Carbon Intensity Reduction
Enhanced Oil Recovery with Carbon Dioxide
Oil production historically has been accomplished through natural drive mechanisms (i.e. primary) or after the natural reservoir drive decreases, through water injection into the reservoir, which acts as an “artificial” reservoir drive. Enhanced oil recovery is the process of improving the recovery of an existing conventional oil pool through the application of water, chemical, thermal, gas or solvent injection beyond what has been achieved through primary recovery methods. Enhanced oil recovery (EOR) with CO2 is a means of improving oil recovery from a pool, beyond waterflood, by injecting CO2 into an oil reservoir.
Typically under primary recovery, an oil pool is capable of producing 5-20% of the total original oil in place (OOIP). Under a waterflood, the potential recovery factor can increase by an additional 15-30%. With primary and waterflood recovery, an oil pool can see ultimate recovery factors ranging from 20% to 50% of the total OOIP, leaving over half to three-quarters of the total oil untapped. With CO2 EOR, the injection of CO2 can improve recovery factors by an incremental 5-30%, meaning up to 25-80% of total oil in place could be recovered.
Why it works
Typical CO2 flooding works through the process of miscibility. At the proper reservoir temperature and pressure, CO2 acts as a solvent; and when mixed with oil, results in reduced interfacial tension, a net oil volume increase (i.e. swelling), and reduction in the oil viscosity. This all leads to improved mobility of the oil from the reservoir.
Because of the reservoir heterogeneity and to prevent CO2 fingering (or CO2 breakthrough), water is often injected intermittently with CO2 injection, called a water alternating gas (WAG) scheme. The WAG technique forms sequential banks of fluids (water, CO2, and oil) from the injection well to the producing well. The purpose of the WAG technique is to increase ultimate oil recovery.
Source: Ansarizadeh et al, 2015: Carbon Dioxide – Challenges and Opportunities, BMO Capital Markets.
How it works
For every CO2 injection well, there are often between two and four producing wells. CO2 is compressed or delivered compressed as a liquid via a pipeline to the injection well. The CO2 is injected down to the reservoir and fills the pore space of the rock. When encountering oil, the CO2 becomes miscible and eventually forms an oil bank that is pushed or “swept” through to the producing wells. As mentioned above, the WAG process can also be applied, whereby water is injected following CO2 injection resulting in higher ultimate oil recovery.