Hertanto Adidharma , University of Wyoming
The results of the study of the brine salinity effect on WAG injection performance has been reported in the Annual Technical Report 2009. The study of the effects of CO2 and water half-cycle slug size and miscibility conditions on Water Alternating Gas (WAG) performance in tertiary CO2 flooding has been reported in the Annual Technical Report 2010.
At the end of the project, the effects of timing of cyclic injections on CO2 WAG performance were experimentally investigated. As in our previous study, coreflood experiments were performed in Berea sandstone core, from which the WAG performance, such as percent oil recovery, tertiary recovery factor, and CO2/Gas utilization factor were determined. The cores used, 1-in diameter and 10.5-in long, were drilled from a homogeneous Berea sandstone block, the permeability of which was about 150 mD. It was water wet and had low clay content. The core was saturated with an artificial connate brine and then aged for over 12 hours before flooded with oil to obtain certain oil saturation. It was finally aged in the oven again at 60 for at least 36 hours.
In the study of timing of cyclic injections, a crude oil from South Slattery field in Wyoming was used in all experiments. The core flooding experiments were conducted at the formation temperature of South Slattery field, i.e., 57 oC , and at miscible condition, i.e., at a pressure of 2960 psi (20% above the minimum miscible pressure/MMP of the oil sample). The experiments also utilized artificial brines. The injection and connate brines contained 33.33 wt% CaCl2 and 66.67 wt% NaCl with salinities of 16000 ppm (mg/L) and 30000 ppm, respectively. The injection rates of water and WAG flooding were 0.3 mL/min to minimize the viscous instabilities and discontinuities at the inlet and outlet of the core. WAG injection was introduced at different stages of water flooding, i.e., when secondary water flooding recoveries were 0%, 20%, 40%, and 55% of original oil in place (OOIP). Ten cycles were injected with a half cycle slug size of 0.1 PV and a WAG ratio of 1:1.
The results showed that injecting WAG too early or too late resulted in either low macro sweep efficiency or low micro displacement efficiency. This could be explained by considering the water saturation (both mobile and immobile water saturations). If the water saturation is too high, such as in the tertiary mode, mobile water will keep CO2 from contacting with the oil (water shielding) and immobile water will make the relative permeability of oil lower (oil trapping). On the other hand, if the water saturation is too low, such as in the secondary mode, water inefficiently controls CO2 mobility (no or little gas trapping). In both cases, the oil recovery will be small. When the water saturation is proper, the gas saturation reaches an optimum value, and thus the oil recovery would be higher. The results also showed that the best timing to inject miscible CO2 WAG was when the water flood front roughly passed through the middle of the core, i.e., when water flooding had produced roughly half of the oil that could be recovered by secondary water flooding. By using the best timing, the oil recovery achieved an optimum value.
This experimental study is an essential effort to obtain better understanding the effects of timing of cyclic injections on WAG performance, which have never been experimentally investigated before. The understanding is critical for optimizing the WAG performance and has a great impact on enhanced oil recovery. This study is perfectly in accordance with my research focus, the products of which include not only fundamental understanding and theories that underpin the behavior of complex fluids and solids, but also practical understanding and engineering models that are needed to design optimal recovery/separation strategies and develop new materials and processes. This study also provides valuable experience in WAG flooding and enhanced oil recovery for our students and postdoc, which gives a great impact for their future career.