Laboratory Measurements Of Permeability Research Articles

Hydraulic properties of sediments from the GC955 gas hydrate reservoir in the Gulf of Mexico

The economic feasibility of gas production from hydrate deposits is critical for hydrate to become an energy resource. Permeability in hydrate-bearing sediments dictates gas and water flow rates and needs to be accurately evaluated. Published permeability studies of hydrate-bearing sediments mostly quantify vertical permeability; however, the flow is mainly horizontal during gas production in layered reservoirs. Additionally, ASTM standards require a hydraulic gradient of 10–30 to be used during laboratory permeability measurements, but the gradient is much higher in the field, particularly near a production well. To address these issues, this study focuses on the hydraulic properties of a sandy silt subsample of the hydrate reservoir and a clayey silt subsample of the fine-grained, hydrate-free interbed recovered from a GC955 deep-water Gulf of Mexico gas hydrate reservoir. We characterize the sediment pore space with water retention curves for both hydrate-free and hydrate-bearing samples (hydrate saturation, Sh =80 %). Vertical deformation with increasing stress is also quantified while consolidating the samples to the 4 MPa in situ vertical effective stress. The customized permeameter measures both the horizontal and vertical permeability with increasing stress. Results show that high hydraulic gradients lower permeability in the flow direction, possibly due to increased flow tortuosity and local sediment compaction from the high seepage force. Assuming a single permeability value, even though hydraulic gradients decrease with distance from the well, is not realistic for field estimations. The results highlight that permeability anisotropy, hydrate saturation, stress conditions, and hydraulic gradient all substantially impact reservoir permeability during production.

Laboratory Measurement of Permeability in Postseismic Flow

Permeability is a critical parameter that reflects the physical and mechanical states of soil. The variation in permeability through the liquefaction and postliquefaction phases is important in evaluating dynamic soil behavior and establishing adequate numerical models. This aspect has been addressed by several empirical methods. Conversely, in this study, the postseismic behavior of soil was investigated experimentally. Several undrained cyclic strain-controlled and permeability tests on Ottawa C-109 sand, 1–1.3 mm calibrated beads, and blank samples were performed. An experimental strategy is proposed after highlighting the major committed mistakes and factors affecting the permeability variation in a triaxial cell, i.e., the sample’s diameter-to-height ratio, stone porosity, flow trajectory, tube diameter, and flow rate. This paper provides suggestions on the experimental setup signature, which frequently results in unreliable measurements. The permeability variation in the postseismic phase was measured during the time history of the excess pore pressure dissipation by following the falling head test rule. Permeability increased, reaching a peak of 2.5–3 times the initial value, and then decreased after a 50% regain of effective stress. The test results were verified by performing permeability tests before cyclic loading and after the dissipation process in triaxial conditions.

Reassessment of coal permeability evolution using steady-state flow methods: The role of flow regime transition

Steady-state flow methods are widely applied in the laboratory where permeability evolution is typically evaluated assuming a uniform pressure gradient along the sample. The accuracy of this approach is questionable for tight geomaterials when gas is used as the injecting fluid, due to: (i) the nonlinear distribution of pore pressure and gradient along the sample, suggesting that (ii) both slip and viscous flow may occur concurrently within samples containing nano-pores. The following presents laboratory permeability measurements integrated with numerical analysis to investigate the evolution of coal permeability under different flow regimes. Measured coal permeability first decreases and then rebounds as gas injection pressure is reduced, indicating the transition in flow regime from viscous to slip dominant. Numerical results chart the nonlinear distribution of pore pressure along the sample and the spatial transition of flow regimes determined by controlling the magnitude of the injection pressure. When injection pressure is below the threshold for slip flow, then slip flow dominates throughout the entire coal sample and apparent permeability increases significantly along the gas flow direction. The relative contribution of the slip flow to total flow increases with the reduction in pore pressure, increasing from 0.02 to 0.18 for the tested sample. Results also show that the conventional method of plotting apparent permeability against the mean experimental pressure always gives a greater permeability than the two alternate methods proposed in this work, and the discrepancy increases with increasing injection pressures (up to 7.66% when pore pressure = 6 MPa). Uncertainty analysis is strongly recommended when measuring permeability of tight rocks using this experimental method.

The influence of sample geometry on the permeability of a porous sandstone

Abstract. Although detailed guidelines exist for measuring the physical and mechanical properties of laboratory rock samples, guidelines for laboratory measurements of permeability are sparse. Provided herein are gas permeability measurements of cylindrical samples of Darley Dale sandstone (with a connected porosity of 0.135 and a pore and grain size of 0.2–0.3 mm) with different diameters (10, 20, and 25 mm) and lengths (from 60 to 10 mm), corresponding to aspect (length ∕ diameter) ratios between 6.2 and 0.4. These data show that, despite the large range in sample length, aspect ratio, and bulk volume (from 29.7 to 1.9 cm3), the permeabilities of the Darley Dale sandstone samples are near identical (3–4×10-15 m2). The near-identical permeability of these samples is considered the consequence of the homogeneous porosity structure typical of porous sandstones and the small grain and pore size of Darley Dale sandstone with respect to the minimum tested diameter and length (both 10 mm). Laboratory permeability measurements on rock samples with inhomogeneous porosity structures or with larger grain and pore sizes may still provide erroneous values if their length, diameter, and/or aspect ratio is low. Permeability measurements on rocks with vastly different microstructural properties should now be conducted in a similar manner to help develop detailed guidelines for laboratory measurements of permeability.

Imaging and computational considerations for image computed permeability: Operating envelope of Digital Rock Physics

Abstract This study defines the optimal operating envelope of the Digital Rock technology from the perspective of imaging and numerical simulations of transport properties. Imaging larger volumes of rocks for Digital Rock Physics (DRP) analysis improves the chances of achieving a Representative Elementary Volume (REV) at which flow-based simulations (1) do not vary with change in rock volume, and (2) is insensitive to the choice of boundary conditions. However, this often comes at the expense of image resolution. This trade-off exists due to the finiteness of current state-of-the-art imaging detectors. Imaging and analyzing digital rocks that sample the REV and still sufficiently resolve pore throats is critical to ensure simulation quality and robustness of rock property trends for further analysis. We find that at least 10 voxels are needed to sufficiently resolve pore throats for single phase fluid flow simulations. If this condition is not met, additional analyses and corrections may allow for meaningful comparisons between simulation results and laboratory measurements of permeability, but some cases may fall outside the current technical feasibility of DRP. On the other hand, we find that the ratio of field of view and effective grain size provides a reliable measure of the REV for siliciclastic rocks. If this ratio is greater than 5, the coefficient of variation for single-phase permeability simulations drops below 15%. These imaging considerations are crucial when comparing digitally computed rock flow properties with those measured in the laboratory. We find that the current imaging methods are sufficient to achieve both REV (with respect to numerical boundary conditions) and required image resolution to perform digital core analysis for coarse to fine-grained sandstones.

Correction of source-rock permeability measurements owing to slip flow and Knudsen diffusion: a method and its evaluation

Source-rock permeability is a key parameter that controls the gas production rate from unconventional reservoirs. Measured source-rock permeability in the laboratory, however, is not an intrinsic property of a rock sample, but depends on pore pressure and temperature as a result of the relative importance of slip flow and diffusion in gas flow in low-permeability media. To estimate the intrinsic permeability which is required to determine effective permeability values for the reservoir conditions, this study presents a simple approach to correct the laboratory permeability measurements based on the theory of gas flow in a micro/nano-tube that includes effects of viscous flow, slip flow and Knudsen diffusion under different pore pressure and temperature conditions. The approach has been verified using published shale laboratory data. The “corrected” (or intrinsic) permeability is considerably smaller than the measured permeability. A larger measured permeability generally corresponds to a smaller relative difference between measured and corrected permeability values. A plot based on our approach is presented to describe the relationships between measured and corrected permeability for typical Gas Research Institute permeability test conditions. The developed approach also allows estimating the effective permeability in reservoir conditions from a laboratory permeability measurement.

An optimized transient technique and flow modeling for laboratory permeability measurements of unconventional gas reservoirs with tight structure

Considering the significance of pressure-dependent-permeability in characterizing fluid flow and production from gas reservoirs and the inaccuracy of the conventional transient technique in laboratory permeability measurements of core samples from unconventional reservoirs with tight structure and strong sorption potential, the conventional transient technique is optimized by using two equal-sized gas reservoirs mounted in the up and downstream sides of the experimental setup and creating same magnitude of pressure pulses in each reservoir concurrently. Applicability of the optimized transient technique is examined through both numerical simulation and laboratory tests. A mathematical model is firstly developed to numerically investigate fluid flow behavior in time and space domain. The results from numerical simulation showed that the optimized technique can greatly improve the efficiency and accuracy of the measurement, because a steady flow state as a function of time can be achieved much faster in comparison with the conventional transient technique when the same magnitude of initial pressure difference is established between the two ends of the sample. Given the difference in experimental design and procedures between the conventional and the optimized technique, it is questionable whether currently used analytical solutions developed for the conventional technique for permeability calculation can be applied for the optimized technique. Therefore, an analytical solution, corresponding to the optimized transient technique, has been then theoretically derived. It is finally experimentally verified that accurate permeability results can be obtained based on late-time pressure responses by the derived analytical solution. The optimized experimental method is capable of extending the application of the transient technique into permeability measurements of rocks with tight structure and strong gas sorption potential, providing a more reliable approach for laboratory permeability tests.

Correction of laboratory gas permeability measurements using Klinkenberg-type correction models

Routine laboratory permeability measurements require both overburden correction and in the case of lower permeability gas measurements also Klinkenberg-type correction, accounting for slippage of gas when flowing through a porous medium. These corrections are necessary for obtaining representative permeability values for dynamic simulation. The objective of this paper is to determine the most suitable technique for determining representative, equivalent reservoir permeability. Laboratory permeability is routinely measured using different types of gases, most often helium and air, less often liquid. Single phase permeability measurements should be independent of the measuring fluid. However, laboratory permeability measurements using gas tend to overestimate sample permeability due to gas slippage. This effect was first reported by Klinkenberg (1941). Influencing factors are type of gas, mean experimental pressure and rock properties. The so called ‘Square-root model’ (Florence et al. 2007) accounts for all of these factors and is an extension of Klinkenberg’s original equation. The applicability of the Square-root model and earlier Klinkenberg-type models of Jones and Owens (1979) and Sampath and Keighin (1981) for correcting single-point laboratory gas permeability measurements are investigated on a comparative basis. Furthermore, Klinkenberg-type corrections are best made after overburden correction. The study presented involves a parametric approach of the gas slippage influencing factors, in addition to a comparison of alternative formulations. In comparing various Klinkenberg-type corrections, it is shown that the Square-root model compares most favourably and is most suitable for correcting laboratory data in the absence of specific measurements, as validated by comparison with laboratory deduced measurements. Datasets from the Asia-Pacific region and elsewhere are used to exemplify the methodology.

Application of critical path analysis for permeability prediction in natural porous media

Critical path analysis (CPA), originally developed to describe electrical conductance in semiconductors, has been shown recently to hold some promise in describing transport properties of porous media. I applied some previously developed concepts in CPA and percolation theory to predict permeability in a suite of sandstone, carbonate, and clay-rich samples. I assumed that the pore sizes in my samples exhibited fractal scaling and expressed the electrical formation factor as a function of porosity using universal scaling from percolation theory. The resulting CPA permeability predictions match the measured values very well. In addition, I show how considering the scale-dependence of the percolation threshold yields two characteristic length scales for transport properties: the critical pore size, and the sample size. This work suggests that the CPA framework is appropriate for describing transport properties of natural porous media, and provides a theoretical basis for understanding the permeability of tight rocks like shale in which laboratory permeability measurements are difficult.

Exploring the scale-dependent permeability of fractured andesite

Extension fractures in volcanic systems exist on all scales, from microscopic fractures to large fissures. They play a fundamental role in the movement of fluids and distribution of pore pressure, and therefore exert considerable influence over volcanic eruption recurrence. We present here laboratory permeability measurements for porous (porosity = 0.03–0.6) andesites before (i.e., intact) and after failure in tension (i.e., the samples host a throughgoing tensile fracture). The permeability of the intact andesites increases with increasing porosity, from 2×10−17 to 5×10−11 m2. Following fracture formation, the permeability of the samples (the equivalent permeability) falls within a narrow range, 2–6×10−11 m2, regardless of their initial porosity. However, laboratory measurements on fractured samples likely overestimate the equivalent permeability due to the inherent scale-dependence of permeability. To explore this scale-dependence, we first determined the permeability of the tensile fractures using a two-dimensional model that considers flow in parallel layers. Our calculations highlight that tensile fractures in low-porosity samples are more permeable (as high as 3.5×10−9 m2) than those in high-porosity samples (as low as 4.1×10−10 m2), a difference that can be explained by an increase in fracture tortuosity with porosity. We then use our fracture permeability data to model the equivalent permeability of fractured rock (with different host rock permeabilities, from 10−17 to 10−11 m2) with increasing lengthscale. We highlight that our modelling approach can be used to estimate the equivalent permeability of numerous scenarios at andesitic stratovolcanoes in which the fracture density and width and host rock porosity or permeability are known. The model shows that the equivalent permeability of fractured andesite depends heavily on the initial host rock permeability and the scale of interest. At a given lengthscale, the equivalent permeability of high-permeability rock (10−12 to 10−11 m2) is essentially unaffected by the presence of numerous tensile fractures. By contrast, a single tensile fracture increases the equivalent permeability of low-permeability rock (<10−15 m2) by many orders of magnitude. We also find that fractured, low-permeability rock (e.g., 10−17 m2) can have an equivalent permeability higher than that of similarly fractured rock with higher host rock permeability (e.g., 10−15 m2) due to the low-tortuosity of fractures in low-porosity andesite. Our modelling therefore outlines the importance of fractures in low-porosity, low-permeability volcanic systems. While our laboratory measurements show that, regardless of the initial porosity, the equivalent permeability of fractured rock on the laboratory scale is 2–6×10−11 m2, the equivalent permeability of low-permeability rock is significantly reduced as the scale of interest is increased. Therefore, due to the scale-dependence of permeability, laboratory measurements on pristine, low-permeability rocks significantly underestimate the equivalent permeability of fractured volcanic rock. Further, measurements on fractured rock samples can significantly overestimate the equivalent permeability. As a result, care must be taken when selecting samples in the field and when using laboratory data in volcano outgassing models. The data and modelling presented herein provide insight into the scale-dependence of the permeability of fractured volcanic rock, a prerequisite for understanding outgassing at active volcanoes.

A new interpretation of the response of coal permeability to changes in pore pressure, stress and matrix shrinkage

Coal permeability is often modelled as an exponential function of the Terzaghi effective stress, that is the difference between stress and pore pressure. However, laboratory measurements of permeability have found that, under constant effective stress conditions, shrinkage with gas desorption or swelling with adsorption (known as the sorption strain) influences the measured permeability. In the derivation of the exponential model these shrinkage and swelling processes were neglected. In applying the exponential model to the reservoir, the stress is replaced by a geomechanical solution for uniaxial strain, constant vertical stress conditions that includes the sorption strain. However, laboratory results indicate that there is a direct influence from sorption strain on permeability. This paper also demonstrates that pore and confining pressures can also operate independently on permeability rather than together as the Terzaghi effective stress. Based on well established theoretical treatments and without invoking new conceptual models, this paper develops a simple extension of the exponential model to include the independent effects of pore and confining pressures and the sorption strain. This new model introduces two new properties over the existing exponential function with effective stress. The new model is tested by applying it to a data base of laboratory measurements of coal core permeability with helium, nitrogen, methane, and carbon dioxide over a range of pore and confining pressures where it was shown to be able to accurately match the observations with generally consistent model properties. Forms of the new model are then developed for uniaxial strain and constant vertical stress reservoir conditions using the Shi-Durucan geomechanical model. The model predictions of reservoir permeability show that the additional sorption strain term acts to increase the rebound of permeability during pore pressure drawdown. The reservoir form of the model is applied to observations of permeability from the San Juan Basin and compared to equivalent Shi-Durucan model results. Additional processes operating within the San Juan act to reduce the permeability rebound below that predicted by both of the permeability models. One explanation for this behaviour is formation damage due to fines production.

Nanoscale grain boundary channels in fracture cement enhance flow in mudrocks

AbstractHydrocarbon production from mudrock or shale reservoirs typically exceeds estimates based on mudrock laboratory permeability measurements, with the difference attributed to natural fractures. However, natural fractures in these reservoirs are frequently completely cemented and thus assumed not to contribute to flow. We quantify the permeability of nanoscale grain boundary channels with mean apertures of 50–130 nm in otherwise completely cemented natural fractures of the Eagle Ford Formation and estimate their contribution to production. Using scanning electron imaging of grain boundary channel network geometry and a digital rock physics workflow of image reconstruction and direct flow modeling, we estimate cement permeability to be 38–750 nd, higher than reported permeability of Eagle Ford host rock (~2 nd) based on laboratory measurements. Our results suggest that effective fracture‐parallel mudrock permeability can exceed laboratory values by upward of 1 order of magnitude in shale reservoirs of high macroscopic cemented fracture volume fraction.

Laboratory investigations of gas flow behaviors in tight anthracite and evaluation of different pulse-decay methods on permeability estimation

Permeability evolution in coal is critical for the prediction of coalbed methane (CBM) production and CO2-enhanced-CBM. The anthracite, as the highest rank coal, has ultra-tight structure and the gas flow dynamics is complicated and influenced by multi-mechanistic flow components. Gas transport in anthracite will be a nonlinear multi-mechanistic process also including non-Darcy components like gas as-/desorption, gas slippage and diffusion flow. In this study, a series of laboratory permeability measurements were conducted on an anthracite sample for helium and CO2 depletions under both constant stress and uniaxial strain boundary conditions. The different transient pulse-decay methods were utilized to estimate the permeability and Klinkenberg correction accounting for slip effect was also used to calculate the intrinsic permeability. The helium permeability results indicate that the overall permeability under uniaxial strain condition is higher than that under constant stress condition because of larger effective stress reduction during gas depletion. At low pressure under constant stress condition, CO2 permeability enhancement due to sorption-induced matrix shrinkage effect is significant, which can be either clearly observed from the pulse-decay pressure response curves or the data reduced by Cui et al.'s method. But within the same pressure range, there is almost no difference between Brace's method and Dicker & Smits's method. Gas slippage effect is also significant at low pressure for low permeability coal based on the obtained experimental data.

Vertical and horizontal permeability measurements in organic soils

Abstract The paper is referring to vertical and horizontal laboratory permeability measurements in soft organic soils. The estimation of anisotropic permeability in soft organic soils, as peats, requires to use a special apparatus and the knowledge of proper analysis of the test results. During loading the void ratio decreases substantially that causes the changeability of the permeability. The change of permeability during the compression is very important because of the influence of the consolidation co-efficient. Initial strain in soft organic soils appears very quickly, just after loading, and brings immediately the decrease of permeability. In most of the estimations, it is assumed that during the consolidation process the water flows just in the vertical direction. In soft organic soils, like peats, the consolidation theory should consider the changes of mechanical and physical properties in consolidation period, in both directions. The direct measurement of vertical and horizontal permeability of organic soil and the non-Darcian flow theory may be of considerable importance in estimating pore water pressure dissipation, and settlement rates in the consolidation model. In the paper, the method of investigation and the test results of the vertical and horizontal permeability are presented. The Modified Rowe Cell Set for obtaining consolidation and flow characteristics in different directions is used.

Assessment of water and air permeability of chernozem supported by image analysis

We aimed at the assessment of water and air permeability of a Haplic Chernozem developed from loess. The laboratory permeability measurements were presented against a number of morphometric indices that characterize the pore system in this soil, and were obtained by computer-aided image analysis on the basis of large resin-impregnated soil opaque blocks. Two indices of soil pore connectivity are proposed, i.e. the index of soil pore network growth rate and the percolation number. Basic soil properties were evaluated (soil texture, TOC, carbonates, pH, particle and soil bulk density, total porosity). The saturated hydraulic conductivity, and soil water content, air capacity and permeability at −15.54kPa and −9.81kPa were measured. From the samples with preserved structure, resin-impregnated soil opaque blocks 8cm×9cm in size were prepared and then used for morphological and morphometric structure analysis. The preliminary image analysis was made to find the best representation of the actual chernozem pore system. The images were modified by applying the morphological closing with or without the spike noise, which modelled tiny pores missed during scanning. Consequently, the macroporosity of the images approximated the air capacity at potentials −15.54 and −9.81kPa. During the extended image analysis, we calculated: the index of soil pore network growth rate; the percolation number; the average cross-sectional size of the pore; the total length of pore path; the relative volume of pores overlapping the left and right, and the top and bottom image edge; the relative volume of pores connecting the left and right and the top and bottom edge of the image. The correlation coefficients between the parameters’ values obtained from image analysis and from laboratory permeability measurements were calculated.The water and air permeability and the air capacity of the chernozem decreased with depth into the soil pedon. With decrease in the saturated hydraulic conductivity, measured in the laboratory, there was a decrease in the relative volumes of pores overlapping the left and right and upper and lower edges of the image, obtained from image analysis. The air permeability was positively correlated with the index of pore network growth rate. Morphological and morphometric image analysis confirmed that the most important parameters determining the transport of fluids in the soil are continuity of the pore system and pore volume. Based on the results obtained from image analysis one can formulate qualitative conclusions concerning the water and air permeability of soil.

Hydraulic and acoustic properties of the active Alpine Fault, New Zealand: Laboratory measurements on DFDP-1 drill core

We report on laboratory measurements of permeability and elastic wavespeed for a suite of samples obtained by drilling across the active Alpine Fault on the South Island of New Zealand, as part of the first phase of the Deep Fault Drilling Project (DFDP-1). We find that clay-rich cataclasite and principal slip zone (PSZ) samples exhibit low permeabilities (⩽10−18 m2), and that the permeability of hanging-wall cataclasites increases (from c. 10−18 m2 to 10−15 m2) with distance from the fault. Additionally, the PSZ exhibits a markedly lower P-wave velocity and Young's modulus relative to the wall rocks. Our laboratory data are in good agreement with in situ wireline logging measurements and are consistent with the identification of an alteration zone surrounding the PSZ defined by observations of core samples. The properties of this zone and the low permeability of the PSZ likely govern transient hydrologic processes during earthquake slip, including thermal pressurization and dilatancy strengthening.

Drilling reveals fluid control on architecture and rupture of the Alpine fault, New Zealand

Rock damage during earthquake slip affects fluid migration within the fault core and the surrounding damage zone, and consequently coseismic and postseismic strength evolution. Results from the first two boreholes (Deep Fault Drilling Project DFDP-1) drilled through the Alpine fault, New Zealand, which is late in its 200–400 yr earthquake cycle, reveal a >50-m-thick “alteration zone” formed by fluid-rock interaction and mineralization above background regional levels. The alteration zone comprises cemented low-permeability cataclasite and ultramylonite dissected by clay-filled fractures, and obscures the boundary between the damage zone and fault core. The fault core contains a <0.5-m-thick principal slip zone (PSZ) of low electrical resistivity and high spontaneous potential within a 2-m-thick layer of gouge and ultracataclasite. A 0.53 MPa step in fluid pressure measured across this zone confirms a hydraulic seal, and is consistent with laboratory permeability measurements on the order of 10?20 m2. Slug tests in the upper part of the boreholes yield a permeability within the distal damage zone of ?10?14 m2, implying a six-orders-of-magnitude reduction in permeability within the alteration zone. Low permeability within 20 m of the PSZ is confirmed by a subhydrostatic pressure gradient, pressure relaxation times, and laboratory measurements. The low-permeability rocks suggest that dynamic pressurization likely promotes earthquake slip, and motivates the hypothesis that fault zones may be regional barriers to fluid flow and sites of high fluid pressure gradient. We suggest that hydrogeological processes within the alteration zone modify the permeability, strength, and seismic properties of major faults throughout their earthquake cycles.

間隙水圧・変形連成問題への最近の取り組み 地下深部における新第三紀泥質軟岩中の亀裂の透水特性―室内試験による推定―

We operated laboratory permeability measurements with Neogene mudstone specimens in order to investigate an applicability of laboratory tests to estimate fracture permeability in depth. We collected rock samples from drill cores of Koetoi Formation diatomaceous mudstone and Wakkanai Formation siliceous mudstone at Horonobe area, northern Hokkaido, Japan. We prepared specimens with a saw-cut discontinuity, simulating a fracture in rock, and measured permeability in the direction parallel to the discontinuity under hydrostatic stress condition up to 80 MPa. The measured permeability of a specimen with a saw-cut discontinuity fracture was larger than that of an intact specimen when confining pressure is less than 1 MPa (comparable to the depth of approximately 150 m) in the case of Koetoi Formation diatomaceous mudstone, but, when confining pressure is 1 MPa or more, the permeability values of a saw-cut specimen and an intact specimen became a similar to each other. On the other hand, in the case of Wakkanai Formation siliceous mudstone, the confining pressure under which the permeability of a saw-cut specimen became similar to the permeability of an intact specimen was close to 80 MPa (comparable to the depth of several km) . These results can explain the difference between Koetoi and Wakkanai Formations on distribution of in situ permeability measurements operated by Japan Atomic Energy Agency. Numerical simulations of a flow through a fracture under normal stress could explain the results of the laboratory test for Koetoi Formation diatomaceous mudstone, by using reasonable mechanical properties. These results support the possibility of estimating stress dependency of fracture permeability from rock mechanical properties and permeability of intact part.

Pore pressure development beneath the décollement at the Nankai subduction zone: Implications for plate boundary fault strength and sediment dewatering

Despite its importance for plate boundary fault processes, quantitative constraints on pore pressure are rare, especially within fault zones. Here, we combine laboratory permeability measurements from core samples with a model of loading and pore pressure diffusion to investigate pore fluid pressure evolution within underthrust sediment at the Nankai subduction zone. Independent estimates of pore pressure to ∼20 km from the trench, combined with permeability measurements conducted over a wide range of effective stresses and porosities, allow us to reliably simulate pore pressure development to greater depths than in previous studies and to directly quantify pore pressure within the plate boundary fault zone itself, which acts as the upper boundary of the underthrusting section. Our results suggest that the time‐averaged excess pore pressure (P*) along the décollement ranges from 1.7–2.1 MPa at the trench to 30.2–35.9 MPa by 40 km landward, corresponding to pore pressure ratios of λb = 0.68–0.77. For friction coefficients of 0.30–0.40, the resulting shear strength along the décollement remains &lt;12 MPa over this region. When noncohesive critical taper theory is applied using these values, the required pore pressure ratios within the wedge are near hydrostatic (λw = 0.41–0.59), implying either that pore pressure throughout the wedge is low or that the fault slips only during transient pulses of elevated pore pressure. In addition, simulated downward migration of minima in effective stress during drainage provides a quantitative explanation for down stepping of the décollement that is consistent with observations at Nankai.

Common Evolution of Mechanical and Transport Properties in Thermally Cracked Westerly Granite at Elevated Hydrostatic Pressure

Increasing the damage and crack porosity in crustal rocks can result in significant changes to various key physical properties, including mechanical strength, elastic and mechanical anisotropy, and the enhancement of transport properties. Using a Non-Interactive Crack Effective Medium (NIC) theory as a fundamental tool, we show that elastic wave dispersion can be inverted to evaluate crack density as a function of temperature and is compared with optically determined crack density. Further, we show how the existence of embedded microcrack fabrics in rocks also significantly influences the fracture toughness (KIC) of rocks as measured via a suite of tensile failure experiments (chevron cracked notch Brazilian disk). Finally, we include fluid flow in our analysis via the Guéguen and Dienes crack porosity-permeability model. Using the crack density and aspect ratio recovered from the elastic-wave velocity inversion, we successfully compare permeability evolution with pressure with the laboratory measurements of permeability.