Fractal hydrological-thermal–mechanical analysis of unconventional reservoir: A fracture-matrix structure model for gas extraction

Abstract

The complex fracture-matrix structure of the unconventional reservoir will significantly affect the gas seepage, energy conduction and the extraction properties. A good understanding of the underlying mechanisms controlling such a process is essential for a clear description of the reservoir gas extraction problems. In this study, we propose a fractal dual-porosity permeability model through analytical derivation, and the fracture-matrix microstructural properties of the reservoir is characterized by the following structural parameters: (1) density of natural fractures (Df), (2) density of matrix pores (Dp), (3) maximum natural fracture length (lmax), and (4) maximum matrix pore radius (λmax). These structural parameters evolve with the effective stress through porosity under the combination of hydrological-thermal–mechanical interactions, reservoir deformation, fracture-matrix interactions, and gas adsorption-desorption effect. The results indicate that: (1) the fractal model proposed in this paper is capable of better characterizing the reservoir microstructure at the fracture-matrix scale compared with the homogeneous models (9.45 and 78.07% enhancement in permeability of the fracture system whilst 10.46% and 86.74% enhancement in permeability of the matrix system); (2) the effective stress due to the evolution of microstructure has a significant contribution to reservoir temperature and gas pressure; (3) adsorption-desorption and thermal expansion effects dominate the unconventional reservoir structure; (4) greater initial pressure causes clearer effect on the overall fracture-matrix structure. This study might shed light on the investigation of the gas seepage property and improve the recovery of the unconventional reservoir.