Study and Pilot Test of Multiple Thermal-Fluid Stimulation in Offshore Nanpu Oilfield

Heavy-oil resources whose underground oil viscosity is greater than 350 mPa.s is abundant in the Bohai oilfield. Because of the lack of effective exploitation technology, production performance with cold-production methods was not satisfactory. Seeking an effective method of heavy-oil exploitation, the multiple thermal-fluid stimulation was proposed and studied, in which hot water or steam mixed with carbon dioxide (CO2) and nitrogen (N-2) would be injected into the formation for heating the oil and improving heavy-oil production. Considering the limitation of the offshore platform, a miniaturized multiple thermal-fluid generator was designed and developed. Integrated technologies such as seawater desalinization, heat insulation, and anticorrosion methods were also studied and developed. A pilot test of multiple thermal-fluid stimulation was conducted in Nanpu oilfield, starting in 2008. Until now, the pilot test has lasted for more than 10 years, and a total of 27 cycles of multiple thermal-fluid stimulation have been carried out. The oil-production rate and periodic-oil production amount of the thermal wells were both greatly improved. The cumulative oil produced by multiple thermal-fluid stimulation reached approximately 5.7 x 10(5) m(3), and the incremental oil production is approximately 2.6 x 10(5) m(3). The performance of oil production was satisfactory. As the first offshore thermal pilot area, the application experience showed that multiple thermal-fluid stimulation was effective for exploitation of heavy oilfields. With the increase of the stimulation cycles, a gas-channeling problem emerged and resulted in a decrease in the oil-production rate. New methods need to be studied and used for further enhancing oil recovery in Nanpu oilfield at the late stage of multiple thermal-fluid stimulation.