A Review of Swelling Effect on Shale Permeability: Assessments and Perspectives

Abstract

The significant effect of gas sorption induced swelling on shale permeability has been observed through laboratory measurements and explained through permeability models over the past decades. However, there are lab observations that cannot be explained by these models. This knowledge gap prompts this review. Our goal is to form perspectives on how to resolve this gap through assessing the role of swelling on shale permeability. This goal is achieved through the following three steps: (1) collection of experimental shale permeability data measured under both constant effective stress and constant confining pressure conditions; (2) collection and classification of shale permeability models under the influence of gas sorption induced swelling strains; (3) assessments of co-relations between shale permeability data and permeability models. On the basis of all assessments, we conclude that the discrepancies between model predictions and laboratory measurements depend on the relation between bulk and pore swelling strains, pore size scales, and consistencies of strain treatments in the experiments and models. Models assuming that bulk swelling strain is different from pore swelling strain can better explain lab observations than those assuming that they are the same. When the pore size transitions from the micron-scale to the nano-scale, the effect of swelling on shale permeability gradually diminishes. The inconsistencies between how swelling strains are measured in the lab and treated in the models are common in the literatures and affect the discrepancies between model predictions and lab observations. On the basis of these assessments, we form our perspectives: (1) transformation between bulk and pore swelling strains should be characterized and incorporated both for experiments and in permeability models; (2) shale multiscale pore structural characteristics should be characterized when assessing the swelling effect on shale permeability; (3) the time-dependent nature of swelling strain and permeability evolution should be incorporated into future experiments and permeability models.

Shale Gas Production Evaluation Considering Gas Diffusion and Dispersed Distribution of Kerogen

“…In shale matrix, whether organic or inorganic matter, the pore size is tiny with diameters in the nanometre range. − This means that the pore diameter is typically close to the mean free path of gas, and the collision frequency between gas molecules and pore walls becomes larger. ,, In this case, the traditional continuum flow governed by Darcy’s law is no longer applicable, and flow regimes also play an important role in gas transport. Therefore, relying solely on intrinsic permeability (related to Darcy’s law) is insufficient to accurately describe gas flow characteristics.…”

“…27−29 These two matrix components present significant differences in physical properties, particularly in their sorption capacities. 12,26,30 In this case, Langmuir's strain constant in the shale matrix exhibits a high degree of heterogeneity.…”

“…In general, permeability for fractures is several orders of magnitude larger than that for matrix. As a result, gas is able to flow rapidly in fractures, while gas diffusion in the matrix exhibits a slow pace, lasting from days to months. − This means that during gas production, shale is mainly in nonequilibrium states. ,,, During nonequilibrium periods, matrix pressure exhibits a nonuniform distribution from the fracture wall to the inner matrix. ,, In addition, under the influence of environmental conditions, geological history, and chemical reactions during formation, the shale matrix comprises multiple minerals, which can broadly be classified into organic matter (kerogen inclusions) and inorganic matrix (including clay, quartz, and pyrite). − The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results reveal that organic matter is dispersedly embedded within an inorganic matrix. − These two matrix components present significant differences in physical properties, particularly in their sorption capacities. ,, In this case, Langmuir’s strain constant in the shale matrix exhibits a high degree of heterogeneity.…”

“…Consequently, models derived based on the uniform RVE do not incorporate the mechanisms induced by these two factors. These models struggle to adequately explain current experimental observations, thereby hindering their ability to yield satisfactory results in predicting gas shale production as well. ,− For example, these models suggest that shale permeability is related only to the effective stress and is independent of the gas sorption effect. Nevertheless, these conclusions are not consistent with experimental observations reported in the literature .…”

Study on Multiscale Fluid–Solid Coupling Theoretical Model and Productivity Analysis of Horizontal Well in Shale Gas Reservoirs

“…Industrial oil and gas flow can hardly be obtained from shale reservoir with extremely low porosity and permeability in natural state. − Horizontal drilling and multistage fracturing technology have realized large-scale commercial exploitation of shale oil and gas, which is changing the world energy pattern. − As shown in the Figure , the shale reservoir after fracturing becomes a multiscale complex fracture network structure composed of a matrix, natural microfractures, and hydraulic fractures; the gas flow characteristics in the pore and fracture structures of different scales are quite different. − The Knudsen number K n can be used to divide the gas flow regime based on the pore and fracture size. When K n > 0.2, the flow regime is dominated by the diffusion effect; when K n < 0.01, the flow is dominated by seepage; and when 0.001 < K n < 0.2, the gas flow is in the form of coexistence of multiple flow patterns mainly including slip flow, surface diffusion, Fick diffusion, and Knudsen diffusion. − Shale gas is stored in various states in a shale reservoir, mainly including free gas, adsorbed gas, and dissolved gas; the gas mass transfer is the common result of the above multiple flow regimes.…”