Effect of Gravity On Movement of Water-oil Interface For Bottom Water Driving Upwards to a Horizontal Well

Abstract

Abstract In a previous paper from our laboratory, an equation was developed to predict the movement of the water-oil interface produced by bottom water driving upwards to a horizontal well; the oil and water were assumed to have the same viscosity and density. If the oil has a lower density than the water, as is usually the case, the interface will be flatter due to the effect of gravity. This will improve the performance. In the present study, the effect of gravity on the movement of the interface and on the oil recovery has been studied physically and theoretically. The new theory allows the effect of gravity to be included. Two sets of experiments were carried out in a Hele-Shaw cell to investigate the effect of gravity on the movement of the water-oil interface. The first set involved upward displacement of oil by bottom water to a horizontal well. It was found that the higher the flow rate, the lower the oil recovery at breakthrough even when the two fluids have the same viscosity. This effect is caused by gravity. In the other experiments, a water crest established previously in the Hele-Shaw cell was allowed to fall driven by gravity alone, with both the injection and production wells shut in. The interface fell at the centre and rose at the two ends and thus became progressively flatter. The interface was photographed periodically. Analytical and numerical studies were carried out to predict the movement of the fluids. The predictions from both techniques are in agreement with the experimental results. These equations can be used to predict the relaxation of water crests in the field and a numerical example is given. Introduction Gravitational segregation due to the density difference between displacing and displaced fluids is one of the important factors affecting oil drainage from the reservoir. The overriding of the injected gas or steam and the down-flow of injected water, for example, usually reduce the vertical sweep efficiency. In some cases, the effect of the net gravitational force to assist the oil drainage has been utilized commercially for both conventional and heavy oil reservoirs. The process of steam-assisted gravity drainage (SAGD)(1,2,3,4) by using a horizontal production well at the bottom of the reservoir, with either a parallel horizontal injection well or several vertical injection wells located above in the reservoir, has been applied successfully to the tar sands in AOSTRA's UTF project in Fort McMurry, Alberta, and to the Tangleflags North heavy oil reservoir in Saskatchewan. The Vapex process(5), which uses a vapourized hydrocarbon to extract the bitumen, is an analogue to SAGD but a non-thermal process. Downward displacement of oil from the depleted reservoirs by gas cap or injected gas(6) is another example to take the advantages of the density difference between gas and oil phases. Gravitational forces can act as either a de-stabilizing force or a stabilizing force for the interface(7), depending on the sequences of two fluid layers.