THE MECHANICS OF SUPPLEMENTARY RECOVERY

Supplementary recovery results from increasing the natural energy of the reservoir, usually by displacing the hydrocarbons towards the producing wells with some injected fluid. By far the most common fluid injected is water because of its availability, low cost and high specific gravity which facilitates injection. The basic mechanics of oil displacement by water can be understood by considering the mobilities of the separate fluids. The mobility of any fluid is defined as

which, considering Darcy's law, can be seen to be directly proportional to the velocity of flow. Also included in this expression is the term kr/µ, which is referred to as the relative mobility. The manner in which water displaces oil is illustrated in fig. 4.10 for both an ideal and non-ideal linear horizontal waterflood.

In the ideal case there is a sharp interface between the oil and water. Ahead of this, oil is flowing in the presence of connate water (relative mobility = kro (Sw=Swc )/µo = ro k′ /µo), while behind the interface water alone is flowing in the presence of residual oil (relative mobility = krw(Sw=1 - Sor)/µw = ro k′ /µw). This favourable type of displacement will only occur if the ratio

where M is known as the end point mobility ratio and, since both and ro k′ are rw k′ the end point relative permeabilities, is a constant. If M ≤ 1 it means that, under an imposed pressure differential, the oil is capable of travelling with a velocity equal to, or greater than, that of the water. Since it is the water which is pushing the oil, there is therefore, no tendency for the oil to be by-passed which results in the sharp interface between the fluids.