Immiscible CO2 Flooding, Post Steam Injection in a Shallow, Heavy Oil Reservoir, Forest Reserve field, Trinidad

Abstract Trinidad has significant heavy oil resources in shallow, onshore reservoirs. Steam has been the dominant Enhanced Oil Recovery (EOR) method since the 1960s. In 1994, in the Forest Reserve field, a steam project, Phase 1 East, was implemented in a clay-rich, sandstone reservoir at 900 ft, containing 15 °API oil. However, steam-induced swelling of clays, as confirmed by petrography, combined with operational issues resulted in poor steam injection performance and ultimately to project cessation in January 1999. Immiscible carbon dioxide (CO2) flooding was later implemented in 2001, leveraging existing CO2 supply and field facilities. This paper will assess the historical performance of the Phase 1 East steam and CO2 injection projects. Furthermore, the results will be benchmarked against worldwide heavy oil, immiscible CO2 projects such as those in the Bati Raman, Lick Creek, Midway-Sunset and Wilmington fields where CO2 floods following water/steam injection have yielded incremental recovery. In Phase 1 East, seven (7) injectors delivered approximately 3.0 hydrocarbon pore volumes (HCPV) of CO2 between 2001 and 2004. Production averaged 200 BOPD with a peak of 263 BOPD in March 2003. Incremental recovery totaled 157 MSTB or 1.8% original oil in place (OOIP) at a gross CO2 utilization of 15 Mscf/STB. Positive response was observed in eight (8) producers within months of start-up, with the per-well incremental oil ranging between 6 – 26 MSTB. CO2 is believed to have remediated near-wellbore damage caused by steam-induced clay swelling, enhancing injectivity and contributing to sustained production. Supporting this, mud influx ceased in several wells following the switch to CO2 injection. However, CO2 performance was limited by conformance issues due to unfavourable mobility ratios, possible fracture-induced channeling and adsorption of CO2 by clay minerals. Compared to worldwide heavy oil analogs, Phase 1 East realized low recovery for high CO2 HCPV, which resulted in high CO2 utilization. This paper presents a unique field case where immiscible CO2 flooding, following an unsuccessful steam project, achieved tertiary recovery of heavy oil in a shallow, water sensitive reservoir.