Abstract: There are abundant coal rock resources in Benxi Formation coal seam No.8 in Ordos Basin, which is a realistic replacement field for increasing unconventional gas reserves and production in China. Compared with shallow coalbed methane, deep coal rock gas has the characteristics of “tight matrix, high proportion of isolated pores, well developed cleavages and fissures, coexistence of free gas and adsorbed gas”. Therefore, the large-scale fracturing is an effective means to reduce free gas flow resistance and promote the desorption of adsorbed gas in deep coal rocks. The experiments such as triaxial mechanical compression, in-situ stress test and large physical simulation on fracture propagation have been conducted, which have identified the well-developed cleavages, mechanical properties of low Young’s modulus, high Poisson’s ratio, large longitudinal stress difference of coal rock No.8, and fracture propagation characteristics given the fracturing modes of high displacement low viscosity and low displacement high viscosity. By applying multi-parameter geology–engineering sweet spot identification method and multi-stage and multi-cluster differentiated fine fracture layout technology, the economic and large-scale fracturing technology has been formed with the core of “multi-stage and few clusters, alternating fluid injection, and combined sand addition”. The key points of the technology include: (1) Few clusters in one stage to concentrate energy at the fracture opening and initiate cleavages and fissures; (2) Alternating injection of high and low viscosity fluids to expand fracture propagation zone and increase the fracture complexity; (3) Combined sand addition to increase flow capacity, so as to achieve the effective support of multi-scale fractures. The technology has been applied to 19 wells, with an initial daily gas rate of 4.9×10⁴ m³/d, showing good gas production test results, which has provided important technical support for beneficial development of deep coal rock gas.

Key words: Ordos basin, deep coal gas, Benxi Formation, fracturing stimulation technology, application results