3.7 Reservoir flow relations
Within the reservoir, the flow of fluids is the governing factor for the extraction process. In order to be
produced, the hydrocarbon fluids must reach the production wells and consequently, the rock properties
affecting fluid mobility will have a major influence on the amount that can be extracted and also on how
fast it can be extracted.
Viscosity, gravity drainage and capillary effects are the main forces governing the flow (Satter et al.,
2008). Viscous forces dominate the behaviour of fluids, both produced and injected, in a reservoir. Under
viscous conditions, flow rates are laminar and proportional to the pressure gradient that exists in the res-
ervoir (Satter et al., 2008). However, there are examples of tilted reservoirs and dipping formations,
where gravity drainage is the prime driving force.
Capillary forces are a result of surface tension between the fluid phase and the pore walls, something
that can form sealing conditions if the capillary entry pressure is high. Gravity and capillary forces act in
the opposite directions and can be used to determine the initial distribution and saturation of oil, gas and
water in any hydrocarbon-bearing porous structure (Satter et al., 2008). The movement of fluids in a res-
ervoir depends on the following factors:
Depletion (leading to a decrease in reservoir pressure)
Compressibility of the rock/fluid system
Dissolution of the gas phase into the liquid
Capillary rise through microscopic pores
Additional energy provided from aquifer or gas cap
External fluid injection
Thermal, miscible or similar of manipulation of fluid properties
In most reservoirs, more than one factor is responsible for the flow of fluids and closer discussion on
this can be found in Satter et al. (2008). Some parameters can be affected by man-made measures, while
others cannot. The slope of the hydrocarbon-bearing formation is an example of a flow parameter that is
fixed, while external fluid injection is dynamic and dependent on installed technology and production
Compressibility determines how much the reservoir can be compacted, which is similar to squeezing a
sponge in order to get more fluid out. It is a function of a number of parameters, including the type of
minerals that make up the rock mass, the degree of sorting, the degree of mineral decomposition or altera-
tion, cementation and especially porosity (Nagel, 2001). Highly compactable reservoirs usually have res-
ervoir porosity greater than 30% (Nagel, 2001).
Compaction has been known to cause a significant increase in the available drive energy for hydrocar-
bon recovery. In the Norwegian giant field Valhall, compaction has been claimed to make up 50% of the
total drive energy (Cook et al., 1997). The Norwegian Ekofisk field is estimated to recover an additional
243 to 280 million barrels as a result of increased reservoir compaction (Sulak et al. 1991; Sylte et al.
1999). In the Bolivar Coastal oil fields in Venezuela, compaction drive has been estimated to constitute as
much as 70% of the total drive energy (Escojido, 1981) and if steam flooding is used, compaction drive
contribution can reach 80% of the total energy for the same region (Finol and Sancevic, 1995).
Compaction can also lead to subsidence, which is the sinking of the ground level above the reservoir.
Wilimington and Ekofisk oil fields are both well known examples both due to the magnitude of the subsi-
dence as well as the cost of remediation (Nagel, 2001). Lake Maracaibo and the nearby Bolivar Coastal
Region are other examples of how reservoir depletion has caused severe subsidence and flooding and
similar effects are true for the giant Groningen gas field, where a subsidence of only a few decimetres
poses a significant threat since large portions of the Netherlands are below sea level and protected by
dikes (Nagel, 2001).
Water-injection can solve subsidence problems and has also been shown to be a cost effective way to
control compaction (Piece, 1970). However, injecting water might take the edge off subsidence issues but
this also leads to the loss of compaction drive, a significant energy for driving hydrocarbon flows in the
reservoir. All together this shows how various flow factors can balance each other. An increase in one of
the driving forces can lead to a decrease in another and vice versa. All together, reservoir flows are a
complex problem, with many interdependent variables.