Study On Nonlinear Seepage Mechanism And CO₂ Micro-foam Flooding In Low Permeability Reservoir Of Yanchang Formation

Yanchang oilfield is the birthplace of China's petroleum industry.In 1907,it had the first oil well onshore put into production in China.The main oil development layer of Yanchang formation shows typical characteristics of "low permeability,low pressure and low production".The heterogeneity of the reservoir is quite strong due to well developed natural fractures.The pore throat is narrow and the seepage mechanism is complex.And there are large differences on the oil burial depth and fractures properties between the western and eastern reservoirs in Yanchang.Finally,systematic and deep study has not yet been done on the seepage laws of such kind of reservoirs with middle pores and narrow pore throats.In addition,the oil was always developed naturally in Yanchang oilfield,and waterflooding was conducted recently.But the waterflooding doesn,t work well,especially in fractures zone,serious water channeling and water flooded was occurred.What's worse,northern of Shanxi area is shortage in water resources so as to seriously restrict the economical development of low permeability reservoir in Yanchang oilfield.Therefore,it is necessary to establish a new method to enhance oil recovery tihat is suitable for the low permeability reservoir in Yanchang oilfield.We mutually combined experimental study and numerical simulation,gradually scaled from microscope to macroscope,and organically integral theory with practice in this study.The following five aspects were studied:i)Fractures characteristics of low permeability reservoir;?)Stress sensitivity evaluation of medium and shallow Yanchang reservoirs considering pressure arching effect,?)Nonlinear seepage characteristics experiments,(iv)Nonlinear seepage simulation of dual media in fractured low permeability reservoirs,and(v)CO2 micro-foam EOR.This paper has achieved the following main research results and understanding.(1)Starting from different scales of flow paths,the fracture characteristics of low permeability reservoirs in Yanchang Formation are comprehensively studied by means of thin-section analysis,mercury intrusion analysis,CT scanning,paleomagnetic positioning,micro-seismic monitoring and water drive frontage monitoring.We comprehensively studied the fracture characteristics of low permeability reservoir in Yanchang oilfield and proposed a series of research means on those special characteristics for the low permeability reservoirs of Yanchang oilfield.(2)Introducing the pressure arching effect of rock mechanics,the calculation method of shallow and deep pressure arch ratio in different burial depth for western and eastern areas of Yanchang oilfield is established and so as to evaluate the effects of arch ratio on stress sensitivity of reservoir.Studies have shown that when the reservoir depth parameter(the ratio of reservoir half-width to reservoir depth)is greater than 0.1,the influence of the depth parameter cannot be ignored,especially the reservoir with a large aspect ratio(ratio of reservoir thickness to width).(3)Forcusing on the characteristics of pore structure that behave middle pores and narrow throats existed in the low permeability reservoir of Yanchang oilfield,we carried out abundance of experiments,including non-Darcy flow seepage experiments,micro-scale displacement experiments,starting pressure gradient experiments,oil-water relative permeability experiments and stress sensitivity experiments.Furthermore,we experimentally evaluated the reservoir stress sensitivities for the shallow fractured horizontal fractures in eastern area of Yanchang oilfield and for the deep fractured vertical fractures in western area of Yanchang oilfield,and finally revealed the non-Darcy flow seepage laws for the low permeability reservoir of Yanchang oilfield.(4)Based on the nonlinear seepage physical model of low-permeability reservoirs,a mathematical model of nonlinear seepage in fractured low-permeability reservoirs considering the initiation pressure gradient and fracture system considering stress sensitivity is established,and the independent intellectual property rights are developed.The numerical simulation software of nonlinear seepage in fractured low permeability reservoirs simulates the influence of starting pressure gradient and stress sensitivity on the development dynamics of fractured low permeability reservoirs,and improves the nonlinear seepage theory of fractured low permeability reservoirs.(5)The most optimized CO2 micro-bubble formula was selected from 35 formula schemes.And the corresponding CO2 micro-bubble dynamic flooding micro-etching simulation system was developed to sinulate the micro-foam flooding process of different pore throat structures under different agitation rates.Furthermore,the experimental studies on the injection capability and oil displacement by CO2 micro-bubble were carried out.As a result,the best injection method and the most propriate flowrate of CO2 micro-bubble were found and contributed great in a low permeability reservoir of Yanchang oilfield.