Study On Enhanced Bypassed Oil Recovery In Low-Permeability Zones Through CO₂-Oil Mass Transfer

CO₂ injection,an efficient enhanced oil recovery（EOR）method,has been widely used in oilfield development.However,during CO₂ flooding in real reservoirs,the gas is likely to flow through relatively high-permeability zones or fractures from the injection well to production well and bypass the remaining oil in relatively low-permeability zones,resulting in gas channeling and the formation of bypassed oil,which will seriously affect the performance of CO₂-EOR.Out of previous studies,although CO₂ cannot directly displace bypassed oil after gas channeling,there exists mass transfer between CO₂ and bypassed oil（dissolution-expansion and extraction）.Therefore,it is of great significance to make full use of CO₂-oil mass transfer for improving bypassed oil recovery in low-permeability reservoirs.Firstly,the intension of CO₂-oil mass transfer was determined by extraction and dissolution-expansion experiments.For the shallow bypassed oil,CO₂ has a strong extraction effect on it.At the pressure of 40.0 MPa,the extraction rate is up to 85.2%,and CO₂-oil mass transfer makes the final volume of crude oil shrink.For the deep bypassed oil,the extraction effect is weak and CO₂-oil mass transfer is mainly manifested as expansion of crude oil caused by CO₂ dissolution into oil.Then the characteristics and recovery performance of bypassed oil were observed by CO₂ soaking and subsequent CO₂ huff-n-puff experiments in modified core holder.Under multi-contact miscible condition,only about 20%of bypassed oil could be produced by CO₂ soaking,and the main oil recovery mechanism changes from expansion to extraction.While CO₂ huff-n-puff can further improve the recovery factor of about 65%,and larger pressure depletion contributes to higher oil recovery.Secondly,the influence of miscibility and the size of bypassed oil on the effectiveness of CO₂-EOR were investigated.During CO₂ soaking process,with the size of bypassed oil being the same,first-contact miscible soaking is more effective than both multi-contact soaking and immiscible soaking.Under the same miscible condition,the smaller the size of bypassed oil,the higher the oil recovery factor.During CO₂ huff-n-puff process,with the size of bypassed oil being the same,multi-contact huff-n-puff obtains the maximum recovery factor,whose recovery efficiency is higher than that both at first-contact miscible and immiscible condition.Furthermore,the mass transfer depth of CO₂ huff-n-puff is much deeper than that of CO₂ soaking.Meanwhile,the method of improving bypassed oil recovery by enhancing CO₂-oil mass transfer with chemical modifiers was studied.By dissolving surfactancts（TX100 and TX45）or ethanol into supercritical CO₂,the extraction rate crude oil could be improved and thus the intension of mass transfer be enhanced.Moreover,with the compounding use of TX45 and ethanol,the modified CO₂ system could obtain better extraction efficiency（extraction rate was increased by about 9%）.It has been shown from the oil displacement experiments that for both non-bypassed and bypassed oil,the recovery factor of modified CO₂ was higher than that of pure CO₂ to some extent.