The Transient Productivity Research Of Multi-Stage Fractured Horizontal Wells In Shale Based On Unstructured Grids

Shale gas is a widely distributed unconventional resource, which is regarded as an alternative energy of overwhelm potential. In China, shale gas reservoirs are summarized the characteristics of deep depth, diverse configuration and complex surface condition in reports. The existence of these complexity and particularity make the reservoirs more difficult for exploitation and production. Thus, it is an urgent task to extend numerical simulation method and corresponding fluid flow model to describe the flow dynamic in shale.As a special kind of gas reservoir, shale gas reservoir has significant difference with conventional resources in accumulation, occurrence and percolation mechanism. On the one hand, Shale gas reservoir has complex natural pore space (nano-scale organic pore, micro-pore, micro-fracture). On the other hand, due to the brittleness of shale, after stimulation operation, fracture network system would become a part of the porosity. Obviously, the multimodal pore space may lead to the multi-mechanisms and complicated state equations of the gas flow in shale, such as ad-desorption, diffusion, slippage and stress sensitivity. The development of shale gas reservoir is much more difficult comparing to the conventional gas reservoir. To solve the problems of the unsteady flow in shale, we should built a reasonable fracture network model combined the multiple flow mechanism in multimodal porous media and the consideration of multi-stage fractured horizontal well, then use a reliable and precise method to obtain the solution of the partial differential equations. This thesis carried out an in-depth study on the transient seepage theory of multi-stage fractured horizontal wells in shale gas reservoir and the major contents of this dissertation are summarized as follows:1) Based on the literature review and scanning electron microscopy of the Longmaxi formation in the Sichuan Basin, the gas storage types as well as gas transport mechanisms has been researched. Through calculating the Knudsen numbers under different pore sizes, the flow type of defined as slippage flow and transition flow, gas flow in the macro-pore is dominated by the viscous flow;2) Based on the research of gas storage types and fluid transport mechanisms, and considering the characteristics of ad-desorption, diffusion, slippage and stress sensitivity, two microscopic seepage mechanism models are established.3) According to the seepage mechanism models, with the consideration of different fracture network descriptions and complex internal boundary conditions, three comprehensive flow mathematical models are established. After that, the implicit iterative schemes of the discrete numerical models are obtained by using the un-structure grids, finite element- finite volume theories and the NR method.4) The well model and well operation methods of un-structure grids are presented as an extension of the Peaceman well model in finite difference method. Based on the theory researches, the solver of the comprehensive flow mathematical models is programmed.5) Production performances of multi-stage fractured horizontal wells in shale gas reservoir are calculated by the solver, and the effects of reservoir properties, fracture networks and hydraulic fractures on well behaviors are analyzed.The research and the results obtained in this paper can provide theoretical guidance to efficient development for shale gas reservoir.