Summary
Models for gravity segregation in gas enhanced oil recovery (EOR) indicate
that the distance injected gas and water travel together before complete
segregation scales with the injection rate Q. Therefore, in cases where
injection pressure is limiting, increasing injectivity can improve sweep
efficiency. We examine several strategies. Reducing skin resulting from damage
at the wellbore face directly increases volumetric sweep of gas. Even in the
absence of damage at the wellbore face, most of the injection pressure is
dissipated near the injection well, but most of segregation of gas and water
occurs much further from the well. Therefore, if injection pressure is limited,
increasing mobility near the injection well has a large impact on Q,
with a direct benefit in delaying gravity segregation. There is also a
relatively small increase in gravity segregation in the near-well region. An
analytical model for gravity segregation in homogeneous reservoirs can be
extended to a case where permeability is stimulated within a cylindrical region
inside a larger cylindrical reservoir. The effect of this stimulation in
increasing Q at fixed injection pressure can be estimated as well. One
can increase the volume swept by gas before segregation by as much as 170%,
though a large volume must be stimulated to reach this optimum.
The model represents schematically a number of ways proposed in gas EOR for
delaying segregation beyond the possibilities with uniform, steady coinjection
of Newtonian fluids: alternate injection of gas and liquid [water alternating
gas (WAG) or surfactant alternating gas (SAG) with foam]; injection of gas
above water; and injection of shear-thinning foam. In all these cases, the
process gives higher mobility near the well, allowing an increase in injection
rate, and thereby increases the distance to the point of segregation.
The model can be extended directly to the case of shear-thinning (power-law)
foam. One obtains a differential equation for the segregation process, in place
of the algebraic equation that results for Newtonian fluids. In the limit of
extremely shear-thinning behavior, it is possible to double the distance to the
point of segregation with no increase in injection pressure. The model can also
be applied to foams that are shear-thinning only at high superficial velocity
(i.e., near the well). Simulations fit the theoretical prediction well.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
19 November 2007
- Meeting paper published:
13 February 2008
- Revised manuscript received:
14 April 2009
- Manuscript approved:
14 May 2009
- Published online:
22 October 2009
- Version of record:
12 March 2010
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