Study on Two Component Gas Transport in Nanopores for Enhanced Shale Gas Recovery by Using Carbon Dioxide Injection

Abstract

Injecting carbon dioxide to enhance shale gas recovery (CO2-EGR) is a useful technique that has raised great research interests. Clear understanding of the two-component gas transport mechanisms in shale nanopores is the foundation for the efficient development of shale gas reservoir (SGR) and also the long-term geological storage of CO2. Although extensive studies on single-component gas transport and corresponding models in shale nanopores have been carried out in recent years, limited studies have been conducted on two-component or even multi-component gas transport models in shale nanopores. In this work, the shale nanopores were classified into inorganic and organic nanopores. The corresponding models for two-component gas transport were constructed. Mechanisms including Knudsen diffusion, slip flow, viscous flow, and molecular diffusion are considered in the inorganic pores. In the organic pores, due to existence of adsorption gas, surface diffusion is further considered besides the aforementioned mechanisms. Effects of pressure, temperature, fraction of organic nanopores, and gas concentration were analyzed. Results show that gas apparent permeability is negatively correlated with pressure, and positively correlated with temperature and organic nanopore fraction. As the concentration of CH4 decreases, the apparent permeability of CH4 increases continuously, while the apparent permeability of CO2 decreases. The permeability ratio of CH4 in the total permeability is negatively correlated with pressure and gas concentration ratio. Additionally, the contribution of transport mechanisms to the total gas apparent permeability has been analyzed. It is found that the surface diffusion contributes up to 5.68% to gas apparent permeability under high pressure. The contribution of molecular diffusion can reach up to 88.83% in mesopores under low pressure. Under high pressure and macropores, it contributes less than 1.41%. For all situations, the contribution of viscous flow is more than 46.36%, and its contribution can reach up to 86.07%. Results of this study not only can improve the understanding of two-component gas transport in nanochannels, but also can lay the foundation for more reliable reservoir simulation of CO2-EGR.

A comprehensive proposed model for gas separation prediction of mixed matrix membranes containing tubular fillers

“…where r is nanotube radius, p is the mean pressure within the membrane (Pa), σ is diameter of gas molecules (m), T is temperature (K), and K B is Boltzmann constant equals to 1.38 Â 10 À23 J/K. Gas permeability coefficient of Knudsen diffusion is obtained by following relationship 28 :…”

“… where r is nanotube radius, p is the mean pressure within the membrane (Pa), σ is diameter of gas molecules (m), T is temperature (K), and K B is Boltzmann constant equals to 1.38 × 10 −23 J/K. Gas permeability coefficient of Knudsen diffusion is obtained by following relationship 28 : where M is molecular mass of gas (kg/mol) and R is ideal gas constant equals to 8.314 J/mol·K. Calculation by Equation () gives gas permeability coefficient of viscous flow mechanism 28 …”

Experimental Study of Temperature Effect on Methane Adsorption Dynamic and Isotherm

“…Meanwhile, methane adsorption dynamics have attracted attention from many scholars [1,[20][21][22]. Yuan et al [23] investigated the pore structure characteristics of Lower Silurian shale and the diffusion behavior of methane molecules, as well as the shale sample size influence on the adsorption and diffusion of methane.…”

Anisotropic Permeability Influencing the Performance of Free CH4 and Free CO2 during the Process of CO2 Sequestration and Enhanced Gas Recovery (CS-EGR) from Shale

“…With CH 4 as its main ingredient, shale gas is treated as a clean and high-efficiency energy resource, making the shale gas industry booming over the past decade. − As shale is identified as pretty tight, how to improve shale gas recovery is one of the key issues for meeting the increasing demand for energy all over the world. ,− Therein, current knowledge indicates that the injection of CO 2 into shale reservoirs is a promising technique and is able to achieve CO 2 sequestration and to enhance gas recovery (CS-EGR) from shale at the same time, resulting in the shale-based CS-EGR being paid increasing attention. − Although this promising technique has been investigated extensively, CS-EGR in shale is still under development and has not yet been implemented widely in the field. Therefore, the CS-EGR technique in shale needs to be strengthened further and thus deserves more investigation, aiming to the potential field practice in a responsible manner.…”

“…22 These physical processes are coupled with each other (Figure 1b). Accordingly, to numerically investigate the CS-EGR operation in a shale reservoir, a coupled THM model is developed based on the following assumptions: 15,22,30,32−38 (1) Shale deforms elastically and is described as a homogeneous isotropic material with regard to the mechanical characteristic; especially, for the permeability, the anisotropic characteristic is highlighted in the model; (2) CH 4 and CO 2 conform to the ideal gas law and exist in both the pores and fractures; (3) Competitive adsorption between CH 4 and CO 2 in the matrix follows the modified Langmuir equation, while the diffusion and seepage (the calculation of dissolved gases follows Henry's law) of CH 4 and CO 2 follow Fick's law and Darcy's law, respectively; (4) Water exists only in the fractures, with the transportation of the water following Darcy's law, and the change in water vapor satisfying the Kelvin-Laplace law; (5) The influence of temperature change on gas dynamic viscosity is not considered; (6) The gas seepage occurs in the fracture system, and the relationship between the fracture width and the permeability follows the Cubic Law. Governing Equations.…”