a geochemical perspective and assessment of leakage potential for a mature carbon dioxide enhanced oil recovery project and as a prototype for carbon dioxide sequestration rangely field colorado measurements of co2 and ch4 soil gas concentrations and gas exchange with the atmosphere at a large scale co2 enhanced oil recovery eor operation at rangely colorado united states allowed assessment of the microseepage potential shallow and deep soil gas concentrations and direct transport of co2 and ch4 complemented by carbon isotopes have demonstrated an estimated microseepage rate to the atmosphere of approximately 400 t ch4 yr from the 78 km2 area of the rangely field preliminary estimates of deep sourced co2 losses are in the range of 170 to less than 3800 t yr several holes as much as 9 m deep for nested sampling of gas composition stable carbon isotopic ratios for co2 and ch4 and carbon 14 measurements on co2 indicate that deep sourced co2 microseepage loss was detected methanotrophic oxidation of microseeping ch4 to co2 in soils demonstrates significant contribution to soil gas co2 in high ch4 flux areas necessitating a revision of the estimated direct co2 microseepage rate to less than 170 t yr over the rangely field an evaluation of produced water quality from pre co2 eor to 1999 demonstrates an increase in some parameters particularly of ca2 and hco3 indicating dissolution of ferroan calcite and ferroan dolomite in the weber formation inverse computer modeling suggested carbonate mineral sequestration was possible within the constraints of evolution of produced water quality x ray analyses of well scales however do not support the presence of significant mineral sequestration instead modeling indicates that the bulk of the co2 that has been injected since 1986 is sequestered as dissolved co2

using 3he 4he isotope ratios to identify the source of deep reservoir contributions to shallow fluids and soil gas abstract one strategy to counteract rising atmospheric co 2 levels is the capture and subsequent geological storage of anthropogenic co 2 legislation will require the monitoring and early identification of any leakage to the near surface from the storage site owing to their low concentrations and unreactive nature noble gases are well established as key tracers of crustal fluid systems ballentine et al 2002 the 3 he 4 he and noble gas content of soil gases and groundwater may provide the opportunity to detect identify and quantify a weak deep sourced signal that contributes to a larger near surface biological signal we present 3 he 4 he ne ar kr and xe data from near surface soil gases two of which exhibit hydrocarbon microseepage from deeper oil reservoirs from the teapot dome oil field wyoming usa we also present a noble gas characterisation of the oil reservoirs beneath the seepages a helium excess 4 he excess up to 37 7 ppm relative to air concentrations 4 he air 5 24 ppm is found in soil gases at both microseepage sites error propagation demonstrates that it is possible to unambiguously resolve the 3 he 4 he of the deep helium source and allows us to explore the limits of this technique at one seep the resolved deep 3 he 4 he 0 055 0 009ra where ra 1 40 10 6 is indistinguishable from the range observed in the subsurface hydrocarbon system of 0 046 to 0 109ra notably the soil gas 4 he ch 4 1 8 10 3 at this microseepage site compares similarly with the lower subsurface source 4 he ch 4 1 4 to 3 4 10 4 and demonstrates the conservative nature of helium as a tracer we show that the sensitivity of helium as a deep fluid tracer should be increased by up to two orders of magnitude in groundwater compared to soil gases studied here due to the low solubility of helium in water groundwater rather than soil gas should be the priority in any monitoring strategy that plans to use helium as an early indicator of deep fluid microseepage

review of monitoring issues and technologies associated with the long term underground storage of carbon dioxide large scale underground storage of co2 has the potential to play a key role in reducing global greenhouse gas emissions typical underground storage reservoirs would lie at depths of 1000 mor more and contain tens or even hundreds of millions of tonnes of co2 a likely regulatory requirement is that storage sites would have to be monitored both to prove their efficacy in emissions reduction and to ensure site safety a diverse portfolio of potential monitoring tools is available some tried and tested in the oil industry others as yet unproven shallow focused techniques are likely to be deployed to demonstrate short term site performance and in the longer term to ensure early warning of potential surface leakage deeper focused methods notably time lapse seismic will be used to track co2 migration in the subsurface to assess reservoir performance and to calibrate validate site performance simulation models the duration of a monitoring programme is likely to be highly site specific but conformance between predicted and observed site performance may form an acceptable basis for site closure the geological society of london 2009

predicting the seismic implications of salt anisotropy using numerical simulations of halite deformation in the past the potential for seismic anisotropy in salt structures and its effect on their seismic imaging has received little attention we consider the plausibility of salt anisotropy through linked numerical studies of salt deformation and its seismic consequences numerical models are used to predict lattice preferred orientations lpos in halite polycrystalline aggregates subjected to axial extension and simple shear the elastic constants for the deformed polycrystalline aggregate are then calculated simple models representing a salt sill and the stem of a diapir are created using these elastic constants ray tracing is used to investigate the effects of halite lpo on the propagation of seismic waves the results suggest that salt anisotropy can cause significant traveltime effects and could lead to significant errors in seismic interpretation in salt environments if this anisotropy is ignored we also investigate potential amplitude variation with offset and azimuth avoa for the reflection from the top and bottom of an anisotropic salt sill ray paths with a shear wave leg within the salt display strong avoa effects with a clear four fold symmetry

influence of water saturation on the nonlinear elastic mesoscopic response in earth materials and the implications to the mechanism of nonlinearity 1 much is known empirically about the qualitative and quantitative nonlinear response of rocks still the mechanism of nonlinear response in porous media is only speculation in this work we illustrate evidence from several types of experiments indicating that fluid plays an important role in the nonlinear response for instance measurements at low degrees of water saturation indicate that molecular layers of adsorbed moisture as well as condensation fluids significantly influence the linear and nonlinear response at dynamic strain levels due to the activation of internal molecular forces in rock we measure a significant increase in the nonlinear response especially in the saturation range of 1 20 this is consistent with observed changes in the linear response but the extent of the variation is larger in the nonlinear measurements especially in rocks containing small pore systems these modifications can be attributed to an increased fluid solid interaction upon wetting causing the material to expand and to soften concurrently the microscopic and mesoscopic hysteretic entities which give rise to the macroscopically observed nonlinear response are activated at saturation dependent opening and closing pressures as a consequence of the moisture induced forces the fraction of active hysteretic units increases with saturation the fact that the nonlinear response increases with water saturation especially in the low saturation range implies that the presence of moisture plays a major role in the nonlinear mechanism or more precisely in the activation of that mechanism

elastic piezosensitivity of porous and fractured rocks understanding the stress dependencies of seismic velocities is important for interpreting a variety of seismic data ranging from amplitude versus offset avo and velocity analysis to overpressure prediction and 4d seismic monitoring of reservoirs sometimes rather complex forms of these dependencies based on specific models of porous space geometry are used for example spherical contacts models duffy and mindlin 1957 merkel et al 2001 duffy merkel 2001 and crack contacts models gangi and carlson 1996 carcione and tinivella 2001 have been used in different studies gangi96 however usually the pore pressure velocity dependence along with the velocity dependence on the confining pressure are phenomenologically described by the following simple relationship zimmerman et al 1986 eberhart phillips et al 1989 freund 1992 jones 1992 prasad and manghnani 1997 khaksar et al 1999 carcione and tinivella 2001 kirstetter and macbeth 2001

a classification for the pressure sensitivity properties of a sandstone rock frame a three parameter formula is proposed to describe the sensitivity of the bulk modulus or shear rigidity for a sandstone rock frame under applied isotropic loading the theoretical basis for this relation is the use of excess compliance as a pseudo function to describe all internal weaknesses in the rock regardless of their origin the law is fitted to 179 sets of laboratory measurements on unsaturated reservoir core and outcrop sandstones that have low to moderate porosity and a range of clay fractions and cementation a stress index defined to quantitatively rank the reservoir sands indicates that laboratory derived pressure sensitivity is highest 10 per mpa maximum for the clean moderate porosity paleocene sands from the forties formation and west of shetlands whereas the lower porosity cemented and more consolidated permian sands rank as least pressure sensitive 1 per mpa maximum higher porosity sands are stress sensitive because of the overall rock frame compressibility all of t

microcrack induced elastic wave anisotropy of brittle rocks the failure of brittle rocks during compression is preceded by the formation growth and coalescence of microcracks elastic wave velocities are reduced in the presence of open microcracks and fractures and may therefore be used to monitor the progressive damage of the rock in general these microcracks are not randomly oriented and the rock displays an elastic anisotropy the elastic anisotropy due to cracks can be expressed in terms of a second rank and fourth rank crack density tensor for open cracks the contribution of the fourth rank crack density tensor to the elastic wave velocities is small these results are compared with recent measurements of the ultrasonic compressional and shear wave velocities for propagation parallel and perpendicular to an increasing axial stress applied at constant confining stress to berea sandstone inversion of the velocity measurements indicates that the microcracks propagate parallel to the maximum compressive stress in agreement with current rock mechanics theory a reasonable fit to the data is obtained using only the second rank crack density tensor even though at high confining stress the cracks are expected to be in partial contact along their length this is consistent with the model of elastic wave propagation in a medium containing partially contacting fractures published by white however measurements of off axis wave velocities are required to fully quantify the contribution of the fourth rank crack density tensor

anomalous atmospheric circulation over the weddell sea antarctica during the austral summer of 2001 02 resulting in extreme sea ice conditions during the austral summer of 2001 02 exceptionally heavy sea ice conditions were experienced over the eastern weddell sea satellite microwave imagery showed that large negative positive ice anomalies were present from october 2001 to january 2002 over the north western weddell sea off the coast of dronning maud land these were a result of anomalously high low atmospheric pressure over the south atlantic southern weddell sea and bellingshausen sea which gave strong north to north westerly cyclonic flow over the northern and eastern weddell sea this resulted in convergence of sea ice into the southern weddell sea and inhibited ice advection along the coast of dronning maud land the atmospheric anomalies around the weddell sea were part of an antarctic wide amplification of the mean wavenumber 3 pattern resulting in more intrusions of mid latitude air into the interior of the continent giving rise to near record warm temperatures at several locations

rocks under strain strain induced time lapse time shifts are observed for depleting reservoirs pressure depletion as a result of oil and gas production creates changes in the stress and strain fields of the rock material both inside and outside the reservoir seismic waves propagating through these rocks have different traveltimes before and after depletion because the layer thicknesses change and the seismic velocity is altered by the changes in the stress and strain fields the cumulative change in two way traveltimes with depth is referred to as time lapse time shift

carbon dioxide injection and resultant alteration of the weber sandstone rangely field colorado 1 geologic interpretations made during the current enhanced oil recovery project at rangely field rio blanco county colorado have shown the relationships between alteration of reservoir mineralogy and texture changes in produced water composition and increased problems with oil production the bottom hole ph of weber brine has decreased from approximately 7 5 to 4 5 since the initiation of co 2 injection into the pennsylvania permian weber sandstone in the central portion of the field much of the weber is partially cemented with carbonate minerals including ferroan calcite and ferroan dolomite with authigenic clay minerals predominately illite and mixed layer smectite illite also present these minerals are affected by the lowered ph of the co 2 flood changes in the chemistry of produced water are associated with alteration of reservoir mineralogy the co 2 flood has caused substantial increases in the concentrations of iron calcium and magnesium in the produced brine from the dissolution of carbonate cements and perhaps authigenic clays and detrital feldspars produced water from some wells has experienced large changes in barium and sulfate concentrations an increase in bromide concentration indicates that previously segregated portions of the reservoir which contained very saline original formation brine have been accessed by the carbonated water hypotheses based on the water chemistry changes were confirmed in core flood experiments these experiments also indicated that there was no net change in permeability following co 2 injection

baseline studies of surface gas exchange and soil gas composition in preparation for co2 sequestration research teapot dome wyoming a baseline determination of co2 and ch4 fluxes and soil gas concentrations of co2 and ch4 was made over the teapot dome oil field in the naval petroleum reserve 3 in natrona county wyoming united states this was done in anticipation of the experimentation with co2 sequestration in the pennsylvanian tensleep sandstone underlying the field at a depth of 5500 ft 1680 m the measurements were made in january 2004 to capture the system with minimum biological activity in the soils resulting in a minimum co2 flux and a maximum ch4 flux the co2 fluxes were measured in the field with an infrared spectroscopic method the ch4 fluxes were determined from gas chromatographic measurements on discrete samples from under the flux chambers the co2 and ch4 were determined at 30 60 and 100 cm 11 23 and 39 in depths in soil gas by gas chromatography a total of 40 locations had triplicate flux measurements using 1 00 m2 10 763 ft2 chambers and soil gas was sampled at single points at each of the 40 locations carbon dioxide fluxes averaged 227 1 mg co2 m2 day1 a standard deviation of 186 9 mg m2 day1 and a range of 281 7 to 732 9 mg m2 day1 not including one location with subsurface infrastructure contamination methane fluxes averaged 0 137 mg ch4 m2 day1 standard deviation of 0 326 mg m2 day1 and a range of 0 481 to 1 14 mg m2 day1 not including the same contaminated location soil gas co2 concentrations increased with depth averaging 618 645 and 1010 ppmv at 30 60 and 100 cm 11 23 and 39 in respectively soil gas ch4 concentrations averaged 0 128 0 114 and 0 093 ppmv at 30 60 and 100 cm 11 23 and 39 in respectively the decrease in ch4 with depth reflects a slow rate of methanotrophic oxidation even during winter conditions the 13c of the soil gas co2 was also determined in the soil gas samples and in the atmosphere these data demonstrated that the increased co2 with depth was derived from the biological oxidation of soil organic matter

effective elasticity of fractured rocks production related changes in subsurface stress are known to cause velocity variations that can be detected from time lapse seismic data and may provide valuable information for reservoir surveillance it has been long recognized that the stresses primarily influence the most compliant part of a rock by preferentially opening or closing the fractures walsh 1965 therefore examining wave propagation in fractured formations might give important clues about the stresses acting upon them as the length of seismic waves is usually much larger than that of the fractures deformation of cracked rocks can be treated as static

compressibility of porous rocks four compressibilities are defined for porous rock relating changes in the bulk and pore volumes to changes in the pore and confining pressures using a micromechanical theory based on classical linear elasticity three relations are found between these compressibilities two of these relations are verified experimentally for berea and bandera sandstone bounds are derived for these compressibilities involving only the porosity and the elastic moduli of the rock forming minerals for the strains to be unique functions of the stresses the compressibilities must be functions only of the difference between the confining and pore pressures this dependence is verified for berea bandera and boise sandstone while the strains cannot be expressed as functions of an effective stress regardless of how it is defined the elastic volumetric behavior of porous rocks can be studied through tests conducted at zero pore pressure

exploring trends in microcrack properties of sedimentary rocks an audit of dry core velocity stress measurements rock physics models are used increasingly to link fluid and mechanical deformation parameters for dynamic elastic modeling we explore the input parameters of an analytical stress dependent rock physics model to do this we invert for the stressdependent microcrack parameters of more than 150 sedimentary rock velocity stress core measurements taken from a literature survey theinversionschemeisbasedonamicrostructuraleffective medium formulation defined by a second rank crack density tensor scalar crack model or by a second and fourth rank crack density tensor joint inversion model then the inversion results are used to explore and predict the stress dependent elastic behavior of various sedimentary rock lithologies using an analytical microstructural rock physics model via the initial model inputparameters initialcrackaspectratioandinitialcrackdensity estimates of initial crack aspect ratio are consistent among most lithologies with a mean of 0 0004 but for shales they differ uptoseveraltimesinmagnitudewithameanof0 001 estimates of initial aspect ratio are relatively insensitive to the inversion method although the scalar crack inversion becomes less reliableatlowvaluesofnormal to tangentialcrackcomplianceratio bn bt initialcrackdensityissensitivetothedegreeofdamage as well as the inversion procedure an important implication is thatthefourth rankcrack densitytermisnotnecessarilynegligible for most sedimentary rocks and evaluation of this term or bn bt is necessary for accurate prediction of initial crack density this is especially important because recent studies suggest thatbn btcanindicatefluidcontentincracks

effects of single vertical fluid filled fractures on full waveform dipole sonic logs we conducted a parametric study of the effects of single finite width adjacent vertical fluid filled fractures on full wave borehole dipole logs using the transformed boundary integral equation method we found that a fracture has significant effects on dipole response if it is within half the wavelength or two to three times the borehole diameter from the borehole center dual flexural waves and a leaky fracture mode resulted from the waveguide composed of the borehole and fracture the first flexural wave was controlled by both the borehole and fracture whereas the second flexural wave was controlled primarily by the borehole but influenced by the fracture the separation of these two flexural waves increased and the strength of the first one decreased when the distance between the fracture and the borehole increased the leaky fracture mode was represented by a sharp minimum in the amplitude spectrum and energy leakage to the fracture reached a maximum when the fracture intersected the borehole

modeling combined fluid and stress change effects in the seismic response of a producing hydrocarbon reservoir editor s note a fuller version of this article can be downloaded in pdf format from the gumpa project web site at the following url http www pet hw ac uk research gumpa index html the exploration and production of hydrocarbons are generally accomplished with the aid of 3 d seismic to image reservoir structure and in some instances reservoir properties and direct hydrocarbon indicators repeated seismic surveys over a period are termed time lapse seismic and sometimes 4 d seismic changes observed in the seismic character with time have been attributed to impedance changes as a result of production e g gawith and gutteridge 1996 these changes have been used in a few producing fields to monitor reservoir performance identifying observed differences in repeat 3 d surveys and relating these to either in situ saturation or stress state changes or both has been difficult because of the lack of control data it has been noted that in some fields watts et al 1995 the sensitivity to stress changes can be very much greater than the sensitivity to saturation changes in other fields landro et al 1999 the saturation changes are thought to be more significant to aid understanding a need for greater integration of geophysics and reservoir engineering has been noted and was the motivation for this study jack 2001 there is a limited window of opportunity in a field s producing life when there are sufficient changes saturation or stress in the subsurface to show a surface seismic response these changes have to be monitored before the field has reached significant decline for the observed changes to be exploited for reservoir management through in fill drilling for by passed compartmentalized or attic oil reservoir modeling is an essential tool for managing the development of and production from hydrocarbon reservoirs many technical issues however surround the realism and validity of the models on which management decisions are based

the stress dependent elastic properties of thermally induced microfractures in aeolian rotliegend sandstone the impact of thermally induced microfractures on the stress sensitive elastic wave properties of aeolian rotliegend sandstone samples is analysed it is found that to identify the effects of the microfracture contribution accurately a correction must first be made to account for water loss representing a mass loss of 4 6 from the pore throats and clays due to the heating process despite care being taken to ensure that the thermally fractured samples re adsorb room moisture both the original and thermally fractured rocks are stress sensitive at the ultrasonic wave frequencies of the laboratory however a distinct shift in the estimated distribution of internal rock compliance indicates that the population of thermal microfractures differs in nature from that caused solely by core plug extraction damage in particular the ratio of normal to tangential compliance is observed to be higher for the thermally generated microfractures than for the broken grain grain contacts created by extraction unloading this can be explained by the intragranular thermal fracture surfaces being smoother when compared to the intergranular boundaries mechanical hysteresis is observed between the up and downgoing test cycles for both the original and to a greater extent the thermally fractured rock this indicates that there is compaction induced movement of the fractures in the samples during application of stress in the laboratory

seismic anisotropy of fractured rock a simple method for including the effects of geologically realistic fractures on the seismic propagation through fractured rocks can be obtained by writing the effective compliance tensor of the fractured rock as the sum of the compliance tensor of the unfractured background rock and the compliance tensors for each set of parallel fractures or aligned fractures the compliance tensor of each fracture set is derivable from a second rank fracture compliance tensor for a rotationally symmetric set of fractures the fracture compliance tensor depends on only two fracture compliances one controlling fracture compliance normal the other tangential to the plane of the fractures the stiffness tensor which is more useful in the consideration of elastic wave propagation through rocks can then be obtained by inversion the components of the excess fracture compliance tensor represent the maximum amount of information that can be obtained from seismic data if the background rock is isotropic and the normal a

predicting time lapse stress effects in seismic data time lapse changes in seismic data are commonly evaluated in terms of changes in reservoir properties such as pressure saturation or temperature traditionally the evaluation of time lapse seismic data has focused on changes of seismic signatures within the reservoir interval recent studies however have shown convincingly that time lapse seismic changes occur not only in the reservoir but also in the overburden and generally in the rock mass surrounding the reservoir these time lapse changes can be explained by production induced stress changes in the rocks surrounding the reservoir

relative stability and significance of dawsonite and aluminum minerals in geologic carbon sequestration 1 computer simulations predict dawsonite naalco3 oh 2 will provide long term mineral sequestration of anthropogenic co2 whereas dawsonite rarely occurs in nature or in laboratory experiments that emulate a carbon repository resolving this discrepancy is important to determining the significance of dawsonite mineralization to the long term security of geologic carbon sequestration this study is an equilibrium based experimental and modeling evaluation of underlying causes for inconsistencies between predicted and observed dawsonite stability using established hydrothermal methods 0 05 molal nahco3 aqueous solution and synthetic dawsonite were reacted for 18 7 days 449 2 hours at 50 c 20 mpa temperature was increased to 75 c and the experiment continued for an additional 12 3 days 295 1 hours incongruent dissolution yielded a dawsonite gibbsite nordstrandite assemblage geochemical simulations using geochemist s workbench and the resident database thermo com v8 r6 incorrectly predicted a dawsonite diaspore assemblage and underestimated dissolved aluminum by roughly 100 times higher aqueous aluminum concentrations in the experiment suggest that dawsonite or diaspore is less stable than predicted simulations employing an alternate database thermo dat correctly predict dawsonite and dawsonite gibbsite assemblages at 50 and 75 c respectively although dissolved aluminum concentrations are still two to three times lower than experimentally measured values correctly reproducing dawsonite solubility in standard geochemical simulations requires an as yet undeveloped internally consistent thermodynamic database among dawsonite gibbsite boehmite diaspore aqueous aluminum complexes and other al phases such as albite and kaolinite these discrepancies question the ability of performance assessment models to correctly predict dawsonite mineralization in a sequestration site

seismic anisotropy as an indicator of reservoir quality in siliciclastic rocks improving the accuracy of subsurface imaging is commonly the main incentive for including the effects of anisotropy in seismic processing however the anisotropy itself holds valuable information about rock properties and as such can be viewed as a seismic attribute here we summarize results from an integrated project that explored the potential to use observations of seismic anisotropy to interpret lithological and fluid properties the sail project our approach links detailed petrofabric analyses of reservoir rocks laboratory based measurements of ultrasonic velocities in core samples and reservoir scale seismic observations we present results for the clair field a carboniferous devonian reservoir offshore scotland west of the shetland islands the reservoir rocks are sandstones that are variable in composition and exhibit anisotropy on three length scales the crystal grain and fracture scale we have developed a methodology for assessing crystal preferred orientation cpo using a combination of electron back scattered diffraction ebsd x ray texture goniometry xrtg and image analysis modal proportions of individual minerals are measured using quantitative x ray diffraction qxrd these measurements are used to calculate the intrinsic anisotropy due to cpo via voigt reuss hill averaging of individual crystal elasticities and their orientations the intrinsic anisotropy of the rock is controlled by the phyllosilicate content and to a lesser degree the orientation of quartz and feldspar the latter can serve as a palacoflow indicator our results show remarkable consistency in cpo throughout the reservoir and allow us to construct a mathematical model of reservoir anisotropy a comparison of cpo predicted velocities and those derived from laboratory measurements of ultrasonic signals allows the estimation of additional elastic compliance terms due to grain boundary interactions the results show that the cpo estimates are good proxies for the intrinsic anisotropy of the clean sandstones the more micaceous rocks exhibit enhanced anisotropy due to interactions between the phyllosilicate grains we then compare the lab scale predictions with reservoir scale measurements of seismic anisotropy based on amplitude variation with offset and azimuth avoa analysis and non hyperbolic moveout our mathematical model provides a foundation for interpreting the reservoir scale seismic data and improving the geological modelling of complex reservoirs the observed increases in avoa signal with depth can only be explained with an increase in fracturing beneath the major unit boundaries rather than a chan e in intrinsic cpo properties in general the style and magnitude of anisotropy in the clair field appears to be indicative of reservoir quality less

compressional to shear wave velocity ratio of granular rocks role of rough grain contacts experimental data indicate that although the compressional vp and shear vs wave velocities of unsaturated monomineralic granular sedimentary rocks vary with grain mineralogy degree of compaction porosity and confining pressure their ratio vp vs is to various degrees insensitive to these factors a rough contact model for the elastic properties of a granular aggregate is presented which is consistent with these observations the relevant assumptions of the model are 1 the medium is a random aggregate of monomineralic elastically isotropic equal size grains and 2 the intergrain contacts are rough and their elastic properties are described by the results of yoshioka and scholz 1989 the model predicts that vp vs the bulk to shear frame modulus ratio and the poisson s ratio of the aggregate are independent of porosity and confining pressure and only weakly dependent on mineral poisson s ratio as the mineral poisson s ratio varies between 0 08 and 0 40 vp vs and are essentially constant at 1 55 and 1 07 respectively and the poisson s ratio varies between 0 141 and 0 144 these values are in good agreement with experimental data on dry monomineralic granular rocks

kerogen maturation data in the uinta basin utah usa constrain predictions of natural hydrocarbon seepage into the atmosphere natural seepage of methane from the lithosphere to the atmosphere occurs in regions with large natural gas deposits according to some authors it accounts for roughly 5 of the global methane budget i explore a new approach to estimate methane fluxes based on the maturation of kerogen which is the hydrocarbon polymer present in petroleum source rocks and whose decomposition leads to the formation of oil and natural gas the temporal change in the atomic h c ratio of kerogen lets us estimate the total carbon mass released by it in the form of oil and natural gas then the time interval of active kerogen decomposition lets us estimate the average annual formation rate of oil and natural gas in any given petroleum system which i demonstrate here using the uinta basin of eastern utah as an example obviously this is an upper bound to the average annual rate at which natural gas seeps into the atmosphere after adjusting for biooxidation of natural gas i conclude that the average annual seepage rate in the uinta basin is not greater than 3100 900 tonne yr 1 this is 0 5 0 15 of the total flux of methane into the atmosphere over the basin as measured during aircraft flights i speculate about the difference between the regional 0 5 and the global 5 estimates

seismic anisotropy of shales the seismic properties and preferred clay mineral orientation of a suite of shales are investigated using laboratory velocity measurements as a function of confining pressure x ray diffraction techniques and electron microprobe backscatter bse imaging the velocity measurements indicate that these shales are transversely isotropic with the main symmetry axis perpendicular to bedding anisotropy at elevated pressures caused mainly by preferred orientation of clays illite parallel to bedding ranges from 20 vp and 19 vs for a sample of new albany shale to 30 vp and 35 vs for a sample of chattanooga shale the degree of clay mineral alignment in the shales is constrained by orientation indices produced using simple x ray diffraction techniques a strong positive correlation is found between the degree of preferred orientation as expressed in the orientation indices and seismic anisotropy bse images of the shale fabrics confirm in a qualitative manner the results of the x ray study to investigate wave propagation in the shales elastic constants of each sample are calculated and used to produce phase and group wave velocity surfaces which describe variation in velocity as a function of angle to the bedding normal the calculated velocity surfaces constrained by independent velocity measurements display a lack of shear wave splitting at near normal incidence in even the most anisotropic shales for the highly anisotropic chattanooga shales group velocity surfaces differ significantly from corresponding phase velocity surfaces

dispersion and anisotropy of elastic waves in cracked rocks 1 rocks in the earth s crust contain variable amount of cracks depending on the deviatoric and confining stress levels pore pressure and temperature conditions crack damage results in effects that have been investigated for a long time in particular a decrease in elastic wave velocity and the development of anisotropy in this paper we focus on cracked rocks and develop a method to calculate both the elastic wave anisotropy and the dispersion in a fluid saturated cracked rock we show that analytic expressions can be derived for both properties in terms of the crack density tensor components two fundamental theoretical tools are used in this purpose the first one is the effective medium theory and the second one is the anisotropic poroelastic theory using both it is possible from first principles to predict anisotropy and dispersion results are shown to be valid up to a total crack density of 0 5 the failure threshold appears close to a total crack density of 1 between 0 5 and 1 the model still gives reasonably good results anisotropy can increase up to 60 and dispersion up to 30 depending on the crack distribution these results point out important differences that could exist in fluid saturated rocks between laboratory high frequency data and field low frequency results

petrology and isotopic geochemistry of dawsonite bearing sandstones in hailaer basin northeastern china abstract the co2 gas reservoir sandstones in the hailaer basin contain abundant dawsonite and provide an ideal laboratory to study whether any genetic relationship exists between dawsonite and the modern gas phase of co2 the origins of dawsonite and co2 in these sandstones were studied by petrographic and isotopic analysis according to the paragenetic sequence of the sandstones dawsonite grew later than co2 charging at 110 85 ma the dawsonite 18o value is 7 4 smow and the calculated 18o values of the water present during dawsonite growth are from 11 4 to 9 2 smow this combined with the nahco3 dominated water linked to dawsonite growth suggests meteoric water being responsible for dawsonite growth the 13c values of gas phase co2 and the ratios of 3he 4he of the associated he suggest a mantle magmatic origin of co2 rich natural gas in hailaer basin dawsonite 13c values are 5 3 to 1 5 average 3 4 and the calculated 13c values of co2 gas in isotopic equilibrium with dawsonite are 11 4 to 7 3 these c isotopic values are ambiguous for the dawsonite c source from the geological context the timing of events together with formation water conditions for dawsonite growth dawsonite possibly grew in meteoric derived water atmospherically derived co2 maybe or at least the dominant c source for dawsonite it seems that there are few relationships between dawsonite and the modern gas phase of co2 in the hailaer basin

porosity and elastic anisotropy of rocks under tectonic stress and pore pressure changes elastic properties of rocks depend on tectonic stress using the theory of poroelasticity as a constraint we analyze features of these dependencies related to changes in rock pore space geometry we develop a formalism describing elastic moduli and anisotropy of rocks as nonlinear functions of confining stress and pore pressure this formalism appears to agree with laboratory observations to a first approximation elastic moduli and seismic velocities as well as porosity depend only on the difference between the confining tectonic stress and pore pressure however in general both the confining stress tensor and the pore pressure must be taken into account as independent variables the stress dependent geometry of the pore space fully controls the stress induced changes in elastic moduli and seismic velocities specifically the compliant porosity plays the most important role despite the fact that in many rocks the compliant porosity is a very small part of total porosity changes in compliant porosi

nonlinear rock physics model for estimation of 3d subsurface stress in anisotropic formations theory and laboratory verification we develop a rock physics model based on nonlinear elasticity that describes the dependence of the effective stiffness tensor as a function of a 3d stress field in intrinsically anisotropic formations this model predicts the seismic velocity of both p and s waves in any direction for an arbitrary 3d stress state therefore the model overcomes the limitations of existing empirical velocity stress models that link p wave velocity in isotropic rocks to uniaxial or hydrostatic stress to validate this model we analyze ultrasonic velocity measurements on stressed anisotropic samples of shale and sandstone with only three nonlinear constants we are able to predict the stress dependence of all five elastic medium parameters comprising the transversely isotropic stiffness tensor we also show that the horizontal stress affects vertical s wave velocity with the same order of magnitude as vertical stress does we develop a weakanisotropy approximation that directly links commonly measured anisotropic thomsen parameters to the principal stresses each thomsen parameter is simply a sum of corresponding background intrinsic anisotropy and stressinduced contribution the stress induced part is controlled by the difference between horizontal and vertical stresses and coefficients depending on nonlinear constants thus isotropic rock stays isotropic under varying but hydrostatic load whereas transversely isotropic rock retains the same values of dimensionless thomsen parameters only unequal horizontal and vertical stresses alter anisotropy since thomsen parameters conveniently describe seismic signatures such as normal moveout velocities and amplitudevariation with offset gradients this approximation is suitable for designing new methods for the estimation of 3d subsurface stress from multicomponent seismic data

investigation of core data reliability to support time lapse interpretation in campos basin brazil quantitative interpretation of time lapse seismic data requires knowledge of saturation and pressure effects on seismic velocities although the former can usually be modeled adequately using the gassmann equation the latter is obtained mainly by laboratory measurements which can be affected by core damage we investigate the magnitude of this effect on compressional wave velocities by comparing laboratory experiments and log measurements we use gassmann fluid substitution to obtain low frequency saturated velocities from dry core measurements thus mitigating the dispersion effects taken at reservoir pressure the analysis is performed for an unusual densely cored well from which 43 cores were extracted over a 45 m thick turbidite reservoir these computed velocities show very good agreement with the sonic log measurements this confirms that for this particular region the effect of core damage on ultrasonic measurements is less than the measurement error consequently stress sensitivity of elast

lattice preferred orientation and seismic anisotropy in sedimentary rocks summary although it is well known that sedimentary rocks can be seismically anisotropic there have been few detailed investigations of the underlying cause of such anisotropy here we investigate anisotropy due to the preferred orientation of minerals or lattice preferred orientation lpo in a suite of sedimentary rocks seismic properties are predicted by averaging single crystal elastic constants of minerals according to their crystal orientation and modal volume fraction in the rock aggregate both electron backscattered diffraction ebsd and x ray texture goniometry xtg are tested as quantitative techniques for measuring the lpo of sedimentary rocks although ebsd has promise for future lpo measurements in polymineralic sedimentary rocks problems currently remain in measuring low symmetry phases e g feldspars and very small clay or mica particles however the lpo of very fine grained phyllosilicates can be measured using xtg and the lpo of low symmetry minerals can be measured using manual ebsd pattern analysis here we use such a hybrid approach to estimate lpo in a suite of sedimentary samples the seismic properties calculated from lpo data show anisotropy values for p waves ranging from 1 5 per cent in sandstones to over 3 5 per cent in a siltstone to 12 per cent in a shale the effect of thin multilayering on long wavelength propagation in a siltstone is predicted by applying backus type averaging the layering does not enhance the anisotropy because of the small differences in density and in elasticity between the two layer types the lpo of phyllosilicates and to a lesser extent dolomite and siderite seem to contribute significantly to the seismic anisotropy of phyllosilicate rich rocks siltstones and shales the weak lpo of quartz in sandstones causes a few per cent anisotropy cumulatively our results suggest that field observations of seismic anisotropy have the potential to be used as an indicator of rock lithology

petrological record of co 2 influx in the dongying sag bohai bay basin ne china abstract dawsonite occurs in sandstones of the fourth member of the shahejie formation dongying sag bohai bay basin northeastern china the paragenesis sequence formation temperatures carbon dioxide co2 and aqueous medium origin of dawsonite from the binnan area were studied to understand the formation of dawsonite as a possible mechanism for mineral trapping of co2 in carbon capture and storage ccs programs the petrological and geochemical characteristics of the dawsonite bearing sandstones were identified using polarizing microscope examination scanning electron microscope x ray fluorescence x ray diffraction and stable isotopic analyses the paragenesis sequence of the authigenic minerals are mainly quartz overgrowth early stage calcite a ferrocalcite dawsonite late stage calcite b ankerite siderite the carbon and oxygen isotope compositions of dawsonite reveal that 13c values of dawsonite range from 2 27 to 3 50 indicating inorganic co2 the calculated 13c values of co2 gas in isotopic equilibrium with dawsonite range from 7 2 to 1 6 suggesting that co2 originated mainly from degassing of mantle derived magmas and partly from thermal decomposition of carbonate rocks as supported by regional magmatic activities rifting and observation of carbonate during drilling in well bin76 the 18o values of dawsonite vary from 17 2 to 19 8 and the calculated 18o values of h2o in isotopic equilibrium with dawsonite range from 2 1 to 0 4 the mixed aqueous medium was mainly connate water influenced by magmatic activities and meteoric water influx during the formation of dawsonite according to oxygen isotope data the formation temperatures of dawsonite in the binnan area range from 77 c to 95 c which are consistent with the estimated paleo temperatures 79 c to 85 c indicating that the dawsonite formed at the late stage of the diagenetic phase

the effect of microstructure and nonlinear stress on anisotropic seismic velocities recent work in hydrocarbon reservoir monitoring has focused on developing coupled geomechanical fluid flow simulations to allow production related geomechanical effects such as compaction and subsidence to be included in reservoir models to predict realistic time lapse seismic signatures generation of appropriate elastic models from geomechanical output is required these elastic models should include not only the fluid saturation effects of intrinsic shape induced and stress induced anisotropy but also should incorporate nonlinear stress dependent elasticity to model nonlinear elasticity we use a microstructural effective medium approach in which elasticity is considered as a function of mineral stiffness and additional compliance is caused by the presence of low aspect ratio displacement discontinuities by jointly inverting observed ultrasonic p and s wave velocities to determine the distribution of such discontinuities we assessed the appropriateness of modeling them as simple planar penny shaped features by using this approximation we developed a simple analytical approach to predict how seismic velocities will vary with stress we tested our approach by analyzing the elasticity of various sandstone samples from a united kingdom continental shelf ukcs reservoir some of which display significant anisotropy as well as two data sets taken from the literature

supercritical carbon dioxide and sulfur in the madison limestone a natural analog in southwest wyoming for geologic carbon sulfur co sequestration article i nfo the madison limestone on the moxa arch southwest wyoming usa contains large volumes 65 95 of supercritical co2 that it has stored naturally for 50 million years this reservoir also contains supercritical h2s aqueous sulfur complexes so4 2 and hs and sulfur bearing minerals anhydrite and pyrite although so2 is not present these sulfur bearing phases are known products of so2 disproportionation in other water rock systems the natural co occurrence of so4 2 s 2 supercritical co2 and brine affords the opportunity to evaluate the fate of a carbon sulfur co sequestration scenario mineralogic data was obtained from drill core and aqueous geochemical data from wells outside and within the current supercritical co2 sulfur brine rock system in addition to dolomite calcite and accessory sulfur bearing minerals the madison limestone contains accessory quartz and the aluminum bearing minerals feldspar illite and analcime dawsonite naalco3 oh 2 predicted as an important carbon sink in sequestration modeling studies is not present after confirming equilibrium conditions for the madison limestone system reaction path models were constructed with initial conditions based on data from outside the reservoir addition of supercritical co2 to the madison limestone was simulated and the results compared to data from inside the reservoir the model accurately predicts the observed mineralogy and captures the fundamental changes expected in a madison limestone brine system into which co2 is added ph decreases from 5 7 to 4 5 at 90 c and to 4 0 at 110 c as expected from dissolution of supercritical co2 creation of carbonic acid and buffering by the carbonate rock the calculated redox potential increases by 0 1 v at 90 c and 0 15 v at 110 c due to equilibrium among co2 anhydrite and pyrite final calculated eh and ph match conditions for the co existing sulfur phases present in produced waters and core from within the reservoir total dissolved solids increase with reaction progress mostly due to dissolution of calcite with an accompanying increase in dissolved bicarbonate the madison limestone is a natural example of the thermodynamic end point that similar fluid rock systems will develop following emplacement of a supercritical co2 sulfur mixture and is a natural analog for geologic carbon sulfur co sequestration