The Effect of Initial CO2 Content for Recovery Optimization and Sequestration

Abstract Carbon dioxide (CO2) emissions are increasing due to the hydrocarbons produced increasingly around the world. This is contrary to the Kyoto protocol which states that the signators have agreed to reduce such emissions. This work looks at one such scenario where CO2 is injected prior to steam flooding in a heavy oil reservoir. Field applications have established that CO2 can be an efficient oil-displacing agent for both heavy and light oils as long as they are mobile at reservoir conditions. Using steam with horizontal injection-production well pairs is another recovery method for non-mobile heavy oils. This process can result in relatively higher oil production rates as compared to other steam-based processes not using horizontal wells. However, the higher production rates are associated with excessive energy requirements, CO2 generation, and postproduction water treatment. For understanding and analyzing the effect of CO2 gas storage prior to steam flooding and optimizing recovery by varying the spacing between the horizontal injectors, a series of simulations were carried out. Unlike traditional sequestration of CO2, which is usually limited to exhausted reservoirs, the CO2 is injected, at high pressure, from horizontal well pairs into the heavy oil reservoir. This is done prior to the steam flood to enable viscosity reduction by swelling and to store the anthropogenic CO2, which is the main cause of global warming. To enable diffusion of the injected gas around the well bore, the system is shut in for some time. Then, steam flooding is conducted using the horizontal steam well pairs (SWP). For the heavy oil field model used in this study, which had no gas, the optimum injection rate with respect to distance, recovery and cumulative water-oil ratio (CWOR) was found to be 100 sm3/day/well. An analysis of the results obtained showed that the performance of the wells at the optimum steam injection rate, both with and without CO2 injection prior to the steam flood, were similar. In particular, over the 20-year life of the project, the recoveries were essentially the same. However, during the production period, when CO2, was injected, the CWOR decreased, resulting in an environmentally and economically attractive process. Moreover, 100% of the CO2was stored, which resulted in a reduction in the amount of water that needed to be treated. The sequestration results showed that, after producing to the economic limit, 40 to 50 times the reservoir pore volume (PV) can be employed for sequestration purposes, depending on the amount of residual fluids. Given the volume of oil contained in heavy oil reservoirs, this represents a huge sink for future CO2 sequestration, and an opportunity not only to tap into a huge energy source for the future, but also an opportunity to reduce greenhouse gas emissions. Introduction Heavy oil and bitumen represent a huge amount of natural resources; the world's total estimated original oil in place (OIP) in these forms is approximately 6 trillion barrels, which corresponds to six times the conventional oil reserves.

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