Experimental investigation of permeability evolution in deep reservoirs under true triaxial stress: A review

Despite its economic significance, hydrocarbon production from deep and ultra-deep formations poses considerable technological and scientific challenges. The permeability of reservoirs is a critical indicator for assessing the feasibility of commercial oil and gas exploitation, particularly for deep and ultra-deep reservoirs characterized by high geo-stress and an anisotropic stress state (σ₁ > σ₂ > σ₃). This review examines recent studies on permeability assessments under true triaxial stress conditions and highlighted the latest advancements in this field. We first outline current mainstream true triaxial permeability testing apparatuses and methodologies. Thereafter, the permeability response of rocks, particularly homogeneous rocks, under true triaxial stress conditions is analyzed, with particular emphasis on the influence of the intermediate principal stress. Furthermore, various true triaxial stress loading paths aimed at different research objectives and their corresponding permeability evolution behaviors are thoroughly introduced. Subsequently, we investigate the permeability characteristics of anisotropic rocks such as shale and coal, considering the coupling effect of textural features such as bedding and true triaxial stress on permeability evolution. Throughout this investigation, the permeability evolution laws exhibited by various types of rocks across the full stress-strain stage under true triaxial stress are comprehensively analyzed. Finally, specific future research directions and challenges in true triaxial permeability studies are outlined, with a focus on their applicability to deep resource exploitation engineering. This review presents an overview of the current state of true triaxial permeability testing and offers novel insights into permeability characterization in deep and ultra-deep reservoirs.