Mechanism Of Complex Fracture Extension With Hydraulic Fracturing In Coal Bed

Coal bed methane in China is usually characterized by low porosity and low permeability,so measures are needed to stimulate production.This can be done by creating flow channels between the wellbore and the coal formation or connecting the fractures and natural cracks in order to achieve an optimal and reasonable scale and ratio of the desorption-diffusion-permeation process and thus an economic production.The most effective and commonly used operation is hydraulic fracturing.This technology can effectively increase the permeability near the wellbore and boost gas production.However,due to the low strength and internal developed fracture system within coal,the fracturing mechanism differs greatly between coal bed and traditional reservoirs.The hydraulic fracture can be fractured along weak structures,which may lead to an extremely complicated extension pattern of fractures.It also accounts for the formation of a noticeable area of complicated fractures or an extension area of fractures.Current research on the mechanism of coal fracture extension and its propagation pattern is still limited.An accurate and reasonable method is yet to be found to illustrate its extension mechanism and to describe its propagation pattern of coal hydraulic fractures.The distribution of natural cleats system in coal bed is analyzed firstly in this paper.The basic mechanical parameters such as tensile strength,shear strength and compressive strength of coal are tested by laboratory tests.Consider the relationship between coal reservoir permeability and effective stress to establish well circumferential stress calculation formula,besides,consider the non-planar characteristics of the hydraulic fractures extension and the dynamic change of net pressure,and establish the induced stress field calculation model in the extension process of the hydraulic fractures combined with displacement discontinuity.Then the calculation model of the dynamic induced stress field of hydraulic fractures is deduced on the basis of the above models.The special tip element of the displacement model near the crack tip is established considering the dynamic change of the stress intensity factor of top-down cracks in the extension process of hydraulic fractures.Thus the dynamic stress intensity factor in the extension process of the hydraulic fractures and the dynamic stress distribution model can be obtained.The mechanical analysis of natural fractures,such as coal cleats and crannies is carried out.And the crossing and turning criterion of the natural fractures,such as hydraulic fractures,cleats,cracks,etc.,is derived.Besides,the extension model of complex fracture and net fracture of coal fracturing is established and we obtain the calculation model of complex fracture zone of coal fracturing combined with corresponding fracture criterion.Finally,the physical simulation experiment of coal fracturing is carried out.Then the extended evolutionary forms of fractures under different ground stress and development direction in cleats in coal fracturing is simulated.And comparing with the complex fracture morphology under the theoretical study analysis,the correctness of the theoretical model is verified.This paper begins with the stress distribution during fracture extension of coal hydraulic fractures,conducts corresponding research on the mechanism of fracture extension of complicated fractures and finally reveals the physical meaning of the development of coal hydraulic fracture network.The research results provide a scientific basis for the design of fracturing operations and production forecast after the fracturing.This paper can also be used as a theoretical reference for a detailed investigation on the extension pattern of coal hydraulic fractures.