Changes in the sources of velocity anisotropy and their relative magnitude as maturation progresses in organic-rich shale are still incompletely characterized in the rock-physics literature. As a result of the increasing importance of organic-rich shale as unconventional reservoirs, a more thorough understanding of the elastic behavior of shale is needed. We have formulated a comprehensive, multiphysics, multiscale experimental methodology for the characterization of the intrinsic (syn-lithification) and extrinsic (postlithification) factors contributing to velocity anisotropy. Application of this methodology to unsaturated samples also enabled the characterization of the shale frame for fluid substitution modeling. The methodological framework was then tested on a set of five naturally matured organic-rich shale samples. In this experimental methodology, we combined classical rock-physics measurements, e.g., ultrasonic velocity and emergent high-resolution imaging techniques, such as X-ray diffraction (XRD), scanning electron microscopy, confocal laser scanning microscopy, and X-ray microtomography to better characterize the heterogeneous and microstructurally complex shale at all scales. The use of XRD-based lattice-preferred orientation measurements in conjunction with conventional ultrasonic velocity experiments confirmed that the degree of alignment of the mineral matrix governed the intrinsic anisotropy of organic-rich shale. The closure of soft, crack-like porosity, as identified from axial strain data, was identified as the extrinsic source governing the pressure sensitivity of velocity anisotropy. We determined, for the set of samples included in this study, that the intrinsic anisotropy was the dominant source of anisotropy at all confining pressures. Indeed, at low confining pressures, the opening of microcracks contributed no more than 30% of the total velocity anisotropy. Applying these results to saturated rocks at depth indicated that, for these shales, the extrinsic, crack-based sources, will contribute no more than 30% of the shale anisotropy in situ.

The sources of elastic anisotropy in organic-rich shale and their relative contribution therein remain poorly understood in the rock-physics literature. Given the importance of organic-rich shale as source rocks and unconventional reservoirs, it is imperative that a thorough understanding of shale rock physics is developed. We made a first attempt at establishing cause-and-effect relationships between geochemical parameters and microstructure/rock physics as organic-rich shales thermally mature. To minimize auxiliary effects, e.g., mineralogical variations among samples, we studied the induced evolution of three pairs of vertical and horizontal shale plugs through dry pyrolysis experiments in lieu of traditional samples from a range of in situ thermal maturities. The sensitivity of P-wave velocity to pressure showed a significant increase post-pyrolysis indicating the development of considerable soft porosity, e.g., microcracks. Time-lapse, high-resolution backscattered electron-scanning electron microscope images complemented this analysis through the identification of extensive microcracking within and proximally to kerogen bodies. As a result of the extensive microcracking, the P-wave velocity anisotropy, as defined by the Thomsen parameter epsilon, increased by up to 0.60 at low confining pressures. Additionally, the degree of microcracking was shown to increase as a function of the hydrocarbon generative potential of each shale. At 50 MPa confining pressure, P-wave anisotropy values increased by 0.29–0.35 over those measured at the baseline — i.e., the immature window. The increase in anisotropy at high confining pressure may indicate a source of anisotropy in addition to microcracking — potentially clay mineralogical transformation or the development of intrinsic anisotropy in the organic matter through aromatization. Furthermore, the evolution of acoustic properties and microstructure upon further pyrolysis to the dry-gas window was shown to be negligible.

The evolution of the elastic properties of organic-rich shale as a function of thermal maturity remains poorly constrained. This understanding is pivotal to the characterization of source rocks and unconventional reservoirs. To better constrain the evolution of the elastic properties and microstructure of organic-rich shale, we have studied the acoustic velocities and elastic anisotropy of samples from two microstructurally different organic-rich shales before and after pyrolysis-induced thermal maturation. To more physically imitate in situ thermal maturation, we performed the pyrolysis experiments on intact core plugs under applied reservoir-magnitude confining pressures. Iterative characterization of the elastic properties of a clay-rich, laminar Barnett Shale sample documents the development of subparallel to bedding cracks by an increase in velocity sensitivity to pressure perpendicular to the bedding. These cracks, however, are not visible through time-lapse scanning electron microscope imaging, indicating either submicrometer crack apertures or predominant development within the core of the sample. At elevated confining pressures, in the absence of pore pressure, these induced cracks close, at which point, the sample is acoustically indistinguishable from the prepyrolysis sample. Conversely, a micritic Green River sample does not exhibit the formation of aligned compliant features. Rather, the sample exhibits a largely directionally independent decrease in velocity as load-bearing, pore-filling kerogen is removed from the sample. Due to the weak alignment of minerals, there is comparatively little intrinsic anisotropy; further, due to the relatively directionally independent evolution of velocity, the evolution of the anisotropy as a function of thermal maturity is not indicative of aligned compliant features. Our results have indicated that horizons of greater thermal maturity may be acoustically detectable in situ through increases in the elastic anisotropy of laminar shales or decreases in the acoustic velocities of nonlaminar shales, micritic rocks, or siltstones.

Microporous rocks are being increasingly researched as novel exploration and development technologies facilitate production of the reserves confined in the low-permeability reservoir. The ability to numerically estimate effective permeability is pivotal to characterizing the production capability of microporous reservoirs. In this study, a novel methodology is presented for estimating the steady-state effective permeability from FIB-SEM volumes. We quantify the effect of a static adsorbed monolayer and Knudsen diffusion on effective permeability as a function of pore pressure to better model production of microporous rock volumes. The adsorbed layer is incorporated by generating an effective pore geometry with a pore pressure-dependent layer of immobile voxels at the fluid-solid interface. Due to the steadystate nature of this study, surface diffusion within the adsorbed layer and topological variations of the layer within pores are neglected, potentially resulting in underestimation of effective permeability over extended production time periods. Knudsen diffusion and gas slippage is incorporated through computation of an apparent permeability that accounts for the rarefaction of the pore fluid. We determine that at syn-production pore pressures, permeability varies significantly as a function of the phase of the pore fluid. Simulation of methane transport in micropores indicates that, in the supercritical regime, the effect of Knudsen diffusion relative to adsorption is significantly reduced resulting in effective permeability values up to 10 nanodarcies (9.87 × 10 −21 m 2) less or 40% lower than the continuum prediction. Contrastingly, at subcritical pore pressures, the effective permeability is significantly greater than the continuum prediction due to rarefaction of the gas and the onset of Knudsen diffusion. For example, at 1 MPa, the effective permeability of the kerogen body is five times the continuum prediction. This study demonstrates the importance of, and provides a novel methodology for, incorporating noncontinuum effects in the estimation of the transport properties of microporous rocks.

The management of the mine at Mount Isa, Queensland, Australia decided to enquire into the following questions with regard to men working underground in hot conditions: (a) Which of the various heat stress indices predicts most accurately the effects on workmen of the various heat stress factors which occur in the mine at Mount Isa? (b) How best should the limits of heat stress be judged at which the normal 8-hour shift should be reduced to a 6-hour shift, or at which work should be stopped? With these objects in mind, oral temperatures were measured on 86 workmen after three hours of ordinary work in the mine and also on 36 occasions on 29 volunteers after three hours of stepping on and off a stool at a work rate of 1,560 ft. lb./min. These men were studied in different environmental heat stresses over the range that occurs in the mine. Dry bulb air temperatures (D.B.), wet bulb temperatures (W.B.), velocity of air movements, and globe temperatures (G.T.) were measured in the micro-climate in which each man worked. An estimate was made of the work rate of the 86 workmen. From these estimates and measurements, the predicted 4-hourly sweat rate (P4SR) and corrected effective temperature (C.E.T.) values were determined for each heat stress condition. P4SR values varied between 0·9 and 6·5 and C.E.T. between 70° and 95°F. Correlation coefficients were calculated between oral temperatures and W.B.s, C.E.T.s, and P4SRs and are 0·51, 0·64, and 0·75 respectively. Further analysis was confined to C.E.T. and P4SR. Plots of oral temperature on P4SR for conditions where G.T. was more than 10°F. above D.B. were found to fall well below the rest of the plots, indicating that P4SR exaggerates the effect of mean radiant temperature. These data were therefore excluded from the rest of the analysis. Regression equations were calculated for oral temperature on P4SR and for oral temperature on C.E.T. for (a) men `on the job', for (i) conditions where D.B. was more than 10°F. above W.B. and (ii) for conditions where D.B. was less than 10°F. above W.B., and (b) for men `stepping'. This analysis showed that one overall regression line can be used for all three conditions for oral temperature on P4SR, but for oral temperature on C.E.T. at least two different regression lines would be needed. Also the correlation coefficients between oral temperature and P4SR were generally higher than between oral temperature and C.E.T. For the prediction of oral temperature in the mine at Mount Isa the P4SR index is to be preferred to the C.E.T. scale. These results indicate that the emphasis given to G.T. in the P4SR index is too great. A multi-variance analysis of the P4SR index shows that, in the middle of the range of heat stress conditions examined, a unit change in P4SR would be obtained by about the same change in W.B. and G.T. This is at variance with the present results and also with the experimental findings of the M.R.C. Climatic Physiology Unit at Singapore. It appears, therefore, that the P4SR index should be revised in this regard. When ...