Study On Well Test Mathematical Model Of Fractured-cavity Reservoir

Fractured-vuggy carbonate reservoir space can be considered as a continuous medium for fracture and cavity characterization unit,and large scale fractures and cavities independent system which can be regarded as a discontinuous medium.The diameter of the cave is several meters or even tens of meters.The types of the fluid flow in the fractured-vuggy reservoir may be seepage,pipe flow,cavity flow or coupled with a variety of flow modes,and the flow mechanism is very complex.Based on the extremely strong reservoir heterogeneity and the diversity of the size of the fractures and cavities,it is difficult to explain the reservoir parameters of fractured-vuggy reservoirs,and the development is very difficult.Using well test analysis method to explain reservoir and get a lot of parameters of fractures and cavities,its significance is to provide some help for the development and production of large-scale fractured-vuggy carbonate reservoirs.The single wells that are typical of fracture-cavity units are analyzed in the Tarim Oilfield,according to the geological data of large scale fracture-cavity reservoirs,the paper presents four typical fracture-cavity units can be divided into four kinds of fracture-cavity reservoir types:large scale single cavity model(well drilling cavity),well-fracture-cavity model(the well is drilled on fracture next to the cavity),well-fracture-cavity-fracture-cavity model(double fractures and cavities in series),cavity-fracture-well-fracture-cavity model(double fractures and cavities in parallel connection).In order to clarify the law of fluid flow in the fractured-vuggy reservoir,the flow experiment and flow theoretical model of the two fractured-vuggy reservoirs were established,which verified the feasibility of the experiment to reflect the flow law of the underground fracture-cavity system.The flow in a single fracture can be regarded as Darcy seepage.The flow in a single karst cavity can be regarded as elastic expansion.The flow of fractures and karst cavities is the seepage of fractures,which controls the elastic expansion flow of the cave system.A large number of wells are basically independent fractured-vuggy system,and the wellbore communicated with cavity through fracture.So the actual flow process of the reservoir does not match the result are obtained by adopting traditional continuum explanation theory.An analytical mathematic model was presented by considering the fracture network between well-cavity and cavity-cavity as one-dimensional linear fracture.Thus,the three linear-in-series well test mathematic models were established,with the introduction of conception of transient flow and interporosity flow of multiple-porosity.The Transient conduction principle and Laplace transformation are used to solve the mathematical model analytically in Laplace space,and the bottom-hole pressure type curves are plotted in real space by using the improved Stehfest numerical inversion.The sensitivity of cave parameters was analyzed and discussed,which the model could correctly reflect the relation between flow control factor and geological factor.Finally,experimental verification and explanation and comparison of application examples indicated:the parameters of the wellbore storage effect,skin effect,the permeability of fracture system and the permeability of cavity system,and the geometric parameters of fracture and cavity are obtained by using the model.This paper provides a reliable guarantee for the parameters of the large scale fractured-vuggy carbonate reservoir.