Experimental Studies On CO₂ Sweep Efficiencies For Conglomerate Reservoirs

The pore structure of conglomerate reservoir is complex and its porosity and permeability are low.Improving the recovery of conglomerate reservoir is one of the difficult problems to be solved urgently.Based on the analysis of reservoir geological characteristics and reservoir physical properties in the target block,single core and parallel core CO₂flooding experiments were carried out by field coring,and the effects of temperature,backpressure,displacement speed and permeability combination on oil recovery were studied.The cast slices of field samples were observed by electron microscope,and the most representative conglomerate reservoir characteristics were screened out.Cast slice and corroded the microscopic transparent simulation model at the moment;Carbon dioxide micro-flooding experiment was carried out by using microscopic transparent simulation model,and the sweep state of carbon dioxide in pore and throat at at different times was observed.The effects of temperature,backpressure,model characteristics and displacement speed on the final oil recovery were compared and analyzed,and the image gray analysis software was used to calculate the final oil recovery.The recovery factor of the model is calculated.The experimental results show that:(1)Under the experimental conditions,the recovery rate of single core CO₂ flooding is positively correlated with temperature and backpressure,and negatively correlated with displacement velocity;in parallel core CO₂ flooding experiment,the greater the permeability gradient difference,the greater the difference of ultimate recovery rate.(2)The reservoir rocks in the target block are feldspar-quartz sandstone and conglomerate with high feldspar content;According to the characteristic grade of conglomerate,two casting slices of Well FN-15 and Well MH-8 were selected as samples to make micro-model.(3)In micro-displacement of carbon dioxide,carbon dioxide preferentially flows into the diagonal mainstream channel with good fluidity,and residual oil mainly exists in the corner and the pore with poor connectivity;carbon dioxide improves oil recovery through exfoliation,deformation,extrusion,blocking and connectivity mechanisms;carbon dioxide is affected by pore structure,connectivity and injection parameters of conglomerate.