The Roles of Fluid Properties, Half-Cycle Slug Size, and Timing of Cyclic Injection on Water Alternating Gas Injection Performance in Near-Miscible and Miscible CO2 Flooding

Reports: DNI9 49016-DNI9: The Roles of Fluid Properties, Half-Cycle Slug Size, and Timing of Cyclic Injection on Water Alternating Gas Injection Performance in Near-Miscible and Miscible CO2 Flooding

Hertanto Adidharma, University of Wyoming

The overall goal of the research is to investigate the roles of unexplored yet important factors that affect Water Alternating Gas (WAG) injection performance. Specifically, the effects of fluid properties (brine salinity and gas composition), CO₂ and water half-cycle slug size, and timing of cyclic injection are experimentally studied in near-miscible and miscible flooding.

The results of the study of the brine salinity effect on WAG injection performance has been reported in the Annual Technical Report 2009. In the past 12 months, the effects of CO₂ and water half-cycle slug size and miscibility conditions on Water Alternating Gas (WAG) performance in tertiary CO₂ flooding were investigated. As in the study of brine salinity, 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 a certain oil saturation. It was finally aged in the oven again at 60^oC for at least 36 hours.

In the study of the effect of half-cycle slug size (HCSS), the core flooding experiments were conducted at 60^oC and at miscible condition, i.e., at a pressure 20% above the minimum miscible pressure (MMP) of the oil sample. The experiments utilized Cottonwood Creek crude oil and artificial brines. The injection and connate brine contained 33.33 wt% CaCl₂ and 66.67 wt% NaCl with salinities of 16000 ppm (mg/L) and 30000 ppm, respectively. The injection rates for secondary water flooding and tertiary WAG flooding were 0.3 mL/min to minimize the viscous instabilities and discontinuities at the inlet and outlet of the core. In every core flood test, for tertiary WAG flooding, alternate cycles of brine and CO₂ with a WAG ratio of 1:1 were injected with half cycle slug size ranging from 0.05 to 0.75 pore volumes (PV).

The results showed that there was an optimum HCSS, by which the oil tertiary recovery reaches a maximum value and the CO₂ utilization factor reaches a minimum value. In the core flooding experiments conducted, the optimum HCSS was 0.1 PV, in which the CO₂ usage was only 0.6 PV to reach a high tertiary oil recovery of 40.63%. An HCSS higher or lower than 0.1 PV gave lower oil recovery. With an HCSS lower than 0.1 PV, the system gave lower oil recovery because some of the gas was trapped by water and kept staying in the core instead of displacing oil. Meanwhile, an HCSS higher than 0.1 PV resulted in larger clusters of CO₂ and water, thus water inefficiently controlled CO₂ mobility, which made the CO₂-oil contact time and interfacial area for mass transfer decrease and gas breakthrough occurred prematurely.

In the study of the effect of miscibility conditions, two more miscibility conditions were investigated. The HCSS study was then repeated for near miscible condition, i.e., at a pressure 5% lower than MMP, and immiscible condition, i.e., at a pressure 50% lower than MMP of the oil sample.

Each miscibility condition had its own behavior in displacing oil through porous media, leading to distinct WAG performance. The optimum HCSS was consistent with the results in the HCSS study, i.e., 0.1 PV. For tertiary flooding (post waterflooding) using this optimum HCSS, immiscible WAG gave 29.90% oil recovery while near miscible and miscible gave 40.32% and 40.63% oil recovery, respectively. Immiscible flooding did not perform as well as near miscible and miscible flooding, but near miscible and miscible flooding performed comparably in water wet system, which demonstrated that requiring miscible condition could be overdesign.

This experimental study is an essential effort to obtain better understanding the effects of half cycle slug size and miscibility condition 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.