seismic velocity anisotropy of phyllosilicate rich rocks characteristics inferred from experimental and crack model studies of biotite rich schist summary seismic velocity anisotropy of biotite schist 30 per cent mode biotite was measured under confining pressures up to 150 mpa the rock shows weak orthotropy which was altered from transverse isotropy ti generated by biotite preferred orientation the orthotropy was caused by microfolding in the rock the velocity increase under confining pressure indicates that most crack planes are aligned parallel to the cleavage planes silicate sheet of the oriented biotite minerals the anisotropy of the rock is basically ti due to both the aligned biotite minerals and cracks which have a common symmetry axis we found that other sheet silicate rich rocks have a similar anisotropy with the biotite schist in which the ti type anisotropy is characterized by the slow p and s wave velocities along the symmetry axis this is caused by the preferred orientation of sheet silicate minerals and the extremely slow p and s wave velocities along the axis perpendicular to the silicate sheet compared to the directions along the silicate sheet when rock contains a large percentage of highly oriented sheet silicates the fast and slow shear waves exchange their polarities at some off symmetry axis directions indicating that the qswave quasi s wave velocity exceeds the sh wave velocity the phase velocity distribution of qs wave shows an asymmetry with respect to the angle from the symmetry axis which is characterized by a bulge in this distribution located near the symmetric axis this is inherent to most sheet silicate minerals when crack density of aligned cracks increases the p wave velocity along the symmetry axis decreases considerably the qs wave phase velocity in the off axis directions also decreases in accordance with the decrease of the p velocity along the symmetry axis the asymmetry of the qs wave phase velocity distribution increases as the p wave velocity decreases along the symmetry axis this relationship can be well understood by means of berryman s extended thomsen approach

frequency dependent seismic anisotropy due to fractures fluid flow versus scattering abstractanisotropy is a useful attribute for the detection and characterization of aligned fracture sets in petroleum reservoirs unfortunately many of the traditional effective medium theories for modeling the seismic properties of fractured rock are insensitive to the size of the constituent fractures for example the same pattern of anisotropy may be produced by a high concentration of small stiff cracks or by a lower concentration of large compliant fractures the distinction between these models is important for assessing permeability anisotropy because fluid flow is dominated by the largest fractures one method to gain further insight is through the analysis of frequency dependent shear wave splitting in microseismic data because fracture compliance is frequency dependent and microseismic data are relatively rich in frequency content we compared two potential mechanisms causing frequency dependent compliance of fractures 1 squirt flow in fractured porous rock and 2 wave scattering over r

shear wave splitting of the 2009 l aquila seismic sequence fluid saturated microcracks and crustal fractures in the abruzzi region central apennines italy

velocity anisotropy of shales and depth estimation in the north sea basin it is known that in the north sea basin the depths to major reflectors as determined from surface seismic data are often larger than the well log depths from a study of data sets which tie 21 wells i found a strong correlation between the occurrence of the depth error and the presence of shales in the subsurface assuming that the error is caused by elliptical velocity anisotropy in shales i measured the anisotropy from a comparison of the well log sonic data and the interval velocity profile obtained from the surface seismic data and also from a comparison of the seismic depth and the well log depth it was found that the two methods of measurements agree with each other and also agree qualitatively with the previous laboratory measurements of anisotropy in shale samples the results strongly suggest that the depth anomaly in the north sea basin is caused by the velocity anisotropy of shales a simple method to correct the seismic depth is given

seismic migration in elliptically anisotropic media1 the study of wave propagation in media with elliptical velocity anisotropy shows that seismic energy is focused according to the horizontal component of the velocity field while the vertical component controls the time to depth relation this implies that the vertical component cannot be determined from surface seismic velocity analysis but must be obtained using borehole or regional geological information both components of the velocity field are required to produce a correctly focused depth image a paraxial wave equation is developed for elliptical anisotropic wave propagation which can be used for modelling or migration this equation is then transformed by a change of variable to a second paraxial equation which only depends on one effective velocity field a complete anisotropic depth migration using this transformed equation involves an imaging step followed by a depth stretching operation this allows an approximate separation or splitting of the focusing and depth conversion steps of depth migration allowing a different velocity model to be used for each step this split anisotropic depth migration produces a more accurate result than that obtained by a time migration using the horizontal velocity field followed by an image ray depth conversion using the vertical velocity field the results are also more more accurate than isotropic depth migration and yield accurate imaging in depth as long as the lateral variations in the anisotropy are slow less

seismic velocity modelling of the carboneras fault zone se spain abstract the carboneras fault zone forms part of a major strike slip fault system in se spain striking ne sw and accommodating up to 40 km displacement it affects basement metamorphic rocks and unconformably overlying upper miocene sediments and volcanic rocks high resolution shallow seismic tomographic sections were made across the fault zone in two localities from the same areas fault rocks and their wallrocks were collected for laboratory seismic velocity measurements the laboratory data were corrected for the substantial effects of near surface crack damage by combining these results with geological cross sections forward velocity models for the fault zone were constructed to compare with field seismic measurements and hence to ground truth the inferences made from them these velocity depth relationships matched moderately well with those extracted from the in situ tomography results aspects of the in situ seismic sections matched features on the forward modelled sections but the comparisons showed that it is important to have some degree of foreknowledge of the geology to be able successfully to interpret seismic tomography sections as an exploration tool

stress induced seismic azimuthal anisotropy sand shale content and depth trends offshore north west australia abstractseismic azimuthal anisotropy is apparent when p wave velocities vary with source receiver azimuth and downward propagating s waves split into two quasi s waves polarized in orthogonal directions not accounting for these effects can degrade seismic image quality and result in erroneous amplitude analysis and geologic interpretations there are currently no physical models available to describe how azimuthal anisotropy induced by differential horizontal stress varies with sand shale lithology and depth we develop a model that does so in unconsolidated sand shale sequences offshore north west australia our method naturally introduces two new concepts critical anisotropy and anisotropic depth limit critical anisotropy is the maximum amount of azimuthal anisotropy expected to be observed at the shallowest sediment burial depth where the confining pressure and sediment compaction are minimal the anisotropic depth limit is the maximum depth where the stress induced azimuthal anisotropy is ex

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

reflection shear wave data collected near the principal axes of azimuthal anisotropy the presence of vertically oriented fractures and or unequal horizontal stresses has created an azimuthally anisotropic earth in which shear wave sh data collected along the principal axes of the anisotropy display time and reflection amplitude anomalies amoco shot two crossing shear wave sh lines that were approximately parallel to the orthogonal principal axes of the azimuthal anisotropy at the tie point these crossing sh lines display a time variant mis tie the tie point also displays reflection coefficient anomalies attributable to azimuthally dependent shear wave velocities field mapping documented a set of fractures striking n69e which are approximately parallel to the line that exhibited greater traveltimes time variant mis ties and reflection coefficient anomalies are two of the seismic responses theoretically expected of an azimuthally anisotropic earth i e one in which the shear wave velocity depends upon the polarization azimuth of the shear wave

anisotropic effective medium modeling of the elastic properties of shales shales are complex porous materials normally consisting of percolating and interpenetrating fluid and solid phases the solid phase is generally comprised of several mineral components and forms an intricate and anisotropic microstructure the shape orientation and connection of the two phases control the anisotropic elastic properties of the composite solid we develop a theoretical framework that allows us to predict the effective elastic properties of shales its usefulness is demonstrated with numerical modeling and by comparison with established ultrasonic laboratory experiments the theory is based on a combination of anisotropic formulations of the self consistent sca and differential effective medium dem approximations this combination guarantees that both the fluid and solid phases percolate at all porosities our modeling of the elastic properties of shales proceeds in four steps first we consider the case of an aligned biconnected clay fluid composite composed of ellipsoidal inclusion

stress induced temporal variations in seismic anisotropy observed in microseismic data summary in situ stress monitoring of crustal rocks is desirable as it yields insights into earthquake mechanisms volcanic eruptions and changes in hydrocarbon reservoirs shear wave splitting induced by stress controlled cracks in the shallow crust provides a way to infer this stress temporal variations in these observations can be difficult to quantify due to scatter in the data and discontinuous observations here we present evidence of temporal variations in shear wave splitting from a continuous time series with a high occurrence of microseismic events recorded in a borehole we interpret these observations in terms of stress controlled cracks and are able to infer changes in stress and via modelling suggest the cause of the anisotropy possible origins of the temporal variation in per cent anisotropy are tidal or oil field production processes our results suggest that shear wave splitting is a viable probe for inferring changes in crustal stress in cracked rock

high resolution x ray texture goniometry high resolution x ray texture goniometry hrxtg significantly improves peak background ratios and allows for the determination of crystallographic preferred fabrics of small regions 1 mm2 in heterogeneously deformed rocks the basic instrument is a commercially available automated single crystal x ray diffractometer equipped with a mo source a stage for transmission mode was constructed that is driven by the original motors so the unit is fully computer controlled absorption corrections are calculated which removes the need for generally difficult to obtain empirical corrections this combined with equipment optimization enables us to use small irradiated volumes of rocks with variable compositions the advantage of hrxtg in heterogeneously deformed rocks is demonstrated in a cleaved mudstone analysis of 1 mm2 areas in a 2 cm bed showing a gradation from micaceous sandstone to slate reveals that fabric intensity of both mica and chlorite increases significantly dm from 0 64 to 1 03 and from 0 45 to 0 78 respectively without appreciable change in the oblate shape km of the fabric thus hrxtg can record a strain state at the mm scale of heterogeneously deformed rocks

the first 3d 4 c ocean bottom seismic surveys in the caspian sea acquisition design and processing strategy bubbling mud volcanoes create havoc for conventional seismic images over the crest of the azeri chirag gunashli acg structure in the caspian sea layers of trapped gas and shallow mud flows in the overburden sediments leave areas of severely degraded data quality at the crest of the reservoir structure fortunately in 2002 the application of the first 3d four component 3d 4 c seismic in the caspian has helped lift the veil of mud and gas attenuation which previously obscured the details of the subsurface reservoir

cracking the fractures seismic anisotropy in an offshore reservoir the clair field lies west of the shetland islands on the u k continental shelf in 140 m of water figure 1

t matrix approach to shale acoustics summary the t matrix approach of quantum scattering theory is used here to place many long wavelength equivalent medium approximations for porous composites polycrystals and cracked media on a common footing and to indicate their limitations but also to derive some new results based on two point statistics in this way we have obtained an insight into the difficult problem of elastic inclusions at finite concentrations which is of foremost relevance when estimating the effective material parameters of porous cracked shales involving stacks of more or less horizontally aligned clay platelets mixed together with more rounded silt minerals and with fluid filling the spaces a rather involved perturbative analysis of the effects of interactions between or structural correlations among the various inclusions minerals and cavities making up a real shale of hexagonal symmetry was performed in an attempt to obtain a better match between theoretical predictions based on a combination of coherent and optical potential approximations and experimental results recovered from ultrasonic wave speeds for the effective elastic stiffness tensor for the particular data set considered in this study the t matrix approach was able to match the data better than the approach of hornby et al but the match was not completely satisfactory further progress in theoretical shale modelling may come from a better knowledge of the elastic properties of pure clay minerals a more detailed knowledge of the microstructure of shales the incorporation of constraints obtained from comparisons between theoretical predictions and experimental results as well as a continuing development of the t matrix approach numerical results also for the effect of bedding parallel microcracks on the elasticity of such a real shale have value in illustrating the importance of taking into account the effects of spatial distribution when trying to deal with non dilute mixtures of highly contrasting material properties

estimating aquifer hydraulic properties from the inversion of surface streaming potential sp anomalies 1 electrokinetic effects of water flow during pumping tests have been shown to generate surface streaming potential sp anomalies of several tens of mv that are well correlated with the geometry of the water table it follows that sp measurements can be used to estimate aquifer hydraulic properties we have developed an inversion scheme for surface sp data generated by flow pumping and found that we are able to estimate the hydraulic conductivity and the depth and the thickness of the aquifer we applied our inversion scheme to the data from bogoslovsky and ogilvy 1973 and found a hydraulic conductivity of 10 5 m s 1 an aquifer thickness of roughly 28 m and an electrokinetic coupling coefficient of 1 mv m 1 these values are in the range of what is expected for this kind of environment

the role of texture cracks and fractures in highly anisotropic shales organic shales generally have low permeability unless fractures are present however how gas oil and water flows into these fractures remains enigmatic the alignment of clay minerals and the alignment of fractures and cracks are effective means to produce seismic anisotropy thus the detection and characterization of this anisotropy can be used to infer details about lithology rock fabric and fracture and crack properties within the subsurface we present a study characterizing anisotropy using s wave splitting from microseismic sources in a highly anisotropic shale we observe very strong anisotropy up to 30 with predominantly vti vertical transverse isotropy symmetry but with evidence of an hti horizontal transverse isotropy overprint due to a ne striking vertical fracture set parallel to the maximum horizontal compressive stress we observe clear evidence of a shear wave triplication due to anisotropy which to our knowledge is one of only a very few observations of such triplications in field scale data we use modal proportions of minerals derived from x ray fluorescence data combined with realistic textures to estimate the contribution of intrinsic anisotropy as well as possible contributions of horizontally aligned cracks we find that aligned clays can explain much of the observed anisotropy and that any cracks contributing to the vertical transverse isotropy vti must have a low ratio of normal to tangential compliance zn zt typical of isolated cracks with low hydraulic connectivity subhorizontal cracks have also been observed in the reservoir and we propose that their reactivation during hydraulic fracturing may be an important mechanism to facilitate gas flow

pre development fracture modelling in the clair field west of shetland abstract the clair oilfield is a large fractured sandstone reservoir lying 75 km west of shetland on the uk continental shelf fracture analysis and modelling was carried out in preparation for the phase 1 development which started production early in 2005 fracture clusters and discrete fluid inflows observed in wells are associated with faults and other localized deformation features tens or hundreds of metres apart the reservoir has moderate to good matrix permeability but well flow rates and profiles are fracture dominated full field geological models were built using conventional object modelling approaches for matrix and discrete fracture networks for fractures and upscaled to populate a reservoir simulation grid dual porosity dual permeability dynamic modelling full field and well test was undertaken to understand the fracture and matrix flow contributions and their interaction fracture models were conditioned to wells and to seismic data including coherency and multi azimuthal velocity information from a four component ocean bottom cable three dimensional seismic survey at this early stage in field development there is insufficient calibration to select a single fracture model instead well and depletion plans have been tested against multiple fracture models chosen to encompass a wide range of plausible outcomes

seismic anisotropy in trinidad a new tool for lithology prediction a great challenge facing explorationists is identification of potential intervals of reservoir quality sand in areas with little well control this is especially acute in deep water where reservoir presence can be elusive and well costs are considerable substantial evidence suggests the occurrence of transversely isotropic ti anisotropy in the subsurface associated primarily with shales while intervals of clastic material show greatly reduced and or near zero ti anisotropy although this has been demonstrated conclusively on core samples the need exists to translate these concepts into a geologic framework on a basinwide and prospect scale this article presents a method for estimating interval anisotropy from nonhyperbolic moveout on common midpoint gathers and relating these values to subsurface geology alkhalifah and tsvankin 1995 demonstrated that for ti media with a vertical symmetry axis vti media only two parameters are sufficient for performing all time related processing such as nmo correction including nonhyperbolic moveout correction if necessary dmo correction and prestack and poststack time migration one of these parameters is dimensionless its departure from zero implies anisotropy the other parameter vnmo is the short spread nmo velocity for horizontal reflectors these two parameters can be obtained solely from surface p wave data using estimates of stacking velocity for reflections from interfaces having two distinct dips alkhalifah and tsvankin 1995 vti media induce nonhyperbolic moveout of reflections that depends on vnmo and at large offsets alkhalifah 1997 showed that these parameters can be obtained by analyzing the moveout at far offsets the direct output from such an inversion or velocity analysis is the effective values of vnmo and that include the combined average influence of the overburden these effective values are converted to interval values using layer stripping techniques using data from offshore africa alkhalifah et al 1996 showed that anisotropic

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

fracture characterization at valhall application of p wave amplitude variation with offset and azimuth avoa analysis to a 3d ocean bottom data set the delineation and characterization of fracturing is important in the successful exploitation of many hydrocarbon reservoirs such fracturing often occurs in preferentially aligned sets if the fractures are of subseismic scale this may result in seismic anisotropy thus measurements of anisotropy from seismic data may be used to delineate fracture patterns and investigate their properties here fracture induced anisotropy is investigated in the valhall field which lies in the norwegian sector of the north sea this field is a chalk reservoir with good porosity but variable permeability where fractures may significantly impact production e g during waterflooding to investigate the nature of fracturing in this reservoir p wave amplitude variation with offset and azimuth avoa is analyzed in a 3d ocean bottom cable obc data set in general 3d ocean bottom seismic obs acquisition leads to patchy coverage in offset and azimuth and this must be addressed when considering such data to overcome this challenge and others associated with 3d obs acquisition a new method for processing and analysis is presented for example a surface fitting approach which involves analyzing azimuthal variations in avo gradients is used to estimate the orientation and magnitude of the fracture induced anisotropy this approach is also more widely applicable to offset azimuth analysis of other attributes e g traveltimes and any data set where there has been true 3d data acquisition land or marine using this new methodology we derive high resolution maps of p wave anisotropy from the avoa analysis for the top chalk reflection at valhall these anisotropy maps show coherent but laterally varying trends synthetic avoa modeling using effective medium models indicates that if this anisotropy is from aligned fracturing the fractures are likely liquid filled with small aspect ratios and the fracture density must be high furthermore we show that the fracture normal direction is parallel to the direction of most positive avo gradient in other situations the reverse can be true i e the fracture normal direction can be parallel to the direction of the most negative avo gradient effective medium modeling or comparisons with anisotropy estimates from other approaches e g azimuthal variations in velocity must therefore be used to resolve this ambiguity the inferred fracture orientations and anisotropy magnitudes show a degree of correlation with the positions and alignments of larger scale faults which are estimated from 3d coherency analysis overall this work demonstrates that significant insight may be gained into the alignment and character of fracturing and the stress field variations throughout a field using this high resolution avoa method

3 d prestack depth migration in anisotropic media a case study at the lodgepole reef play in north dakota a key problem in interpreting depth migrated sections prestack or poststack is that seismic horizons almost never agree with well tops often seismic events are on the order of 1000 ft deeper seismic anisotropy is mostly blamed for the discrepancy in this paper we show a case study at the lodgepole reef play in north dakota where by incorporating strong seismic anisotropy in a 3 d velocity model we can achieve good depth migration results and excellent well ties within 50 ft our estimate of anisotropy parameters agrees with laboratory measurements on core samples in 1996 shell acquired 200 miles2 of 3 d seismic data in north dakota to explore for mississippian age lodgepole mounds structure in williston basin is relatively uncomplicated but prospecting for subtle structures and for stratigraphic traps which include the lodgepole is made more difficult by a number of geologic factors that affect both time and depth imaging exploration targets in the lodgepole play near dickinson north dakota lie at 9500 ft drill depths about 1 9 s two way time the geologic section is relatively flat lying and includes a broad range of lithologies and depositional environments the upper half section is dominated by shales and sandstones and the lower half by carbonates typical succession and alternation of lithologies at a closer scale shows great variation numerous formations consist of anhydrite and halite and in many areas salts have undergone partial or complete dissolution the dissolution shows both time and areal variation and the overlying formations carry this imprint into their original depositional and later diagenetic patterns other formations show significant lateral thickness variations due to sand dune distribution or to local patterns of thins and thicks which result from widespread and intermittently active vertical fault systems interacting with frequent episodes of uplift and subsidence success in the lodgepole exploration play has

geochemistry of the early proterozoic metasedimentary rocks of the alligator rivers region northern territory australia abstract the early proterozoic metasedimentary sequence of the alligator rivers region a part of the pine creek geosyncline in the northern territory australia overlies an archaean granitoid basement early proterozoic sedimentary sequences in general record important changes in the composition of the upper continental crust about the archaean proterozoic boundary however the geochemistry of only a few of these sequences has been documented the geochemistry of the early proterozoic succession in the alligator rivers region is reported here and the results are interpreted in terms of differences between the stratigraphic units their provenance particularly in relation to crustal evolution and their subsequent metamorphism and weathering clastic metasedimentary rocks throughout the alligator rivers region have a remarkably uniform major and trace element geochemistry the kakadu group and upper member of the cahill formation are relatively more enriched in sio 2 and correspondingly more depleted in al 2 o 3 than the rest of the sequence reflecting the greater dominance of metapsammitic assemblages the lower member of the cahill formation which hosts the major u deposits of the alligator rivers region and the metasedimentary sequence in general exhibit no significant enrichment in u above normal background values rare earth element ree concentrations in the metasedimentary units within the alligator rivers region are uniform though in detail there are some important differences within and between formations the composition of the early proterozoic clastic metasediments in the alligator rivers region is consistent with the composition of similar material of the same age from other areas and supports current ideas on crustal evolution the alligator rivers metasediments are enriched in si and k and depleted in mg ca and na relative to the archaean average for clastic sedimentary rocks and their ree geochemistry resembles typical post archaean sedimentary rocks having a light ree enriched pattern and a distinct eu eu depletion compared to typical archaean sediments however the ree data indicate that two compositionally distinct sources are involved in the provenance of the kakadu group and possibly the lower member of the cahill formation where two types of ree patterns can be distinguished on their hree concentration and eu eu anomaly

digital rock physics numerical prediction of pressure dependent ultrasonic velocities using micro ct imaging digital rock physics numerical prediction of pressure dependent ultrasonic velocities using micro ct imaging

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

detection of multiple fracture sets using observations of shear wave splitting in microseismic data the hydrocarbon industry is moving increasingly towards tight sandstone and shale gas resources reservoirs that require fractures to be produced economically therefore techniques that can identify sets of aligned fractures are becoming more important fracture identification is also important in the areas of coal bed methane production carbon capture and storage ccs geothermal energy nuclear waste storage and mining in all these settings stress and pore pressure changes induced by engineering activity can generate or reactivate faults and fractures p and s waves are emitted by such microseismic events which can be recorded on downhole geophones the presence of aligned fracture sets generates seismic anisotropy which can be identified by measuring the splitting of the s waves emitted by microseismic events the raypaths of the s waves will have an arbitrary orientation controlled by the event and geophone locations meaning that the anisotropy system may only be partly illuminated by the available arrivals therefore to reliably interpret such splitting measurements it is necessary to construct models that compare splitting observations with modelled values allowing the best fitting rock physics parameters to be determined commonly splitting measurements are inverted for one fracture set and rock fabrics with a vertical axis of symmetry in this paper we address the challenge of identifying multiple aligned fracture sets using splitting measured on microseismic events we analyse data from the weyburn ccs eor reservoir which is known to have multiple fracture sets and from a hydraulic fracture stimulation where it is believed that only one set is present we make splitting measurements on microseismic data recorded on downhole geophone arrays our inversion technique successfully discriminates between the single and multiple fracture cases and in all cases accurately identifies the strikes of fracture sets previously imaged using independent methods borehole image logs core samples microseismic event locations we also generate a synthetic example to highlight the pitfalls that can be encountered if it is assumed that only one fracture set is present when splitting data are interpreted when in fact more than one fracture set is contributing to the anisotropy

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

experimental laboratory determination of the dynamic elastic properties of wet drained shales the ultrasonic anisotropic elastic properties of drained saturated shales were measured as a function of confining pressure two shales were characterized in this study a jurassic outcrop shale retrieved under the sea in a saturated state and a kimmeridge clay shale cut from a north sea borehole both shales were highly anisotropic both texturally as revealed by scanning electron microscopy analysis and elastically as measured ultrasonically in the laboratory the strongest anisotropy was seen in the kimmeridge clay shale where up to 38 compressional wave anisotropy thomsen s parameter e and up to 58 shear wave anisotropy thomsen s parameter were observed in addition for both shales e and were found to decrease as a function of increasing confining pressure with the pore pressure drained to atmosphere while the anellipticity deviation of the slowness surfaces from ellipses was found to be positive and decreased as a function of increasing confining pressure accompanying these changes in elastic properties was a decrease in porosity with increasing confining pressure from 10 5 to 8 5 for the jurassic shale the decrease in overall anisotropy of the shales with increasing confining pressure was found to be consistent with theoretical modeling of shale properties where the shale anisotropy and anellipticity were predicted to decrease as a function of decreasing fluid filled porosity