Page 72 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 11

p i − 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.
dV tot = oil production
= dV + dV + dV g
w
o

aquifer gascap
oil
V w dV W dV g V g

V o

Fig. 1.5 Primary oil recovery resulting from oil, water and gas expansion

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

dV TOT = Oil Production = dV o + dV w + dV g

in which the balance is expressed in fluid volumes at reservoir conditions. Applying
equ. (1.12), this may be expressed as

dV TOT = c o V o ∆p + c w V w ∆p + c g V g ∆p

Considering the following figures as typical for the compressibilities of the three
components at a pressure of 2000 psia:

−6
c o = 15 × 10 /psi

−6
c w = 3 × 10 /psi

1
−6
c g = 500 × 10 /psi ≈ ; refer sec.1.5
p

it is evident that the contribution to dV TOT supplied by the oil and water expansion will
only be significant if both V o and V w, 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.