Analysis of hydraulic fracture behavior and well pattern optimization in anisotropic coal reservoirs

Macrolithotypes control the pore-fracture distribution heterogeneity in coal, which impacts stimulation via hydrofracturing and coalbed methane (CBM) production in the reservoir. Here, the hydraulic fracture was evaluated using the microseismic signal behavior for each macrolithotype with microfracture imaging technology, and the impact of the macrolithotype on hydraulic fracture initiation and propagation was investigated systematically. The result showed that the propagation types of hydraulic fractures are controlled by the macrolithotype. Due to the well-developed natural fracture network, the fracture in the bright coal is more likely to form the complex fracture network, and the simple case often happens in the dull coal. The hydraulic fracture differences are likely to impact the permeability pathways and the well productivity appears to vary when developing different coal macrolithtypes. Thus, considering the difference of hydraulic fracture and permeability, the CBM productivity characteristics controlled by coal petrology were simulated by numerical simulation software, and the rationality of well pattern optimization factors for each coal macrolithotype was demonstrated. The results showed the square well pattern is more suitable for dull coal and semi-dull coal with undeveloped natural fractures, while diamond and rectangular well pattern is more suitable for semi-bright coal and bright coal with more developed natural fractures and more complex fracturing fracture network; the optimum wells spacing of bright coal and semi-bright coal is 300 m and 250 m, while that of semi-dull coal and dull coal is just 200 m.

Automated summary

Macrolithotypes play a crucial role in controlling the distribution heterogeneity of pores and fractures in coal, thereby affecting stimulation techniques like hydrofracturing and coalbed methane production. It was found that the type of macrolithotype dictates the propagation pattern of hydraulic fractures, with bright coals showing more complex fracture networks compared to dull coals. Numerical simulations revealed that square well patterns are optimal for dull and semi-dull coals, while diamond and rectangular patterns are better suited for semi-bright and bright coals with more developed natural fractures. The spacing of optimal wells also varies depending on the coal macrolithotype, with bright coals requiring a spacing of 300 m and dull coals needing just 200 m.