Effective gas permeability of Tight Gas Sandstones as a function of capillary pressure – a non‐steady‐state approach

Single‐ and two‐phase (gas/water) fluid transport in tight sandstones has been studied in a series of permeability tests on core plugs of nine tight sandstones of the southern North Sea. Absolute (Klinkenberg‐corrected) gas permeability coefficients (k_gas__inf) ranged between 3.8 × 10^?16 and 6.2 × 10^?19 m² and decreased with increasing confining pressure (10–30 MPa) by a factor 3–5. Klinkenberg‐corrected (intrinsic) gas permeability coefficients were consistently higher by factors from 1.4 to 10 than permeability coefficients determined with water. Non‐steady‐state two‐phase (He/water) flow experiments conducted up to differential pressures of 10 MPa document the dynamically changing conductivity for the gas phase, which is primarily capillary‐controlled (drainage and imbibition). Effective gas permeability coefficients in the two‐phase flow tests ranged between 1.1 × 10^?17 and 2.5 × 10^?22 m², corresponding to relative gas permeabilities of 0.03% and 10%. In the early phase of the nonstationary flow regime (before establishment of steady‐state conditions), they may be substantially (>50%) lower. Effective gas permeability measurements are affected by the following factors: (i) Capillary‐controlled drainage/imbibition, (ii) viscous–dynamic effects (iii) and slip flow.     

Evolution of the transport properties of fractures subject to thermally and mechanically activated mineral alteration and redistribution

Strong feedbacks link temperature (T), hydrologic flow (H), mechanical deformation (M), and chemical alteration (C) in fractured rock. These processes are interconnected as one process affects the initiation and progress of another. Dissolution and precipitation of minerals are affected by temperature and stress, and can result in significant changes in permeability and solute transport characteristics. Understanding these couplings is important for oil, gas, and geothermal reservoir engineering, for CO2 sequestration, and for waste disposal in underground repositories and reservoirs. To experimentally investigate the interactions between THMC processes in a naturally stressed fracture, we report on heated (25°C up to 150°C) flow‐through experiments on fractured core samples of Westerly granite. These experiments examine the influence of thermally and mechanically activated dissolution of minerals on the mechanical (stress/strain) and transport (permeability) responses of fractures. The evolutions of the permeability and relative hydraulic aperture of the fracture are recorded as thermal and stress conditions' change during the experiments. Furthermore, the efflux of dissolved mineral mass is measured periodically and provides a record of the net mass removal, which is correlated with observed changes in relative hydraulic fracture aperture. During the experiments, a significant variation of the effluent fluid chemistry is observed and the fracture shows large changes in permeability to the changing conditions both in stress and in temperature. We argue that at low temperature and high stresses, mechanical crushing of the asperities and the production of gouge explain the permeability decrease although most of the permeability is recoverable as the stress is released. While at high temperature, the permeability changes are governed by mechanical deformation as well as chemical processes, in particular, we infer dissolution of minerals adjacent to the fracture and precipitation of kaolinite.     

Controls on fluid movement in crustal lithologies: evidence from zircon in metaconglomerates from Shetland

An investigation of the morphology of zircon in clasts and matrix of a greenschist facies metaconglomerate from Shetland has revealed a history of alteration of radiation‐damaged grains, partial dissolution and growth of new zircon. These processes are linked to the generation of chemically modified dark backscattered electron (BSE) zircon that is spatially related to fractures generated during radiation damage; embayments and rounding of zircon margins; and late overgrowths of original grains. These late modifications of zircon are all linked to the presence of fluids and so zircon morphology is used to track fluid behaviour in different lithologies in the metaconglomerate. Alteration is unrelated to clast margins and radically different in various clast types. This reflects a difference in permeability and suggests that deformation strongly controls fluid influx into quartzite, whereas zircon alteration in granite is associated with a restricted permeability reflecting the more limited response to deformation events.     

Permeability evolution in sorbing media: analogies between organic‐rich shale and coal

Shale gas reservoirs like coalbed methane (CBM) reservoirs are promising targets for geological sequestration of carbon dioxide (CO₂). However, the evolution of permeability in shale reservoirs on injection of CO₂ is poorly understood unlike CBM reservoirs. In this study, we report measurements of permeability evolution in shales infiltrated separately by nonsorbing (He) and sorbing (CO₂) gases under varying gas pressures and confining stresses. Experiments are completed on Pennsylvanian shales containing both natural and artificial fractures under nonpropped and propped conditions. We use the models for permeability evolution in coal (Journal of Petroleum Science and Engineering, Under Revision) to codify the permeability evolution observed in the shale samples. It is observed that for a naturally fractured shale, the He permeability increases by approximately 15% as effective stress is reduced by increasing the gas pressure from 1 MPa to 6 MPa at constant confining stress of 10 MPa. Conversely, the CO₂ permeability reduces by a factor of two under similar conditions. A second core is split with a fine saw to create a smooth artificial fracture and the permeabilities are measured for both nonpropped and propped fractures. The He permeability of a propped artificial fracture is approximately 2‐ to 3fold that of the nonpropped fracture. The He permeability increases with gas pressure under constant confining stress for both nonpropped and propped cases. However, the CO₂ permeability of the propped fracture decreases by between one‐half to one‐third as the gas pressure increases from 1 to 4 MPa at constant confining stress. Interestingly, the CO₂ permeability of nonpropped fracture increases with gas pressure at constant confining stress. The permeability evolution of nonpropped and propped artificial fractures in shale is found to be similar to those observed in coals but the extent of permeability reduction by swelling is much lower in shale due to its lower organic content. Optical profilometry is used to quantify the surface roughness. The changes in surface roughness indicate significant influence of proppant indentation on fracture surface in the shale sample. The trends of permeability evolution on injection of CO₂ in coals and shales are found analogous; therefore, the permeability evolution models previously developed for coals are adopted to explain the permeability evolution in shales.     

Mechanisms of strain-rate- and reaction-dependent permeability in the mid-crust of the Southern Alps: insight from three-dimensional mechanical models

We used three-dimensional mechanical modelling to explore the interplay of rheology, modes of permeability creation and fluid flow in the mid-crust of an oblique orogen. We used the central Southern Alps and the Otago Schist of New Zealand to constrain our models. We also compared our models with the magnetotelluric survey along the Rangitata–Whataroa rivers that imaged a U-shaped zone of high conductivity, interpreted as interconnected fluids, beneath the central Southern Alps. Modelling was carried out using the numerical code FLAC^3D. We used a number of simple assumptions: an initially homogeneous starting material, deformation boundary conditions based on the tectonics of the South Island, the capability of the modelled material to develop an anisotropic permeability structure, strain rate and reaction-induced permeability increases, initial saturation and lithostatic pore pressures as a basis for our models. The initial isotropic permeability was 10⁻¹⁸ m². We modelled two possible mechanisms of permeability increase: (i) strain-rate dependent and (ii) reaction dependent. For a strong mid-crust, the models showed enhanced permeability and hence fluid interconnectivity in a symmetric region beneath the model main divide, both ends were turned up towards the brittle–ductile transition and fluid flow was the greatest in the across strike direction. For a weak mid-crust, the region of enhanced permeability was asymmetric and turned up towards the brittle–ductile regime close to the Alpine Fault. The strong mid-crust model reproduces the features that are common to all interpretations of the MT soundings and is our preferred model for the central Southern Alps.     

Grain‐scale permeabilities of faceted polycrystalline aggregates

Porous synthetic quartzites and amphibolites, each with faceted pore walls, were synthesized and evaluated to examine the permeability of pore networks similar to those of the lower crust and mantle. Quartzite with a fluid in equilibrium with an Mg–clinopyroxene contained connected networks of pores with a dihedral angle of 30° bounded by walls that were 10–50% faceted. The relationship of their permeability (k) to porosity (φ) is approximated by the previously determined relationship for relatively nonfaceted synthetic quartzite Amphibolite with an HF fluid contained fluorotremolite and a connected network of pores bounded by walls exhibiting 78–90% faceting. These materials showed much lower k for a given φ, with an apparent permeability threshold at φ_c = 0.04. A curve fit to these data yields The results suggest that moderate faceting has little effect on the transmission of fluids through rocks, but extensive faceting significantly alters permeability. This difference is most likely produced through isolation of the fluid to the grain corners at low φ with extensive faceting. Rocks with pores that tend toward faceting may impede the flow of fluids and melt.     

Timescales for reaching steady-state fluid flow in systems perturbed by formation of highly permeable faults

We consider the case of an isothermal, fluid‐saturated, homogeneous rock layer with transverse fluid flow driven by an imposed constant fluid pressure gradient. A rupture in the centre of the rock layer generates a highly permeable fault and results in a change of the initially homogeneous permeability distribution. This leads to a perturbation of the fluid flow field and its gradual transition to a new steady‐state corresponding to the new permeability distribution. An examination of this transitional process permits us to obtain an analytical estimation of the transition stage duration. The application of the results obtained to km‐scale faults in crystalline rock bodies leads to the conclusion that the evolution of the fluid velocity field is rather rapid compared with geological timescales.     

Permeability of continental crust influenced by internal and external forcing

The permeability of continental crust is so highly variable that it is often considered to defy systematic characterization. However, despite this variability, some order has been gleaned from globally compiled data. What accounts for the apparent coherence of mean permeability in the continental crust (and permeability–depth relations) on a very large scale? Here we argue that large‐scale crustal permeability adjusts to accommodate rates of internal and external forcing. In the deeper crust, internal forcing – fluxes induced by metamorphism, magmatism, and mantle degassing – is dominant, whereas in the shallow crust, external forcing – the vigor of the hydrologic cycle – is a primary control. Crustal petrologists have long recognized the likelihood of a causal relation between fluid flux and permeability in the deep, ductile crust, where fluid pressures are typically near‐lithostatic. It is less obvious that such a relation should pertain in the relatively cool, brittle upper crust, where near‐hydrostatic fluid pressures are the norm. We use first‐order calculations and numerical modeling to explore the hypothesis that upper‐crustal permeability is influenced by the magnitude of external fluid sources, much as lower‐crustal permeability is influenced by the magnitude of internal fluid sources. We compare model‐generated permeability structures with various observations of crustal permeability.