SPE 113482 Simulation Evaluation of Gravity Stable CO2 Flooding in the Muddy Reservoir at Grieve Field, Wyoming

Grieve oil field, discovered in 1954, is located in southeastern Wind River Basin, central Wyoming. This Lower Cretaceous, valley-fill and channelized, Muddy sandstone reservoir is a stratigraphic/structural trap with an average structural dip of 15 degrees. Multiple recovery mechanisms have contributed to produce more than 30 million barrels of premium light sweet crude, including gas expansion, down-dip water drive, and re-injection of produced gas into the field’s gas cap. The reservoir depth, at 6,900 ft, and oil gravity, 37API, are considered favorable for miscible gravity stable CO2 flooding to enhance oil recovery. Three distinct reservoir lithofacies are identified within the Muddy channel sand at Grieve Field, which are overlain by a low-permeability sandstone interval of bay-head delta deposition. Wettability tests indicate that the reservoir rock is weakly water-wet. A full-field simulation model was developed to simulate the production history and forecast the performance of various CO2 flooding scenarios. The simulation evaluation concluded that gravity stable CO2 flooding can be an effective EOR method for the Grieve Muddy reservoir. Up to 23 MMSTBO could ultimately be recovered by gravity stable CO2 flooding. To repressurize the reservoir to an operation pressure above the minimum miscibility pressure, a cumulative injection of 90 BSCF or more of CO2 would be needed before any production. Total CO2 purchased is estimated to be in the 119 to 188 BSCF range depending on the operation duration and CO2 injection rate. The net CO2 usage efficiency, the ratio between total purchased CO2 and total produced oil, varies from 7.3 to 8.1 MSCF/BO in the simulated cases. The reservoir has potential to sequester more than 145 BSCF of CO2 at the end of CO2 flooding operation. Introduction Grieve oil field was discovered in August 1954. The field is located in southeastern Wind River Basin, central Wyoming, and is currently operated by Elk Petroleum Inc. (Elk Petroleum), Figure 1. The producing oil reservoir is a stratigraphic/structural trap at a depth of 6,900 ft in the Lower Cretaceous, valley-fill and channelized, Muddy sandstone. The average structural dip in the Grieve area is about 15 degrees to the northeast. By 2006, Grieve Field has produced about 30 million barrels of oil, 33 million barrels of water, and more than 109 billion cubic feet of gas. The oil is a premium light sweet crude of 37API gravity. Figure 2 shows the monthly production history of Grieve Field. The recovery mechanisms include gas expansion, down-dip water drive, and pressure maintenance by re-injecting produced gas into the field’s gas cap. Reservoir blow down started in 1977 when field water-oil production ratio increased to 9:1. Previous estimations of original oil-in-place (OOIP) range from 54 to 103 million barrels of oil (MMSTBO), while estimations based on material balance usually result in higher oil reserves than that from volumetric calculations. In this study, an OOIP estimation of 67 MMSTBO in the Grieve Muddy channel sand has resulted from a simulation history matching based on the full-field material balance. History match also reveals that about one MMSTBO of oil and 8.2 BSCF of gas have moved down from the overlain low-permeability sandstone interval into the Muddy channel sand interval during the reservoir depletion. The simulation work presented in this paper is part of a collaborative study between the University of Wyoming Enhanced Oil Recovery Institute (EORI) and Elk Petroleum. The objective of the study is to assess the EOR potential of CO2 flooding in Grieve Muddy reservoir which, with a large dip angle, is considered favorable for gravity stable CO2 flooding. In addition, stratigraphic and diagenetic studies of the Grieve cores, minimum miscibility pressure (MMP) assessment from slim-tube experiments, and core wettability tests have been performed for this study to provide required inputs for the