深层煤岩气缝网激活无砂压裂理论基础与实践OA

Coal-rock gas in the Ordos Basin represents an abundant,strategically vital resource for enhancing China's natural gas supply capacity.However,these deep coal reservoirs exhibit extremely low permeability under in-situ conditions.Although current commercial production breakthroughs primarily rely on large-scale propped hydraulic fracturing,this method is constrained by high operational costs and marginal economic returns.Focusing on the No.8 coal seam of the Benxi Formation in the eastern Ordos Basin,this study integrates reservoir characterization,rock mechanical testing,water-coal interaction experiments,and post-fracturing conductivity evaluations with field practices for comprehensive analysis.As indicated by the research results,the coal reservoirs are characterized by highly developed natural cleats and pronounced plastic deformation.Under the mechanical boundary conditions of"rigid support"provided by the roof and floor strata,a unique configuration similar to a"stress compartment"is established.The injection of high-pressure fluids induces coupled tensile-shear failure and structural slip within the coal matrix,promoting matrix fragmentation and activation of natural cleats,which ultimately generates a complex fracture network.The resulting fragmented coal significantly expands the available seepage space under in-situ conditions,allowing the unpropped fractures to maintain high conductivity even after fracturing fluid flowback.Based on these insights,the concept of"fracture-network activation and proppant-free stimulation"for coal-rock gas development was proposed,marking a technological paradigm shift from"controlled proppant transport with fluid rate stabilization"to"maximum proppant concentration,"and ultimately to"proppant-free fracturing."Field pilot tests demonstrate that,under conditions of no proppant and similar fluid volumes,the single-well production is on par with that of conventional fracturing,indicating that fluid volume is a primary factor controlling well productivity.Specifically,a horizontal well with a 1 500 m lateral and subjected to proppant-free fracturing with 24 500 m³ of fluid,achieved a stable daily gas production exceeding 40×103 m³.This study concludes that proppant-free fracturing technology successfully capitalizes on the unique geological-engineering attributes of coal-rock reservoirs.While ensuring stable production,it offers substantial cost reduction and environmental advantages,holding profound implications for the commercial,large-scale development of deep coal-rock gas.