Abstract

China Offshore oilfield is rich in heavy oil, M oilfield in Bohai Bay is the most thick heavy oil field in offshore by far with the formation oil viscosity as high as 450 ~ 950 mPa.s.  Using conventional oil field development (depletion and water flooding), the heavy oil field exposed its limits, such as low productivity of single well, low production rate and low predicted recovery. As is well known, the development effect of thermal recovery is not only related with the thermal recovery methods (steam stimulation, steam flooding, etc.), but also related with the thermal medium (steam, hot water, flue gas etc.). A new thermal media called multi-thermal fluid, which contains steam, hot water and flue gas was researched for horizontal well stimulation, which is to inject N₂ and CO₂ at the same time when injecting steam. This multi-thermal fluid could employ various mechanisms of each fluid, including reducing oil viscosity by heating and dissolving gas, increasing pressure by injecting gas, expanding heating range, reducing heat loss and gas assisting gravity drive etc. With the advantages of single thermal injection, this method made up the disadvantages of single thermal fluid injection and can improve steam stimulation effect in some special formation conditions. In this article, the author first analyzed the characterization of multi-thermal fluid, and then researched its stimulation theory. Based on this theory, the single element of geological data, such as dip angle, permeability, rock compression coefficient, formation thickness, oil viscosity, and injection data, such as injection volume, gas-water-ratio and CO₂ concentration was analyzed using numerical simulation software. Based on this result, selecting data that make more contribution in incremental oil, production rate, and enhanced oil recovery (EOR) to make sensitivity study to summarize formation that multi-thermal fluid could be applied. Based on the theoretical research, a multi-thermal fluid stimulation pilot test was carried out at M oilfield applying compact thermal recovery equipment. The first cycle of the pilot test showed some optimistic results. Multi-thermal fluid stimulation becomes a new efficient development model for offshore heavy oil, opening up a domestic precedent of thermal development of offshore heavy oil (except shallow sea).

Key words: Heavy oil, Bohai Bay, flue gas, offshore.