Mechanism And Application Of Transient Inter-porosity Flow Between Matrix And Fracture In Tight Oil Reservoirs

Tight oil reservoirs are mainly composed of micro/nano-pores and throats,which lead to the obvious microscale effect and nonlinear seepage characteristics.Tight formation features complex pore structure and ultra-low permeability,there is generally no productivity under natural condition.Multi-fractured horizontal well(MFHW)which is the most effective technique is widely used to improve productivity.A complex multiscale coupling medium with complicated seepage characteristics is formed after multistage fracturing.It composes of the matrix,natural and induced fractures and artificial fracture.Since the seepage law of fluid in matrix and fracture of the tight reservoir is quite different from that of the conventional reservoir,the inter-porosity flow law between matrix and fracture in the tight reservoir is also different from the conventional reservoir.Hence,it is of great significance to study the mechanism and application of transient inter-porosity between matrix and fracture in the tight reservoir for reasonable exploitation of unconventional resources.First of all,an experimental device of inter-porosity flow has been developed,and the experimental steps and data processing method were established based on the research of transient inter-porosity flow theory.The experimental process was simulated by finite element simulation,and the feasibility of the experiment was analyzed.In addition,the basic parameters of the experiment were determined by the simulation.Then,the occurrence and monitoring of inter-porosity flow phenomenon have been realized by carrying out the experiment.The effects of matrix permeability,matrix characteristic length,fracture with,effective stress and fluid viscosity on inter-porosity flow were studied.And the transient inter-porosity flow mechanism was revealed.Research shows that the worse the physical property of the reservoir,the smaller the value of the shape factor;as the characteristic length of the matrix decreases,shape factor increases due to the higher density the fracture network;the wider the fracture width,the greater the effective volume and conductivity of the fracture,which leads to a larger shape factor;the shape factor is strongly sensitive to the stress sensitivity,which decreases with the increase in effective stress;as the viscosity of the fluid increases,the shape factor decreases.Secondly,a method for obtaining the fractal dimension and tortuosity fractal dimension using mercury intrusion data is established by considering the effect of tortuosity.Based on the capillary bundle model and the fractal theory,a new nonlinear seepage equation was deduced,and a further fractal permeability model was obtained for oil transport in tight formation by taking into account the effect of the boundary layer.The predictions of the model were then compared with experimental data to demonstrate that the model is valid.Considering the effect of roughness and tortuosity of fracture,a fracture permeability model was established.Thirdly,a transient inter-porosity model with comprehensive consideration of multiple mechanisms was established,which makes the characterization of transient interporosity flow in tight reservoirs realized.And then,the effects of stress sensitivity,nonlinear seepage parameters,and fracture pressure depletion on shape factor and interporosity rate were discussed.Finally,analytical and numerical models for the productivity of MFHW in tight reservoirs were established respectively.The stress sensitivity of fractures,transient interporosity flow and the fractal distribution of the fracture network were considered in models.The main factors influencing the productivity were analyzed,and the application of the model was carried out for a typical tight reservoir.The research of this paper has developed and improved the nonlinear seepage theory of tight reservoirs,which provide a theoretical basis for the identification of parameters and productivity evaluation of tight reservoirs.