Waterflooding Increases Gas Recovery
Authors: Cason Lloyd D. 
Published: December 2007

Summary
Injecting water into a low-pressure gas reservoir nearing abandonment will displace gas and increase ultimate recovery. This paper examines the theory and reports the results of a case history. Water was injected into a southern Louisiana gas reservoir for more than 10 years, successfully increasing recovery by 25 Bcf [710×106 m3]. This paper also discusses the importance of considering all possible producing mechanisms early in a reservoir's life to predict future performance properly and shows that straight-line p/z performance does not necessarily indicate that a gas reservoir has a depletion drive, as is frequently assumed in practice.

Introduction
Natural gas is a major source of energy production and consumption in the U.S. As reserves decline, investigators are studying the potential of increased supplies from several unconventional sources1 and from waterdrive reservoirs with high production rates in the gas zones2,3 and the aquifers.4 Production at high rates from a waterdrive reservoir is an attempt to reduce reservoir pressure to a low value, thereby minimizing the actual gas volume occupying residual pore space. A reservoir with little water influx that depletes to low pressure has the opposite problem because all the pore space is still filled with gas.
This paper shows that increased recovery can be obtained by waterflooding a low-pressure gas reservoir and discusses an actual field case. The Discorbis 1 reservoir (Reservoir D-1) in the Duck Lake field of southern Louisiana (St. Martin Parish) initially produced by limited aquifer influx. Water injection was initiated after reservoir pressure had fallen below 1,000 psi [6.9 MPa] and continued for 11 years. An incremental recovery of 25 Bcf [710×106 m3] was attributed to water injection. This project was primarily a low-cost expansion of the field's existing saltwater disposal system, providing a unique opportunity for an economic project at the gas prices of the 1970's, which were about 15% of current prices.
Analysis of Reservoir D-1 also provides the opportunity to show that straight-line p/z performance does not necessarily mean that a gas reservoir has a depletion drive. Theory showing that depletion-drive gas reservoirs will exhibit a straight-line p/z plot has been developed. The corollary - that a straight-line p/z plot proves the existence of a depletion drive - has not been proven, although it is frequently assumed in practice.

Theory
If a gas reservoir produces strictly by pressure depletion, hydrocarbon pore space at abandonment pressure should be equal to that at initial pressure. A significant amount of gas, however, can remain in the reservoir. Injecting water at abandonment pressure displaces a fraction of the PV to production wells, leaving a residual gas saturation that contains a minimal standard volume because of the low trapping pressure. Thus, the good features of pressure depletion and water displacement are combined in a controlled production mechanism to maximize recovery.
Craft and Hawkins5 showed that recovery for a waterdrive reservoir can be expressed asEquation 1
and that recovery for a depletion-drive reservoir can be expressed asEquation 2
For imbibition fluid displacements, Naar and Henderson6 concluded that residual nonwetting-phase saturation should be about one-half the initial nonwetting-phase saturation, so thatEquation 3
Substituting Eq. 3 into Eq. 1 and subtracting Eq. 2 yieldsEquation 4
Eq. 4 presents a simple means to calculate a theoretical incremental recovery, as a percentage of original gas in place (OGIP), that results from waterflooding a gas reservoir that is pressure depleting. This recovery factor is representative of a homogeneous reservoir where permeability is uniform areally and vertically, and water coning is neglected.