The relationship between permeability and porosity for fine-grained clastic sediments (“mudstones”) is a key constitutive equation for modelling subsurface fluid flow and is fundamental to the ...quantification of a range of geological processes. For a given porosity, mudstone permeability varies over a range of 2–5 orders of magnitude. We show here that much of the range can be explained by variations in lithology, which we define simply and pragmatically by clay content (mass fraction of particles less than 2 microns in diameter). Using clay content as the quantitative lithology descriptor, we have used a dataset (clay content range of 12–97%; porosity range of 0.04–0.78; six orders of magnitude permeability range) comprising 376 data points to derive a new bedding perpendicular permeability (K, m
2) – void ratio (e = porosity/(1-porosity)) relationship as a function of clay content (CF):
ln
(
K
)
=
−
69.59
−
26.79
⋅
C
F
+
44.07
⋅
C
F
0.5
+
(
−
53.61
−
80.03
⋅
C
F
+
132.78
⋅
C
F
0.5
)
⋅
ⅇ
+
(
86.61
+
81.91
⋅
C
F
−
163.61
⋅
C
F
0.5
)
⋅
ⅇ
0.5
The coefficient of regression (
r
2) = 0.93. At a given porosity, the inclusion of the quantitative lithological descriptor, clay content reduces the predicted range of permeability from 2 to 5 orders of magnitude to one order.
We present permeability and other petrophysical data (including pore size distribution, porosity, particle size distribution, grain density, specific surface area, total carbon content, organic ...carbon content, and sulphur content) for 30 deeply buried mudstones. Permeabilities were measured at different effective consolidation stresses ranging from 2.5 to 60 MPa with a 30,000 mg L−1 NaCl solution. Samples represent a wide spectrum of mudstone types with clay size particle contents ranging from 13 to 66%. Porosities range from 6 to 27%; pore size data show that porosity loss is driven primarily by collapse of the largest pores. Our results confirm and considerably extend previously reported results indicating the influence of clay content on pore size distributions and the way they evolve as a result of compaction. Vertical permeabilities, measured using the transient pulse decay technique, range from 2.4 × 10−22 m2 to 1.6 × 10−19 m2. Horizontal permeabilities range from 3.9 × 10−21 m2 to 9.5 × 10−19 m2, overlapping with but generally higher than vertical permeabilities. In general, permeability decreases logarithmically with porosity. The relationship between permeability and porosity is strongly influenced by clay content, especially at higher porosities. Ratios of horizontal to vertical permeability measured on four samples range from 1.7 to 11.8, implying the influence of both particle alignment and sedimentological heterogeneity. We have used the data to calibrate two permeability models. For the Kozeny‐Carman model, values of 200 and 1000 for the product of shape and tortuosity factors provide the best fit for the vertical and horizontal permeabilities, respectively. The calibrated Yang‐Aplin model predicts the permeability of almost all the samples to within a factor of ±3 over a 4 orders of magnitude range of permeability.
Sorption capacities and pore characteristics of bulk shales and isolated kerogens have been determined for immature, oil-window, and gas-window mature samples from the Lower Toarcian Posidonia shale ...formation. Dubinin–Radushkevich (DR) micropore volumes, sorption pore volumes, and surface areas of shales and kerogens were determined from CO2 adsorption isotherms at −78 and 0 °C, and from N2 adsorption isotherms at −196 °C. Mercury injection capillary pressure porosimetry, grain density measurements, and helium pycnometry were used to determine shale and kerogen densities and total pore volumes. Total porosities decrease through the oil-window and then increase into the gas-window. High-pressure methane isotherms up to 14 MPa were determined at 45, 65, and 85 °C on dry shale and at 45 and 65 °C on kerogen. Methane excess uptakes at 65 °C and 11.5 MPa were in the range 0.056–0.110 mmol g–1 (40–78 scf t–1) for dry Posidonia shales and 0.36–0.70 mmol g–1 (253–499 scf t–1) for the corresponding dry kerogens. Absolute methane isotherms were calculated by correcting for the gas at bulk gas phase density in the sorption pore volume. The enthalpies of CH4 adsorption for shales and kerogens at zero surface coverage showed no significant variation with maturity, indicating that the sorption pore volume is the primary control on sorption uptake. The sum of pore volumes measured by (a) CO2 sorption at −78 °C and (b) mercury injection, are similar to the total porosity for shales. Since mercury in our experiments occupies pores with constrictions larger than ca. 6 nm, we infer that porosity measured by CO2 adsorption at −78 °C in the samples used in this study is largely within pores with effective diameters smaller than 6 nm. The linear correlation between maximum CH4 surface excess sorption and CO2 sorption pore volume at −78 °C is very strong for both shales and kerogens, and goes through the origin, suggesting that the vast majority of sorbed CH4 occurs in pores smaller than 6 nm. The DR micropore volume obtained from CO2 adsorption at 0 °C was 40%–62% of the corresponding CO2 sorption pore volume. Sorption mass balances using kerogen and shale isotherms showed that approximately half of the CO2 sorption in these dry shales is in organic matter, with the rest likely to be associated with the inorganic phase (mainly clay minerals). A similar distribution was observed for supercritical CH4 adsorption. Mass balances for adsorption isotherms for kerogen and clay minerals do not always account for the total measured sorbed CH4 on dry shales, suggesting that some sorption may not be completely accounted for by the minerals identified and kerogens in the shales.
Low and high resolution petrographic studies have been combined with mineralogical, TOC, RockEval and porosity data to investigate controls on the evolution of porosity in stratigraphically ...equivalent immature, oil-window and gas-window samples from the Lower Toarcian Posidonia Shale formation. A series of 26 samples from three boreholes (Wickensen, Harderode and Haddessen) in the Hils syncline was investigated. The main primary components of the shales are microfossiferous calcite (30–50%), clay minerals (20–30%) and Type II organic matter (TOC = 7–15%, HI = 630–720 mg/gC in immature samples). Characteristic sub-centimetric light and dark lamination reflects rapid changes in the relative supply of these components. Total porosities decrease from 10 to 14% at Ro = 0.5% to 3–5% at Ro = 0.9% and then increase to 9–12% at Ro = 1.45%. These maturity-related porosity changes can be explained by (a) the primary composition of the shales, (b) carbonate diagenesis, (c) compaction and (d) the maturation, micro-migration, local trapping and gasification of heterogeneous organic phases. Calcite undergoes dissolution and reprecipitation reactions throughout the maturation sequence. Pores quantifiable in SEM (>ca. 50 nm) account for 14–25% of total porosity. At Ro = 0.5%, SEM-visible macropores1 are associated mainly with biogenic calcite. At this maturity, clays and organic matter are not visibly porous but nevertheless hold most of the shale porosity. Porosity loss into the oil window reflects (a) compaction, (b) carbonate cementation and (c) perhaps the swelling of kerogen by retained oil. In addition, porosity is occluded by a range of bituminous phases, especially in microfossil macropores and microfractures. In the gas window, mineral-hosted porosity is still the primary form of macroporosity, most commonly observed at the organic-inorganic interface. Increasing porosity into the gas window also coincides with the formation of isolated, spongy and complex meso- and macropores within organic particles, related to thermal cracking and gas generation. This intraorganic porosity is highly heterogeneous: point-counted macroporosity of individual organic particles ranges from 0 to 40%, with 65% of organic particles containing no macropores. We suggest that this reflects the physicochemical heterogeneity of the organic phases plus the variable mechanical protection afforded by the mineral matrix to allow macroporosity to be retained. The development of organic macroporosity cannot alone account for the porosity increase observed from oil to gas window; major contributions also come from the increased volume of organic micro- and meso-porosity, and perhaps by kerogen shrinkage.
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•Porosity of organic-rich, calcareous Posidonia Shale halves in oil window and doubles in gas window.•Porosity changes driven by carbonate diagenesis and retention/gasification of bitumen.•Pores quantifiable by SEM (>ca. 50 nm) only account for 14–25% of total porosity.•Macroporosity of single organic particles in gas window range from 0 to 40%.•Much of porosity generated in gas window is in micro- and mesopores.
Shale gas is becoming an increasingly important energy resource. In this study, the adsorption of methane on a dry, organic-rich Alum shale sample was studied at pressures up to ∼14 MPa and ...temperatures in the range 300–473 K, which are relevant to gas storage under geological conditions. Maximum methane excess uptake was 0.176–0.042 mmol g–1 (125–30 scf t–1) for the temperature range of 300–473 K. The decrease in maximum methane surface excess with increasing temperature can be described with a linear model. An isosteric enthalpy of adsorption 19.2 ± 0.1 kJ mol–1 was determined at 0.025 mmol g–1 using the van’t Hoff equation. Supercritical adsorption was modeled using the modified Dubinin–Radushkevich and the Langmuir equations. The results are compared with absolute isotherms calculated from surface excess and the pore volumes obtained from subcritical gas adsorption (nitrogen (78 K), carbon dioxide (273 and 195 K), and CH4 (112 K)). The subcritical adsorption and the surface excess results allow an upper limit to be put on the amount of gas that can be retained by adsorption during gas generation from petroleum source rocks.
An inter-laboratory study of high-pressure gas sorption measurements on two carbonaceous shales has been conducted in order to assess the reproducibility of the sorption isotherms and identify ...possible sources of error. The measurements were carried out by seven international research laboratories using either in-house or commercial sorption equipment (manometric and gravimetric methods). Excess sorption isotherms for methane, carbon dioxide and ethane were measured at 65°C and at pressures up to 25MPa on two organic-rich shales in the dry state. The samples used in this study were taken from immature Posidonia shale (Germany) and over-mature Upper Chokier Formation (Belgium). Their total organic carbon (TOC) contents were 15.1% and 4.4% , respectively, and their vitrinite reflectance (VRr) values 0.5% and 2.0%.
The objective of this study was to assess the reproducibility of sorption isotherms among laboratories each following their own measurement and data reduction procedures. All labs were asked to follow a predefined sample drying procedure prior to measurement in order to minimize any effects related to moisture. The reproducibility of the methane excess sorption isotherms was better for the high-maturity shale (within 0.02–0.03mmol/g) than for the low-maturity sample (up to 0.1mmol/g), similar to observations in earlier inter-laboratory studies on coals. The reproducibility for CO2 and C2H6 sorption isotherms was satisfactory at pressures below 5MPa, however, the results deviate considerably at higher pressures. Anomalies in the shape of the excess sorption isotherms were observed for CO2 and C2H6 and these are explained as being due to high sensitivity of gas density to temperature and pressure close to the critical point as well as from a limited measurement accuracy and possibly uncertainty in the equation of state (EoS).
The low sorption capacity of carbonaceous shales (as compared to coals and activated carbons) sets very high demands on the accuracy of pressure and temperature measurement and precise temperature control. Furthermore, the sample treatment, measurement and data reduction procedures must be optimized in order to achieve satisfactory inter-laboratory consistency and accuracy. Systematic errors must be minimized first by calibrating the pressure and temperature sensors to high-quality standards. Blank sorption measurements with a non-sorbing sample (e.g. stainless steel) can be used to identify and quantitatively account for measuring artifacts resulting from unknown residual systematic errors or from the limited accuracy of the EoS. The possible sources of error causing the observed discrepancies are discussed.
•First inter-laboratory study of reproducibility of sorption isotherms on shales•Excess sorption isotherms of CH4, CO2 and C2H6 for low- and high-maturity shales•Significant discrepancies observed at high pressures (>5MPa)•Low sorption capacities of shales require the highest measurement standards.•Recommendations for improving accuracy in sorption measurement
Overpressure prediction in tectonic environments is a challenging topic. The available pore pressure prediction methods are designed to work in environments where compaction is mostly one dimensional ...and driven by the vertical effective stress applied by the overburden. Furthermore, the impact of tectonic deformation on stresses, porosity and overpressure is still poorly understood. We use a novel methodology to capture the true compaction phenomena occurring in an evolving 3D stress regime by integrating a fully-coupled geomechanical approach with a critical state constitutive model. To this end, numerical models consisting of 2D plane strain clay columns are developed to account for compaction and overpressure generation during sedimentation and tectonic activity. We demonstrate that a high deviatoric stress is generated in compressional tectonic basins, resulting in a substantial decrease in porosity with continuing overpressure increase. The overpressure predictions from our numerical models are then compared to those estimated by the equivalent depth method (EDM) in order to quantify the error induced when using classical approaches, based on vertical effective stress, in tectonic environments. The stress paths presented here reveal that a deviation from the uniaxial burial trend can substantially reduce the accuracy of the EDM overpressure predictions.
•A fully-coupled geomechanical approach to capture the true compaction occurring in an evolving 3D stress regime is adopted.•2D models of clay columns accounting for sedimentation and tectonic activity are developed.•A high deviatoric stress is generated in compressional tectonic basins resulting in a substantial decrease in porosity.•The more the deviation from the uniaxial burial trend, the more inaccurate the EDM overpressure predictions.
Kerogens were purified from 26 samples of the Kimmeridge Clay Formation over the full range of maturities pertinent to petroleum generation. Most samples comprised >90% amorphous organic matter ...(AOM). Prior to and during the early phase of petroleum generation, kerogen densities range between 1.18 and 1.25 g cm-3. During peak and late stage petroleum generation, densities increase to ∼1.35 g cm-3 as hydrogen indices decrease from ∼350 to 50 mg HC/g C. The data are qualitatively consistent with the loss of alkyl carbon from kerogen to petroleum and the increased aromatization of remaining carbon. The density increase observed for AOM contrasts with the data for vitrinite, which exhibits a decrease in density at maturity levels relevant to petroleum generation. The contrasting behavior of AOM and vitrinite is thought to reflect the differing structural composition of the two kerogen types, most obviously the greater initial aromaticity of vitrinite.
Extracting gas from shale rocks is one of the current engineering challenges but offers the prospect of cheap gas. Part of the development of an effective engineering solution for shale gas ...extraction in the future will be the availability of reliable and efficient methods of modelling the development of a fracture system, and the use of these models to guide operators in locating, drilling and pressurising wells. Numerous research papers have been dedicated to this problem, but the information is still incomplete, since a number of simplifications have been adopted such as the assumption of shale as an isotropic material. Recent works on shale characterisation have proved this assumption to be wrong. The anisotropy of shale depends significantly on the scale at which the problem is tackled (nano, micro or macroscale), suggesting that a multiscale model would be appropriate. Moreover, propagation of hydraulic fractures in such a complex medium can be difficult to model with current numerical discretisation methods. The crack propagation may not be unique, and crack branching can occur during the fracture extension. A number of natural fractures could exist in a shale deposit, so we are dealing with several cracks propagating at once over a considerable range of length scales. For all these reasons, the modelling of the fracking problem deserves considerable attention. The objective of this work is to present an overview of the hydraulic fracture of shale, introducing the most recent investigations concerning the anisotropy of shale rocks, then presenting some of the possible numerical methods that could be used to model the real fracking problem.
The Kimmeridge Clay Formation (KCF) is a laterally extensive, total-organic-carbon-rich succession deposited throughout northwest Europe during the Kimmeridgian–Tithonian (Late Jurassic). It has ...recently been postulated that an expanded Hadley cell, with an intensified but alternating hydrological cycle, heavily influenced sedimentation and total organic carbon (TOC) enrichment by promoting primary productivity and organic matter burial in the UK sectors of the Boreal Seaway. Consistent with such climate boundary conditions, petrographic observations, total organic carbon and carbonate contents, and major and trace element data presented here indicate that the KCF of the Cleveland Basin was deposited in the Laurasian Seaway under the influence of these conditions. Depositional conditions alternated between three states that produced a distinct cyclicity in the lithological and geochemical records: lower-variability mudstone intervals (LVMIs) which comprise clay-rich mudstone and higher-variability mudstone intervals (HVMIs) which comprise TOC-rich sedimentation and carbonate-rich sedimentation. The lower-variability mudstone intervals dominate the studied interval but are punctuated by three ∼ 2–4 m thick intervals of alternating TOC-rich and carbonate-rich sedimentation (HVMIs). During the lower-variability mudstone intervals, conditions were quiescent with oxic to suboxic bottom water conditions. During the higher-variability mudstone intervals, highly dynamic conditions resulted in repeated switching of the redox system in a way similar to the modern deep basins of the Baltic Sea. During carbonate-rich sedimentation, oxic conditions prevailed, most likely due to elevated depositional energies at the seafloor by current–wave action. During TOC-rich sedimentation, intermittent anoxic–euxinic conditions led to an enrichment of redox-sensitive and sulfide-forming trace metals at the seafloor and a preservation of organic matter, and an active Mn–Fe particulate shuttle delivered redox-sensitive and sulfide-forming trace metals to the seafloor. In addition, based on TOC–S–Fe relationships, organic matter sulfurization appears to have increased organic material preservation in about half of the analysed samples throughout the core, while the remaining samples were either dominated by excess Fe input into the system or experienced pyrite oxidation and sulfur loss during oxygenation events. New Hg∕TOC data do not provide evidence of increased volcanism during this time, consistent with previous work. Set in the context of recent climate modelling, our study provides a comprehensive example of the dynamic climate-driven depositional and redox conditions that can control TOC and metal accumulations in a shallow epicontinental sea, and it is therefore key to understanding the formation of similar deposits throughout Earth's history.
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