Abstract
A successful two-well propane-injection field test with a vertical injector and horizontal producer was conducted in the Middle Bakken reservoir. The objective was to demonstrate the technical feasibility of propane to enhance oil recovery from Bakken reservoirs currently under primary depletion.
The benefits of applying enhanced oil recovery (EOR) techniques in unconventional Bakken reservoirs are potentially substantial because a significant portion of the oil left behind after primary depletion can be recovered. EOR by gas injection in fractured tight Bakken reservoirs is challenged by high gas mobility. This leads to early gas breakthrough, in addition to high capillary resistance experienced by the injected gas to contact oil in the matrix.
An advantage of propane is that it is first-contact miscible with Bakken oil (no capillary resistance) at around 650 psi (lower than the current reservoir pressure) at the reservoir temperature. Furthermore, propane is a liquid under field conditions and can be pumped into the injection well without the need for a compressor (operational ease). These factors make propane a highly desirable EOR agent for unconventional reservoirs; however, it must be handled carefully in the field for safety reasons.
A simulation guided injection/soak/production strategy was implemented in the field. It entailed injecting propane in the vertical well, V-1, then waiting for a soaking period. The horizontal producer, H-1, was shut-in during the injection and soaking periods, then put on production at the end of the soaking period. A total of ∼19 MMSCF of propane was injected into well V-1 over two cycles. The injection rate varied from 100 to 150 MSCF/D. The injection/soak/production periods were varied for the two cycles because of operational issues. As a result, well H-1 produced for a short period after the first cycle but was put on sustained production after the second cycle.
In the post-soak production, the well H-1 oil rate increased initially to ∼65 BOPD, more than doubling the pre-injection level of 30 BOPD. It then declined and plateaued at 50–55 BOPD over the next 4 months. The production uplift lasted for over 9 months and is estimated to be 20% over the hypothetical "flush" production that would have resulted from the producer being shut-in for the same period. The produced fluid propane concentration peaked at ∼70 mole% initially, stabilized at 65% for the next 4 months, and then gradually decreased. This indicates that injected propane contacted relatively larger volumes of oil in the matrix and recovered more of it through oil swelling, vaporization, and viscosity reduction. The process was efficient with a reasonable gas utilization factor of ∼12 MSCF/incremental bbl.
This is the first field test using propane to enhance Bakken oil recovery, and it demonstrates the efficacy of propane as an EOR agent for unconventional reservoirs. The test data validated the innovative subsurface modeling workflow process (presented in a companion paper). This work involved a coordinated, cross-discipline effort from an integrated team of engineers, geo-scientists, and operations personnel.
