A Direct-Fired Downhole Steam Generator-From Design to Field Test

Abstract

Introduction The concept of using a direct-fired downhole steam generator (DFDSG) in thermal oil recovery projects was investigated because it offers several advantages over conventional steaming methods. Five are described in this paper. paper. Reduction in Air Pollution In a study that evaluated alternatives in downhole steam generation, funded by the U.S. DOE, interest centered on the premise that a DFDSG would either reduce or postpone air pollution problems. With the injection of exhaust gases from the burner and the generated steam into the well, the need for an expensive, inefficient stack scrubber to remove SO, and a sophisticated combustion technique to control NO is eliminated. Preliminary results of the injection of generator exhaust gases have shown that the formation and produced fluids retain most of the air pollutants. produced fluids retain most of the air pollutants. In steamdrive projects, some of the combustion gases will certainly break through to the producing well, but current air quality regulations require wells to be in a casing recovery system. Because this system already exists and because the combustion gas temperatures will be nearly ambient, only minor additional costs are expected during production operations. Reduction in Heat Losses The majority of the heat (energy) losses associated with conventional steam generators can be eliminated through the application of a DFDSG. If the burner of a DFDSG is placed near the injection interval, the generator stack, flowline, and wellbore heat losses all can be eliminated. The inefficiency of the conventional surface generator results from the large amount of heat released through the stack of the convection section. Nearly 19% of all the fuel consumed in the generator is lost in the stack heat, and 3 to 20% of the fuel is lost in the flowline. It is not uncommon to experience an average flowline loss of 13% in both small and large generator installations. Thus 32% of the energy (fuel) consumed could be lost by the time the steam is delivered to the wellhead, costing an operator with a single 25-MMBtu/hr [7.3-MW] conventional generator $19,000/month or an operator with a bank of twelve 50-MMBtu/hr [14.7-MW] conventional generators $484,000/month. Wellbore heat losses also can vary widely. For shallow, properly completed wells, a 3% heat loss can be experienced, but for deeper (2,000- ft [610-m]) steaming projects, wellbore heat losses approaching 20% are common. Deeper Steaming Potential Currently, the maximum practical steaming depth is approximately 2,500 ft [762 m]. Wellbore heat losses below that depth are generally so large that most of the steam has been reduced to hot water. To deliver enough heat to the producing interval so that heavy-oil viscosities are reduced, excessive quantities of this expensive "hot water" must be injected. A DFDSG operating near the producing interval could extend the economical steaming depth, possibly to 6,000 ft [1829 m] or more. This potential increase in depth can be extremely beneficial in reducing U.S. dependence on foreign oil supplies. The Natl. Petroleum Council reported that there is between 1.6 and 2.1 billion bbl [254 and 334 Mm3] of heavy oil recoverable below 2,500 ft [762 m] in California, Texas, and Louisiana alone that cannot be recovered economically with conventional steaming methods. Offshore Potential During the last 5 years, a large number of offshore heavy-oil reservoirs have been identified. With the total reserves continually growing, many operators are making serious evaluations of steaming operations from offshore platforms. The large size of conventional steam generators, combined with a lack of fresh water and the complicated heat-loss problems, reduces the chances of obtaining an economical thermal project offshore with conventional steam generators. The smaller size of the DFDSG, the ability to use seawater, and the minimal downhole heat losses could open up many offshore heavy-oil reservoirs to economical thermal production. Reservoir Repressurization In either cyclic- or continuous-injection steam projects, simply reducing the oil viscosity by increasing the temperature is not enough. To get the oil to flow to the wellbore so that it can be produced, there must be a sufficient pressure gradient between the reservoir and the wellbore. As the steaming project continues, reduction of the original gas and liquid saturations can cause a severe decline in reservoir pressure, which in turn can reduce productivity. JPT p. 1903