Microstructural evolution and hydraulic response of shale self-propped fracture using X-ray computed tomography and digital volume correlation

Abstract

Methane in-situ explosive fracturing technology produces shale debris particles within fracture channels, enabling a self-propping effect that enhances the fracture network conductivity and long-term stability. This study employs X-ray computed tomography (CT) and digital volume correlation (DVC) to investigate the microstructural evolution and hydromechanical responses of shale self-propped fracture under varying confining pressures, highlighting the critical role of shale particles in maintaining fracture conductivity. Results indicate that the fracture aperture in the self-propped sample is significantly larger than in the unpropped sample throughout the loading process, with shale particles tending to crush rather than embedded into the matrix, thus maintaining flow pathways. As confining pressure increases, contact areas between fracture surfaces and particles expand, enhancing the system’s stability and compressive resistance. Geometric analyses show flow paths becoming increasingly concentrated and branched under high stress. This resulted in a significant reduction in connectivity, restricting fracture permeability and amplifying the nonlinear gas flow behavior. This study introduces a permeability-strain recovery zone and a novel sensitivity parameter m, delineating stress sensitivity boundaries for permeability and normal strain, with m-value increasing with stress, revealing four characteristic regions. These findings offer theoretical support for optimizing fracturing techniques to enhance resource extraction efficiency.

1. Introduction

With the continuous growth of global energy demand, shale gas has emerged as a crucial unconventional oil and gas resource, increasingly becoming an integral part of the global energy supply [1,2]. Shale reservoirs are characterized by low porosity and permeability, making it a significant challenge to enhance their productivity [3,4]. Hydraulic fracturing has been widely used to create flow-conductive fractures in the reservoir by injecting high-pressure fluids, thereby enhancing oil and gas recovery rates [5,6]. However, the conductivity of these fractures is directly impacted by the in-situ stresses, particularly in high-stress environments, where fracture closure and conductivity degradation are significant constraints on recovery efficiency [7].

In conventional hydraulic fracturing, proppants such as ceramic beads, quartz sand, and resin materials are typically injected to maintain fracture aperture and conductivity, mitigating the fracture closure under high-stress conditions [8,9]. However, the transport and use of traditional proppants increase extraction costs and operational complexity. Recently, methane in-situ explosive fracturing technology has garnered attention as a novel approach. This technology exploits high-temperature, high-pressure gas from the in-situ combustion of desorbed methane and oxygen to dynamically impact the shale reservoir, efficiently creating complex fractures and generating substantial amounts of debris [10,11]. Unlike conventional proppants, these debris particles share similar mechanical and physical properties with the shale matrix, filling the fracture and spontaneously forming a self-propping effect. This effect has the potential to enhance the fracture’s resistance to compressive deformation, thereby maintaining fracture conductivity. This phenomenon theoretically reduces reliance on artificial proppants and offers a more cost-effective and efficient stimulation method.