Gas Mass Transport Model for Microfractures Considering the Dynamic Variation of Width in Shale Reservoirs

Abstract

Summary Microfractures are commonly observed in shale reservoirs. During the shale gas–production process, stress sensitivity induces a change in the width of the microfractures, which is a significant factor that affects shale gas mass transport. By using research methods based on desorption theory and elastic–plastic mechanics, a shale gas mass transport model that considers the dynamic variations in the microfracture width is established in this paper. This model comprehensively fuses the surface diffusion model, slip flow model, Knudsen diffusion model, and cubic grid model. The reliability of this model is verified using molecular simulations, which do not include surface diffusion. The shale gas is considered as pure methane. Then, the different contributions of the gas mass transport mechanisms to the total mass transport are discussed in detail. The results demonstrate the following findings: (1) The studied flows are well–simulated by the proposed model. (2) Stress sensitivity results in a decrease in gas mass transport when the formation pressure exceeds 3.4 MPa, and the minimum value is approximately 0.45 times smaller than that when the width change is not considered. Moreover, stress sensitivity results in an increase in gas mass transport when the formation pressure is lower than 3.4 MPa, and the maximum value is approximately 4.5 times higher than that when the width change is not considered. (3) Shale gas mass transport is positively associated with the Young's modulus and Poisson's ratio, whereas it is negatively associated with the microfracture compressibility. When the formation pressure is less than 4 MPa, shale gas transport is positively correlated with the desorption capacity, whereas when the formation pressure exceeds 4 MPa, the effect of different desorption capacities on gas transport is nearly consistent. (4) When the microfracture width is at nanoscale and the reservoir pressure is lower than 15 MPa, surface diffusion has an obvious effect on the shale gas mass transport process. When the contribution of surface diffusion to the total shale gas mass transport is relatively small, the contributions of slip flow and Knudsen flow to shale gas mass transport exhibit the trend of “shifting each other.” When the surface diffusion contribution is larger, a reduction in its contribution leads to simultaneous initial increases in the contributions of slip flow and Knudsen flow to shale gas mass transport, and then these flows begin “shifting each other.”