Page 273 - Introduction to Petroleum Engineering

260 PRODUCTION PERFORMANCE
Given these definitions, the general material balance equation has the form

N D D D D D N B N RB
o go w gw r p o p so g
(13.16)
GB GB GB W W W WB
ps g pc gc i g e i p w
Changes in fluid volume in the reservoir are represented by the terms on the left‐hand
side of Equation 13.16. Fluid production and injection are represented by the terms
on the right‐hand side. The physical significance of each term is summarized in
Table 13.3.
The relative importance of different drive mechanisms can be estimated by
rearranging the general material balance equation. Indices for different drive
mechanisms are shown in Table 13.4 relative to the hydrocarbon production D
HC
defined by
D N B G B G NR B (13.17)
HC p o pc gc ps p so g
The sum of the drive indices shown in Table 13.4 equals one, thus

I sg I gc I w I i I e 1 (13.18)
Equation 13.18 can be derived by rearranging Equation 13.14. A comparison of the
magnitudes of the drive indices indicates which drive is dominating the performance
of the reservoir.
We can illustrate the value of the drive indices by considering an undersaturated
oil reservoir that is attached to an aquifer. The reservoir is in the primary produc-
tion stage, that is, the reservoir is being produced and no fluids are being injected.
What drive mechanisms do we need to consider? The undersaturated oil reservoir
does not have a gas cap, so m = 0. Furthermore, no fluids are being injected.

TABLE 13.3 Physical Significance of Material Balance Terms

Term Physical Significance
ND o Change in volume of initial oil and associated gas
ND go Change in volume of free gas
N(D + D ) Change in volume of initial connate water
gw
w
ND r Change in formation pore volume
N B o Cumulative oil production
p
N R B g Cumulative gas produced in solution with oil
so
p
G B g Cumulative solution gas produced as evolved gas
ps
G B gc Cumulative gas cap gas production
pc
G B ′ Cumulative gas injection
i
g
W B w Cumulative water influx
e
WB w Cumulative water injection
i
W B Cumulative water production
p w