In-Situ Combustion in Naturally Fractured Heavy Oil Reservoirs

In fractured reservoirs primary production of heavy oil is mainly from fractures (secondary porosity). Matrix oil can be produced only at a very low rate because of low oil mobility. In this paper, in-situ combustion is considered as an EOR method. Experiments are described which show that the burning process is governed by diffusion of oxygen from the fractures into the matrix. The main oil-production mechanisms were found to be thermal expansion and evaporation with subsequent condensation of the oil from the matrix. A semi-two-dimensional (2D) numerical simulator has been constructed for modeling the process. It incorporates the main physical mechanisms as found in the experiments; heat losses to cap or base rock and gravitational effects are not included. The model predicts that in-situ combustion in fractured reservoirs is feasible and will have a high recovery efficiency in the swept zone. Oxygen breakthrough was observed when the air-injection rate exceeded a critical value predominantly determined by the fracture spacing. This phenomenon and also the shape and width of the combustion zone can be explained by a simple analytical model, which is in fair agreement with the simulator results. The effect of heat losses on the velocity of the combustion front is estimated and a lower bound for the injection rate can be obtained in this way. Combining this lower bound with the upper bound for oxygen breakthrough leads to a maximum value for fracture spacing. This distance will be on the order of 1 m [3.28 ft]. The conclusion is that in-situ combustion appears to be a feasible process in a naturally fractured reservoir with a high recovery in the swept zone. However, for predictions for a particular reservoir, vertical sweep efficiency should be taken into account. Introduction For the production of oil from heavy-oil reservoirs, thermal methods are applied widely. One of these is the in-situ combustion (ISC) process. In this process, air is injected into the reservoir and the oxygen in the air burns part of the oil, thereby generating heat. In some field trials of this method the combustion process could not be sustained if there were fractures in the reservoir. Since fractures are much more permeable than the surrounding reservoir rock, the injected air will flow almost exclusively through the fractures and will contact only oil present in these fractures or in their immediate vicinity. Evidently this is not sufficient to sustain the combustion process; either the reaction rate is too low because the contact area between air flow and fracture walls is very small, or the total amount of fuel available for combustion might be insufficient. If only the low reaction rate is responsible for the dying out of the combustion process, the question arises whether ISC is feasible in densely fractured reservoirs (such as occur, for example, in the Middle East in Iran and Oman). These reservoirs are believed to contain several vertical fractures per 1 m [3.28 ft] of formation. It is easily tested that in densely fractured reservoirs the contact area between air flow and fracture walls might be sufficiently large to sustain combustion, assuming that sufficient fuel is available. Hence, to predict whether ISC is feasible in such reservoirs, it is necessary to gain predict whether ISC is feasible in such reservoirs, it is necessary to gain insight into the mechanisms by which fuel and oxygen come into contact. Experimental Setup The experimental setup is shown schematically in Fig. 1. Basically it consists of a stack of oil-saturated core plugs in a vertical pressure vessel. This vessel, designed for a working pressure of 4 MPa [580 psi] has an 1D of 27 mm [1 in.] and an internal length of 432 mm [17 in.]. To reduce heat losses from the vessel it can be heated locally by eight independently controlled elements. They maintain the vessel wall at a temperature slightly lower than that of the adjacent core material. The heating elements are separated by 5-mm [0.2-in] wide openings. Circular gas coolers are mounted around these openings. These coolers blow cold nitrogen gas directly onto the vessel wall (if required) to reduce conductive heat transport in the longitudinal direction through the thick wall of the vessel. During an experiment, air (or nitrogen) is fed continuously into the top of the pressure vessel. The flow men passes through the small gap (1 mm [0.04 in]) between the stack of plugs and the vessel wall (this gap is, of course, the simulated fracture). Finally, air and produced fluids leave the vessel at the bottom. Liquid and gas are separated, and the gas is analyzed by means of a mass spectrometer. Typical measurements include oxygen consumption and CO2 production. SPEJ p. 67