Multi-round diverting fracturing technology and its application in deep coalbed methane in the Daning-Jixian block

The Daning-Jixian block on the eastern margin of the Ordos Basin exhibits high-abundance deep coalbed methane (CBM) resources, well-developed natural fractures of coal reservoirs, well-developed cleats and fractures in coals themselves, coals with excellent structures and high mechanical strength, and strong sealing ability of coal roofs and floors. All these create favorable conditions for the formation of a large-scale fracture network through volume fracturing. The ultra-large-scale fracturing process has contributed to a major breakthrough in the single-well output of deep CBM. However, the tracer monitoring results show that various fracturing stages of horizontal wells exhibited different contribution rates to gas production, there exhibited blind zones of resource production, and expected comprehensive benefits were not achieved. This study proposed two major challenges posed to the formation of ultra-large-scale effective fracture networks in deep coal reservoirs: (1) unclear understanding of fracture propagation patterns in deep coal seams and (2) the presence of areas subjected to over and insufficient stimulation using current fracturing technologies. Given these challenges, this study developed a multi-round diverting fracturing technology to form a merged fracture network for the stimulation of deep coal reservoirs. This technology involved: (1) analyzing the feasibility of the formation of a super-large fracture network of deep coal seams. (2) determining the effects of microstructures, such as the curvatures and dip angles of strata, on fracture propagation based on the field fracturing data and microseismic monitoring results. (3) Establishing a stress field calculation method, which laid the foundation for the process optimization and field experiments of multi-round fracturing diverting. This technology was verified through field experiments in the Daning-Jixian block. The results revealed the uniform propagation of hydraulic fractures in areas with nonuniform micro-stress fields around wells. This uniform propagation increased the overall fractured volume, with single-well gas production in the experiment area significantly improving compared to surrounding wells. Well DJ55, experiencing five rounds of fracturing, exhibited a stimulated reservoir volume of up to 243.6×10⁴ m³, 340-day cumulative gas production of 970.5×10⁴ m³, and an average daily gas production of 2.85×10⁴ m³, with daily gas production and pressure remaining stable. These results indicate excellent stimulation results. With an estimated ultimate recovery greater than 3000×10⁴ m³, this well had great potential for gas production. Well JS8-6P05 in the block yielded a daily gas production of 8.59×10⁴ m³ after 2‒3 rounds of fracturing at fracturing stages 1‒7. Compared to well JS8-6P04, which employed single-round fracturing at each fracturing stage, well JS8-6P05 witnessed reductions in the proppant volume and fracturing cost by 21% and 41.9%, respectively. However, the horizontal sections of both wells produced comparable daily gas production. The experimental results indicate that the multi-round diverting fracturing technology, partially solving the problem that fractures propagate on one side of a horizontal well due to the stress differences on both sides, promotes the uniform propagation of induced fractures on both sides of a wellbore and thus ensures a high production degree and post-fracturing production of deep coal reservoirs. This technology serves as a main technical method for reducing the costs and increasing the efficiency of fracturing technology for deep CBM.