Experimental and numerical analyses of apparent gas diffusion coefficient in gas shales (original) (raw)
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Surface Diffusion in Nanopores and Its Effects on Total Mass Transport in Shale Gas Reservoirs
International Journal of Energy and Environmental Science, 2023
In the 21st century, shale gas reservoirs have emerged as a significant and valuable source of natural gas. However, their distinct characteristics, particularly the nanoscale pore throat and pore-size distribution, set them apart from conventional reservoirs. These unique features have a profound impact on the storage and flow behavior of hydrocarbons within the shale, making them challenging to exploit using conventional methods. One of the primary challenges associated with shale gas reservoirs is the confined space phase behavior, which alters the fluid properties compared to what is typically observed in a standard PVT (Pressure-Volume-Temperature) cell. In particular, the increased surface adsorption of gas molecules in the shale leads to deviations in fluid properties. This means that the properties of gas within the shale differ from those predicted by conventional models, making it crucial to understand and account for these differences to efficiently extract gas from these reservoirs. Surface diffusion is a critical parameter in assessing the transport ability of adsorbed gas in shale organic matter. Surface diffusion refers to the movement of gas molecules along the surfaces of organic matter in the shale. It is a complex process influenced by various factors. Recent research has provided some insights, indicating that the shale-methane surface diffusion coefficient has a value of around 10-16 cm 2 /g. However, accurately measuring this coefficient remains a challenge, and there is a need for a definitive and reliable method to do so. Despite the importance of surface diffusion, it has been found that its contribution to total mass transport in shale gas reservoirs is not as significant as previously anticipated. Other mechanisms, such as desorption and matrix diffusion, also play essential roles in the overall transport of gas within shale. To improve our understanding of shale gas reservoirs and optimize gas extraction, this paper proposes an interdisciplinary approach. It suggests combining insights and advances from different industries and fields of research to gain a comprehensive understanding of these complex reservoirs. By bringing together knowledge from geology, engineering, chemistry, and other relevant disciplines, researchers can develop more accurate models and strategies to unlock the full potential of shale gas reservoirs. In summary, shale gas reservoirs have revolutionized the natural gas industry in the 21st century, but their unique characteristics require a specialized approach. Surface diffusion is an important factor affecting gas transport in shale, but its contribution is not as significant as initially thought. Through interdisciplinary research, we can enhance our understanding of these reservoirs and develop more efficient methods for gas extraction.
Characterization and Measurement of Multi-scale Gas Transport in Shale Core Samples
Proceedings of the 2nd Unconventional Resources Technology Conference, 2014
This work introduces an experimental technique to probe simultaneously flow and diffusion of gas through shale. A core-scale pressure-pulse-decay experiment is used to study the upstreamand downstream-pressure responses of Eagle Ford and Haynesville shale samples. With the aid of numerical models, the pressure histories obtained from the experiments are matched and gas and rock properties are obtained. The experiments are conducted at varying pore pressure and net effective stress to understand the sensitivity of the rock porosity and permeability as well as the gas diffusivity. A dual-porosity model is constructed to examine gas transport through a system of micropores and microcracks. In this sense, the role of the two different-sized pore systems is deconvolved. In some cases, the micropore system carries roughly onethird of the gas flow. The porosity, permeability, and diffusivity obtained assign physical properties to the macroscales and microscales simultaneously. Results bridge the gap between these scales and improve our understanding of how to assign transport physics to the correct pore scale.
Gas Multiple Flow Mechanisms and Apparent Permeability Evaluation in Shale Reservoirs
Sustainability, 2019
Gas flow mechanisms and apparent permeability are important factors for predicating gas production in shale reservoirs. In this study, an apparent permeability model for describing gas multiple flow mechanisms in nanopores is developed and incorporated into the COMSOL solver. In addition, a dynamic permeability equation is proposed to analyze the effects of matrix shrinkage and stress sensitivity. The results indicate that pore size enlargement increases gas seepage capacity of a shale reservoir. Compared to conventional reservoirs, the ratio of apparent permeability to Darcy permeability is higher by about 1–2 orders of magnitude in small pores (1–10 nm) and at low pressures (0–5 MPa) due to multiple flow mechanisms. Flow mechanisms mainly include surface diffusion, Knudsen diffusion, and skip flow. Its weight is affected by pore size, reservoir pressure, and temperature, especially pore size ranging from 1 nm to 5 nm and reservoir pressures below 5 MPa. The combined effects of mat...
Transport in Porous Media, 2011
A theoretically improved model incorporating the relevant mechanisms of gas retention and transport in gas-bearing shale formations is presented for determination of intrinsic gas permeability and diffusivity. This is accomplished by considering the various flow regimes according to a unified Hagen-Poiseuille-type equation, fully compressible treatment of gas and shale properties, and numerical solution of the non-linear pressure equation. The present model can accommodate a wide range of fundamental flow mechanisms, such as continuum, slip, transition, and free molecular flow, depending on the prevailing flow conditions characterized by the Knudsen number. The model indicates that rigorous determination of shale-gas permeability and diffusivity requires the characterization of various important parameters included in the present phenomenological modeling approach, many of which are not considered in previous studies. It is demonstrated that the improved model matches a set of experimental data better than a previous attempt. It is concluded that the improved model provides a more accurate means of analysis and interpretation of the pressure-pulse decay tests than the previous models which inherently consider a Darcian flow and neglect the variation of parameters with pressure.
Energy Reports
Gas transport in ultra-tight rock is non-Darcian. In addition to continuum flow, there are multiple other flow mechanisms such as slip flow and pore and surface diffusion. Various multi-physics models have been put forth in the literature to forecast the apparent permeability of gas in shales and ultratight formations. However, a means of accurately describing the relative contributions of physics in multiscale pore systems remains a challenge. Moreover, it is important to explain pore size, pressure dependency, and the relationships among adsorption, diffusion, and permeability in porous media. For these reasons, a semi-analytical model is proposed to predict gas permeability according to the viscous flux, pore diffusion and surface diffusion and establish control of the adsorbed gas layer. The reliability of the equations developed was checked by validation using experimental and molecular simulation data obtained from macropore-and micropore-sized nanotubes systems respectively. Furthermore, the equations' performance for micropores was compared to existing theoretical shale permeability models. The subsequent sensitivity analysis showed that permeability is sensitive to the nanoscale geometry factor and adsorption mechanisms. Moreover, the relevance of the surface diffusion was found to increase as the pore size decreased. For instance, surface diffusion constituted over 50% of the apparent permeability below the 10 MPa and 5.0 MPa conditions in micro-and mesopore systems, respectively, while the Darcy scale phenomenon controlled the transport of gas in macropores. Across all diffusion regimes, the microstructure geometry and sorption dynamics significantly influenced the total diffusion of methane, particularly at low pressures and decreased pore sizes. The decline in reservoir pressure during production shifted the relative importance of the adsorption and diffusion mechanisms, consequently altering the apparent gas permeability. Therefore, reservoir management teams should take into account the dynamics of gas permeability at different pressures and representative pore sizes throughout the life cycle of the asset.
Gas Flow Models of Shale: A Review
Energy & Fuels
Conventional flow models based on Darcy's flow physics fail to model shale gas production data accurately. The failure to match field data and laboratory-scale evidence of non-Darcy flow has led researchers to propose various gas-flow models for the shale reservoirs. There is extensive evidence that suggests the size of the pores in shale is microscopic in the range of a few to hundreds of nanometers (also known as nanopores). These small pores are mostly associated with the shale's organic matter portion, resulting in a dual pore system that adds to the gas flow complexity. Unlike Darcy's law, which assumes that a dominant viscous flux determines a rock's permeability, shale's permeability leads to other flow processes besides viscous flow such as gas slippage and Knudsen diffusion. This work reviews the dominant gas-flow processes in a single nanopore on the basis of theoretical models and molecular dynamics simulations, and lattice Boltzmann modeling. We extend the review to pore network models used to study the gas permeability of shale.
Some key technical issues in modelling of gas transport process in shales: a review
Geomechanics and Geophysics for Geo-Energy and Geo-Resources, 2016
As a result of small pore sizes and property heterogeneities at different scales, flow processes and the related physical mechanisms in shales can be dramatically different from those in conventional gas reservoirs. To accurately capture the ''unconventional'' flow and transport in shales requires reevaluation of dominant physics controlling flow in shales, as well as innovative hardware technologies to estimate critical material and flow properties. To do so, we need to quantify the current knowledge and identify technology gaps especially as related to the modeling fluid flow in shale gas reservoirs. While fluid flow in shale includes many important aspects, this paper focuses on fluid flow in complex heterogeneous shale matrix. It discusses the recent progress in the areas of multiscale fluid flow, fracturing fluid imbibition, and stressdependent shale matrix properties. Future research topics in the related areas are also suggested based on the identified technology gaps.
Gas transport and storage capacity in shale gas reservoirs – A review. Part A: Transport processes
Journal of Unconventional Oil and Gas Resources, 2015
For decades, scientists and engineers have been investigating and describing storage and transport mechanisms in geological porous media such as reservoir rocks. This effort has resulted in the development of concepts such as single-phase and multi-phase flow, which describe the storage and transport of fluids in conventional reservoir rock types such as sandstones and carbonates. However, many of these concepts are not directly applicable to unconventional reservoirs. For example, shale gas reservoirs consist of organic-rich lithotypes, which have high compressibility, very small pore throats, low porosities and extremely low and anisotropic permeabilities, and relatively low gas storage capacities. The models developed to describe conventional reservoirs do not accurately describe the hydrocarbon transport processes involved in these rocks. In this part A of the review paper, we aim to provide a concise and complete review on characterizing the fluid transport processes in unconventional reservoirs. We will examine processes occurring at various spatial scales, ranging from fracture flow on the centimeter scale down to slip-flow on the nanometer scale. Due to the softer nature of tight shales, many processes, such as slip-flow and the pore-throat compressibility, will have to be considered as coupled. We also develop a detailed description of the coupling between slip-flow, which is a fluid dynamic effect, and the pore-throat compressibility, which is a poroelastic effect, in unconventional reservoirs, and interpret experimental observations in light of this description. Furthermore, we discuss in detail how these transport properties depend on organic content, clay content and type, amount of pre-adsorbed water and pore compressibility.