Experimental Investigation of Oil Recovery by Different Injection Scenarios under Low Oil/Gas IFT and Mixed-Wet Condition: Water-Flood, Gas Injection, WAG and SWAG Injection

Abstract

Abstract Water flooding and gas injection are two widely used improved oil recovery techniques that can be applied individually or combined as water alternating gas (WAG) or simultaneous gas and water (SWAG) injection. To do reservoir development planning, for possible implementation of these oil recovery schemes, reliable reservoir performance prediction is needed. Most of the existing reservoir simulators are unable to adequately account for all the complex multi-phase and multi-physics processes involved in these oil recovery techniques. That is particularly the case under mixed-wet and low gas-oil IFT (near-miscible) conditions. Performing reliable laboratory experiments is the key to evaluating the performance of these oil recovery techniques under reservoir conditions. We present the results of a comprehensive series of well-controlled coreflood experiments carried out under mixed-wet condition using a very low IFT gas-oil system. The experiments include oil recovery by water flood (WF), continues gas injection (CGI), two series of WAG, and two series of SWAG injection tests. The difference between the two WAG experiments is the order in which gas and water injections are carried out. The first WAG test started with water injection whereas the second WAG experiment started with gas injection. The difference between the two SWAG experiments is the gas/water (SWAG) ratio, which was 0.25 for the first one and 1.0 for the second SWAG test. The results show that in both mixed-wet cores, WAG injection has a superior performance over WF, CGI and SWAG injection. Oil recovery by the WAG test which had started with water injection was higher than the WAG test started with gas injection. SWAG performed better compare to CGI. However, surprisingly, SWAG resulted in lower oil recovery compared to primary waterflood in these mixed-wet systems. It was observed that increasing the gas/water ratio in SWAG leads to faster gas breakthrough, higher produced gas/oil ratio and further reduction in the recovery performance of SWAG. Compared to the other injection strategies, a very high pressure drop across the core was observed during SWAG injection indicating injectivity problems with the application of the process in mixed-wet rocks. The results show that for mixed-wet rocks, amongst the studied injection strategies, SWAG is the worst and alternating injection of water and gas (WAG), starting with a water flood period, is the best injection strategy.