Background Deep coalbed methane (CBM) reservoirs generally exhibit ultra-low porosity and permeability, high stress, predominance of nanopores, and complex fracture propagation. These characteristics complicate reservoir stimulation, thereby restricting the enhancement in single-well productivity and commercial exploitation. Since 2019, technologies for deep CBM recovery have evolved from volumetric acid fracturing, which highlights the enhancement of matrix permeability, into large-scale volume fracturing focusing on the elevation of both the fracture network scale and proppant efficiency. In 2023, ultra-large-scale volume fracturing aimed at maximizing estimated ultimate recovery (EUR) was further developed. However, the implementation of this novel technique poses challenges such as a surge in water consumption, elevated costs, and difficulties in treating flowback fluids.
Methods and Results This study investigated the application of large-scale volume fracturing in the Daning-Jixian block. The impacts of geological characteristics, including the physical properties of deep coals, in situ stress distribution, natural fracture development, and temperature-pressure conditions, on reservoir stimulation in the application were systematically analyzed. Accordingly, three innovative techniques were proposed: diverting fracturing, long-section few-cluster diverting fracturing, and roof-floor control. Regarding the diverting fracturing technique, low-cost proppant integrating temporary plugging, diverting, and propping was developed, facilitating fracture diversion and increasing fracture network complexity. By combining a single cluster, a high injection rate of fracturing fluids, and fracture diversion, the long-section few-cluster diverting fracturing technique can increase fracture density, the uniformity of fracture propagation, and fracture network volume. For the roof-floor control technique, continuous diverting agent injection, combined with liquid nitrogen co-injection and pump shutdown, is employed to govern the out-of-control risk of fracture height caused by poor integrity of roofs and floors. This technique can enhance the fracture network propagation efficiency within coal seams and improve the far-field migration capability of proppants. Field test results indicate that the application of the three innovative techniques contributed to an increase of 27% in the stimulated fracture lengths and a decrease of 10.7% in the inter-section re-stimulation rate, with a fracture length difference on both sides of the wellbore determined at merely 26 m. Owing to fracturing with the floor control technique, the CBM productivity of well H19 was more than 100% higher than that of its neighboring wells.
Conclusion This study provides valuable experience and technical support for progress in fracturing techniques and the sustainable, efficient exploitation of deep CBM reservoirs. Based on field practices, two research directions are further proposed: (1) accurate parameter design for geological-engineering integrated, intelligent fracturing and (2) synergistic recovery enhancement through fracturing and supercritical gas-based thermal flooding. These efforts will contribute to the constant iteration and innovations of technologies for deep CBM exploitation.
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