Reservoir Change And CO₂Flooding Mode Optimization In Low Permeability Reservoir After Waterlfooding

Eservoir properties in low permeability oil reservoir changed after water injectiondevelopment, the original reservoir physical parameters will not be able to meet the designrequirements of existing development scheme. Through water injection modelingexperiment,X-ray diffraction (XRD) phase analysis, scanning electron microscopy (SEM)analysis, determination of capillary pressure curve and the oil-water relative permeabilitycurve experiments, the variation of reservoir properties and changes in the fluid percolationcharacteristics before and after water flooding in low permeability reservoir were studied,based on the application of low permeability natural cores in Group Chang6th of Yanchanglayers in Yanchang Oilfield. Combined with the existing research results, the reasons for thechanges of reservoir properties were analyzed from the mechanism. Compared this researchwith the change characteristics of the Longhupao reservoir properties after water flooding inDaqing Oilfield, the different reasons for changes of those two reservoirs after water injectionwere further discussed. Besides, suitable injection timing after water flooding was selected incarbon dioxide flooding which is a suitable oil displacement way in low permeabilityreservoirs. The conclusion that permeability of low permeability cores in Group Chang6thhada decreasing trend caused by Long-term water erosion was made, but the trend was notobvious; Content of the main clay minerals like illite, kaolinite, chlorite and green fudge layerreduced a litter；There was a polarization change trend of pore structure: big pore throatbecame bigger, small pore throat became smaller, heterogeneity enhanced, connectivity andseepage capacity of pore throat weakened. Comprehensive analysis found that causes of rockpermeability change in Group Chang6threservoir were mineral particles grain size, differentreservoir thickness and initial permeability, strength and time length of injected wate r, thechange of the form and content of clay minerals caused by water erosion, and the change ofpore structure; Causes of pore structure change after the water flooding were theform,structure and content change of clay minerals in pores, clay minerals p hysical migrationcaused by dynamic effect in long period water washing,dissolution caused by compatibilitybetween property of injected water and minerals in the reservoir, and the size composition ofreservoir, the initial permeability and characteristics of sedimentary microfacies. It wasbelieved that reasons for different permeability change after water flooding between Sapugaoreservoir in Daqing Longhupao Oilfield Saertu and Yanchang Group Changqing6thwere that the reservoir particle size differences and different injected water effect on reservoir afterwater flushing; reasons for different pore structure changes were different clay mineralcomposition and difference reservoir composition. Finally,conclusion that injecting CO2afterwater flooding in low permeability reservoir could improve oil recovery degree which will behigher when inject CO2at lower water cut was made.