Water Expansion Term in Gas Reservoirs

Water expansion term in gas reservoir engineering is the effect of water influx on the overall material balance of a gas reservoir.  As the pressure in a reservoir drops (due to production of hydrocarbons like gas or oil), the water in the formation expands, and if there's an aquifer connected to the reservoir, water may flow into the reservoir to balance the pressure drop.  The water expansion effect is important because it can provide additional pressure support to a reservoir during production, which can sustain or slow the decline in reservoir pressure over time, affecting the recovery of gas or oil.

Key Points about Water Expansion Term

Compressibility of Water  -  Water has low but non-zero compressibility, meaning its volume increases as pressure decreases.
Aquifer Support  -  In gas or oil reservoirs connected to aquifers, the expansion of water from the aquifer into the reservoir can maintain reservoir pressure, supporting hydrocarbon production.
Material Balance Equation  -  The water expansion term appears as an additional term in the material balance equation to account for water influx from an aquifer or water-bearing zones.

 

Water Expansion Term in Gas Reservoirs Formula
\( E_w \;=\; G_f \; ( \beta_w \; S_w \; \beta_f ) \; \Delta p \;/\; 1 - S_w \)
Symbol	English	Metric
\( E_w \) = Water Expansion Term	\(dimensionless\)	-
\( G_f \) = Initial Gas Formation Factor	\(ft^3 \;/\; bbl\)	-
\( \beta_w \)  (Greek symbol beta) = Water Compressibility (psi)	\(lbf \;/\; in^2\)	-
\( S_w \) = Initial Water Saturation	\(dimensionless\)	-
\( \beta_f \)  (Greek symbol beta) =  Formation Compressibility (psi)	\(lbf \;/\; in^2\)	-
\( \Delta p \) = Pressure Differential (psi)	\(lbf \;/\; in^2\)	-