Page 93 - Fundamentals of Reservoir Engineering

SOME BASIC CONCEPTS IN RESERVOIR ENGINEERING 32

as shown in fig. 1.12, and the proper value of G can be determined by linear
extrapolation to the ordinate. Selecting the correct aquifer model (aquifer fitting) is a
trial and error business which continues until a straight line is obtained.

One other interesting feature shown in fig. 1.11 is that the maximum possible gas
recovery, shown by the circled points, depends on the degree of pressure maintenance
afforded by the aquifer, being smaller for the more responsive aquifers. The reason for
this has already been mentioned in sec. 1.2; that in the immiscible displacement of one
fluid by another not all of the displaced fluid can be removed from each pore space.
Thus as the water advances through the reservoir a residual gas saturation is trapped
behind the front. This gas saturation, S gr, is rather high being of the order of 30−50% of
the pore volume 7,17 , and is largely independent of the pressure at which the gas is
trapped. This being the case, then applying the equation of state, equ. (1.15), to the
gas trapped per cu.ft of pore volume behind the flood front, gives

p
S = nRT
Z gr

and, since S gr is independent of pressure, then for isothermal depletion

p
n ∝
Z

which indicates that a greater quantity of gas is trapped at high pressure than at low.

The ultimate gas recovery depends both on the nature of the aquifer and the
abandonment pressure. For the value of (p/Z) ab shown in fig. 1.11, the aquifer giving
the pressure response corresponding to line B is the most favourable. While choice of
the abandonment pressure is under the control of the engineer, the choice of the
aquifer, unfortunately, is not. It is, therefore, extremely important to accurately measure
both pressures and gas production to enable a reliable aquifer model to be built which,
in turn, can be used for performance predictions.

One of the more adventurous aspects of gas reservoir engineering is that gas sales
contracts, specifying the market rate and pipeline pressure, are usually agreed
between operator and purchaser very early in the life of the field, when the amount of
history data is minimal. The operator is then forced to make important decisions on
how long he will be able to meet the market demand, based on the rather scant data.
Sensitivity studies are usually conducted at this stage, using the simple material
balance equations presented in this chapter, and varying the principal parameters, i.e.

− the GIIP

− the aquifer model, based on the possible geometrical configurations of the aquifer
− abandonment pressure; whether to apply surface compression or not

− the number of producing wells and their mechanical design.