Summary
A simple mobility-ratio model was used to predict cleanup times for both
fractured and unfractured production wells after a gel treatment. The time to
restore productivity to a gel-treated oil zone (1) was similar for radial vs.
linear flow, (2) varied approximately with the cube of distance of gel
penetration, (3) varied inversely with pressure drawdown, (4) varied inversely
with the kw at Sor in the gel-treated region, and (5) was not sensitive to the
final ko at Swr. Although ko at Swr (after gel placement) had no effect on the
cleanup time, it strongly affected how much of the original oil productivity
was ultimately regained.
Introduction
Utility of Disproportionate Permeability Reduction. In mature
reservoirs, wells typically produce more water than hydrocarbon. In many wells,
hydrocarbon productivity could be increased significantly if the water
production rate could be reduced. For these cases, the water and hydrocarbon
must flow to the wellbore through different pathways (i.e., some zones have
high fractional hydrocarbon flow, while other zones have high fractional water
flow) (Liang et al. 1993). Because of physical or economic constraints,
remedial chemical treatments (e.g., gel treatments) that are intended to plug
water strata are often placed without zone isolation. Consequently, the
injected fluids and chemicals penetrate into both hydrocarbon and water zones,
and the operator must be concerned about damage to hydrocarbon productivity
(Liang et al. 1993; Seright 1988). Certain water-based gels and water-soluble
polymers (after adsorption or entrapment in rock) can reduce permeability to
water much more than that to hydrocarbon (Seright et al. 2006; Zaitoun and
Kohler 1988). Basic engineering calculations reveal that materials that provide
“relative permeability modification” or “disproportionate permeability
reduction” are currently of far more practical use when treating linear flow
features (e.g., fractures) than when treating radial matrix flow problems
(e.g., wells without fractures) (Seright website; Seright et al. 1998; Marin et
al. 2002). For these materials to effectively treat radial matrix flow, they
should reduce permeability to water by more than a factor of 10 (and preferably
by more than a factor of 20). At the same time, they must reduce permeability
to oil by less than a factor of two if oil zones are not protected during
placement (Seright website). In contrast, when treating fractures, a
significant oil residual resistance factor (permeability reduction value for
oil) can be tolerated so long as (1) the permeability to water is reduced much
more (e.g., >50 times more) than that to oil and (2) the distances of
gelant leakoff from the fracture faces are controlled (Seright website; Seright
et al. 1998; Marin et al. 2002).
© 2006. Society of Petroleum Engineers
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