TREATING CARBON SEQUESTRATION IN HETEROGENEOUS CARBONATE RESERVOIRS AS A WATER ALTERNATING GAS (WAG) CO2 ENHANCED OIL RECOVERY FLOOD

As the need for identifying secure geologic storage for carbon dioxide (CO₂) increases, the Permian Basin will become a primary target for development of CO₂ storage hubs. The presence of multiple large volume basin-wide storage targets, reservoir knowledge, existing infrastructure, and an understanding CO₂ behavior in enhanced oil recovery (EOR) projects provides opportunities for development of CO₂ Storage Hubs. The heterogeneous nature of Permian Basin reservoirs not only makes them great oil and gas reservoirs but also makes them excellent candidates for CO₂ storage. As new projects are developed, treating CO₂ sequestration in heterogeneous carbonate reservoirs as if they were a Water Alternating Gas (WAG) CO₂ EOR flood will be necessary.

Carbonate reservoirs in the Permian Basin have historically produced <50% of the original-oil-in-place (OOIP) for one simple reason: they are inherently heterogeneous and challenging to secondary and tertiary recovery. Estimated ultimate recovery is, on average, 25–35% in Permian age carbonate reservoirs. With CO₂ EOR, recovery seldom approaches 50% of OOIP because of variability in depositional environment, diagenetic overprint, nature of the traps and seals, and pore to pore throat ratios. Carbonate reservoirs are deposited as complex cycles, cycle sets, and high frequency sequences which form multiple baffles and barriers within the reservoir that enhance storage potential when combined with traditional trap and seal geometries. Additionally, the presence of “thief” zones, large volumes of secondary porosity, dual or multi porosity/permeability systems, diagenetic overprinting, late-stage tectonics, and variable pore to pore throat ratios in heterogeneous reservoirs all work to create a system where employing a WAG flooding methodology is essential to successful sweep and to limit early breakthrough. Almost all CO₂ floods in the heterogeneous reservoirs of the Permian Basin rely on WAG to maintain conformance (vertical sweep efficiency) within the target formation and requires balance between the CO₂ in the flood and the CO₂ produced. These reservoir properties suggest that for successful CO₂ sequestration in heterogeneous carbonate reservoirs employing a WAG methodology will be necessary.