Research On Numerical Well Testing And Production Decline Analysis Theory For Multi-fractured Horizontal Wells In Shale Gas Reservoirs

Due to the huge demands for energy and the environmental pressures,shale gas has become an important strategic resource because of its advantages of large resources,low-carbon and clean,etc.Compared with conventional gas reservoirs,shale gas reservoirs belong to a typical ultra-low porosity and permeability unconventional gas reservoirs.Because of the unfavorable porosity and permeability,horizontal well drilling and hydraulic fracturing are effective measures to attain economic production capacity..However,complex fracture network generated by mass hydraulic fracturing and complicated multi-transport mechanisms exist in shale gas reservoirs,bring a new challenge for shale gas reservoir engineering theories and methods.For the reservoir engineering theories,determination and evaluation of the parameters of shale gas reservoir and stimulate reservoir volume(SRV),etc is one of the critical technologic bottlenecks of shale gas reservoir engineering theory and one of the key scientific issues for efficient development of shale gas reservoirs.Therefore,to analysis and evaluate the gas reservoir parameters,flow characteristics and stimulating effect etc.,it is necessary and urgent to establish a suitable set of theory and method of theory and method of well testing and dynamic analysis of production decline for multi-fractured horizontal well(MFHW)in shale gas reservoirs.In this study,based on the literature reviews,the adsorption/desorption rule and the multi-transport mechanisms of shale gas are firstly identified by analyzing the core flowing tests.And then,the dusty gas model(DGM)coupled the gas diffusion,Darcy flow and adsorption/desorption is used to establish the transport mathematical model.Subsequently,the unstructured perpendicular bisection(PEBI)grids which can accurately describe the MFHW flowing behaviors are constructed by using the indirect method.For different fracturing effects of shale gas reservoirs,four MFHW physical concept models including symmetrical wings fracture model,two-region composite model,SRV multi-region composite model and discrete fracture network model are proposed.In addition,considering the nonlinearity caused by gas desorption,diffusion and gas PVT,four different numerical mathematical models for the numerical well testing and production decline analysis of MFHW physical concept models are derived by using the control volume finite element method and the fully implicit method,respectively.Then the well testing curves and Blasingame production decline type curves of the corresponding models are obtained,and the effects of relevant parameters including shale gas flow mechanism,SRV,etc.on type curves are analyzed as well.Further,for the "well pad" development mode in shale gas reservoirs,the optimized parameters of the horizontal well and the well placement principle considering the interference of well group are obtained by utilizing PEBI grids of the MFHW group.Finally,a comprehensive set method of numerical well testing and production decline analysis of MFHW in shale gas reservoirs is developed,which can provide a theoretical basis for efficient development of shale gas reservoirs.According to the study of the dissertation,the major results are as follows:(1)The research on shale gas flowing experiments and its flowing mechanisms are conducted.A strong nonlinear characteristics between shale gas diffusion,adsorption/desorption and reservoir pressure are identified based on the results of flowing mechanisms analysis,unsteady pulse permeability tests,gas diffusion coefficient tests and nitrogen adsorption isotherms tests.In addition,the results also indicate that,compared to the use of analytical/semi-analytical methods,the numerical simulation method for well testing analysis and production decline analysis of MFHW in shale gas reservoirs can simulate the gas flowing mechanisms more accurately.(2)The unstructured PEBI grids of MFHW describing its flowing behavior accurately are constructed.Based on the theory of Voronoi grids generation,the practicability of PEBI grids is studied from the distributing point strategy of the gas reservoir module and the MFHW module,respectively.The unstructured PEBI grids procedures are programmed so as to provide the basis of utilization of numerical well testing and production decline analysis theory.(3)The numerical well testing analysis theory&method for MFHWs in shale gas reservoirs are developed.Based on the unstructured PEBI grids of MFHW and DGM,the MFHW numerical well testing mathematical models including symmetrical wings fracture model,two-region composite model,SRV multi-region composite model and discrete fracture network model are established by combining the control volume finite element method and the fully implicit method.Furthermore,the well testing type curves of the corresponding models are obtained and the flow behaviors of the type curves are characterized accurately.Additionally,the effects of the parameters on the curves are quantitatively analyzed as well.Finally,the method of interpretation of the parameters is proposed.(4)According to the material balance theory,the material balance equation considering the effect of adsorption/desorption for shale gas reservoirs is derived and the material balance pseudo-time of Blasingame decline type curves is modified.Furthermore,the Blasingame production decline analysis theory is extended to shale gas reservoirs for the first time.Based on the unstructured PEBI grids and DGM,the MFHW production decline mathematical models including symmetrical wings fracture model,two-region composite model,SRV multi-region composite model and discrete fracture network model are established by combining the control volume finite element method and the fully implicit method.The modified Blasingame production decline type curves of corresponding models are calculated and obtained.Combing the Blasingame production decline type curves and pressure cloud chart,the flow characteristics of different models are recognized and the effects of the parameters on the curves are quantitatively analyzed as well.In addition,the diagnosis and quantitative evaluation of the parameters(such as SRV,discrete fracture network and permeability,etc.)can be achieved by matching the Blasingame type curves.(5)The field data of the MFHW pressure build up and dynamic production in shale gas reservoirs are used to validate the theoretical models of the numerical well testing and the Blasingame production decline analysis proposed in this thesis,respectively.In this work,the theory/method of the MFHW numerical well testing and advanced production decline analysis for shale gas reservoirs has been developed.It significantly improve the understanding of the multi-flow mechanisms including the adsorption/desorption,diffusion and Darcy flow etc.during the MFHW production period in shale gas reservoirs.At the same time,it provides a diagnose tool to evaluate the macroscopic parameters in shale gas reservoirs,such as permeability and SRV etc.Furthermore,it offers the solid theoretical guidelines for the dynamic analysis of the MFHWs and well placement program,which can promote the progress of the research and application of the shale gas reservoirs engineering theory.