cenozoic deformation of the tarim plate and the implications for mountain building in the tibetan plateau and the tian shan 1 the tarim basin in nw china developed as a complex foreland basin in the cenozoic in association with mountain building in the tibetan plateau to the south and the tian shan orogen to the north we reconstructed the cenozoic deformation history of the tarim basement by backstripping the sedimentary rocks two dimensional and three dimensional finite element models are then used to simulate the flexural deformation of the tarim basement in response to the sedimentary loads and additional tectonic loads associated with overthrusting of the surrounding mountain belts the results suggest that uplift of both the western kunlun shan and the tian shan orogens started during or before oligocene since the miocene mountain building accelerated in the western kunlun shan but showed segmented development in the tian shan the results also indicate reduced tectonic load along the altyn tagh fault since late miocene consistent with geological and geophysical evidence of the fault cutting the entire lithosphere and migrating southward during its cause of the cenozoic evolution

time lapse sonic logs reveal patchy co2 saturation in situ 1 based on time lapse sonic and neutron porosity logs from the nagaoka co2 sequestration experiment a p wave velocity saturation relation at reservoir depth is retrieved it does not coincide with either of the end member models of uniform and patchy saturation but falls in between even if realistic error estimates for the host rock properties are considered assuming a random distribution of co2 patches it is shown that the mechanism of wave induced flow can be evoked to explain this velocity saturation relation characteristic co2 patch size estimates range from 1 to 5 mm such mesoscopic heterogeneity can be responsible for attenuation and dispersion in the well logging frequency band

comparison of p wave attenuation models of wave induced flow abstract wave induced oscillatory fluid flow in the vicinity of inclusions embedded in porous rocks is one of the main causes for p wave dispersion and attenuation at seismic frequencies hence the p wave velocity depends on wave frequency porosity saturation and other rock parameters several analytical models quantify this wave induced flow attenuation and result in characteristic velocity saturation relations here we compare some of these models by analyzing their low and high frequency asymptotic behaviours and by applying them to measured velocity saturation relations specifically the biot rayleigh model considering spherical inclusions embedded in an isotropic rock matrix is compared with white s and johnson s models of patchy saturation the modeling of laboratory data for tight sandstone and limestone indicates that by selecting appropriate inclusion size the biot rayleigh predictions are close to the measured values particularly for intermediate and high water saturations

seismic attenuation and pore fluid viscosity in clay rich reservoir sandstones the frequency dependence of seismic attenuation in a suite of clay rich reservoir sandstones was investigated in the laboratory compressional and shear wave velocities vp and vs and quality factors qp and qs were measured as functions of pore fluid viscosity at an effective pressure of 50 mpa and at an experimental frequency of about 0 8 mhz using the pulse echo technique the experimental viscosity ranged from 0 3 to 1000 centipoise which gives equivalent frequencies for a water saturated sandstone of 2 6 mhz to 780 hz assuming a global flow loss mechanism two types of behavior were observed high permeability greater than 100 millidarcies sandstones tend to show variable qp and qs which are similar in magnitude to those predicted by the biot theory over the viscosity range 0 3 to about 20 centipoise equivalent frequency range 2 6 mhz to about 39 khz low permeability less than 50 millidarcies sandstones tend to show almost constant qp and qs over the experimental viscosity range that are

the effect of methane hydrate morphology and water saturation on seismic wave attenuation in sand under shallow sub seafloor conditions a better understanding of seismic wave attenuation in hydrate bearing sediments is needed for the improved geophysical quantification of seafloor methane hydrates important for climate change geohazard and economic resource assessment hence we conducted a series of small strain 10 6 seismic frequency 50 550 hz laboratory resonant column experiments on synthetic methane hydrate bearing sands under excess water seafloor conditions the results show a complex dependence of p and s wave attenuation on hydrate saturation and morphology p and s wave attenuation in excess water hydrate bearing sand is much higher than in excess gas hydrate bearing sand and increases with hydrate saturation between 0 and 0 44 the experimental range theoretical modelling suggests that load bearing hydrate is an important cause of heightened attenuation for both p and s waves in gas and water saturated sands while pore filling hydrate also contributes significantly to p wave attenuation in water saturated sands a squirt flow attenuation mechanism related to microporous hydrate and low aspect ratio pores at the interface between sand grains and hydrate is thought to be responsible for the heightened levels of attenuation in hydrate bearing sands at low hydrate saturations 0 44

velocity dispersion between ultrasonic and seismic frequencies in brine saturated reservoir sandstones ultrasonic p and s wave velocities have been measured on 44 specimens from core samples of relatively clean sandstones recovered from hydrocarbon reservoirs ten specimens have porosities less than 10 and 34 have porosities in the range 20 30 velocities were measured with the specimens in both their dry and brine saturated states under hydrostatic effective stresses to 60 mpa for the low porosity specimens and to 40 mpa for those of the high porosity set gassmann s poroelastic theory is found to account for changes in velocity for the low porosity set at 60 mpa effective stress when the dry specimens are fully saturated with brine the velocities used for dry specimens in gassmann s theory must however be those measured after the dry specimens have first adsorbed a small quantity of moisture for saturated specimens at effective stresses of 40 mpa or less local flow phenomena due to the presence of open microcracks are assumed to be responsible for the measured velocities being appreciably hig

biot dispersion for p and s wave velocities in partially and fully saturated sandstones ultrasonic compressional and shear wave velocities have been measured on 34 samples of sandstones from hydrocarbon reservoirs the sandstones are all of low clay content high porosity and cover a wide range of permeabilities they were measured dry and brine saturated under hydrostatic effective stresses of 10 20 and 40 mpa for eight of the sandstones ultrasonic velocity measurements were made at different partial water saturations in the range from dry to fully saturated the gassmann biot theory is found to account for most of the changes in velocities at high effective stress levels when the dry sandstones are fully saturated with brine provided the lower velocities resulting when the dry sandstone initially adsorbs small amounts of moisture are used to determine the elastic properties of the dry sandstone at lower effective stress levels local flow phenomena due to the presence of open microcracks are assumed to be responsible for measured velocities higher than those predicted by the theory the partial saturation results are modelled fairly closely by the gassmann biot theory assuming heterogeneous saturation for p waves

seismic attenuation due to wave induced flow analytical expressions for three p wave attenuation mechanisms in sedimentary rocks are given a unified theoretical framework two of the models concern wave induced flow due to heterogeneity in the elastic moduli at mesoscopic scales scales greater than grain sizes but smaller than wavelengths in the first model the heterogeneity is due to lithological variations e g mixtures of sands and clays with a single fluid saturating all the pores in the second model a single uniform lithology is saturated in mesoscopic patches by two immiscible fluids e g air and water in the third model the heterogeneity is at microscopic grain scales broken grain contacts and or micro cracks in the grains and the associated fluid response corresponds to squirt flow the model of squirt flow derived here reduces to proper limits as any of the fluid bulk modulus crack porosity and or frequency is reduced to zero it is shown that squirt flow is incapable of explaining the measured level of loss 10 sup 2 q sup 1 10 sup 1 within the seismic band of frequencies 1 to 10 sup 4 hz however either of the two mesoscopic scale models easily produce enough attenuation to explain the field data

compressional wave dispersion due to rock matrix stiffening by clay squirt flow the standard biot gassmann theory of poroelasticity fails to explain strong compressional wave velocity dispersion experimentally observed in 12 tight siltstone with clay filled pores in order to analyze and understand the results we developed a new double porosity model of clay squirt flow where wave induced local fluid flow occurs between the micropores in clay aggregates and intergranular macropores the model is validated based on the combined study of ultrasonic experiments on specimens at different saturation conditions and theoretical predictions the presence of a sub pore scale structure of clay micropores contained in intergranular macropores where the fluid does not have enough time to achieve mechanical equilibrium at ultrasonic frequencies and thus stiffens the rock matrix provides a suitable explanation of the experimental data moreover the model provides a new bound for estimating the compressional wave velocity of tight rocks saturated with two immiscible liquids the theoretical predictions indicate that the velocity variation between gas and liquid saturated specimens is predominantly induced by the clay squirt stiffening effect on the rock matrix and not by fluid substitution the effect contributes more than 90 to the variation in the porosity range of 0 5 thus clay squirt flow dominates the relationships between compressional wave velocity and pore fluid in tight rocks

seismic attenuation due to heterogeneities of rock fabric and fluid distribution jing ba 1 2 jose m carcione3 and weitao sun4 1school of mathematics and statistics xi an jiaotong university 710049 xi an china e mail jba uh edu 2department of earth and atmospheric sciences university of houston houston tx 77204 usa 3istituto nazionale di oceanografia e di geofisica sperimentale ogs borgo grotta gigante 42c sgonico trieste i 34010 italy 4zhou pei yuan center for applied mathematics tsinghua university 100083 beijing china

attenuation in fine grained marine sediments extension of the biot stoll model by the effective grain model egm it is postulated that the presence of swelling clay minerals in unconsolidated fine grained saturated marine sediments leads to deviations from the normally assumed ideal elasticity of the solid phase in the proposed effective grain model egm the elastic grain material is consequently replaced by an effective medium made up of a homogeneous elastic mineral phase that is isotropically interspersed with cylindrical penny shaped inclusions of low aspect ratio representing the intracrystalline water layers in the swelling clay minerals the two phase nature of the grain material thus specified results in a wave energy consuming squirt flow process from the inclusions into the pore space introducing the egm into the biot stoll model bsm via the complex bulk modulus of the dissipative grain material leads to a better fit to literature data on the attenuation of compressional waves than does the original bsm alone since swelling clay minerals occur in nearly all clay bearing sediments it is conclud

the squirt flow mechanism macroscopic description we introduce a poroelasticity model that incorporates the two most important mechanisms of solid fluid interaction in rocks the biot mechanism and the squirt flow mechanism this combined biot squirt bisq model relates compressional velocity and attenuation to the elastic constants of the drained skeleton and of the solid phase porosity permeability saturation fluid viscosity and compressibility and the characteristic squirt flow length squirt flow length is a fundamental rock property that does not depend on frequency fluid viscosity or compressibility and is determined experimentally we find that the viscoelastic response of many sandstones is dominated by the squirt flow component of the bisq mechanism and that the viscoelastic properties of these rocks can be expressed through a single dimensionless parameter omega r 2 kappa where omega is angular frequency r is the characteristic squirt flow length and kappa is hydraulic diffusivity the biot mechanism alone does not give an adequate explanation of the observed velocity dispersion and attenuation and the viscoelastic behavior of many sandstones

seismic wave attenuation and dispersion resulting from wave induced flow in porous rocks a review one major cause of elastic wave attenuation in heterogeneous porous media is wave induced flow of the pore fluid between heterogeneities of various scales it is believed that for frequencies below 1 khz the most important cause is the wave induced flow between mesoscopic inhomogeneities which are large compared with the typical individual pore size but small compared to the wavelength various laboratory experiments in some natural porous materials provide evidence for the presence of centimeter scale mesoscopic heterogeneities laboratory and field measurements of seismic attenuation in fluid saturated rocks provide indications of the role of the wave induced flow signatures of wave induced flow include the frequency and saturation dependence of p wave attenuation and its associated velocity dispersion frequency dependent shear wave splitting and attenuation anisotropy during the last four decades numerous models for attenuation and velocity dispersion from wave induced flow have been developed with varying degrees of rigor and complexity these models can be categorized roughly into three groups according to their underlying theoretical framework the first group of models is based on biot s theory of poroelasticity the second group is based on elastodynamic theory where local fluid flow is incorporated through an additional hydrodynamic equation another group of models is derived using the theory of viscoelasticity though all models predict attenuation and velocity dispersion typical for a relaxation process there exist differences that can be related to the type of disorder periodic random space dimension and to the way the local flow is incorporated the differences manifest themselves in different asymptotic scaling laws for attenuation and in different expressions for characteristic frequencies in recent years some theoretical models of wave induced fluid flow have been validated numerically using finite difference finite element and reflectivity algorithms applied to biot s equations of poroelasticity application of theoretical models to real seismic data requires further studies using broadband laboratory and field measurements of attenuation and dispersion for different rocks as well as development of more robust methods for estimating dissipation attributes from field data

seismic dispersion and attenuation in layered porous rocks with fractures of varying orientations abstract wave induced fluid flow plays an important role in affecting the seismic dispersion and attenuation of fractured porous rocks while numerous theoretical models have been proposed for the seismic dispersion and attenuation in fractured porous rocks most of them neglect the wave induced fluid flow resulting from the background anisotropy e g the interlayer fluid flow between different layers that can be normal in real reservoirs here according to the theories of poroelasticity we present an approach to study the frequency dependent seismic properties of more realistic and complicated rocks i e horizontally and periodically layered porous rock with horizontal and randomly orienting fractures respectively distributed in one of the two periodical layers the approach accounts for the dual effects of the wave induced fluid flow between the fractures and the background pores and between different layers the interlayer fluid flow because c33 i e the modulus of the normally incident p wave is directly related to the p wave velocity widely measured in the seismic exploration and its comprehensive dispersion and attenuation are found to be most significant we study mainly the effects of fracture properties and the stiffness contrast between the different layers on the seismic dispersion and attenuation of c33 the results show that the increasing stiffness contrast enhances the interlayer fluid flow of the layered porous rocks with both horizontal and randomly orienting fractures and weakens the wave induced fluid flow between the fractures and the background pores especially for the layered porous rock with horizontal fractures the modelling results also demonstrate that for the considered rock construction the increasing fracture density reduces the interlayer fluid flow while improves the dispersion and attenuation in the fracture relevant frequency band increasing fracture aspect ratio is found to reduce the dispersion and attenuation in the fracture relevant frequency band only especially for the layered porous rock with horizontal fractures

analysis of dynamic fracture compliance based on poroelastic theory part ii results of numerical and experimental tests in part i a dynamic fracture compliance model dfcm was derived based on the poroelastic theory the normal compliance of fractures is frequency dependent and closely associated with the connectivity of porous media in this paper we first compare the dfcm with previous fractured media theories in the literature in a full frequency range furthermore experimental tests are performed on synthetic rock specimens and the dfcm is compared with the experimental data in the ultrasonic frequency band synthetic rock specimens saturated with water have more realistic mineral compositions and pore structures relative to previous works in comparison with natural reservoir rocks the fracture pore geometrical and physical parameters can be controlled to replicate approximately those of natural rocks p and s wave anisotropy characteristics with different fracture and pore properties are calculated and numerical results are compared with experimental data although the measurement frequency is relatively high the results of dfcm are appropriate for explaining the experimental data the characteristic frequency of fluid pressure equilibration calculated based on the specimen parameters is not substantially less than the measurement frequency in the dynamic fracture model the wave induced fluid flow behavior is an important factor for the fracture wave interaction process which differs from the models at the high frequency limits for instance hudson s un relaxed model

nonlinear acoustoelastic constants of dry and saturated rocks 1 conventional descriptions of the stress dependence of acoustic velocities in rock are often based on microcrack models a more general model independent description that ignores microstructure is third order elasticity theory this theory has had much success in metals and crystals and has recently been applied to rocks as well we measured third order elastic constants in several rocks both dry and water saturated a five velocity measurement technique was used where a rock sample is stressed either hydrostatically or uniaxially and compressional and shear velocities are measured as functions of stress in dry rocks we find that third order elasticity theory provides a good description of velocity versus stress however in water saturated rocks agreement between theoretical predictions and measurements is significantly worse than in dry rocks this indicates that third order elasticity theory does not fully describe the stress dependence of velocities in water saturated rock under our experimental conditions detailed analysis of the velocity data suggests that rock dilatancy may be responsible for the discrepancies between theory and experiment

application of the bisq model to clay squirt flow in reservoir sandstones 1 an understanding of the variations of compressional wave velocity and attenuation in sandstones as a function of pore fluid properties is important for the identification and monitoring of hydrocarbon reservoirs in order for seismic inversion to be well constrained a rock physics model that describes accurately the observed behavior is required a number of such models have been proposed but have not been extensively tested against laboratory data the unified biot and squirt flow model was applied to data obtained from ultrasonic experiments conducted on a wide range of sandstone types when saturated with different pore fluids with viscosities ranging from 0 3 to 1000 cp this model was seen to match well all the observed velocity and attenuation values as opposed to the biot model which was only successful for clean sandstones saturated with low viscosity fluids this has led to the conclusion that a squirt flow mechanism is important at the grain scale in addition an increase in the dynamic uniaxial strain modulus of the dry frame with increasing pore fluid viscosity was observed in the absence of free gas or grain contact microcracks the laboratory data were collected at 60 mpa effective pressure this can be linked to decreasing local flow and decreasing relaxation in the soft pore space of the sandstone the extent of which is in turn connected to the clay content and pore structure of the sandstone these results imply that at ultrasonic frequencies the squirt flow is due to the presence of clay

dynamic poroelasticity a unified model with the squirt and the biot mechanisms the velocities and attenuation of seismic and acoustic waves in rocks with fluids are affected by the two most important modes of fluid solid interaction 1 the biot mechanism where the fluid is forced to participate in the solid s motion by viscous friction and inertial coupling and 2 the squirt flow mechanism where the fluid is squeezed out of thin pores deformed by a passing wave traditionally both modes have been modeled separately with the biot mechanism treated in a macroscopic framework and the squirt flow examined at the individual pore level we offer a model which treats both mechanisms as coupled processes and relates p velocity and attenuation to macroscopic parameters the biot poroelastic constants porosity permeability fluid com pressibility and viscosity and a newly introduced microscale parameter a characteristic squirt flow length the latter is referred to as a fundamental rock property that can be determined experimentally we show that the squirt flow mechanism dominates the biot mechanism and is responsible for measured large velocity dispersion and attenuation values the model directly relates p velocity and attenuation to measurable rock and fluid properties therefore it allows one to realistically interpret velocity dispersion and or attenuation in terms of fluid properties changes e g viscosity during thermal enhanced oil recovery eor or to link seismic measurements to reservoir properties as an example of the latter transformation we relate permeability to attenuation and achieve good qualitative correlation with experimental data

squirt flow in fully saturated rocks we estimate velocity frequency dispersion and attenuation in fully saturated rocks by employing the squirt flow mechanism of solid fluid interaction in this model pore fluid is squeezed from thin soft cracks into the surrounding large pores information about the compliance of these soft cracks at low confining pressures is extracted from high pressure velocity data the frequency dependence of squirt induced pressure in the soft cracks is linked with the porosity and permeability of the soft pore space and the characteristic squirt flow length these unknown parameters are combined into one expression that is assumed to be a fundamental rock property that does not depend on frequency the appropriate value of this expression for a given rock can be found by matching our theoretical predictions with the experimental measurements of attenuation or velocity the low frequency velocity limits as given by our model are identical to those predicted by gassmann s formula the high frequency limits may sign