Experimental Study On Water And CO₂ Injection In Inter-Salt Shale Oil Reservoirs

Inter-salt shale is a particular type of shale reservoir characterized by poor physical properties,high content of soluble salt minerals and complex lithology.During its development,the fracturing or injected fluid may dissolve the salt minerals distributed in the reservoir pore spaces.The salts dissolution may improve the physical properties of the reservoir by converting the spaces occupied by the salt minerals into pores and by creating new flow channels,while the salts’recrystallization and precipitation may severely damage the reservoir in the well vicinity.And since oil production through conventional methods in inter-salt shale reservoirs is challenging due to the reservoir’s tight properties,the utilization of enhanced oil recovery（EOR）techniques is of great importance.Therefore,this work investigates the application of EOR methods such as water huff and puff,CO₂ huff and puff,and water alternating gas（WAG）in inter-salt shale reservoirs with respect to the rock lithology and pressure variation,as well as the effect of the injection pressure on inter-salt shale reservoirs.To accomplish this study,a set of laboratory experiments are conducted using inter-salt shale cores taken from the Qianjiang reservoir in Jianghan basin,China.The wettability of the core is assessed using sessile drop and contact angle method,prior to the main experiments.Cores with relatively high porosity and low salt content are used for the evaluation of the pressure change effect on recovery during the soaking time in each injection scenario.Results of wettability measurement have shown strong hydrophilicity which provides better perspectives for water imbibition experiments.In the water huff and puff and water alternating gas process,the argillaceous glauberite exhibited the highest oil recovery degree as a result of salt dissolution,precipitation,and temporary plugging effects.While in the CO₂ huff and puff process argillaceous dolomite’s recovery degree was the highest.Unlike water huff and puff,there is no salt dissolution and the recovery mechanism is dominated by the light components’extraction,oil swelling,and volume expansion effects in CO₂ huff and puff process.In terms of oil contribution,NMR results pointed out that most of the produced oil is recovered from large and medium pores.Regarding the pressure variation,results indicated that the optimum soaking pressure is between 36 and 39 MPa for both water and CO₂ huff and puff injections.When the soaking pressure is lower than this range,it may result in a low oil recovery degree and pressures higher than the above range requires more injection fluid while the recovery degree remains unchanged.