Multi-transport Mechanism Shale Gas Reservoir Fracturing Horizontal Well Productivity Study

Shale gas is a kind of self-sealing and self-souring, unconventional gas which is mainly stored in clay shale and its interlayers with free or adsorbed state. Multi-stage fracturing of horizontal wells (MFHW) is the main effective means to develop shale gas reservoir. The fractured horizontal well productivity study of shale gas reservoir is the foundation of fracture parameter optimization and productivity assessment.It exists wing fracture and complex fracture network under different geology and engineering condition. At present, the productivity study for the two kinds of fracture morphology has limitations at domestic and foreign. Most of the productivity models are converted from Well Test Analysis. But they often neglect non-Darcy flow with high velocity, Skin effect in different shapes and asymmetry in wing fracture. There is a large of fracture network around hydraulic fracture. But there are few fractured horizontal well productivity studies about local fracture network. This paper carried out fractured horizontal well productivity study in shale gas reservoir, based on different fracture morphology.According to shale gas reservoir characteristics, considering the gas adsorption, desorption and diffusion in matrix system, the stress sensitivity in fracture system, and interference between fractures, unsteady flow model is established by use of point source function based on double media theory. The semi-analytical solution of reservoir flow model is obtained by mathematical methods such as Laplace transformation, Perturbation method, Bessel function, Superposition principle and Duhamel principle.Under the considering of high-speed non-Darcy flow in wing fracture and desorption, diffusion and stress sensitivity in fracture network, the wing fracture and SRV fracture network flow model are built respectively by use of single and double linear flow theory. Through discrete fracture and finite difference, the numerical solution of hydraulic fracture flow model is obtained.Based on the equal flow and continuous pressure principle, coupling the shale reservoir model respectively with wing fracture and SRV fracture network, the fractured horizontal well productivity model under the two different fracture morphology.The productivity study under wing fracture shows that the traffic of discrete units is "U" profile, and from the infinite to finite conductivity, and to the high-speed non-Darcy, the width of "U" is narrower and narrower. When the angle of fracture is big, the productivity of mid-stage crossflow becomes high and it happens later. When the fracture distributes symmetryly, productivity is high in early-stage. The pollution thickness of fracture toe is narrow, the productivity becomes high. The main impact of stress sensitivity is preliminary linear flow stage capacity. As for SRV fracture network, in order to separate the accurate flow stage, the productivity model is transferred to dynamic negative epidermis model. The result shows that the fracture network permeability, porosity and width mainly effect the pre-skin. Its desorption coefficients, crossflow coefficient and the length mainly effect the mid-skin. The fracture network parameters of CY-H2 well from Changning-Weiyuan are optimized by use of fracture network model whose permeability is 20-25mD, width is 150-180m, length is 230-260m, number is 16?18, spacing is 70-80m.The fractured horizontal well productivity model under two different hydraulic fracture morphology has theoretical guidance for shale gas reservoir fracture parameter optimization.