Reasearch On Unsteady Percolation Theory And Rate Transient Analysis In Tight Gas Reservoirs

With a good development prospect,tight gas reservoirs accounts for more than half of unconventional gas resources in China.Acceleration of tight gas development has become increasingly important to China's energy supply and economic growth.However,due to the special geologic characteristics and complicated percolation mechanism,the tight gas reservoirs had worse properties,lower natural productivity,greater production decline and shorter stable production period than conventional gas reservoirs.Moreover,it usually requires horizontal well technology and hydraulic fracturing stimulation to achieve an industrial flow rate.Therefore,it is of great theoretical and practical significance to deeply study the characteristics of fluid flow mechanisms and quantify the influence of formation parameters during the exploitation of tight gas reservoirs.Based on the literature review,this dissertation carried out the tight gas unsteady percolation theory and rate transient decline analysis for both horizontal wells and multistage fractured horizontal wells.It mainly contains three types of mathematical models considering stress-sensitivity,threshold pressure gradient,and combined action of both.The pressure solution under constant rate and the rate solution under constant bottom-hole pressure of every model were generated by employing gas reservoir percolation mechanics,gas reservoir engineering theory,the method of mathematical physics,complex mathematical algorithm(Laplace transformation,Duhamel principle,Sturm Liouville eigenvalue,orthogonal transformation,perturbation theory and Stehfest numerical inversion).Correspondingly,pressure transient analysis and rate transient analysis were separately performed with relevant parameters.The work here enriches the relevant theories of unstable percolation,and provides a certain of theoretical guidance for reasonable and efficient development of tight gas reservoirs.The the major contents of this dissertation are summarized as follows:(1)The mechanisms of stress sensitivity(expressed by permeability variation exponential model)and threshold pressure gradient(expressed by low-velocity non-Darcy motion equation)in tight gas reservoirs were further studied by summarizing lots of literatures relating to laboratory experiments,physical simulation,theoretical model,and production effect,etc.(2)On the basis of penneability variation exponential model and fundamental of percolation mechanics,the horizontal well and multistage fractured horizontal well models were seperately established considering the effect of stress sensivity,and solved by employing Pedrosa substitution,perturbation theory,Laplace transformation and orthogonal transformation.(3)On the basis of low-velocity non-Darcy motion equation and fundamental of percolation mechanics,the horizontal well and multistage fractured horizontal well models were seperately established considering the effect of threshold pressure gradient,and solved by employing Laplace transformation,orthogonal transformation and Green function.(4)Taking both stress sensivity and threshold pressure gradient into account,the horizontal well and multistage fractured horizontal well models also were derived,and the pressure solution under constant rate and the rate solution under constant bottom-hole pressure were generated.(5)The exact solutions have been transformed to real space by Stehfest numerical inversion method and Duhamel's principle.The type curves of pseudo-pressure and its derivative,the type curves of dimensionless production and its derivative have been plotted for each model.The flow periods of each model have been divided in details.(6)The effect of permeability modulus and threshold pressure gradient on the rate decline performance were emphatically discussed.Besides,the sensitivities of each model have been analysised considering the effect of relevant parameters such as wellbore storage coefficient,skin factor,fracture number,fracture spacing,fracture half-length,storativity ratio and inter-porosity.(7)Field data is interpreted to verify the correctness and utility of the theoretical models developed in this dissertation.On the basis of tight gas reservoirs fluid flow theory,the researches in this dissertation deepen gas well testing and rate transient decline analysis,which could provide some theoretical support for predicting production dynamics and directing fracturing job design in tight gas reservoirs.