Oil production grew significantly from 0.2 million barrels per day (bpd) to 1.1 million bpd in the Bakken petroleum system from 2009 to 2014. A large volume of associated gas (1.6 billion cubic feet per day) has also been produced with the oil. A substantial part (>10%) of this produced gas is flared off because of the low natural gas price and limited infrastructure for gathering and transporting the gas from the well sites. Such a large scale of gas flaring not only wastes energy but also emits contaminants such as SOx, NOx, and CO2 to the atmosphere. Reduction of flaring and utilization of produced gas are important steps toward sustained development of the Bakken.
The potential for recycled gas enhanced oil recovery (EOR) is being investigated as a method of reducing flaring through utilization. However, large-scale gas flooding might be difficult for the Bakken because of the difference between the low-permeability matrix and the highly conductive hydraulic and natural fracture networks, which may lead to low sweep efficiency. Instead, this research by the Energy & Environmental Research Center (EERC) has aimed to investigate, through a series of laboratory experiments and numerical simulation activities, the potential to extract oil from the tight rocks by taking advantage of diffusion-based processes. Oil and gas produced from Bakken wells were characterized, and the reservoir formation properties were analyzed based upon core samples. A series of oil extraction experiments with varying gas (solvent) compositions were conducted. The minimum miscibility pressure (MMP) of various produced gas components and oil was measured to determine the pressure required for effective extraction. Based on the experimental results, a well-scale model was developed to simulate the performance of recycled gas EOR.
Results showed CO2 and produced Bakken gas to be miscible with the oil in reservoir conditions (>5000 psi, 230°F). The measured MMPs for pure CO2 and ethane with typical Bakken oil samples were 2528 and 1344 psi, respectively. The presence of methane in the gas increased MMP, but miscibility was still achievable under reservoir conditions. CO2 and ethane enabled extraction of most oil components from the rocks during a 24-hour experimental period, but methane exhibited strong molecular selectivity for light-end components. Simulation results showed that a single-well CO2 and methane/ethane huff ‘n’ puff operation could increase cumulative oil production as much as 50% for the multistage fractured wells in the Bakken.
The results of this study clearly showed that produced Bakken gas could be effectively used for recycled gas EOR. Implementation of EOR may have potential to compensate for the production decline of Bakken wells while reducing the quantity of flared gas.