“Clean Up” of Oil Zones after a Gel Treatment R. S. Seright, SPE, New Mexico Petroleum Recovery Research Center Also see SPE paper 92772

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 “clean-up 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 eight months. An explanation is provided. We also noted that a minimum pressure gradient was required for oil to initiate flow through a gel bank (1.3-1.7 psi/ft for our Cr(III)-acetate-HPAM gel in an 8-darcy core).

A simple displacement model was used to predict clean-up 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 clean-up 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.

Introduction Utility of Disproportionate Permeability Reduction “Clean Up” Behavior

Behavior of a Cr(III)-Acetate-HPAM Gel

Mobility Ratios

Permeability to Water during Many Experiments Typical Behavior Breakdown after High Pressure Gradients Stability of kw with Time

Modeling of ko and kw Behavior

Permeability to Oil during Many Experiments Typical Behavior Minimum Pressure Gradient for Oil Flow

Predicting Oil-Zone Clean-up for Field Applications Effect of Distance of Gelant Penetration Effect of Pressure Drawdown Effect of kw and ko Summary

Effect on Ultimate Oil Productivity Gel Penetration from Fracture Faces Gel Penetration in Unfractured Wells

Behavior of an Adsorbed Polymer

Note on Field Applications

Conclusions

Nomenclature

References