Evaluation Of CO 2 Throughput In The Middle And Deep Reservoirs Of N Oilfield

N oilfield started CO2 huff and puff test in shallow oil reservoir from 2010, and achieved great success in oil enhencement. Whereas when CO2 huff and puffs are applied to deep reservoirs in the end of 2011, the effect is not very satisfactory. Most wells in deep reservoir can not give economical output due to lack of energy, while the oil recovery is low. Most of the wells which are applied CO2 huff and puffs tests observe no effects. A few of them observed obvious effect, but still lack clear understanding of oil recovery mechanisms and scientific theory. Therefore, this paper choose Gao block of N oilfield as object, researched on effect evaluation of CO2 huff and puffs in deep reservoirs in N oilfield, aimed to provide reference for similar deep reservoirs CO2 huff and puff applications. The following shows what we have found in this research:(1)Gao block is a tectonic lithologic fault block reservoir with low porosities and low permeabilities, and also multiple uniform oil-water contacts. The typical light reservoir oil has good mobility and expansion capacity, CO2 injection can cause oil expansion and viscosity reduction, in the meantime reduce oil density and interfacial tension, thereby increase the mobility of residual oil from intermediate pores and pore walls.(2)Injecting CO2 gas causes residual oil expand primarily by diffusion and dissolution, increasing oil recovery. The further the CO2 spread, the more residual oil may be taken out. The higher the formation permeability, the further CO2 spread, and more oil increment by CO2 huff and puff.(3) During the gas injection and soak period in CO2 affected area, water saturation reduces and oil saturation increases, because injecting CO2 Gas and expanded oil replace the water, and push it away from wellbore. Injecting gas dissolved into the oil, cause the oil expansion, increase oil mobility. CO2 continue diffusing during soak period.(4)Water injection before CO2 injection into the formation will push oil away from the wellbore reduce oil saturation near the wellbore, thereby a large number of CO2 dissolved into water, few can arrive at the oil, which cause poor throughput. Water injection after the injection of CO2 gas will push the CO2 gas far forward away from wellbore, making contact with more oil, and it will help improve oil increment.(5)Effect evaluation of CO2 huff and puff in G12 block shows that high oil saturation will cause low oil increment. And that there should be waterflooding before CO2 huff and puff, because less movable oil is benefit to oil stimulation;Obtained the optimum injection volume injection rate and soak time for valid wells of CO2 huff and puff, providing a reference for on-site application.