Abstract

There are abundant heavy oil resources in Bohai Offshore Oilfield. In past, only limited heavy oil reservoirs are recovered by cold production such as ESP, and uneconomic oil production(less than 30 m3/day in general) is obtained. Although conventional steam injection such as cycle steam stimulation is an effective onshore enhancing heavy oil recovery method, it is challenging to setup conventional steam generator on production platform for offshore heavy oil production. In consideration of the pothential of Thermal production for offshore heavy oil recovery, a portable and compact steam and flue gas generator is developed to employ cyclic steam-gas-chemical stimulation, and the whole injection system consists of gas generator, water treatment unit, fuel and chemical tanks, insulated surface pipe and tubing. Steam and flue gas are coinjected at the pressure up to 20MPa, content of flue gas is 13.1wt%-28.3wt%. The stimulation or EOR principles of cyclic steam-gas-chemical co-stimulation based on the synergetic effect of fluid injected include reduction in oil viscosity by heating and gas solution, enlargement of heat sweeping efficiency by coinjected gas, decrease in heat loss, expansion of heated porous rock and saturated fluid, improvement in fluid injectivity and sweep efficiency, and reservoir protection or injectivity enhancement and other favorable reaction by chemical additives such as foaming agent. Based on experiment and numerical simulation, the rational temperature of steam and gas coinjected is 300°C, and the volume ratio of gas to steam is about 0.3-1.0 under barometric pressure depending on reservoir condition.

In past two years, cyclic steam-gas-chemical co-stimulation are applied to five offshore heavy oil wells completed thermally or non-thermally with screen pipe, periodical steam injection is about 3000-5000 tons, and flue gas generated is injected together, chemical additives such as corrosion retarder injected alternatively, and a certain amount of nitrogen could be injected through annulus between tubing and casing to insulate tubing and vary injected fluid composition. As a result, up to 3 times of oil production rate in average compared to that of cold production is observed, and the peak oil production is more than 100 m3/day, and cyclic oil production increase is more than 5000 m3. In this paper, EOR principles, injection optimization, engineering design and field testing of cyclic steam-gas-chemical co-stimulation are introduced in detail.

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