Geologic considerations for enhanced oil recovery in Elm Coulee field, Richland County, Montana, Williston Basin

Abstract

The Elm Coulee field in Richland County, Montana, was discovered in 2000 and historically is the largest field producing from the Devonian – Mississippian Bakken Formation in the Williston Basin. The 530 square mile field has an estimated ultimate recovery of 300 million barrels or more of oil. Significant reserves are still present in the reservoir, making the Bakken Formation in the Elm Coulee Field an attractive target for enhanced oil recovery (EOR). The Middle Bakken Member (MBM) is the most significant reservoir of the Bakken Petroleum System in the Elm Coulee Field and will be the focus of this study. The MBM was most likely deposited in a shallow to distal shelf environment and then dolomitized according to the seepage-reflux model. There is a NW – SE trending MBM thick in the Elm Coulee Field, which was most likely formed as a result of the multistage Prairie Formation evaporite dissolution. Hydrocarbon production from the Elm Coulee Field also has a NW – SE trend but does not directly correlate with the MBM thick. Preferential dolomitization of the MBM is the driving force for reservoir quality, and is the likely cause for the discrepancy between these two NW – SE trends. Hydrocarbon production in the Elm Coulee Field is characterized by liquids-rich wells that produce comparatively very little water. The average estimated ultimate recovery (EUR) of a well in the field is 269,000 bbl of oil. The original oil in place (OOIP) of the MBM reservoir is calculated to be 2.02 Bbbl. The recovery for the field factor was calculated to be 13%. Water saturation in the pay interval is extremely low, however there is a sharp contact in the reservoir between pay, and decidedly much more water saturated non-reservoir rock. Fluid saturation in the MBM is a function of pore throat size distribution; intervals with larger pore throats are more likely to be oil saturated and intervals with smaller pore throats are more likely to be water saturated. Pore throat size distribution appears to be related to mineralogy, specifically dolomite. The pay interval in the MBM reservoir in the Elm Coulee Field appears to be preferentially oil – wet. This is supported by high oil saturations in the pay, preferential adsorption of organic material onto dolomite crystals and pore lining clays, and experimental core tests. The wettability of the MBM reservoir has direct consequences on historic production, as well as current and future EOR attempts. Most importantly, an oil – wet reservoir will have more oil adsorbed onto matrix rock, and as a result, will have lower recovery factors compared to an equivalent preferentially water – wet reservoir. This study examines the three most viable EOR methods for an unconvetinoal: solvent flooding (miscible CO2 and hydrocarbon gas), surfactant flooding, and fresh water flooding (also known as low – sal). Experimental and pilot well studies were reviewed to evaluate potential methods for EOR in the MBM. Surfactant solutions lowered the interfacial tension (IFT) between the Bakken crude and water. These solutions also successfully recovered significant amounts of oil from cores saturated in oil. Surfactants dissolved in fresh water were much more effective than those dissolved in produced water. Additionally, fresh water successfully recovered amounts similar amounts of oil, compared to surfactants dissolved in fresh water, from an oil saturated core. Three mechanisms describing hydrocarbon recovery from fresh water injection are proposed. Two of these methods also provide an explanation for the preferential adsorption of organic material onto pore lining clays. The Burning Tree 36-2H well was a CO2 “huff and puff” pilot test in the MBM reservoir in the Elm Coulee Field. The CO2 injection from this well could be responsible for an additional production of 8,596 bbl of oil with a carbon utilization ratio of 5.23 mcf/bbl. Water breakthrough was reported in the Staci 1-11H following water injection in the adjacent 3-11H well. This breakthrough could have been due to extensive hydraulic fracturing, however it is more likely due to a pre-existing preferential flow path related to regional structure and fracture trends. The results of this study argue that the MBM reservoir in the Elm Coulee Field is a viable target for EOR. The large amounts of residual oil present in the MBM reservoir can be mobilized through chemical, solvent, or low salinity water injections. However, more characterization is necessary prior to any large-scale economic endeavor. The nature of the fracture network must be better understood in order to completely optimize any future injections. Pending further characterization, and under the right economic conditions, the Elm Coulee Field could serve as a good example of EOR in an unconventional reservoir.