Study On Imbibition Mechanism And Time Optimization Of Shut-in After Tight Reservoir Fracturing

In recent years,unconventional oil and gas production has increased year by year.Among them,the production of tight sandstone accounts for more than 75%,and its strategic significance is increasingly prominent.Tight reservoirs have complex pore throat structure,small pore size and poor physical properties(φ<10%,K<0.1m D).It is necessary to connect micro-nano pores through large-scale hydraulic fracturing to enhance oil recovery.During the hydraulic fracturing process,a large amount of fracturing fluid will be injected into the reservoir,which does harm the reservoir and lead to poor development results.However,the results obtained through the development of tight reservoirs show that the imbibition of fracturing fluid,generated during the shut-in,is helpful to enhance oil recovery under certain conditions.Unfortunately,the microcosmic mechanism of fracturing fluid imbibition in different types of tight reservoirs is still unclear.In turn,it is not possible to establish an effective model to predict fracturing fluid imbibition behavior and develop an optimal shut-in scheme.Based on the classification of tight reservoirs,macroscopic imbibition experiments were carried out.Moreover,the basic law of fracturing fluid imbibition in tight reservoirs and the influencing factors are further elaborated.In addition,the effect and mechanism of fracturing fluid stripping on crude oil in tight reservoirs at the microscopic scale are studied and analyzed by using the molecular dynamics method.In this way,at the microscopic mechanism level,the positive impact of fracturing fluid imbibition on tight reservoirs is clarified.Based on Fick’s law and oil-water non-piston fluid flow,a mathematical model of fracturing fluid spontaneously imbibition into the tight reservoir under capillary force and gravity during shut-in was established.Then,the characteristics of reservoir stress and temperature field distribution during shut-in were analyzed.Consequently,the optimal shutin duration was determined based on the fracturing fluid imbibition distance,which lays a foundation for the determination of shut-in schemes for different types of tight reservoirs.Specific researches and results are as follows.Firstly,based on the cast thin sections and scanning electron microscope,combined with various experimental data such as constant rate mercury injection,high-pressure mercury injection,CT scanning and NMR,the microscopic pore throat characteristics of tight sandstone reservoirs in the study area were comprehensively analyzed.Moreover,taking into account the complexity of multiple factors affecting seepage characteristics,Qtype clustering is selected the comprehensive influence factor F synthesized together with physical parameters,microscopic pore throat structure parameters,and permeability characteristics parameters,the tight reservoir can be classified into three categories from good to poor.And a classification and evaluation criteria suitable for tight sandstone reservoirs in the study area were established.Secondly,NMR experiments are conducted to quantitatively characterize the variation of imbibition capacity of pores at different scales.Consequently,the fracturing fluid imbibition law was revealed from a macroscopic perspective through fluid transport in pores.Meanwhile,the influence of temperature,fracture,porosity and minerals on the imbibition and displacement of crude oil by fracturing fluid is analyzed.Combined with the influencing factors,the suitable reservoir conditions for shut-in wells are analyzed.Thirdly,the molecular dynamics method is applied,it is clear from the microscopic point of view that diffusion is the main role in the imbibition process of fracturing fluid in tight reservoirs;It reveals the micro-mechanism that fracturing fluid has stronger interaction with pore surface and competitive adsorption with adsorbed hydrocarbon,thus replacing adsorbed hydrocarbon with free hydrocarbon.Furthermore,the fracturing fluid was evaluated by combining pore volume and hydrocarbon density,and the crude oil replacement rate was calculated,which increased with the increase of pressure and pore size.Based on the microscopic mechanism of imbibition and Fick’s law,a mathematical model to describe the fracturing fluid imbibition and replacement process were established.Combined with the fracture distribution characteristics and wellhead pressure in the study area,the optimal shut-in time of three types of reservoirs under different conditions is determined.When the wellhead pressure of I-type reservoir is 45 MPa,the shut-in time is about 15 days;When the pressure of II-type reservoir is 50 MPa,the shut-in time is about 20days;When the pressure of III-type reservoir is 60 MPa,the shut-in time is about 30 days.The research on the imbibition law,microscopic mechanism and prediction model of fracturing fluid in tight reservoirs is helpful to deeply understand the imbibition mechanism of fracturing fluid in tight reservoirs and provide theoretical basis for the optimization of tight oil and gas shut-in time scheme.