Resea Rch On Well Test Models Of Reinjection Well In Gas Reservoirs Based On Tree-shaped Fractal Theory

In the modern age with rapidly development of science and technology,more and more new energy resources are gradually developed and utilized,but it cannot shake the dominant position of oil and gas in the world energy pattern.For the past few years,the global natural gas production has been increasing steadily,and the following formation water has been produced in some gas fields.It is necessary to treat sewage from gas wells with no pollution,dilling reinjection wells is one of the beast ways to store those water.In order to prevent water flooding leakage in reinjection wells and further causing environmental pollution while water-flooding,we need a method to describe fluid seepage dynamics in the reservoir,and grasp the trend of water reinjection flow.On the basis of a large number of literature investigation,the dual porosity reservoir of water reinjection well is taken as the main research object in this paper.A complex model that considers the characters of pore microstructure and fluid in porous media is developed for the permeability and porosity of porous media in water reinjection well.For the description of fracture network in water reinjection well,a tree-shaped fractal fracture network is was utilized for simulating the artificial fractures.By generating the network with double bifurcation structure and numbering each fracture for different levels,the the flow characteristics of the fluid in the network can be analyzed.Based on the tree-shaped fractal theory,the well test model with single-phase fluid and compound fluid were established respectively to analyze the reservoir fluid occurrence state in water reinjection well.The dimensionless bottomhole pressure of the well test model in the Laplace space is solved by using Laplace transform and dimensionless transform.The influence of each parameter on the injection capacity of the water reinjection well was obtained by analyzing the influencing factors.Finally,an example is given to verify the correctness of the models,which provides some theoretical guidance for the safe and effective water injection.The main target tasks should be accomplished in this paper are as follows:(1)Aiming at the complexity of porous media in reinjection wells,a composite model of permeability and porosity of porous media based on capillary bundle hypothesis was established to describe the percolation characteristics of reinjection reservoirs in reinjection Wells.The flow expression of the composite model is compared with that of the previous model,which proves that the model is consistent with the previous research results.The permeability-porosity relationship calculated by the combined permeability in this paper.After comparing the porosity model and the experimental data,the results are in agreement with the experimental data preferably,which further verifies the efficacious of the model.(2)The basic physical model of the injected water in the formation is analyzed based on the seepage theory.By analyzing the columnar flow in cylindrical coordinate system,the mathematical model of seepage flow in single medium and double medium are established.(3)Based on the tree-shaped fractal theory,the fractal network generation method and the micro-fracture numbering rule are established to construct the fractal structure in water reinjection well.The micro-scale percolation characteristics of fractured reservoirs are studied further.The basic fractal parameters are defined based on the assumption that the rectangular fracture extends outward,which extends from the flow characteristics analysis of a single fracture to the flow characteristics analysis of the entire fracture network.(4)A tree-shaped fractal well test model without considering water saturation distribution was established for single-phase fluid in pure water layer,and the the same model which considering water saturation distribution was established for composite fluid.(5)The established well test models are solved and the typical well test curves are drawn.Flow segments are divided according to curve characteristics,and the influence factors of typical curve are discussed.Based on previous studies on seepage theory of water reinjection wells and tree-shaped fractal network theory,well test models of reinjection well is formed in his paper,which helps to develop water injection reasonably and efficiently.