Multiple injection techniques to stimulate complex fracture networks in coal reservoirs: A case study on gas disaster reduction of subsurface coal mine in southwest China

Abstract

The development and utilization of coalbed methane or coal mine gas has been a key way to ensure safe production and solve gas disaster reduction issues in coal mines. As an effective method, the hydraulic fracturing technique can increase both the number of fractures and their degree of connectivity, thereby improving the gas permeability in coal reservoirs. This study concentrates on the stimulation effectiveness of multiple fracturing techniques in coal reservoirs. First, with a novel self-constructed true tri-axial experimental setup, the indoor hydraulic fracturing experiments were conducted on sandstone, similar material to coal, and conglomerate cement under various injection schemes. The results show that long-interval multiple fracturing cannot promote the production of complex fractures but helps to communicate natural cracks, whether it is high (1206.4 mL/min) or low (120.6 mL/min) displacement. In contrast, short-interval multiple fracturing has a benefit in increasing the number of fractures and communicating natural fractures. Subsequently, the stress criterion of composite fracturing cracks caused by multiple fracturing was derived based on fracture mechanics. The theory analysis implies that the elasto-plastic zone at the hydraulic fracture tip possibly exists, and the stress shadow significantly affects fracture initiation and propagation. Besides the geostress and reservoir physical properties, sensitivity analysis shows that pore pressure caused by water and gas can also affect the stimulation effectiveness in the coal reservoirs. Finally, the in situ case studies demonstrate that multiple fracturing can change a large portion of the structure of the roof and floor strata, leading to better stimulation effectiveness by hydraulic fracturing. The extraction rate of a single borehole can be increased by more than 2.64–20.60 times compared to the original. These findings from case studies and experimental studies can provide enhanced insight into the fracturing behaviors in coal reservoirs and promote subsequent on-site application of multiple injection techniques.