Research On Flow Characteristics And Production Prediction Method Of Multi-Stage Fractured Horizonta Wells In Tight Oil Reservoirs

The tight oil reservoirs,which accounting for more than 75%of our China's new proven reserves,are of great significance.Owing to the poor quality of reservoirs properties,difficulties in supplementary formation pressure and rapid decline of production rate challenges the economic development,e.g.,the first-year decline rate of oil can reach 40%to 90%.In recent years,the use of multi-stage fracturing and long horizontal section technology has gradually become the main mode of tight reservoir development,which form a certain range of fracture network near horizontal wellbore,obviously reduced the flow resistance and increased the productivity.Multi-scale flow characteristic is observed in horizontal well production,including the flow in matrix pores(nano scale),secondary micro-fractures(micron scale),hydraulic fractures(millimeter scale),and horizontal wellbore(centimeter scale).Therefore,with the support of the 13th five-year plan national major science and technology project"tight oil development demonstration project in Ordos Basin"(2017ZX05069),aiming at the problems such as unclear understanding of multi-scale complex flow characteristics and flow field variety law formed by tight oil development and rapid production decline.Systematic research was performed,including laboratory experiments,theoretical analysis and numerical simulations.It mainly includes:(1)By improving the force application mode of Brazilian splitting and using acoustic emission real-time monitoring technology in parallel,the nonlinear flow characteristic in matrix core,core with micro-fracture,and fractured core are systematically carried out,revealing the multi-scale flow through different medium.A multi-region coupled flow model for horizontal wellbore,transformed region,and untransformed region in tight oil reservoirs was developed.On the basis of this flow model,the multi-scale nonlinear flow in matrix pores,secondary micro-fractures,hydraulic fractures,and horizontal wellbore were accurately characterized and simulated.(2)Based on the multi-region coupled flow model,a method was established to calculate the dynamic propagation boundary of pressure and the effective utilization range in multiple regions.According to the pressure propagation area,the development process was divided into three stages,including the drainage stage of strong transformed region,the flow stage of weak transformed region and the energy supply stage of matrix region,respectively.Then the full life cycle production prediction method was developed for horizontal wells in tight oil reservoirs.(3)A flow model of oil-water two-phase considering the effect of dynamic capillary force was developed,and a semi-analytical method was introduced to solve the related equations.Then the variation of flow field in soaking stage and production stage was revealed to optimize the soaking time after hydraulic fracturing.Particularly,the contribution of imbibition process was quantified to draw the contribution rate chart and the variation of imbibition contribution to oil production in different development stages and different development areas were finally clarified.(4)A production prediction method for multi-stage fracturing of horizontal wells in tight oil reservoirs developed,which considered the influence of dissolved gas.The main controlling factors affecting tight oil reservoir production were revealed among different rock-fluid properties and hydraulic fracturing parameters.The "bottom hole pressure step-controlled method" was proposed from the basic perspectives of suppressing stress sensitivity,increasing contribution rate of imbibition displacement,and improving oil phase fluidity.It has been well validated that this method realized the effect of increasing oil production rate and decreasing the water production rate from the observation of oilfield.(5)Based on the market price estimation index of Changqing Oilfield Company,an evaluation method for the economic benefit of tight reservoir development was established,and the optimal technique parameters was screened through the index of final net profit and investment payback period.Finally,the production wells in the target block were classified according to the development status,and the corresponding development suggestions were put forward.This research provides a theoretical basis for efficient development of tight oil reservoirs and has effectively supported the demonstration production project of tight oil reservoir in Ordos Basin.