Study On Seepage Law Of Multilayer Fractured Vertical Well In Low Permeability Tight Oil Reservoir

At present,multilayer production and fracturing operation are mostly applied in vertical well development of low permeability tight oil reservoir.After fracturing,the porous structure of multiple media is formed,and the flow of crude oil in multiple media is mainly affected by three aspects,which affects the effectiveness of yield prediction.Firstly,with the decrease of formation pressure in the development process,the increase of effective stress will cause the decrease of pore volume and the influence of oil seepage velocity,thus aggravating the degree of fluid nonlinear seepage.Secondly,a large amount of fracturing fluid is trapped in the formation and interacts with the crude oil in the pores of different sizes,and the effect of imbibition replacing the crude oil in the pores is the most obvious;Finally,in the process of commingled production,the fracture length and width of each layer are different,which will lead to the heterogeneity of pore size of each layer,thereby affecting the flow of crude oil in single or even multilayer,the production prediction,development plan formulation and development technology decision are directly affected by these unclear seepage laws.To provide theoretical and technical support for the development of low permeability tight reservoirs,the nonlinear seepage characteristics of multiple media,the imbibition of fracturing fluid with different pore sizes and the multilayer flow law after fracturing are comprehensively studied,and the relevant mathematical model is established.The Yan 222 block of Shengli Oilfield is taken as the research background,a series of indoor physical simulation and numerical simulation calculation research work have been carried out,the main contents and conclusions are as follows:(1)The matrix rock and artificial fractured rock samples are selected to simulate the multiple media,and a series of laboratory experiments on nonlinear seepage characteristics are carried out.Taking the original effective stress of a reservoir as a starting point,the threshold pressure gradient and permeability stress sensitivity of multiple media are systematically evaluated.The threshold pressure gradient of the matrix rock sample and the micro-fractured rock sample has a power function relationship with the effective stress,and the difference of threshold pressure gradient between two types of media is nearly an order of magnitude,while the permeability decline of the two media rock samples affected by the effective stress is close.Therefore,the dynamic threshold pressure gradient is the main reason for the two types of medium showing different nonlinear seepage characteristics.(2)Through the combination of nuclear magnetic resonance and high-pressure mercury intrusion,the imbibition degree of different scale pores are studied.A new method combining visual porous media and microscope particle image velocimetry technology reveals the microscopic mechanism of the gemini surfactant solution in the oil-wet pores:the surfactant solution molecules enter the porous medium through diffusion and the Marangoni effect induced the oil-water interfacial tension gradient to displaces the crude oil,and the dispersed oil phase and the surfactant solution flow in opposite directions.The uniformly dispersed "water-in-oil" small droplets formed by emulsification improve the fluidity migration.Meanwhile,the microscope visual capillary and macroscope core experiments show the way of replacing crude oil with lipopeptide surfactants is countercurrent imbibition with capillary force.(3)Using indoor parallel physical simulation experiments of unfractured rock samples and artificially fractured rock samples,the impact of different fracturing degrees on reservoir development can be systematically evaluated through the contribution of each layer to the total recovery.The generalized Gaussian distribution function is used to characterize the permeability distribution of the full flow field after fracturing,and a mathematical model for steady state productivity of multilayer fractured vertical wells is constructed considering the dynamic threshold pressure gradient and permeability stress sensitivity,the impacts of fracture half-length,thickness of each layer and span between layers on the productivity of multi-layer vertical wells are mainly discussed.(4)The pressure effect between adjacent layers of the fracture system is used to characterize the interlayer leakage,and a mathematical model of oil-water two-phase seepage flow is established.The results show that after the fracture of a multilayer homogeneous planar reservoir,when the permeability of the fracture zone increases from top layer to bottom layer,the effect of leakage on production increases with the longitudinal permeability difference;when the permeability of the fracture zone decreases from top layer to bottom layer,the effect of leakage on production increases with the longitudinal permeability difference.It is recommended to weaken the halflength of the fracture layer by layer from top to bottom,and the fracturing fluid is selected to add a proper concentration of Gemini and lipopeptide surfactant separately or in combination,thus increasing reservoir production.The research can provide theoretical and technical guidance for the production prediction and the design of development schemes of vertical wells in low permeability tight oil reservoirs.