Comprehensive Evaluation Study on the Fracturability of Deep Coal Rock in the Northern Margin of Qaidam Basin

Abstract In recent years, with the decline in resource quality, the Qaidam Basin has changed its exploration approach and gradually explored deeper and deeper into the source. Coal rocks were interpreted from the Jurassic in the northern margin of the Qaidam Basin, and experimental evaluations were conducted to determine that the area belongs to a low porosity and low-permeability reservoir that requires hydraulic fracturing to obtain gas flow. Through research and combined with reservoir characteristics, a comprehensive evaluation study was conducted on the compressibility of coal rock before gas pressure, and eight main controlling factors affecting fracturability are preferably selected, including coal body structure, brittleness index, fracture toughness, the ratio of Young’s modulus between surrounding rock and coal seam, horizontal stress difference coefficient, minimum horizontal principal stress, pressure measurement coefficient and the stress difference between coal seam and confining pressure. The weights of various factors were calculated through mathematical methods, and a quantitative evaluation method for fracability was established. Combined with the domestic experience in coal rock transformation and the characteristics of the Qaidam coal rock gas reservoir, a classification standard for the fracability of deep coal reservoirs was formulated. It effectively evaluates the “sweet spot” locations of coal rock reservoirs and provides technical support for the optimization of reservoir transformation plans. When this evaluation method was applied in the on-site coal rock gas fracturing tests, gas was found in all three wells, achieving the purpose of gas testing and effectively enhancing the pertinence and effectiveness of the reservoir fracturing solutions.