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This study developed a one-dimensional numerical model of sedimentation and compaction based on the equivalent isochrone framework to investigate the impact of creep on geopressure during burial. In this framework, the void ratio is a function of effective stress and strain rate; the change in void ratio is the same with each order of magnitude decrease in strain rate at a constant effective stress. We simulated lower void ratio and higher overpressure when creep was included compared to cases where no creep was present and void ratio is only a function of effective stress. Creep causes apparent overconsolidation. The apparent overconsolidation ratio is used to quantify the magnitude of creep; this is the vertical distance from the normal compression curve in a void ratio v. effective stress plot. The magnitude of creep depends on the loading rate, and increases with depth at sites with low sedimentation rates. These findings bridge the gap between laboratory and field observations on rock compression behaviours. For example, it provides one explanation why laboratory-derived compression curves have a higher void ratio at a given effective stress. In addition, it suggests under what conditions the rock will behave elastically. Thematic collection: This article is part of the Geopressure collection available at: https://www.lyellcollection.org/cc/geopressure

We measured steady-state liquid (dodecane) permeability in four horizontal core plugs from the middle member of the Bakken Formation at multiple effective stress conditions to investigate how permeability evolves with confining stress and to infer the matrix permeability. Three of the four tested samples behaved almost perfectly elastically as the hysteresis effect was negligible. In contrast, the fourth sample showed a permeability decrease of ~40% at the end of the test program. Our interpretation is that the closure of open artificial micro-fractures initially present in the sample (based on micro-CT imaging) caused that permeability hysteresis. The matrix permeability to dodecane (oil) of the tested samples is between ~50 nD and ~520 nD at the confining pressure of 9500 psi. The 520 nD sample exhibited the lowest porosity, the highest calcite content, and the largest dominant pore throat radii. In contrast, the 50 nD sample was more porous, and exhibited the highest dolomite content and the smallest dominant pore throat radii. This study shows that our multi-stress testing protocol allows the study of the permeability hysteresis effect to interpret the matrix permeability. We also document the presence of middle Bakken lithologies with permeabilities up to one order of magnitude greater than others. These permeable lithologies may have a significant contribution to well production rates.

Argon gas and liquid (dodecane) permeability measurements in two mixed quality siliceous mudstone samples within the Wolfcamp formation demonstrate that it is possible to close multiple bedding parallel open artificial micro-fractures and obtain representative matrix permeability by applying two confining stress cycles at a constant pore pressure under effective stresses ranging from 6.9 MPa to 59.7 MPa. The clearly micro-fractured sample exhibited a three-order decrease in permeability from 4.4 × 10−17 m2 to 2.1 × 10−20 m2. In contrast, the relatively intact sample exhibited initial liquid permeability of 1.6 × 10−19 m2 that declined gradually with stress to 3.0 × 10−20 m2. In this study, we developed a new permeability testing protocol and analytical approaches to interpret the evolution of micro-fractures and estimate the matrix permeability based on initial pulse-decay gas permeability measurements. The insights gained are useful to identify pore-scale controls on fluid flow behavior using SEM petrography and pore-space characterization and validate fundamental fluid flow mechanisms examined using pore-scale numerical models.

We measured argon gas permeability in three intact and one partially fractured Eagle Ford Shale samples documenting the stress dependence of horizontal (bedding parallel) in situ permeability of intact samples which varies between 1 and 10 nD (1 nD = 0.9869233 × 10−21 m2), while the permeability of partially fractured sample varies between 18 and 37 nD. For all samples, permeability decreases by up to an order of magnitude while cycling the confining pressure (PC) between 27.7 and 55.2 MPa at a constant pore pressure (PP) of 14.4 MPa. Most of the permeability decrease is within the first loading and unloading cycle. During this first cycle, we also observe less than 2% decline in permeability over ~ 10 days when we held the PC constant at 51.6–55.2 MPa, respectively. This suggests that the ongoing creep plays a relatively minor role. The subsequent PC cycles result in a small decrease in permeability (~ 6 to 26% variation between the start and the end of each cycle). We interpret that the initial permeability loss is due to the closing of micro-fractures—which we infer are caused by stress relief and gas expansion during sample retrieval and/or preparation. We interpret that the higher permeability of the partially fractured sample is mainly due to incomplete closure of a preexisting fracture, which extends nearly two-third the sample length. We document this dual-permeability structure from the observation of a dual-timescale pressure response behavior during the experiments at lower PC–PP. We find permeability decreases with increasing PC–PP; stress dependency of permeability follows an exponential relationship with a stress-sensitive gradient of 0.019–0.040 MPa−1. A better understanding of permeability variation with stress will help to reliably estimate in situ permeability and to better understand production evolution from unconventional shale reservoirs.

We examined the permeability behavior of two Marcellus Shale samples and two Eagle Ford Shale samples using argon gas under multiple confining stresses (PC = 13.8–68.9 MPa) and pore pressures (PP = 1.6–28.2 MPa). We corrected our measured permeability values for the Klinkenberg effect and contoured permeability as a function of PP and PC to obtain the effective stress coefficient for permeability (nk), which scales the influence of PP on permeability relative to PC. We explored the underlying mechanisms controlling nk using two idealized models of a cylindrical capillary tube consisting of low and high compressibility material to model the unique pore structure present in our samples. Finally, we investigated the impact of PP reduction on permeability during production. We show that the permeability is stress-dependent and that nk is formation-dependent. Marcellus Shale samples, with predominantly organic matter hosted pores, have a Klinkenberg-corrected permeability of 11.0 to 1.9 nD (PC - PP = 5.3–33.8 MPa) and nk value of 1.11 and 1.60, respectively. In contrast, Eagle Ford Shale samples, with predominantly inter-particle pores, have Klinkenberg-corrected permeability of 33.6 to 2.0 nD (PC - PP = 13.3–40.8 MPa) and nk value of 0.96 and 0.84, respectively. We infer that the PP change produces considerable pore wall strain relative to the PC change in the Marcellus Shale organic matter pores resulting in nk > 1. In contrast, we infer a smaller pore wall strain for the same PP and PC changes in the Eagle Ford Shale inter-particle mineral pores giving in nk

We observed multi-scale porosity and permeability at the cm-scale in a Barnett Shale core through a pulse-decay permeability test. The core is composed of alternating layers of silty-claystone and claystone. We interpret the silty-claystone has a permeability of 3.41×10 −20 m 2 (34.6 nD) and a porosity of 5.6% and that the claystone has a permeability of 1.80×10 −23 m 2 (0.0182 nD) and a porosity of 4.8%. The horizontal effective permeability is 2.05×10 −20 m 2 (20.8 nD) and we estimate the vertical effective permeability to be 4.58×10 −23 m 2 (0.0452 nD). The effective permeability anisotropy ratio is approximately 450. These results suggest that relatively high-permeability carrier beds drain organic rich lower permeability beds. The microstratigraphy of mudstones has a fundamental control on flow, and may provide an explanation for recent studies that have suggested either pervasive natural fracturing or extraordinary levels of induced fracturing are necessary to explain shale production behavior.

We document vertical permeability of $$2.3 \times 10^{-21}\, \hbox {m}^{2}$$ (2.3 nd) and horizontal permeability of $$9.5 \times 10^{-20}\, \hbox {m}^{2}$$ (96.3 nd) in two Barnett Shale samples. The samples are composed predominantly of quartz, calcite, and clay; have a porosity and a total organic content of $$\sim $$ 4 % each; and have a thermal maturity of 1.9 % vitrinite reflectance. Both samples exhibit stress-dependent permeability when the confining pressure is increased from 10.3 to 41.4 MPa. We measure a permeability anisotropy, the ratio of the horizontal to the vertical permeability, of $$\sim $$ 40. We find that the permeability anisotropy does not vary with effective stress. Multiscale permeability, as demonstrated by pressure dissipation, is related to millimeter-scale stratigraphic variation. We attribute the permeability anisotropy to preferential flow along more permeable layers and attribute the stress dependence to pore closure. A determination of permeability anisotropy allows us to understand flow properties in horizontal and vertical directions and assists our understanding of upscaling. Characterization of stress dependency allows us to predict permeability evolution during production.

This paper describes the results of triaxial compression tests carried out at effective cell pressures ranging from 12.5 to 100 kPa to investigate the influence of fabric structure on the yield and failure of intact Reigate silver sand. In some of the tests, a digital image-based technique was used to determine the instant of onset of strain localisation, and the distribution of strain localisations within the specimen as overall deformation progressed. Comparative tests on intact and reconstituted specimens showed that fabric structure in the intact material allows the mobilisation of stress ratios close to peak before the onset of dilation, and increases the shear modulus at a given effective cell pressure and strain. Localisation was found to start at or after the onset of dilation, with a tendency to delay at increasing effective cell pressure. More localised deformation was observed at low effective cell pressures. Consistency between the critical state strengths of intact and reconstituted specimens is demonstrated, provided that the effect of shear band geometry is taken into account in stress analysis.

Strain localization developing inside soft rock specimens is examined through experimental observation and numerical simulation. In the experimental study, soft rock specimens are sheared at different strain rates under plane strain conditions and deformation and strain localization characteristics are analysed. Transition of localization mode from highly localized mode for higher strain rate to distributed and diffused mode of strain localization for lower strain rates was observed. In the numerical study, simulations of plane strain compression tests are carried out at different strain rates by using an overstressed-type elasto-viscoplastic model in finite element computations. The role of strain rates on setting gradients of strain fields across shear band is clarified. The probable mechanism for transition of localization mode is discussed. Copyright © 2005 John Wiley & Sons, Ltd.

This paper describes model tests used to investigate how ballast shoulder width and height contribute to a railway sleeper’s resistance to lateral movement for a range of shoulder widths and heights. Deflection and resistance were measured and photographs taken during the tests.The photographs were analyzed using a digital image correlation technique to identify the zones of ballast surface disturbance, which demonstrated that a bulbed failure volume was mobilized at the ultimate limit state. An idealized three-dimensional failure mechanism is proposed, and resistances are calculated using the limit equilibrium approach. The calculation provides a reliable estimate of the measured resistance. The work identifies the optimum shoulder width and height. The calculations are extended to demonstrate that when a number of sleepers are moved simultaneously, the sleeper end resistance may be one-third less per sleeper than that indicated in tests on an isolated sleeper. Image analysis and limit equilibrium calculations show that this is caused by overlapping of mobilized failure volumes from adjacent sleepers.

Legislation in some parts of the world now requires municipal solid waste (MSW) to be processed prior to landfilling to reduce its biodegradability and hence its polluting potential through leachate and fugitive emission of greenhouse gases. This pre-processing may be achieved through what is generically termed mechanical–biological-treatment (MBT). One of the major concerns relating to MBT wastes is that the strength of the material may be less than for raw MSW, owing to the removal of sheet, stick and string-like reinforcing elements during processing. Also, the gradual increase in mobilized strength over strains of 30% or so commonly associated with unprocessed municipal solid waste may not occur with treated wastes. This paper describes a series of triaxial tests carried out to investigate the stress–strain–strength characteristics of an MBT waste, using a novel digital image analysis technique for the determination of detailed displacement fields over the whole specimen. New insights gained into the mechanical behavior of MBT waste include the effect of density on the stress–strain response, the initial 1-D compression of lightly consolidated specimens, and the likely reinforcing effect of small sheet like particles remaining in the waste.