The Study On Mechanism About Increasing Oil Recovery By CO₂ Flooding In Hua 26 Low Permeability Fault Block Oil Reservoirs

Special geologic conditions of low permeability oilfield,i.e.low porosity and permeability,lead to serious problems during oil/gas recovery by water injection,including water sensitive formation,difficult injection,and small amount of water injected.A series of fields prove that CO2 flooding in low permeability oilfields can effectively enhance the oil recovery.This study focused on fluid phase behaviors in formation,EOR mechanism of CO2 flooding,and optimization of horizontal well development parameters in Hua 26 low permeability fault block oil reservoirs,comprehensive use of indoor CO2 flooding test and numerical simulation for basic research of CO2 flooding characteristics of low permeability fault block oil reservoir,and correct understanding of permeability change law of underground oil/gas system,and provides an important basis for subsequent CO2 flooding in Hua 26 oil reservois.This study gives the following conclusions:(1)The formation oil has low elastic energy and low gas-oil ratio,so the production by elastic energy is not feasible,and it is necessary to supply external energy.CO2 injection not only maintains the formation pressure,but also increases solubility,reduces viscosity,seepage resistance and interfacial tension,and helps oil production.(2)During CO2 injection to fluid,Hua 26 fault block formation has the minimum miscibility pressure of 29.48MPa,which is within the formation pressure range,so near-miscible flooding can be achieved.It was found through the viewing window that a two-phase interface still existed during gas injection at the formation pressure,but the oil displacement efficiency had reached 90%.Thus,near-miscible flooding can be achieved at the present formation pressure.(3)Reservoir cores mainly have medium porosity and low permeability cores,and provide good correlativity between porosity and permeability,moderate heterogeneity,and final displacement efficiency between 37.97%and 67.21%during non miscible flooding,i.e.medium to high efficiency.Long core was broken through during gas injection at 0.7HCPV,with the final displacement efficiency of 77.23%.The placement efficiency was increased by 2.32%during depletion development and by 4.60%during steam huff and puff after depletion.Thus,the flooding process was a placement efficiency rise process.(4)It was found duding gas injection parameter bound research that horizontal well located in the good reservoir property zone could effectively enhance the oil recovery;high location injection and low production provided good recovery degree than low location injection and high production;with decreasing horizontal well spacing,the horizontal section shortened,the displacement increased,and the recovery degree reduced;for horizontal wells at high and medium positions,high location injection and low location production was better than low location injection and high location production;for horizontal wells at medium and low positions,low location injection and high production was better than high location injection and low location production.(5)Four projects for development of horizontal wells were designed.Project 4(CO2 was injected to two horizontal wells at high and low positions,and oil was recovered from two horizontal wells in the middle)has the highest swept volume and displacement efficiency,and therefore the best gas injection effect and the highest recovery degree,according with the bound research results of CO2 injection to horizontal wells.