In consideration of the challenges of SAGD process such as the high watercut, the elevated temperature of produced fluid, and the heat loss to the overburden layer, combined CO2 sequestration and electrical heating assisted gravity drainage (CSEAGD in short) is taken into consideration to enhance heavy oil production, especially for water sensitive reservoirs as well, the condition of water short such as offshore and desert. In a typical CSEAGD process, two parallel horizontal wells are drilled as injection well and production well respectively, both wells serve as electrodes or two electrical heaters located in the wells separately, CO2 is injected in the upper injection well, and heated oil and gas are drained to the lower production well by gravity enforced by CO2. At the same time, part of the injected CO2 is sequestrated or stored underground.
Both experiment and numerical simulation were carried out using conventional heavy oil and extra-heavy oil (the viscosities at 50°C are 1640 mPa•s and 12500 mPa•s, respectively) sampled from Bohai offshore oilfield to evaluate the feasibility of CSEAGD process. The study include the effect of electrical heating capacity, CO2 injectivity, distance between injection well and production well. The results show that a similar and even higher oil production rate could be achieved by CSEAGD compared to SAGD or SAGP processes for the heavy oils used (the average oil production rate is 2 to 6 times higher than cold production). The temperature distribution of CSEAGD is an asymmetric dumbbell, different from the inverse triangle in SAGD and the ellipse in SAGP, which is derived from the difference of heating chamber development between injection well and production well. A larger heating chamber could be developed based on the synergetic effect of electrical heating and CO2 injection of the new process. It is found that CSEAGD is more feasible for conventional heavy oil (viscosity lower than 10000 mPa·s under reservoir condition) production, accordingly 3 – 5 PV CO2 could be sequestrated in place depending on the reservoir condition, and input electrical power is larger than 1.0 MW, rational CO2 injection intensity is 50 – 100 m3/d·m, and a larger well distance is recommended.
In the CSEAGD process, CO2 sequestration and electrical heating are combined to enhance heavy oil production due to its environment-friendly feature, water free injection, and competent oil recovery compared to conventional SAGD process. Moreover, flue gas, nitrogen, or natural gas could be injected to replace CO2 or with CO2 together according to the actual condition. It has already been selected as the potentional and prospective offshore heavy oil recovery method in Bohai oilfield.