Page 70 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 9

and solving this simultaneously with equ. (1.7), for the condition that p o = p w, gives the
oil-water contact at a depth of 5640 ft. This is marked on fig. 1.4 as the deepest
possible oil-water contact (DPOWC) and corresponds to the maximum possible oil
column. Therefore, in spite of the fact that the well has been carefully tested, there
remains high degree of uncertainty as to the extent of any oil column. It could indeed
be zero (DPGWC − 5281 ft) or, in the most optimistic case, could extend for 490 ft
(DPOWC − 5640 ft), or, alternatively, assume any value in between these limits. Also
shown in fig. 1.4 is the actual oil column from fig. 1.3.

Therefore, the question is always posed, on penetrating a reservoir containing only
gas; is there a significant oil column, or oil rim, down-dip which could be developed?
The only sure way to find out is to drill another well further down-dip on the structure or,
if mechanically feasible, plug back and deviate from the original hole. When planning
the drilling of an exploration well it is therefore, not always expedient to aim the well at
the highest point on the structure. Doing so will tend to maximise the chance of finding
hydrocarbons but will oppose one of the primary aims in drilling exploration wells,
which is to gain as much information about the reservoirs and their contents as
possible.

Having determined the fluid contacts in the reservoir, using the methods described in
this section, the engineer is then in a position to calculate the net bulk volume V
required to calculate the hydrocarbons in place. In fig. 1.1 (a), for instance, this can be
7,8
done by planimetering the contours above the OWC .
Finally, with regard to the application of equ. (1.2), the correct figure for the STOIIP will
only be obtained if all the parameters in the equation are truly representative of their
average values throughout the reservoir. Since it is impossible to obtain such figures it
is more common to represent each parameter in the STOIIP equation by a probability
distribution rather than a determinate value. For instance, there may be several
different geological interpretations of the structure giving a spread in values of the net
bulk volume V, which could be expressed as a probability distribution of the value of
this parameter.

The STOIIP equation is then evaluated using some statistical calculation procedure,
commensurate with the quality of the input data, and the results expressed in terms of
a probability distribution of the STOIIP. The advantage of this method is that while a
mean value of the STOIIP can be extracted from the final distribution, the results can
also be formulated in terms of the uncertainty attached to this figure, expressed, for
instance, as a standard deviation about the mean 9,10 . If the uncertainty is very large it
may be necessary to drill an additional well, or wells, to narrow the range before
proceeding to develop the field.

1.4 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
Ultimate Recovery (UR) (V (1 S )/B ) RF (1.10)
×
−
φ
=
wc
oi