Measured Crude Oil MMPs with Pure and Mixed CO2, Methane, and Ethane, and Their Relevance to Enhanced Oil Recovery from Middle Bakken and Bakken Shales

Abstract

Abstract Enhanced oil recovery (EOR) processes using CO2 in tight unconventional plays like the Bakken Formation are expected to be very different from the processes which control EOR in conventional reservoirs. During CO2 EOR in conventional reservoirs, CO2 flows through the permeable rock, and the minimum miscibility pressure (MMP) is an important operational parameter for achieving a successful "miscible" flood. In contrast, in tight fractured systems like the Bakken, CO2 flow may be dominated by fracture flow, and not by CO2 flowing through the rock matrix as in a conventional reservoir flood. Since fracture-dominated CO2 flow could essentially eliminate the "flushing" mechanisms responsible for increased recovery in conventional reservoirs, operation at or slightly above MMP may or may not be relevant for the success of an EOR flood in such tight fractured reservoirs. To investigate this concept, capillary-rise vanishing interfacial tension (VIT) was used to measure MMP values for a typical Bakken crude oil (API gravity 41.5) with CO2, methane, and ethane at 110°C (230°F) typical for the reservoir. The effect of these different fluids, as well as the effect of pressures at, above, and well above MMP on recovering crude oil hydrocarbons was determined for small rock core samples from the productive Middle Bakken laminated zone as well as from Upper and Lower Bakken samples. Compared to the MMP value for CO2 of 2520 psi, MMP with methane nearly doubled at 4510 psi, but was nearly cut in 1/2 for ethane at 1360 psi. The recovery of crude oil hydrocarbons from both the Middle Bakken and Lower Bakken shale samples with 24-hour exposures to these fluids at reservoir pressures showed efficiencies that parallel the MMPs determined with each fluid; i.e., ethane yielded faster and more efficient recovery of the crude oil than CO2, but both CO2 and ethane were much more efficient than methane at recovering the crude oil from the 11-mm round rod rock samples. Although hydrocarbon recoveries from the rock samples paralleled each injectant's respective MMP values, extractions with CO2 at the MMP, and at ca. double and triple the MMP pressure showed much more efficient crude oil recoveries at higher pressures from both the Middle Bakken and Lower Bakken shale, demonstrating that EOR pressures much higher than the MMP could substantially increase oil recoveries in tight unconventional systems like the Bakken.