Adaptive Study Of In-situ CO₂ Generation Technology For EOR In Bohai Bay Oilfield

CO₂ flooding is a very effective technology to enhance oil recovery because CO₂dissolved in water can increase the viscosity of the aqueous phase,moreover,CO₂dissolved in oil can reduce the oil viscosity,thus reducing the oil-water interfacial tension and increasing oil displacement efficiency.However,there are limitations for wide application of CO₂ flooding in China,especially in some offshore oilfields,because the CO₂ gas sources are limited,most of the oilfields are far away from gas sources,and the transportation cost of CO₂ is very high.In-situ CO₂ generation technology is injecting in-situ CO₂ generating system into the formation.It can produce a lot of CO₂ and heat in-situ by chemical reactions,and thus increasing the formation energy and lowering oil viscosity,and finally improving oil recovery.This study mainly focused on the performance evaluation,adaptive study and injection methods of the in-situ CO₂ generation technology.The results from sand pack flooding showed that the pressure of the system increased 400kPa,the system temperature increased 2.5℃,oil viscosity decreased by more than 34%.In addition,gas forming（GY）solution decreased the oil-water interfacial tension to 0.6mN·m and gas yielding（GY）solution removed that blockage of the clay in porous bodies.As shown in the adaptive study,the in-situ CO₂ technique could improve oil recovery by more than 20%OOIP.With the increase of the permeability of the homogeneous model,the recovery efficiency improved.With the increase of the permeability average value of the interlayer heterogeneous model,the oil production increased.On the whole,the system can improve the recovery in the higher permeability layer;however,the profile control effect is limited.Moreover,this system is adaptive to various oil viscosity,formation temperature,and salinity.As for the injection methods,injecting this system at higher water cut is more favorable because it is more benefit for an adequate reaction.Exchanging the injection sequence of the GY and GF solution had little effect on the recovery efficiency of the system;however,the effect of that injected GF first was slightly better.When the injection was kept to be 1PV,there is no effect after two and three cycles’injection;however,if combined with strong starch gel sealing systems,this system can effectively improve its application of multiple cycles’injection.