Abstract This paper describes numerically simulated reservoir performance during the injection of hot water and flue gas at North Sea reservoir conditions. Studies of hydrocarbon/water phase behaviour at elevated temperatures and reservoir simulations showed that a hot water flood in a light-oil reservoir could increase the recovery by 3-9 % of the original oil in place. A theoretical study of flue gas injection concluded that minimum miscibility pressure (MMP) for this gas was equal to MMP for an oil/nitrogen system. A nitrogen displacement was estimated to be immiscible at initial reservoir pressure and temperature, but miscibility would be developed at a slightly higher temperature.
The results predict that the combination of hot water and flue gas injection has a substantial potential for increasing the recovery from light-oil reservoirs.
Introduction Hot water and flue gas injection have both been topics of EOR research for the past decades. Generally, high viscosity oil is the target of hot water injection. As typical North Sea oils are rather light, this EOR method has not been given much attention by North Sea operators. Flue gas and CO2 injection are EOR processes where the solvent is injected as either a single slug or in several Water Alternating Gas (WAG) cycles and then followed by water injection to improve the microscopic displacement efficiency. There are no natural sources of CO2 in the North Sea area, and flue gas is generally not miscible at typical North Sea reservoir conditions. Consequently, flue gas injection is usually eliminated in the initial screening phase of an EOR evaluation.
As a consequence of the difficulties in connection with selling associated gas and high gas transportation costs, produced gas will in many cases represent a problem and an expense rather than an asset in the development of North Sea oil fields. Many operators have therefore started to investigate possible ways of using the gas in the field. The usual option would be to implement a hydrocarbon gas injection scheme, either as continuous gas injection or as a WAG process. An alternative strategy would be to bum the gas offshore to generate electric power. This process generates flue gas and exhaust heat, which are normally emitted to the atmosphere and thus lost.
In this paper an EOR method is discussed where the flue gas is reinjected and exhaust heat is used to prewarm the injection water, thereby conserving energy and reducing emissions of pollutant gases. This injection strategy therefore combines the effects of a thermal process and solvent flooding. If such a combined strategy should be of interest, each separate process should yield a positive contribution to the recovery mechanism. Each EOR process is therefore first studied separately, after which the combined effects are discussed.
SIMULATION OF HOT WATER INJECTION Simulations of hot water injection under North Sea reservoir conditions have not been reported previously. This paper describes several tasks necessary for generating input data to thermal simulations. The behaviour of a light-oil system at high temperatures was investigated. The initial reservoir pressure and temperature were 38.3 MPa and 90C, respectively. The high pressure and the light-oil composition made the investigation complicated, because phase behaviour models for these conditions are not fully established. Initial oil viscosity was 0.810 Pas, and oil density at standard conditions was 835 kg/m. Initial reservoir temperature 90 C (T) and 250 C (T) were chosen as reference temperatures, and fluid properties have been evaluated at these temperatures.
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