Abstract
Because polymers and gels can reduce permeability to water much more than that to oil, an unfavorable displacement (high mobility ratio) usually occurs in oil zones when wells are returned to production after bull-headed gel treatments. Since oil displacement through gel is unfavorable, the permeability to oil requires a large throughput to stabilize. Consequently, oil zones often exhibit a significant "cleanup time" after field applications of gel treatments. The oil and water throughput requirements for stabilization of permeabilities were studied for a relatively "strong" pore-filling Cr(III)-acetate-HPAM gel and for a "weak" adsorbing polymer. As oil throughput increased from 1 to 100 PV, permeability to oil gradually increased by factors from 5 to 10 for cores treated with the Cr(III)-acetate-HPAM gel and from 2 to 3 for cores treated with the adsorbing polymer. In contrast, after treatment with gel, permeability to water stabilized rapidly and remained very low for over six months. An explanation is provided.
A simple mobility-ratio model was used to predict cleanup times for both fractured and unfractured wells after a gel treatment. The time to restore productivity to a gel-treated oil zone (1) was similar for radial versus linear flow, (2) varied 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. Earlier work demonstrated that an acceptable gel or polymer placement can be achieved by unrestricted injection of gelants during linear flow (e.g., vertically fractured wells). However, in radial flow through matrix (e.g., unfractured wells), hydrocarbon productive zones must be protected during gelant or polymer placement. These conclusions were confirmed by our new results for both the Cr(III)-acetate-HPAM gel and the adsorbing polymer.
