak i e slip planes subparallel to principal stress planes based on the observed stress state in the southern san joaquin valley much of the seismicity may be the result of elevated fluid pressures within these active fault zones

time variability of the earth s gravity field hydrological and oceanic effects and their possible detection using grace the grace satellite mission scheduled for launch in 2001 is designed to map out the earth s gravity field to high accuracy every 2 4 weeks over a nominal lifetime of 5 years changes in the gravity field are caused by the redistribution of mass within the earth and on or above its surface grace will thus be able to constrain processes that involve mass redistribution in this paper we use output from hydrological oceanographic and atmospheric models to estimate the variability in the gravity field i e in the geoid due to those sources we develop a method for constructing surface mass estimates from the grace gravity coefficients we show the results of simulations where we use synthetic grace gravity data constructed by combining estimated geophysical signals and simulated grace measurement errors to attempt to recover hydrological and oceanographic signals we show that grace may be able to recover changes in continental water storage and in seafloor pressure at scales of a few hundred kilometers and larger and at timescales of a few weeks and longer with accuracies approaching 2 mm in water thickness over land and 0 1 mbar or better in seafloor pressure

stress induced borehole elongation a comparison between the four arm dipmeter and the borehole televiewer in the auburn geothermal well the nature and origin of borehole elongation recorded by the four arm dipmeter calipers is studied utilizing information obtained from hydraulic fracturing stress measurements and borehole televiewer data taken in a well located in auburn new york a preferred orientation n10 w s10 e 10 and a less prominant e w orientation of borehole elongation was observed on two runs of the dipmeter comparisons of borehole geometry determined using the televiewer and the dipmeter show that both tools give the same orientation of borehole elongtion provided that the zone of elongation is longer than 30 cm comparisons of dipmeter caliper data with orientation of in situ stress and natural fractures obtained from hydrofracturing tests and televiewer data show that the n10 w s10 e borehole elongations 1 are axisymmetric 2 are aligned with the minimum horizontal stress sh and 3 are not associated with natural fractures intersecting the well these elongations are interpreted as stress induced well bore breakouts the e w elongation direction is characterized by an asymmetric borehole cross section in thinly bedded rocks and is not caused by breakouts this asymmetric geometry can be discriminated from breakouts using the oriented electric measurements provided by the dipmeter this study demonstrates that the dipmeter can be used to determine the orientation of sh by mapping breakouts confirming the results of earlier less detailed studies and provides a firm basis for mapping regional stress patterns using existing dipmeter data

hydrate melting in soil around hot conductor there is ice in the gulf of mexico and this type of ice called gas hydrates burns when ignited gas hydrates form slowly within the soil mass when the gas seeping up from offshore oil reservoirs mixes with water under high pressures 500 m of water and low temperatures a few degrees celsius the oil travels from the well to the platform through a conductor pipe the oil and therefore the conductor are very hot and melt the existing hydrates that are within the depth of the foundation piles the melting process generates a large amount of gas that can endanger the stability of the foundation the rate at which the temperature rises around the hot conductor in the hydrate rich soil is studied using the finite element method fem a detailed thermodynamic analysis is performed it includes a laboratory experiment to help validate fem a study of the mesh size the thermodynamic analysis results a study of the conductor size and of the latent heat influence the results can be used to evaluate the temperature rise around a hot pipe buried in soil and therefore the propagation of the hydrate melting front around the conductor and toward the piles

ongoing glacial isostatic contributions to observations of sea level change studies determining the contribution of water fluxes to sea level rise typically remove the ongoing effects of glacial isostatic adjustment gia unfortunately use of inconsistent terminology between various disciplines has caused confusion as to how contributions from gia should be removed from altimetry and grace measurements in this paper we review the physics of the gia corrections applicable to these measurements and discuss the differing nomenclature between the gia literature and other studies of sea level change we then examine a range of estimates for the gia contribution derived by varying the earth and ice models employed in the prediction we find similar to early studies that gia produces a small compared to the observed value but systematic contribution to the altimetry estimates with a maximum range of 0 15 to 0 5 mm yr 1 moreover we also find that the gia contribution to the mass change measured by grace over the ocean is significant in this regard we demonstrate that confusion in nomenclature between the terms absolute sea level and geoid has led to an overestimation of this contribution in some previous studies a component of this overestimation is the incorrect inclusion of the direct effect of the contemporaneous perturbations of the rotation vector which leads to a factor of two larger value of the degree two order one spherical harmonic component of the model results aside from this confusion uncertainties in earth model structure and ice sheet history yield a spread of up to 1 4 mm yr 1 in the estimates of this contribution however even if the ice and earth models were perfectly known the processing techniques used in grace data analysis can introduce variations of up to 0 4 mm yr 1 thus we conclude that a single valued gia correction is not appropriate for sea level studies based on gravity data each study must estimate a bound on the gia correction consistent with the adopted data analysis scheme

shear wave polarizations and subsurface stress directions at lost hills field 2 2 s wave data matrix accomplished by computationally rotating sources and receivers although polarization directions obtained by assuming a homogeneous subsurface were moderately consistent with depth considerable improvement in consistency resulted from analytically stripping off a thin near surface layer whose fast s wave polarization direction was about n 6 e s wave birefringence for vertical travel averaged 3 percent in two zones 200 700 ft and 1200 2100 ft 60 210 m and 370 640 m which had closely similar s wave polarizations between those zones the polarization direction changed and the birefringence magnitude was not well defined s wave polarizations from two concentric rings of offset vsps were consistent in azimuth with one another and with polarizations of the near offset vsp this consistency argues strongly for the robustness of the s wave polarization technique as applied in this area the s wave polarization pattern in offset data fits a model of vertical cracks striking n 55 e

stress fields a key to oil migration a coherent oil migration model based on geomechanical considerations includes both the high molecular kerogen structure and the capillary properties of source rocks oil is squeezed from kerogen by compaction following oil generation this squeezing effect should be created by the differential stress maximum compressive stress minus least compressive stress acting on the kerogen which has been chemically broken up by oil formation in sedimentary bodies whose water is at hydrostatic pressure the migration of oil seems to involve two processes 1 lateral transfer by channeling into the more coarsely microporous layers of the source rocks from the oil generation site toward the geologic structure or lower pressured zone and 2 vertical transfer from source rock to eservoir by the opening or reopening of vertical fractures in the few areas such as structural tops where the least compressive stress is slightly greater than or equal to the pore pressure and where the capillary pressure increment 2 ggr r of oil in the microporosity exceeds the tensile strength of the rock in sedimentary bodies whose water is overpressured the pore pressure should be governed by the least compressive stress and thus migration should begin by oil transfers in a system of small open fractures and eventually in larger fractures the theory demonstrates the impossibility of oil being transferred to the reservoir under true tensile conditions negative effective compressive stress and thus explains the large asphaltic veins of southeastern turkey and the well known bitumen veins of the uinta basin

the style of late cenozoic deformation at the eastern front of the california coast ranges the 1983 coalinga earthquake occurred at the eastern boundary of the california coast ranges in response to northeast directed thrusting such movements over the past 2 ma have produced coalinga anticline by folding above the blind eastern tip of the coalinga thrust zone the 600 km length of the coast ranges boundary shares a common structural setting that involves westward upturn of cenozoic and cretaceous strata at the eastern front of the coast ranges and a major southwest facing step in the basement surface beneath the western great valley like coalinga anticline pliocene and quaternary folding and faulting along the rest of the boundary also result from northeast southwest compression acting nearly perpendicular to the strike of the san andreas fault we suggest that much of this deformation is related to active thrusts beneath the eastern coast ranges the step in the basement surface beneath the great valley seems to have controlled the distribution of this deformation and the shape of the coast ranges boundary

an international code comparison study on coupled thermal hydrologic and geomechanical processes of natural gas hydrate bearing sediments abstract geologic reservoirs containing gas hydrate occur beneath permafrost environments and within marine continental slope sediments representing a potentially vast natural gas source numerical simulators provide scientists and engineers with tools for understanding how production efficiency depends on the numerous interdependent coupled processes associated with potential production strategies for these gas hydrate reservoirs confidence in the modeling and forecasting abilities of these gas hydrate reservoir simulators ghrss grows with successful comparisons against laboratory and field test results but such results are rare particularly in natural settings the hydrate community recognized another approach to building confidence in the ghrs comparing simulation results between independently developed and executed computer codes on structured problems specifically tailored to the interdependent processes relevant for gas hydrate bearing systems the united states department of energy national energy technology laboratory doe netl sponsored the first international gas hydrate code comparison study ighccs1 in the early 2000s ighccs1 focused on coupled thermal and hydrologic processes associated with producing gas hydrates from geologic reservoirs via depressurization and thermal stimulation subsequently ghrss have advanced to model more complex production technologies and incorporate geomechanical processes into the existing framework of coupled thermal and hydrologic modeling this paper contributes to the validation of these recent ghrs developments by providing results from a second ghrs code comparison study ighccs2 also sponsored by doe netl ighccs2 includes participants from an international collection of universities research institutes industry national laboratories and national geologic surveys study participants developed a series of five benchmark problems principally involving gas hydrate processes with geomechanical components the five problems range from simple geometries with analytical solutions to a representation of the world s first offshore production test of methane hydrates which was conducted with the depressurization method off the coast of japan to identify strengths and limitations in the various ghrss study participants submitted solutions for the benchmark problems and discussed differing results via teleconferences the ghrss evolved over the course of ighccs2 as researchers modified their simulators to reflect new insights lessons learned and suggested performance enhancements the five benchmark problems final sample solutions and lessons learned that are presented here document the study outcomes and serve as a reference guide for developing and testing gas hydrate reservoir simulators

variations in the earth s oblateness during the past 28 years 1 analysis of satellite laser ranging slr data indicates that the earth s dynamic oblateness j2 has undergone significant variations during the past 28 years the dominant signatures in the observed variations in j2 are 1 a secular decrease with a rate of approximately 2 75 10 11 yr 1 2 seasonal annual variations with a mean amplitude of 2 9 10 10 3 significant interannual variations with timescales of 4 6 years and 4 a variation with period of 21 years and an amplitude of 1 4 10 10 with minimum in december 1988 two large interannual variations are related to the strong el nino southern oscillation events during the periods of 1986 1991 and 1996 2002 and it appears that another interannual cycle may have started in late 2002 the superposition of the decadal variation on the interannual signal makes the j2 fluctuation appear to be anomalously large during the 1996 2002 period contemporary models of the mass redistributions in the atmosphere ocean and surface water can explain a major part of the 4 to 6 year fluctuations however the cause of the decadal variation remains unknown

poroelastic stressing and induced seismicity near the lacq gas field southwestern france o hundreds of shallow small to moderate earthquakes have occurred near the lacq deep gas field in southwestern france since 1969 these earthquakes are clearly separated from tectonic seismicity occurring in the pyrenees 25 km to the southwest the induced seismicity began when the reservoir pressure had declined by 30 mpa repeated levehng over the field shows localized subsidence reaching a maximum of 60 mm in 1989 segall 1989 suggested that poroelastic stressing associated with volumetric contraction of the reservoir rocks is responsible for induced seismicity associated with fluid extraction to test this model we compare the observed subsidence and hypocentral distributions with the predicted displacement and stress fields we find that the relationship between average reservoir pressure drop and subsidence is remarkably linear lending support to the linear poroelastic model displacements and stresses are computed based on a priori knowledge of the reservoir geometry materim properties and reservoir pressure changes the computed vertical displacements are found to be in excellent agreement with the subsidence observed from leveling stress perturbations accompanying gas extraction computed using the same parameters are found to be 0 2 mpa or less changes in coulomb failure stress are computed assuming that shp occurs on optimally oriented planes the predicted failure zones correlate very well with the spatial distribution of earthquakes if the perturbing stresses are small in comparison to the ambient regional deviatoric stresses and if the minimum regional compresslye stress axis is vertical accurate determination of focal mechanisms of the induced events would allow a more rigorous test of the poroelastic model and could lead to important inferences about the crustal stress state

spells of low frequency oscillations and weather regimes in the northern hemisphere abstract the low frequency variability in the midlatitudes is described through an analysis of the oscillatory phenomena in order to isolate nearly periodic components of the atmospheric flow the multichannel version of the singular spectrum analysis m ssa is developed and applied to an nmc 32 year long set of 700 hpa geopotential heights in the same way that principal component analysis identifies the spatial patterns dominating the variability m ssa identifies dynamically relevant space time patterns and provides an adaptive filtering technique three major low frequency oscillations lfos are found with periods of 70 days 40 45 days and 30 35 days the 70 day oscillation consists of fluctuations in both position and amplitude of the atlantic jet with a poleward propagating anomaly pattern the 40 45 day oscillation is specific to the pacific sector and has a pronounced pacific north american pna structure in its high amplitude phase the 30 35 day mode is confined over the atlantic region

induced stresses due to fluid extraction from axisymmetric reservoirs earthquakes can be induced by fluid extraction as well as by fluid injection segall 1989 proposed that poroelastic stresses are responsible for inducing earthquakes associated with fluid extraction here i present methods for computing poroelastic stress changes due to fluid extraction for general axisymmetric reservoir geometries the results ofgeertsma 1973 for a thin disk reservoir with uniform pressure drop are recovered as a special case predicted surface subsidence agrees very well with measured leveling changes over the deep lacq gas field in southwestern france the induced stresses are finite if the reservoir pressure changes are continuous computed stress changes are on the order of several bars suggesting that the preexisting stress states in regions of extraction induced seismicity are very close to frictional instability prior to production

analytical theory relating the depth of the sulfate methane transition to gas hydrate distribution and saturation 1 we develop a theory that relates gas hydrate saturation in marine sediments to the depth of the sulfate methane transition smt zone below the seafloor using steady state analytical expressi

well bore breakouts and in situ stress the detailed cross sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data we show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression this work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ

identification of gas hydrate dissociation from wireline log data in the shenhu area south china sea wireline logs were acquired in eight wells during china s first etas hydrate drilling expedition gmgs 1 in april june of 2007 well logs obtained from site sh3 indicated gas hydrate was present in the depth range of 195 206 in below seafloor with a maximum pore space gas hydrate saturation calculated from pore water freshening of about 26 assuming gas hydrate is uniformly distributed in the sediments resistivity calculations using archie s equation yielded hydrate saturation trends similar to those from chloride concentrations however the measured compressional p wave velocities decreased sharply at the depth between 194 and 199 mbsf dropping as low as 1 3 km s indicating the presence of free gas in the pore space possibly caused by the dissociation of gas hydrate during drilling because surface seismic data acquired prior to drilling were not influenced by the in situ gas hydrate dissociation surface seismic data could be used to identify the cause of the low p wave velocity observed in the well log to determine whether the low well log p wave velocity was caused by in situ free gas or by gas hydrate dissociation synthetic seismograms were generated using the measured well log p wave velocity along with velocities calculated assuming both gas hydrate and free gas in the pore space comparing the surface seismic data with various synthetic seismograms suggested that low p wave velocities were likely caused by the dissociation of in situ gas hydrate during drilling

progress in stability analysis of submarine slopes considering dissociation of gas hydrates gas hydrates have the potential to be a new energy source and a submarine geohazard though researchers generally agree about the association between gas hydrate dissociation and submarine slope failures the processes and mechanism of submarine slope failure caused by gas hydrate dissociation are not clearly understood in the last few years some authors have tried to analyse submarine slope stability by considering the existence and dissociation of gas hydrate and a few researchers have presented quantitative models this paper presents a review of the various causes of submarine slope failures associated with gas hydrate dissociation also analysis models of submarine slope stability associated with gas hydrate dissociation that are documented from the literatures including the infinite slope model wedge model slump and retrogressive failure model are interpreted and illustrated respectively

transient fluid flow through the toe of the barbados accretionary complex constraints from ocean drilling program leg 110 heat row studies and simple models thirty four sediment and mud line temperatures were collected from six drill holes on ocean drilling program odp leg 110 near the toe of the barbados accretionary complex when combined with thermal conductivity measurements from sediment cores and results from earlier surveys these data delineate the complicated thermal structure on the edge of this convergent margin heat flow values at the seafloor of 92 192 mw m sup 2 are 80 300 higher than those predicted by standard heat flow versus age models for oceanic crust but are compatible with earlier seafloor measurements made in this area at the same latitude heat flow tends to decrease downhole at four sites suggesting the presence of heat sources within the sediments these results are consistent with the flow of warm fluids through the complex along high permeability conduits including thrust faults the major decollement zone and sandy intervals simple calculations suggest that this fluid flow is transient occurring on time scales of tens of thousands of years fluid flow velocities along the decollement zone estimated with a simple thermal model are about 10 sup minus 7 m s 100 times faster than predicted by numerical steady state analyses the estimated maximum sediment permeability within the decollement zone which was basedmore on the above fluid flow velocity is about 10 sup minus 12 m sup 2 also 100 times higher that that calculated numerically high heat flow in the vicinity of 15 degree 30 prime n and not elsewhere along the deformation front suggests that the leg 110 drill sites may be situated over a prism water discharge zone with dewatering more active here than elsewhere along the accretionary complex less

utilization of observations of well bore failure to constrain the orientation and magnitude of crustal stresses application to continental deep sea drilling project and ocean drilling program boreholes

influence of poroelastic behavior on the magnitude of minimum horizontal stress sh in overpressured parts of sedimentary basins in many sedimentary basins of the world the minimum hori zontal stress s h is greater in overpressured zones than in normally pressured zones at equivalent depths a common explanation is that the frictional slip on listric normal faults keeps the difference between vertical stress s v and s h within certain bounds and the difference is smaller under lower effective stress i e higher pore pressure p p however in the overpressured parts of the central north sea graben united kingdom and the sable subbasin of the scotian shelf canada conventional friction envelopes underestimate the magnitude of s h these data instead indicate that s h increases at a rate proportional to but less than the rate of increase of p p a condition consistent with a p p induced deformation of the rock called poroelastic behavior this paper argues that whereas friction may govern s h in normally pressured basins poroelastic behavior is responsible for the unusually high s h in the overpres sured parts of these same basins data on the p p and s h gradients from these basins suggest that s h p p 0 7

physical properties of hydrate bearing sediments 1 methane gas hydrates crystalline inclusion compounds formed from methane and water are found in marine continental margin and permafrost sediments worldwide this article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate bearing sediments formation phenomena include pore scale habit solubility spatial variability and host sediment aggregate properties physical properties include thermal properties permeability electrical conductivity and permittivity small strain elastic p and s wave velocities shear strength and volume changes resulting from hydrate dissociation the magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate bearing sediments to changes in mechanical thermal or chemical boundary conditions these predictions are vital for mitigating borehole local and regional slope stability hazards optimizing recovery techniques for extracting methane from hydrate bearing sediments or sequestering carbon dioxide in gas hydrate and evaluating the role of gas hydrate in the global carbon cycle

optimized smoothing of gravity recovery and climate experiment grace time variable gravity observations 1 high degree and high order spherical harmonics of time variable gravity fields observed by the gravity recovery and climate experiment grace gravity mission are dominated by noise we develop two smoothing methods that suppress these high degree and high order errors with results superior to more commonly used gaussian smoothing these optimized smoothing methods considerably improve signal to noise levels of grace terrestrial water storage estimates relative to residual signal and noise over the oceans and show significantly better spatial resolution and lower leakage error on the basis of analysis using an advanced land surface model the equivalent spatial resolution from these optimized smoothing estimates is about 500 km compared to the roughly 800 1000 km gaussian smoothing that is required to suppress high degree noise in the grace fields

basement exploration west of shetlands progress in opening a new play on the ukcs abstract commercial production of hydrocarbon from fractured crystalline basement is well documented with petroleum basins across the globe hosting fractured basement fields the uk is an anomaly within this global phenomenon as despite numerous serendipitous discoveries of basement oil in the north sea and the west of shetlands there is currently no commercial field on the united kingdom continental shelf ukcs that is reliant on oil production from fractured basement recognizing that this situation presented an exploration niche hurricane energy plc hurricane was formed to focus on ukcs basement exploration and concentrated its efforts in acquiring exploration acreage in the west of shetlands in 2009 hurricane drilled the first well designed to explore the basement play on the ukcs leading to the lancaster discovery with a contingent resource range 1c 3c of 62 456 million barrels of oil equivalent mmboe the lancaster discovery is presented to summarize the challenges and processes that have been applied in the exploration and evaluation of the west of shetlands basement play conclusions from this work indicate that basement hydrocarbon resource potential is of such significance that it may represent a strategic resource for the uk with over 1154 mmboe of 2c and mean unrisked prospective resources so far identified

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

accelerated antarctic ice loss from satellite gravity measurements accurate quantification of antarctic ice sheet mass balance and its contribution to global sea level rise remains challenging gravity recovery and climate experiment data spanning the period april 2002 to january 2009 confirm earlier estimates of ice loss for antarctica and indicate that east antarctica started losing mass in about 2006