Experimental Study of the Fracture Network Expansion Mechanism and Three-Dimensional Reconstruction Characteristic of High-Rank Coal in Guizhou Province, China

Abstract

To better understand the fracture propagation characteristics and spatial distribution pattern of a high-rank coal reservoir during hydraulic fracturing, a true triaxial physical simulation device was used to conduct a hydraulic fracturing experiment on large-sized raw coal from Zhijin, Guizhou Province, China. Computed tomography technology was used to scan the three-dimensional morphology of the fracture network before and after fracturing, then AVIZO software was used to reconstruct the internal fractures of the coal sample, and fractal theory was used to quantify the fractures. The results show that (1) the sudden increase of the pump pressure curve and acoustic emission signal is an important identification feature of hydraulic fractures, and the in situ stress difference coefficient plays a leading role in the complexity of coal and rock fractures. (2) When a hydraulic fracture encounters a primary fracture in the process of expansion, the opening of the primary fracture, the penetration, bifurcation, and turning of the hydraulic fracture are the main reasons for the formation of complex fractures, and the existence of a large number of primary fractures is the basis for the formation of complex fractures. (3) The fracture shape of coal hydraulic fracturing can be divided into three categories: complex fracture, plane fracture + cross fracture, and inverted T-shaped fracture. The fracture shape is closely related to the original fracture shape. The research results of this paper provide strong theoretical and technical support for coalbed methane mining design such as Zhijin high-rank coal reservoirs.

Cracks Propagation and Permeability Enhancement Caused by Supercritical CO2 and Water Fracturing in Natural Coal–Rock Combination

“…In addition, permeability stands as the pivotal determinant influencing the extraction of coal-measured gas within reservoirs. The intricate geological characteristics inherent in deep coal measure reservoirs lead to notably diminished coal permeability. , Numerous fracturing tests regrading sandstone, shale, and coal mass have demonstrated that creating complex cracks is critical to enhance the overall permeability of reservoirs. − A large number of studies have confirmed the effects of supercritical carbon dioxide (ScCO 2 ) on pore and fracture structures at different scales, confirming that its unique physical and chemical properties enable it to enhance the permeability of coal. , Noteworthy contributions to this field include comparative analysis of shale permeability (φ25 × 50 mm) following ScCO 2 fracturing versus hydraulic fracturing under the same stress conditions . The results show that ScCO 2 -induced fracturing increases the shale permeability by up to three times compared to hydraulic fracturing.…”