Facies-Dependent Evaluation of Compressibility in Shale Reservoirs: The Role of Pore-Fracture Structures and Connectivity

This study addresses the evaluation of shale compressibility in unconventional reservoirs, with a particular focus on the influence of sedimentary facies and pore-fracture structures. While numerous methods exist for assessing compressibility in unconventional reservoirs, systematic investigations into facies-specific characteristics remain limited. We propose a novel framework that integrates porosity and pore-fracture structural attributes to evaluate shale compressibility across distinct sedimentary environments. Our analysis emphasizes the critical role of pore-fracture structures in dictating compressibility behaviors in both marine and transitional shale systems. The results reveal that porosity positively influences the compressibility of marine shale but negatively affects transitional shale. In marine shale, micropores, macropores, and interconnected pores and fractures in pyrite and carbonate minerals promote compressibility by reducing the energy required for fracture extension during fracturing. Conversely, in transitional shale, mesopores and interconnected fractures in clay have a weaker negative effect on compressibility due to the ductile of clay, which will be closed during fracturing. Additionally, the study emphasizes the dual impact of pore-fracture connectivity on shale compressibility. The connectivity of marine shale is predominantly governed by pore-within-pore structures within the organic matter (OM) at micro- to mesoscales, where OM ductility consequently reduces matrix compressibility. Similarly, transitional shale exhibits equivalent compressibility trends through connectivity controlled by interconnected fracture networks and oriented clay-hosted fractures. These findings establish fundamental theoretical frameworks for optimizing hydraulic fracturing treatment parameters in shale reservoirs.