An Evaluation of EOR Potential in the Elm Coulee Bakken Formation, Richland County, Montana

Abstract

Low primary recovery percentages from unconventional reservoirs have long motivated interest in Enhanced Oil Recovery (EOR) for these reservoirs, resulting in numerous simulation studies and injection pilots. However, performance from injections pilots has typically been disappointing compared to the simulations, suggesting that reservoir permeability and heterogeneity are not adequately described in the reservoir simulation models. In this study, a simulation and history-matching approach was used to quantify the permeability matrix over a six-section, nine-well area. Twelve years of production data were history-matched, using a combination of pressure-dependent permeability and enhanced permeability to represent natural fractures or other high-permeability features. Also, the performance of a failed injection pilot was history-matched to determine the level of reservoir heterogeneity needed to explain the pilot failure. Based on this study, a reservoir description capable of matching twelve years of production and injection history has been developed. Formation properties in the high-permeability streaks capable of causing the disappointing injection pilot performance have been quantified. Recovery has been forecast to depletion, and EOR under hydrocarbon gas injection has been forecast for a variety of scenarios. Optimal operating strategies and recommendations for technology development to mitigate early breakthrough are made. Realistic cost estimates were made for each scenario, and economics were run for each recovery method. These results give insight into the economic potential of enhanced oil recovery in the Elm Coulee Bakken formation. Recommendations for favorable tax treatment and scheduling of expenses/investments are made. Developing the permeability matrix using the history matching approach is a novel and versatile way of quantifying unconventional reservoir properties. However, it is important to match both injection and production data, since the permeability vector appears to have pressure-dependent effects. The effect of controlling injection thief zones by controlling local wellbore outflow is quantified, and a need for in situ permeability modification of fracture thief zones has been determined.

Recovery Mechanisms for Cyclic (Huff-n-Puff) Gas Injection in Unconventional Reservoirs: A Quantitative Evaluation Using Numerical Simulation

“…Therefore, the porosity, ∅ gb , used in the fracture gridblocks was 0.00005 (0.005%) resulting in a 50 ft gridblock representing a 1/8-inch fracture. Pressure-dependent permeability is often used to simulate changes in permeability resulting from pressure depletion in unconventional reservoirs [7,[27][28][29]. This accounts for the micro and meso fractures (less than 50 ft) in the reservoir that close and reduce permeability as pressure is reduced.…”

Investigation of CO2/N2 injection in tight oil reservoirs with confinement effect

“…This makes CO 2 the best candidate for improving the tight oil recovery . Several studies report that it can achieve an incremental oil recovery in the range of 5%‐20% in the Bakken and Eagle ford reservoirs using the huff‐n‐puff gas process . However, some studies also indicate that CO 2 injection does have several drawbacks, including the rapid decline of oil production.…”

“…2,[15][16][17][18] Several studies report that it can achieve an incremental oil recovery in the range of 5%-20% in the Bakken and Eagle ford reservoirs using the huff-n-puff gas process. [19][20][21][22][23] However, some studies also indicate that CO 2 injection does have several drawbacks, 24,25 including the rapid decline of oil production. Some have reported that the reservoir pressure during production cycle sharply drops after the first cycle with linear depletion to the minimum pressure.…”