Function Of Effective Pressure Research Articles

Rock Anelasticity, Pore Geometry and the Biot–Gardner Effect

The anelastic properties of porous rocks depend on the pore characteristics, specifically, the pore aspect ratio and the pore fraction (related to the soft porosity). At high frequencies, there is no fluid pressure communication throughout the pore space and the rock becomes stiffer than at low frequencies, where the pore pressure is fully equilibrated. The models considered here include explicit pore geometry information in determining the poroelastic parameters. They are extensions of the EIAS (equivalent inclusion-average stress) and CPEM (cracks and pores effective medium) models to the whole frequency range, based on the Zener model. Knowing the degree of stiffness dispersion between the low- and high-frequency limits, we fit experimental data in the whole frequency range and obtain the average crack aspect ratio and soft porosity as a function of effective pressure. Then, we compute the dispersion and quality factor of the bulk, shear and Young moduli, and the P- and S-wave seismic velocities and quality factors as a function of frequency. However, when measuring axial or volumetric motions along a cylindrical sample, there is fluid flow at the ends of the sample in the experiments considered here. This generates dispersion and attenuation due to axial flow of the pore fluid, which does not occur for a plane wave in unbounded media. This phenomenon is called “drained/undrained transition" Pimienta et al. (J Geophys Res Solid Earth; https://doi.org/10.1002/2017JB014645 , 2017). Actually, it is an axial version of the Biot–Gardner (BG) effect, and implies an “artificial" (mesoscopic) attenuation peak (and dispersion) due to the generation of slow (diffusion) Biot modes at the cylinder boundary, inducing a global flow at the scale of the sample. The classical BG effect is due to fluid flow along the radial direction, on the basis of open-pore conditions at the sides of the sample. In this case, the sides are sealed. To use the EIAS and CPEM models, the BG effect has to be removed to obtain the intrinsic Q of the rock. The models are applied here for measurements on sandstone. The axial BG effect is more evident if the intrinsic attenuation is weak or absent. An example is Lavoux limestone, which has a bimodal porosity distribution, with an equal proportion of intragranular microporosity and intergranular macroporosity (round pores). In this case, the attenuation and dispersion are related to the BG effect, since no squirt flow is detected due to the absence of cracks. We verified that the bulk and Young moduli obtained from the axial and hydrostatic oscillations are consistent with each other, and that the theoretical description of the axial BG effect shows some discrepancies with the data.

P1158DOES THE PERITONEUM SWELLS OR SHRINK DURING PERITONEAL DIALYSIS?

Abstract Background and Aims The width of the peritoneum (composed mainly of connective tissue and relatively free of vasculature) is increased in patients on peritoneal dialysis compared to healthy subjects. We investigated to what extent increased intraperitoneal (ip) hydrostatic and osmotic pressures following the infusion of dialysis fluid will change hydration status and width of the peritoneum. Method Using linear theory of poroelasticty, clinical data on transport parameters and experimental data on elastic characteristics of the interstitium, the relative change of the width of the poroelastic layer subject to the combined effect of external hydrostatic and effective osmotic pressures (that is, ideal osmotic pressure multiplied by reflection coefficient for osmotic agent) can be described as a function of effective pressure and elastic modulus of the layer: Lmod/L0 = 1/(1-deltaP*/lambda*), where L0 is initial thickness of the tissue, Lmod is modified thickness of the layer, deltaP* is change in effective combined pressure, and lambda* is the elastic modulus of the poroelastic material. The same formula describes also the change in fractional free fluid volume ratio, thetaF. The elastic modulus of the connective tissue was assumed to be 110 mmHg, as measured for the subcutaneous layer of the tip of mouse tail by Swartz et al (J Biomech, 1999), and reflection coefficient for glucose in the interstitium of 0.0035 as estimated by Stachowska-Pietka et al (NDT, 2019) from clinical data for patients on peritoneal dialysis. Results The ip hydrostatic pressure increases by 2-3 mmHg to 15 mmHg at rest depending on infused volume of dialysis fluid (and posture, body weight and location in abdominal cavity), and may increase to 100 mmHg during activities as coughing, whereas the osmotic pressure of glucose 3.86% dialysis fluid is around 400 mmHg above the osmotic pressure of plasma and interstitial fluid (in equilibrium with plasma). However, due to the low reflection coefficient of interstitium, the effective osmotic pressure of dialysis fluid minus the physiological value of interstitial osmotic pressure is only 1.4 mmHg, and is quickly decreasing with dwell time. Therefore, hydrostatic pressure is the dominant factor for interstitial hydration. Assuming ip pressure of 15 mmHg, the stretch of the peritoneum increases its equilibrium width (at 0 mmHg and isotonic interstitial fluid) by 15%. During physical activities peritoneum may transiently thicken even more. Conclusion The peritoneum becomes overhydrated after infusion of dialysis fluid, which increases interstitial hydrostatic pressure; the thickness and fractional free fluid volume of the peritoneum increase by 15% although transiently higher increases may occur following activities that increase intraperitoneal pressure. The mechanical changes in the peritoneum may contribute to the biological changes in cells present there, as fibroblasts and mesothelial cells. The swelling of the peritoneum is in agreement with the increase in the fractional free fluid volume of the intramuscular interstitium behind the peritoneum as reported by Zakaria et al (Am J Physiol Heart Circ Physiol, 1999).

Stress controls on transport properties of the Mercia Mudstone Group: Importance for hydrocarbon depletion and CO2 injection

Abstract The physical properties of the Mercia Mudstone Group (MMG) are of interest to Carbon Capture and Storage (CCS) in the UK, both in terms of the sealing capacity of certain horizons and in order to assess scenarios involving CO2 migration in the overburden above potential CCS sites. In this study, the hydromechanical properties of MMG samples from the Larne Basin, Northern Ireland, were directly measured under steady-state conditions. Test samples were found to be good seals, with hydraulic permeabilities ranging from ≈2.1 x 10−18 to 8.4 x 10−21 m2 (2.1 x 10−3 to 8.5 x 10−6 mD). A detailed examination of the consolidation behaviour of the material yielded values for compressibility, hydraulic permeability and specific storage, as a function of effective pressure. Consolidation testing also provided preconsolidation pressure values of between 30 and 37 MPa. Test data were fitted to a linear elastic model using a two-dimension finite element approach, yielding hydraulic permeability and Young's Modulus, as a function of effective pressure. Findings suggest localisation of flow, due to small-scale heterogeneity, may play a role even in relatively large test samples. Results also highlight the impact of methodology on resulting permeabilities and the importance of using values measured at boundary conditions appropriate for the specific application. Critical state envelopes were derived from test data and used to conduct a scenario analysis, considering a range of stress paths, to examine the impacts of depletion and reinflation during CO2 injection. Initial stress conditions, stress path gradient and caprock heterogeneity were all found to be influencing factors on the potential for yield during depletion and the resulting deformation mode. The response to CO2 injection is less clear, but will be impacted by the initial caprock permeability and resulting drainage response. An awareness of these controls on caprock performance during stress path changes may aid in the selection of depleted CCS sites with geomechanically favourable characteristics for reinjection.

Interplay between permeability and compressibility in shale samples

In this study, we present a new approach to evaluate shale permeability as a function of effective pressure using Mercury Injection Capillary Pressure (MICP) experiments. Traditional approaches to describe permeability from MICP data often use pore size distribution and porosity and ignore intrusion effects. However, recent studies suggest that significant misinterpretation of pore size distribution and porosity can be realized owing to ignorance of pore compressibility in shale samples. We evaluate permeability variation with pressure using MICP data while considering for pore compressibility and intrusion correction.A shale sample often undergoes three stages during MICP tests: conformance, compression and intrusion. By identifying different pressure regions associated with these stages, we characterize pore compressibility as a function of pressure using compression stage and re-evaluate pore size distribution and porosity based on intrusion stage. We then integrate intrusion correction and pore compressibility values to Katz-Thompson correlation to evaluate permeability as a function of pressure.Identifying true intrusion and applying pore compressibility allow us to alter key variables in Katz-Thompson correlation, thus significantly lowers the final estimation of permeability. We observe 10–30% permeability reduction in the samples analyzed after corrections.This paper improves the accuracy of permeability predictions from MICP data; the outcome can be further applied to improve predictions of reservoir performance.

Prediction of rocks thermal conductivity from elastic wave velocities, mineralogy and microstructure

While knowledge on Thermal Conductivity (TC) of rocks is of interest in many fields, determining this property remains challenging. In this paper, a modelling approach for TC prediction from Elastic Wave Velocity (EWV) measurements is reported. To this end, a new effective TC model for a typical sedimentary rock is introduced that explicitly accounts for the presence of pores, pressure-sensitive microcracks (or grain contacts) and formation fluids. A model of effective elasticity is also devised for this same rock that links its microstructural characteristics to the velocity of elastic waves. The two models are based on the same effective medium approach and involve the same microstructural parameters. A workflow based on this explicit modelling approach is devised that allows for the prediction of the TC of a reservoir rock using (i) the elastic waves velocities, (ii) the dominant mineral content and (iii) the bulk porosity. This workflow is validated using experimental data reported in the literature for dry and water-saturated Fontainebleau and Berea sandstones. The datasets include measurements of TC and EWV as a function of effective pressure. In addition, it is shown that the dependence of TC on the rock microstructure is formally and practically similar to that of EWV. It is also demonstrated that the accuracy of TC predictions from EWV increases with effective pressure (burial depth). The underlying assumptions and limitations of the present approach together with the effect of burial are discussed.

Effective Confining stress Depending on Fluid Flow and Capillary Sealing Properties of Mudstones for CO2 Geological Sequestration

Abstract In this study, we investigated effective pressure of dependence on hydraulic properties (i.e. threshold pressure and permeability) of Kazusa group mudstones, which is taken from Boso peninsula in Japan, at 40 °C and effective pressures up to 20 MPa. Additionally, to assess a correlation between these properties, a porosity change depending on effective stress is also examined. Our results demonstrates that for mudstones tested, the relationship between threshold pressure and permeability depends strongly on changes in pore structures as a function of effective pressure.

Modeling the pressure sensitivity of uncemented sediments using a modified grain contact theory: Incorporating grain relaxation and porosity effects

We develop a modified grain contact theory to better describe the pressure-dependent elastic properties of uncemented sediments. The Hertz-Mindlin (HM) theory typically predicts shear moduli that are much higher than observed laboratory measurements, resulting in inaccurate estimates of the dry bulk to shear modulus ratio [Formula: see text]. The HM theory further predicts that the [Formula: see text] ratio is constant with pressure, whereas ultrasonic core measurements typically show an increasing [Formula: see text] ratio as effective pressure decreases. Laboratory data also suggest that the dry bulk and shear moduli variation with effective pressure is greater than the cube-root power law predicted by the HM theory. We introduce two new pressure-dependent calibration parameters to account for the shortcomings in effective medium theory, and we develop a new method to predict pressure-dependent elastic properties. Our calibration parameters agree with the results of published granular dynamics simulations, and they incorporate grain relaxation and porosity effects not included in existing effective medium theories. Our new model provides improved fits to laboratory data when compared with existing models, and it can be used for improved prediction of elastic properties as a function of effective pressure. Our new theory can also be used to model uncemented sediments with values of Poisson’s ratio [Formula: see text], where many existing grain contact and effective medium theories currently fail.

The velocity and attenuation anisotropy of shale at ultrasonic frequency

The velocity and attenuation anisotropy of dry and oil-saturated shale were measured at laboratory ultrasonic frequency by using the transmission technique. Our purpose is to find the variation rules of wave velocity and attenuation in different directions as a function of effective pressure in dry and oil-saturated situations, and those intrinsic factors that result in such anisotropy. Using x-ray diffraction techniques and a scanning electron microscope, the causative factors for the velocity anisotropy are found to be mainly due to the alignment of clay mineral and microcracks. The attenuation of P- and S-waves also shows apparent directional dependence, but different from that of velocities. Attenuation anisotropy can be interpreted in terms of grain and pore geometry. For dry shale samples, the dominant attenuation mechanism is phase hysteresis due to static friction. On one hand, the Biot flow plays a key role in causing wave attenuation for the P-wave propagating parallel to bedding for fluid-saturated samples. On the other hand, the fluid-related attenuation is mainly attributed to the mechanisms of squirt flow for the P-wave propagating vertical to bedding.

Anisotropy of permeability and complex resistivity of tight sandstones subjected to hydrostatic pressure

The dependence of permeability and complex electrical resistivity on direction was measured for low-permeable sandstone samples from a tight gas reservoir. Both properties were measured in three directions at hydrostatic pressures up to 100 MPa. The decrease of permeability as a function of effective pressure (measured with a modified pressure-transient method) can be described by a power function. The pressure dependence is more controlled by the closure of thin aspect ratio pores and cracks than by the minor reduction of porosity. The anisotropy of permeability is also a function of pressure. For some samples the preferred direction of flow changes with increasing pressure. The Cole–Cole response-function can be fitted well to the complex resistivity spectra (kHz–MHz). Interfacial polarization is the dominant polarization effect in this frequency range. The relaxation time of the Cole–Cole model increases with increasing effective pressure, whereas the frequency exponent does not show any continuous behavior. According to the model of Lysne (1983) the geometrical distribution of pore shapes and their orientation can be derived from these quantities. The formation resistivity factor, taken from the real part of resistivity at 10 kHz, also increases with pressure. As the porosity does not change significantly, this increase means an increase of Archie's cementation exponent. Both, relaxation time and formation factor are also a function of the considered direction. But an overall relationship between these quantities and permeability could not be observed; neither for absolute values nor for their anisotropy.

Viscoelasticity of precompacted unconsolidated sand with viscous cement

The elastic properties of sand strongly depend on the grains’ contact stiffness, which can be increased significantly by solid matter and, depending on frequency, viscous fluid acting as contact cement. To calculate seismic velocities in precompacted fluid-cemented sand, we examine how a small amount of viscous fluid at the grain contacts influences their normal and tangential stiffnesses as a function of effective pressure. Using the Hertz-Mindlin approach and considering oscillatory loading in addition to precompaction of a combination of two elastic spheres, we extend the dry-contact elastic theory by a viscoelastic formulation. Here, we describe the radial flow of the fluid cement induced by the oscillations of the grains’ surfaces around the direct contact, a process that leads to a complex normal stiffness and stiffness/frequency dispersion. In the resulting combined model, the low-frequency real part of the complex normal stiffness identical is to the original Hertz-Mindlin expression. The magnitude of the dispersion is governed by the amount of viscous cement; magnitude decreases as effective pressure increases. The frequency of the maximum imaginary part of the normal stiffness is determined mainly by cement viscosity and contact geometry. The tangential contact stiffness virtually is not influenced by the viscous fluid. Comparison of predicted results with data from pulse transmission experiments (500 kHz) on glass beads with two different fluids shows an excellent fit in P-wave velocities [Formula: see text], whereas S-wave velocities [Formula: see text] are systematically overestimated by the model. The experimental results confirm, however, the predicted change with effective pressure in the [Formula: see text] ratio for both examined cases as well as reflect the predicted increase in [Formula: see text] and [Formula: see text], respectively, between the two cases. This implies that our viscoelastic formulation represents a reasonable way to describe the role of viscous cement in sand.

Microstructural control on the elastic properties of thermally cracked granite

Compressional waves velocity V P was measured during long-term experiments in a high-pressure vessel (in the range [10–75] MPa for confining and pore pressures). Experiments were carried out on a granite specimen prepared by a controlled heating treatment at 510 °C, which generated thermal cracks. Data analysis is proposed by using an effective medium approach based on Kachanov's [Appl. Mech. Rev. 45 (1992)304] model. The elastic behaviour of the cracked rock is controlled by the crack density parameter which varies with confining and pore pressures due to crack closure. In order to model the progressive closure of cracks, we assume elliptical cracks with major axis 2 c and aspect ratio α. By using a conformal mapping technique, we derive the variation of the crack aspect ratio as a function of effective pressure, the effective pressure coefficient η depending on α and Poisson's ratio ν 0. As a result, we compute the crack density parameter and the elastic moduli of the cracked rock as a function of confining and pore pressures. To take into account the heterogeneity of the rock sample, a peak-like distribution of crack aspect ratios is introduced, which allows us to calculate the acoustic velocity V P for various effective pressures. Comparison is made between theoretical and experimental values and shows that this simple model captures the essential features of the acoustic velocity variation: an increase of V P when pore pressure is decreased followed by a plateau for a threshold pore pressure. Best consistency between theoretical and experimental velocity values is obtained by introducing a second crack population with a higher mean aspect ratio and an irreversible closure mechanism as effective pressure is cycled.

Acoustic velocities as a function of effective pressure in low to moderate porosity shaly sandstones, Part 1 - experimental results

Compressional and shear wave velocities were measured as a function of effective pressure on a set of cores from gas producing reservoirs in the Cooper Basin in South Australia. The aim of this study was to investigate the relationship between the stress sensitivity of acoustic velocities and geological and petrophysical characters of low to medium porosity shaly sandstones. The suite of samples consists of 22 core plugs from depths between 1917 m and 2564 m with helium porosity ranging from 2.6% to 16.5%. The samples are mainly fine to medium-grained sublitharenite. The echo-pulse technique is used to measure ultrasonic velocity and attenuation in dry and fully water-saturated cores under effective pressures of 5 MPa to 60 MPa. Experimental results show that compressional and shear wave velocities increase non-linearly with effective pressure. The influence of pressure was more pronounced below 40 MPa for both compressional and shear wave velocities. The rapid increase in wave velocities with effective pressure between 5 MPa and 40 MPa is attributed mainly to the closure of low aspect ratio pores such as micro-cracks and loose grain contacts within the rock framework. Compressional wave velocity in dry samples shows stronger pressure sensitivity, whereas there is no significant difference between the stress dependency of shear wave velocity in dry and water saturated samples. There is a positive correlation between change in velocity with pressure and core porosity and permeability. However, this association is weak and diminishes with increasing effective pressure. The pressure dependence of acoustic velocity in the Cooper Basin samples is not related to total clay content, but samples with a greater amount of pore filling and grain-coating clay particles appear to be less pressure sensitive at dry condition. Cooper Basin sandstones show higher pressure sensitivity compared to data from other studies.

Acoustic velocities as a function of effective pressure in low to moderate porosity shaly sandstones, Part 2- Implications for hydrocarbon exploration

Hydrocarbon production may cause changes in dynamic reservoir properties including pressure and fluid saturation. Understanding the magnitude of such variations is essential for the exploration of new reserves and optimising the performance of existing fields. Laboratory measurements of acoustic properties of representative rock samples, simulating in situ pressure and fluid saturation, provide a useful guide for calibrating and interpreting seismic and sonic log data. A petro-acoustic study was carried out to investigate factors influencing acoustic properties of Cooper Basin sandstones. This study integrated field data and petrography with laboratory measurements of acoustic properties of representative rock samples. In a previous paper we highlighted experimental data; here we elaborate on implications of these results in predicting rock property and pore fluid type from sonic logs and seismic reservoir monitoring. Analysis of data from acoustic measurements at ultrasonic frequencies on samples from the Cooper Basin reveals that the pressure dependency of the Cooper Basin rocks is very large. The velocity-pressure relationship obtained from laboratory data is consistent with the sonic log anomaly observed in partially pressure-depleted reservoirs in the Cooper Basin. Neglecting the pressure effect on velocity results in the overestimation of rock porosity by the sonic log in overpressured formations, and underestimation of porosity in pressure depleted zones. A method is proposed to correct the sonic log reading for pressure variation in the study area. At in-situ reservoir pressure Vs and Vp are strongly correlated and dry and water-saturated samples show significantly different velocity ratios (Vp/Vs). The Vp/Vs ratio is not affected by porosity and clay content and therefore has potential as a gas indicator in the region. The strong stress sensitivity and the distinct Vp/Vs. values for dry and water saturated Cooper Basin cores suggest that the dynamic changes in pressure and saturation of the reservoir rocks may also be detectable from acoustic impedance or travel time at seismic and sonic log frequencies.

Compressional wave velocity and attenuation at ultrasonic and sonic frequencies in near‐surface sedimentary rocks

Laboratory ultrasonic measurements of compressional wave velocity and attenuation were made as a function of effective pressure on samples of limestone, sandstone and siltstone taken from a shallow borehole test site. The results indicate that the sandstones are pervaded by grain contact microcracks which dramatically affect their compressional wave attenuations. Clean sandstone shows a compressional wave quality factor (Q{sub p}) of 24 {+-} 2 at 5 MPa effective pressure (close to the estimated in situ burial pressure) and a Q{sub p} of 83 {+-} 29 at 60 MPa. The Q{sub p} of limestones and siltstones at the site show negligible and small increases with pressure in the laboratory, respectively. The strong pressure dependence of Q{sub p} in clean sandstone was used to infer the presence of in situ microcracks. Sediment velocities measured in the laboratory at about 1 MHz were compared with those from the full waveform sonic log at about 10 kHz implies that they must also be highly attenuating over a significant part of the frequency range 10 kHz to 1 MHz, to account for the magnitude of the observed velocity dispersion. Assuming the laboratory Q{sub p} values measured at 5 MPa remain constant down to 10 kHzmore » predicts the observed dispersion quite well. Furthermore, the sonic log velocities of sandstones, limestones and siltstones (after normalizing each lithology for porosity and clay content) were found to reflect the same pressure (depth) trends observed in the laboratory. The results provide evidence for the existence of in situ microcracks in near-surface sediments.« less

The effect of pressure on ultrasonic velocity and attenuation in near‐surface sedimentary rocks

Accurate laboratory ultrasonic measurements of velocity and attenuation were made on water-saturated samples of limestone, sandstone and siltstone as a function of effective pressure (5--60 MPa) using the pulse-echo method. Core samples were taken from Carboniferous sediments at the Whitchester shallow borehole test site; they were cut perpendicular to the horizontal bedding planes. The compressional- and shear-wave velocities and quality factors of the limestones generally showed negligible increases with pressure, while those of the siltstones and sandstones showed intermediate and significant increases with pressure, respectively. At 60 MPa, the sandstone samples generally showed a range of quality factors attributed to their differing porosities and differing proportions of rigid skeletal minerals (quartz grains) and compliant matrix materials (clay, kerogenic organic matter); at 5 MPa, they generally showed similar quality factors attributed to the presence of microcracks, which occur predominantly at grain contacts. The results show that kerogenic organic matter behaves in a similar way to other compliant matrix minerals in causing high attenuation at ultrasonic frequencies, and that caution must be exercised when applying high pressure geostatistical relationships to reservoir rocks at low effective pressures. The results indicate the presence of two types of squirt flow loss mechanisms: clay squirt flowmore » dominates at high pressures, while microcrack squirt flow dominates at low pressures. Clean sandstones and limestones can be distinguished from those with significant amounts of compliant minerals, and sandstones with cracks can be distinguished from those without cracks.« less

Bandwidth-control-induced insulator-metal transition inPr0.65(Ca1−ySry)0.35MnO3andPr0.7Ca0.3MnO3

Both internal chemical pressure by ion substitution and external hydrostatic pressure induce an insulator-to-metal (I-M) transition in the ${\mathrm{Pr}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ca}}_{\mathrm{x}}$${\mathrm{MnO}}_{3}$ system. The behavior of spin and charge ordering through the I-M transition has been studied with use of neutron diffraction techniques. As a function of effective pressure, we identify three different pressure regions for the low-temperature state.

Tectonic sediment thickening, fluid expulsion, and the thermal regime of subduction zone accretionary prisms: The Cascadia Margin off Vancouver Island

Tectonic sediment thickening, fluid expulsion, and the resulting thermal regime of the northern Cascadia subduction zone accretionary prism have been described by numerical models. Both the sediment thickening and the fluid expulsion are found to have important thermal effects. Constraints on the models are provided by (1) detailed heat flow profiles across the continental slope accretion zone from variations in the thermally controlled depth to a gas hydrate bottom‐simulating reflector (BSR), and (2) porosity changes across the accretion zone inferred from the landward variation in the P wave velocity‐depth profiles. The heat flow in the region of rapid prism thickening centered 15 km landward of the deformation front is 20% below that predicted by a larger scale thermal model that ignores the thermal effects of the sediment accretion processes. The porosity also is substantially higher in this region compared to at the same depths in the adjacent deep sea Cascadia basin. The model results are in agreement with both the thermal and velocity data if the incoming sediment section is initially simply tectonically shortened horizontally and thickened vertically, generating a high porosity underconsolidated section. Reconsolidation and fluid expulsion in response to the tectonic thickening is slow, occurring mainly 20–30 km landward of the deformation front. The maximum predicted fluid expulsion rate is about 4 × 10−11 m s−1 (1.3 mm a−1). The pore pressure distribution across the deformation front region has been estimated from the velocity data using the simplifying assumption that velocity is only a function of effective pressure; pore pressures are high, reaching 80% of lithostatic 15 to 20 km landward of the deformation front. An estimate of the bulk permeability in the seaward tens of kilometers of the prism ranging from 10−16 m2 near the top to about 10−18 m2 near the base has been obtained through matching the pore pressure distribution in the numerical model to that obtained from the velocity data. The prism sediments are inferred to be substantially underconsolidated and pore pressures to be high for at least the seaward 30 km of the accretionary prism.

Diagenesis of Marine Sands to Tight Gas Sands: Effects on Acoustic and Hydraulic Properties: ABSTRACT

Figure The primary stages of diagenesis of marine sands usually increase total grain contact area, increase the number of contacts, decrease porosity, and increase pore network tortuosity. These microstructural factors strongly affect the continuum acoustic and hydraulic properties of the sands. To better understand diagenetic effects on these properties, compressional and shear wave velocities, Vp and Vs, and their specific attenuation, Qp-1 and Qs-1, were measured in an unconsolidated quartz sand (Ottawa sand, porosity ^approx 35%), a quartzarenite sandstone (St. Peter sandstone, porosity ^approx 22%), and a lithic-quartz arenite (Spirit River tight gas sand, porosity ^approx 5%). Measurements were made under effective press res, Pe, up to 350 bars as a continuous function of partial gas saturation, at frequencies from 10 to 15,000 Hz, and from 150 to 500 kHz. Experimental results are explained with relations derived from contact and packing theories. Results suggest that Vp/Vs and Qp-1/Qs-1 may be used to distinguish between gas and consolidation effects in reflection seismology and borehole sonic logs. Gas permeability was also measured in Spirit River tight gas sands from the Alberta Deep Basin as a function of effective pressure and partial gas saturation. These values are compared with known values for quartz sands and sandstones, and the results are examined with simple pore network models. While porosity decreases by a factor 7 from Ottawa sand to Spirit River sandstone, gas permeability drops by as much as 8 orders of magnitude. Gas permeability in all granular sedimentary materials, especially the tight sands, is a strong function of clay content, partial gas saturation, and effective pressure. End_of_Article - Last_Page 963------------

Frequency dependence of seismic dissipation in saturated rocks

Measurements of the specific dissipation factor, Q−1, have been made in saturated rock as a function of effective pressure. The measurements were made at two different frequencies, one in the range of 100 to 250 Hz and the other in the range of 7 to 9 kHz, using the resonant bar method. In saturated rock, Q−1 is lower at the lower frequency, and the difference between the values of Q−1 measured at the high and low frequencies decreases as effective pressure increases. At effective pressures greater than 200 bars, the Q−1 of saturated Berea sandstone measured at 200 Hz approaches that of dry Berea sandstone measured at 7 kHz.