Favorable Target Optimization for CO2 Flooding in Deep Block Sandstone Reservoir with Bottom Water: A Case Study of Northwest Oilfield M Reservoir

Abstract

Northwest oilfield M reservoir is deep block sandstone reservoir with strong aquifer, having thin layer, large water-oil volume ratio, high temperature and salinity, strong heterogeneity, being developed more than 20 years by depletion method, influenced by bottom water, reservoir comes into high water cut period, recovery degree is only 26%, requiring for further enhanced oil recovery technology. CO2 is easily miscible with oil, but whether CO2 is applicable for the reservoir with strong aquifer or not is still unclear. No foundation can be referred to in area optimization for CO2 flooding. It is necessary to conduct research on influencing factors of CO2 flooding in the sandstone reservoir with aquifer and set up the favorable target optimization technology. Reservoir engineering method was applied to study M reservoir production pattern. Based on G&G, the geological-dynamic response law was established and productivity influencing factors were studied. Considering flowing differences between block reservoir and layered reservoir, the flowing field is affected by vertical sweep of bottom water, internal small faults have sealing effect on plane sweep, development unit division method was formed on the basis of vertical hydrodynamic characteristics. According to CO2 occurrence state and dynamic distribution in oil layer and strong aquifer, three types of favorable target have been classified, favorable target optimization technology is established for CO2 flooding in deep block sandstone reservoir with strong aquifer. Research results show that the south and middle block are structurally high, edge water intrusion is weak, main layer is relatively thick and interlayer is developed, I and II type well are the main production well. While the north block is structurally low, reservoir quality is poorer, II and III type wells amount a lot. CO2 density is higher than the oil, in the interlayer undeveloped block, CO2 migrates vertically to the oil-water interface, then spreads laterally forced by bottom water, resulting in an increase in the oil-water interface tension and a decrease in the diffusion rate to the water. The more developed the interlayer is, the larger the sweep volume of CO2 is. In high structural position, fault sealing, interlayer developed and thick layer, the enrichment degree of remaining oil is still high, important to ensure CO2 lateral sweep. As results, favorable target for CO2 flooding is classified into I, II and III type. CO2 flooding pilot test was carried out in type I favorable target. Till now, 18,000 tons of CO2 had been injected, oil exchange ratio in 1st year was 0.13. Research results lay the foundation for the efficient utilization of the remaining oil, so as to explore an effective development method for deep, ultra-deep sandstone reservoir to greatly enhanced oil recovery.