OIL RECOVERY: RECOVERY FACTOR

Equation (1.2), for the STOIIP, can be converted into an equation for calculating the ultimate oil recovery simply by multiplying by the recovery factor (RF), which is a number between zero and unity representing the fraction of recoverable oil, thus

And, while it is easy to say, "simply by multiplying by the recovery factor", it is much less easy to determine what the recovery factor should be for any given reservoir and, indeed, it is the determination of this figure which is the most important single task of the reservoir engineer. For a start, one can clearly distinguish between two types of recovery factor.

There is one which is governed by current economic circumstances and, ever increasingly, by environmental and ecological considerations, while the second can be classed as a purely technical recovery factor depending on the physics of the reservoir-fluid system. Regrettably, the former, although possibly the more interesting, is not a subject for this book.

The two main categories of hydrocarbon recovery are called primary and supplementary. Primary recovery is the volume of hydrocarbons which can be produced by virtue of utilising the natural energy available in the reservoir and its adjacent aquifer. In contrast, supplementary recovery is the oil obtained by adding energy to the reservoir-fluid system. The most common type of supplementary recovery is water flooding in which water is injected into the reservoir and displaces oil towards the producing wells, thus increasing the natural energy of the system.

The mechanics of supplementary recovery will be described later, in Chapter 4, sec. 9 and in Chapter 10; for the moment only primary recovery will be considered. The entire mechanics of primary recovery relies on the expansion of fluids in the reservoir and can best be appreciated by considering the definition of isothermal compressibility.

The isothermal compressibility is commonly applied in the majority of reservoir engineering calculations because it is considered a reasonable approximation that as fluids are produced, and so remove heat from the reservoir by convection, the cap and base rock, which are assumed to act as heat sources of infinite extent, immediately replace this heat by conduction so that the reservoir temperature remains constant.

Therefore, compressibility, when referred to in this text, should always be interpreted as the isothermal compressibility. The negative sign convention is required in equ. (1.11) because compressibility is defined as a positive number, whereas the differential, ∂V/∂p, is negative, since fluids expand when their confining pressure is decreased. When using the compressibility definition in isolation, to describe reservoir depletion, it is more illustrative to express it in the form

where dV is an expansion and ∆p a pressure drop, both of which are positive. This is the very basic equation underlying all forms of primary recovery mechanism. In the reservoir, if ∆p is taken as the pressure drop from initial to some lower pressure, pi − p, then dV will be the corresponding fluid expansion, which manifests itself as production.

The skill in engineering a high primary recovery factor, utilising the natural reservoir energy, is to ensure that the dV, which is the production, is the most commercially valuable fluid in the reservoir, namely, the oil. The way in which this can be done is shown schematically in fig. 1.5

The diagram illustrates the fairly obvious fact that to produce an oil reservoir, wells should be drilled into the oil zone. If the reservoir is in contact with a gascap and aquifer, the oil production due to a uniform pressure drop, ∆p, in the entire system, will have components due to the separate expansion of the oil gas and water, thus

it is evident that the contribution to dVTOT supplied by the oil and water expansion will only be significant if both Vo and Vw, the initial volumes of oil and water, are large. In contrast, because of its very high compressibility, even a relatively small volume of gascap gas will contribute significantly to the oil production.