Research On Seepage Theory Of Fractured Wells In Shale Gas Reservoir Based On Multi-Scale Flow Mechanisms

The recoverable resource of shale gas is rich in China, domestic shale gas resources in main basins and districts of China are estimated for more than 26 trillion cubic meters, which is among the world’s highest and has enormous economic value. With rapid production of Weiyuan Gas Field, Changning Gas Field and Fuling Gas Field, the cumulative production of shale gas exceeded 6 billion cubic meters, which made China become the world’s third largest shale gas producer. Although the successful development of shale gas in US, the study on the complex storage mechanism and percolation theories of shale gas is far behind the field practice, which results in unable to accurately predict dynamic development and formulate development schemes for shale gas reservoirs. Therefore, to develop shale gas resources more efficiently, it is of great theoretical and practical significance to deep study the complex storage and multi-scale flow mechanisms during exploitation of shale gas reservoirs.Compared to conventional oil and gas reservoirs, shale gas reservoirs mainly have the following differences:(a) peculiar storage mechanism, in addition to free gas stored in pores and microfracture and adsorved gas stored on pore walls, there are also some gas dissolved in kerogen; (b) multi-scale transport mechanism, nanopores are found abundant in shale and gas flow in nanopores is not Darcy type. There are also a lot of micropores and microfracture in shale, and gas flow in such pores is different from flow in nanopores; (c) shale is too tight, so horizontal well technology and hydraulic fracturing are needed to increase economic yield. Owing to the big differences, the existing development theories and methods of conventional gas reservoirs can not be applied to guide the development of shale gas reservoirs directly.According to the problems of shale gas reservoirs, the dissertation carried out the physical description and mathematical representation of shale gas transport mechanism from different scales based on the pore structure of shale and the storage mechanism of shale gas. The steady seepage model and unsteady seepage model when sigle-phase shale gas flow in formation are established based on complex storage and multi-scale flow mechanisms. A three dimensional, multi-phase flow numerical model of a fractured horizontal well producing in shale gas reservoirs under complex flow mechanisms is presented, which is valid for the entire Knudsen’s range (continuum flow, slip flow, transition flow and free molecular flow) in shale gas reservoirs. After the developed modle is verified by other models and actual field data, the effects of critical factors such as Knudsen diffusion and slippage effect, gas desorption, multi-phase flow and fracturing treatment on gas production rate are discussed as well. The major conclusions of this dissertation can be summarized as follows:(1) The characteristics of microscopic pore structure of shales and gas storage mechanism are studied based on laboratory experiments and theoretical research, the physical description and mathematical representation of shale gas transport mechanism from different scales is illustrated.(2) Gas flow regimes in shale gas reservoir are studied through calculating Knudsen number under different pressure and different pore sizes, results show that gas flow in shale gas reservoirs are mainly continuum flow, slippage flow and transition flow, which cannot be described by Darcy fomula simply.(3) A multi-scale comprehensive seepage model which can simulate different flow regimes based on Beskok-Karniadakis equation is established, and the effect of Knudsen number on permeability correction factor is analyzed. The productivity equation of fractured well is developled based on this multi-scale seepage model, and Knudsen diffusive coefficient and slip factor changing with pressure and pore size is considered in simulation. Inflow performance relationship curves of fractured well for shale gas reservoirs are obtained.(4) An unsteady multi-scale seepage mathematical model for single-phase gas flowing in formation is established, which considered the effects of gas desorption, Knudsen diffusion and slip flow in nanopores based on the research of hydraulic fracturing. A triple porosity model for shale gas reservoirs which considers dissolved gas diffusion in kerogen is developed. The type curves of transient pressure and rate decline of fractured well are obtained, and the effects of dissolved gas diffusion, gas desorption, Knudsen diffusion and slip flow on type curves are analyzed.(5) A three dimensional, multi-phase flow numerical model of a fractured horizontal well producing in shale gas reservoirs is presented, which is valid for the entire Knudsen’s range (continuum flow, slip flow, transition flow and free molecular flow) in shale gas reservoirs, and the effects of Knudsen diffusion and slip flow, gas desorption are taken into consideration. Grid refinement and equivalent fracture conductivity method are used to simulate hydraulic fracturing cracks.(6) The numerical model is validated with commercial software, other model and actural field data, and the effects of Knudsen diffusion and slip flow, gas desorption, multi-phase flow and fracturing treatment on gas production rate of fractured horizontal are analyzed.The presented theoretical models provide theoretical basis for evaluating and predicting the productivity of shale gas well, and is of theoretical and practical significance to promote the efficiently development of shale gas reservoirs.