Multi-well synchronous hydraulic conformance fracturing technology used for deep coal beds and its field application in the Southern Qinshui Basin

For deep CBM productivity improvement, a technical concept of conducting multi-well synchronous hydraulic conformance fracturing was proposed based on its geological characteristics. First, a mathematical model for multi-fracture induced stress was established by using the boundary element displacement discontinuity method, to simulate the distribution of induced stress field in deep coal beds and analyze the possibility of the formation of complex fracture networks induced by the hydraulic conformance fracturing. Then, the propagation situation of fracture networks interfered by stress and its influencing factors were studied by using the discrete element method. And finally, the feasibility of synchronous hydraulic conformance fracturing technology was verified through tri-axial fracturing experiment and field application. It is shown that by virtue of synchronous hydraulic conformance fracturing technology, stress interference area and strength are increased, so horizontal major stress difference is decreased and even the direction of earth stress is changed regionally, which is conducive to the connection of developed face cleats and butt cleats in coal rocks, so as to form large, efficient and complex fracture networks. Furthermore, the favorable conditions for the formation of complex fracture networks by hydraulic conformance fracturing include lower initial horizontal major stress difference, low Poisson's ratio, short well spacing and low fracturing fluid viscosity and high net pressure inside the fractures. Finally, it is shown from the 3D true physical simulation experiments that by virtue of this synchronous hydraulic conformance fracturing technology, natural fractures in coal rocks can be connected sufficiently, and consequently complex fracture networks composed of hydraulic fractures, face cleats and butt cleats are created. Based on these research results, a set of optimization design method for the synchronous hydraulic conformance fracturing of deep coal beds was proposed. Five vertical wells located in the deep coal beds of North Shizhuang Block in the Southern Qinshui Basin were chosen for the pilot test. It is indicated from fracture monitoring and drainage/production data that the stimulated reservoir volume (SRV) of synchronous hydraulic fractured wells is large and its fracture network is complex; and that compared with the conventionally fractured wells, the synchronous hydraulic fractured well is earlier in gas breakthrough, and higher and more stable in production rates and casing pressure and its regional pressure drop even spreads to the adjacent wells, so that their production rates are remarkably raised.