Page 219 - Hydrocarbon Exploration and Production Second Edition

206 Reservoir Drive Mechanisms

continues until the abandonment rate is reached. Abandonment, better referred to as
decommissioning, occurs when the cost of production is greater than the revenues
from the production.
In the solution gas drive case, once production starts the reservoir pressure drops
very quickly, especially above the bubble point, since the compressibility of the
system is low. Consequently, the producing wells rapidly lose the potential to flow
to surface, and not only is the plateau period short, but the decline is rapid.
The producing GOR starts at the initial solution GOR (R si ), decreases until the
critical gas saturation is reached, and then increases rapidly as the liberated gas is
produced into the wells, either directly as it is liberated, or pulled into the producing
wells from the secondary gas cap. The secondary gas cap expands with time, as
more gas is liberated, and therefore moves closer to the producing wells, increasing
the likelihood of gas being pulled in from the secondary gas cap. The producing
GOR may decline in later years as the remaining volume of gas in the reservoir
diminishes.
Commonly the water cut remains small in solution gas drive reservoirs, assuming
that there is little pressure support provided by the underlying aquifer. Water cut is
also referred to as BS&W (base sediment and water), and is defined as

Water production ðstbÞ
Water cut ðor BS&WÞ¼ 100ð%Þ
Oil plus water production ðstbÞ
The typical RF from a reservoir developed by solution gas drive is in the range
5–30%, depending largely on the absolute reservoir pressure, the solution GOR
of the crude, the abandonment conditions and the reservoir dip. The upper end of
this range may be achieved by a high dip reservoir (allowing segregation of the
secondary gas cap and the oil), with a high GOR, light crude and a high initial
reservoir pressure. Abandonment conditions are caused by high producing GORs
and lack of reservoir pressure to sustain production.
This rather low RF may be boosted by implementing secondary recovery
techniques, particularly water injection, or gas injection, with the aim of
maintaining reservoir pressure and prolonging both plateau and decline periods.
The decision to implement these techniques (only one of which would be selected)
is both technical and economic. Technical considerations would be the external
supply of gas, and the feasibility of injecting the fluids into the reservoir. Figure 9.4
indicates how these techniques may be applied. Note again, that it is unlikely that
both gas and water injection would be simultaneously adopted – one or the other
secondary recovery technique would normally be chosen.

9.2.2. Gas cap drive
The initial condition required for gas cap drive is an initial gas cap. The high
compressibility of the gas provides drive energy for production, and the larger the
gas cap, the more energy is available. The well positioning follows the same
reasoning as for solution gas drive; the objective being to locate the producing wells