Page 63 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 2

engineering calculations is to reduce the reservoir volume which can be occupied by
hydrocarbons.

The oil volume calculated using equ. (1.1) is expressed as a reservoir volume. Since all
oils, at the high prevailing pressures and temperatures in reservoirs, contain different
amounts of dissolved gas per unit volume, it is more meaningful to express oil volumes
at stock tank (surface) conditions, at which the oil and gas will have separated. Thus
the stock tank oil initially in place is

STOIIP = n = vφ (1 S− wc ) /B oi (1.2)

where B oi is the oil formation volume factor, under initial conditions, and has the units
reservoir volume/stock tank volume, usually, reservoir barrels/stock tank barrel (rb/stb).
Thus a volume of B oi rb of oil will produce one stb of oil at the surface together with the
volume of gas which was originally dissolved in the oil in the reservoir. The
determination of the oil formation volume factor and its general application in reservoir
engineering will be described in detail in Chapter 2.

In equ. (1.2), the parameters φ and S wc are normally determined by petrophysical
1
analysis and the manner of their evaluation will not be described in this text . The net
bulk volume, V, is obtained from geological and fluid pressure analysis.

The geologist provides contour maps of the top and base of the reservoir, as shown in
fig. 1.1. Such maps have contour lines drawn for every 50 feet, or so, of elevation

Well
Y X Y

5000 OIL
5050 WATER OWC
5150

X OWC 5250
(a) (b)

Fig. 1.1 (a) Structural contour map of the top of the reservoir, and (b) cross section
through the reservoir, along the line X− −− −Y

and the problem is to determine the level at which the oil water contact (OWC) is to be
located. Measurement of the enclosed reservoir rock volume above this level will then
give the net bulk volume V. For the situation depicted in fig. 1.1 (b) it would not be
possible to determine this contact by inspection of logs run in the well since only the oil
zone has been penetrated. Such a technique could be applied, however, if the OWC
were somewhat higher in the reservoir.