Page 94 - Fundamentals of Reservoir Engineering
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SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 33
(the latter point has not been discussed so far, since it requires the development of well
inflow equations, which will be described in Chapter 8). The results of such a study can
give initial guidance on the best way to develop the gas reserve.
EXERCISE 1.2 GAS MATERIAL BALANCE
The following data are available for a newly discovered gas reservoir:
GWC = 9700 ft
Centroid depth = 9537 ft
10
Net bulk volume (V) = 1.776 × 10 cu.ft
φ = 0.19
S wc = 0 20
γ g = 0.85
Although a gas sample was collected during a brief production test the reservoir
pressure was not recorded because of tool failure. It is known, however, that the water
pressure regime in the locality is
p w = 0.441D + 31 psia
and that the temperature gradient is 1.258°F/100 ft, with ambient surface temperature
80° F.
1) Calculate the volume of the GIIP.
2) It is intended to enter a gas sales contract in which the following points have
been stipulated by the purchaser.
a) during the first two years, a production rate build-up from zero
−100 MMscf/d (million) must be achieved while developing the field
b) the plateau rate must be continued for 15 years at a sales point delivery
pressure which corresponds to a minimum reservoir pressure of 1200 psia.
Can this latter requirement be fulfilled? (Assume that the aquifer is small so
that the depletion material balance equation can be used).
3) Once the market requirement can no longer be satisfied the field rate will decline
exponentially by 20% per annum until it is reduced to 20 MMscf/d. (This gas will
either be used as fuel in the company's operations or compressed to supply part
of any current market requirement).
What will be the total recovery factor for the reservoir and what is the length of the
entire project life?
EXERCISE 1.2 SOLUTION
a) In order to determine the GIIP it is first necessary to calculate the initial gas
pressure at the centroid depth of the reservoir. That is, the depth at which there is
