| At present,China ’s dependence on foreign oil is increasing year by year.In order to ensure the sustainable development and utilization of oil and gas resources,the upstream oil and gas business has gradually promoted from conventional oil and gas to unconventional oil and gas.The exploration and development of shale oil and gas reservoirs is in the key moment of technological breakthrough.As the core technology of shale oil and gas reservoir development,horizontal well multi-stage fracturing directly affects the productivity of shale oil and gas resources.Different from conventional oil and gas reservoirs,fracturing fluid flowback in shale oil resources is characterized by slow flowback speed and low flowback efficiency,and the production data in the flowback process also implies the characteristic information of the reconstruction fracture network.Through the analysis of fracturing fluid flowback data,the law of fracturing fluid flowback can be deeply understood and the fracturing effect can be timely evaluated,which is of great significance for the next reasonable development and improvement of productivity.From the perspective of material and energy balance,this paper systematically analyzes the flowback process of fracturing fluid in shale reservoirs around the retention mechanism and flowback stage of fracturing fluid in the flowback process.At the same time,based on the theory of unstable seepage,combined with the material balance equation,an early fracturing fluid flowback data analysis model was established.According to the fracturing fluid flowback data of shale oil fracturing horizontal wells,the flow law of fracturing fluid flowback process was analyzed,and the fracturing effect of 27 shale oil fracturing horizontal wells was evaluated.The results show that:(1)The fracturing fluid of shale oil fractured horizontal wells mainly flows in the complex fracture network caused by large-scale volume fracturing,and there are three obvious flow stages in the flowback process,namely,early radial flow/linear flow,boundary control flow and unsteady linear flow.(2)In the flowback process of shale oil fractured horizontal wells,the boundary control flow stage usually occurs after the bottom hole flow pressure drops by 10-20 MPa,and the duration is affected by the fracture parameters.After this stage,the oil and gas is about to break through the shale matrix into the fracture,and the flowback enters the unsteady linear flow stage.(3)The flowback production data of 27 shale oil fractured horizontal wells are analyzed.The fracturing fluid efficiency of 10 wells in test area B is between 62% and 91%,with an average of 77.09%.The effective fracture pore volume loss rate is between 19% and 59%,with an average of 42.04%.The fracturing fluid efficiency of 6 wells in test area C is between 57% and 84%,with an average of 71.74%.The effective fracture pore volume loss rate is between 5% and 24%,with an average of 16.87%.The fracturing fluid efficiency of other exploration wells is between 63%-90%,with an average of73.69%.(4)The total amount of fracturing fluid is positively correlated with the effective fracture pore volume.The larger the total amount of fracturing fluid,the larger the effective fracture pore volume after fracturing.The strength of fracturing fluid is positively correlated with the initial effective fracture pore volume.(5)In the early stage of fracturing fluid flowback,the shale oil fractured horizontal well chooses large displacement to flowback rapidly,which will cause rapid pressure drop,rapid energy loss,and increase the effective fracture pore volume loss rate.The purpose of this paper is to understand the flow characteristics and fracture network characteristics in the process of fracturing fluid backflow in shale reservoirs,and then evaluate the fracturing effect,so as to provide a theoretical basis for improving the volume fracturing design and provide a basis for the formulation and adjustment of subsequent development policies. |
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