Experimental investigation of miscible and immiscible Water-Alternating-Gas(WAG)process performance

摘要

Gas injection is the second largest enhanced oil recovery process,next only to thermal processes used in heavy oil fields.To increase the extent of the reservoir contacted by the injected gas,the gas is generally injected intermittently with water.This mode of injection,called water-alternating-gas(WAG),is being widely practiced in the oil fields.This experimental study is aimed at evaluating the performance of the WAG process as a function of gas-oil miscibility and brine composition.This performance evaluation has been carried out by comparing oil recoveries from WAG injection with those from continuous gas injection(CGI).Miscible floods were conducted at 17.24 MPa(2500 psi)and immiscible floods at 3.45 MPa(500 psi)using rock-fluids systems consisting of Berea cores,n-Decane and two different brines,namely the commonly used 5% NaCl solution and another being the multi-component reservoir brine from the Yates field in West Texas.The coreflood protocol consisted of a series of steps including brine saturation,absolute permeability determination,flooding with oil(drainage)to initial oil saturation,end-point oil permeability determination,flooding with brine(imbibition)to residual oil saturation,end-point water permeability determination,and finally,tertiary gas injection(in both continuous injection and WAG modes)to recover the waterflood residual oil.When oil recovery per unit volume of gas injection was used as a parameter to evaluate the floods,the WAG mode of injection out-performed the CGI.As expected the miscible floods were found to out-perform the immiscible floods.At increased volumes of gas injection,the performance of miscible CGI flood,in spite of the high injection pressure,approached that of the low-pressure immiscible floods.A change in brine composition from 5% NaCl to 0.926% multivalent brine from Yates reservoir showed a slight adverse effect on tertiary gas flood recovery due to increased solubility of CO_2 in the latter.The results of this study suggest that the optimum mode of gas injection is a combination process between CGI and WAG modes of gas injection.