Laboratory And Numerical Simulation Study On CO₂ Flooding In Ultra Low Permeability Reservoir

The reserve and production of high quality oil-gas field are decreasing with the improvement of exploration and development. The low permeability oil-gas field accounts for larger and larger proportion of oil production. The low permeability deposits have become the main resource to increase reserve and production now and in future. Due to the poor physical property, small size of pore and throat, high capillary force, a threshold pressure gradient exists in the low permeability reservoir. As a result, the water injection pressure is high, the water absorption capacity is poor, and the speed of water injection response is slow. With breakthrough of water in oil wells, the oil and liquid productivity index will decrease rapidly, which cause great difficulty to the stabilization and enhancement of oil production. How to develop effectively the ultra low permeability reservoir is a major problem confronted in the world at presentThe feasibility of CO₂ flooding in Gao Taizi ultra low permeability reservoir of Long Hupao oilfield were studied by laboratory experiment and numerical simulation in this dissertation. At the simulated conditions of Gao Taizi formation, the variation law of oil-gas phase behavior was studied through the combination of laboratory experiment and computation on phase state equation. The critical temperature of the formation crude from Gao Taizi formation was 600℃and the critical pressure was 39.846 MPa. With the increase of CO₂ injection volume, the bubble point pressure, dew point pressure and the critical pressure increased, the area of two-phase region increased, the interval between equivalent fluid lines enlarged. As the injected volume of CO₂ increasing, the GOR, saturation pressure, formation volume factor, volume shrinkage, compressibility coefficient, average solubility factor and expansion coefficient of the formation crude from Gao Taizi oil layer increased, while the viscosity and density declined.Slim tube experiment showed the MMP of crude oil and CO₂ was 20.3 MPa, higher than the formation pressure. Experiments of water flooding, CO₂ miscible and immiscible displacement were carried out, obtaining the following results: the recovery at CO₂ breakthrough, ultimate recovery, conversion rate from CO₂ to oil, CO₂ injectivity increased gradually with the increase of CO₂ injection pressure; After the CO₂ breakthrough, the GOR would increase, and the higher the injection pressure, the greater the GOR; the recovery of CO₂ miscible was higher than that of the water flooding, while the recovery of CO₂ immiscible was lower than that of the water flooding; the injectivity of CO₂ flooding was greater greatly than that of the water flooding, the injectivity ratio of gas to water was more than 16.463.A series of experiments were carried out to investigate the mechanism of CO₂ flooding in Gao Taizi formation, and the contributions to oil displacement efficiency of some mechanisms were decided quantificationally. With the injection of CO₂ into the formation, the oil and the irreducible water dissolved large amount of CO₂. At the formation condition, the viscosity of oil decreased 36.73%, the volume of oil increased 20.53%, the volume of irreducible water increased 5.97%, this portion of water became into flowing water; along with the reduction of formation pressure, CO₂ became into free gas, generating expansion energy; with the increasing of CO₂ injection pressure, the interfacial tension reduced. The interfacial tension was 3.868mN/m when CO₂ was injected at the formation pressure. When the pressure rised to miscibility pressure, the interfacial tension decreased to 2.598mN/m, and misible flooding occurred when the pressure exceeded the miscible pressure. The thereshhold pressure gradient was determined according to the relationship curve of the displacing pressure differential and the displaced fluid volume. The thereshhold pressure gradient of water flooding was 2.9 times to the CO₂ flooding, and the injection of CO₂ decreased the thereshhold pressure, increasing the injectivity; the rock pore structure changed after CO₂ flooding, the permeability increased 4¹3%, the rock hydrophilicity became stronger; the relative permeability was improved, the relative permeability at the end-point of gas phase was more than 2.58 times lower than that at the end-point of water phase, the two phase region of gas flooding was wider than that of the water flooding.According to the geological characteristic, well pattern and development status of the test area of Gao Taizi oil layer in Long Hupao oilfield, facies-controled geological model was established by using PETREL geological modeling software. The CO₂ injection mode, well pattern arrangement, injection and production pressure, injection-production pattern, etc. were simulated by the compositional model in Eclipse. The recommended scheme was inter-well infill, inverted nine-spot gas injection; gas injection pressure being above 30MPa; the flowing pressure in oil well being controlled at about 2³ MPa; synchronous and continuous gas injection. The simulated recovery could arrive at 33.07% in 10 years according to the recommended CO₂ flooding scheme, more than 20% higher than that of the water flooding scheme. Therefore, it is feasible to produce Gao Taizi ultra low permeability reservoir of Long Hupao oilfield by CO₂ flooding.