Field Result of a Short-Setting-Time Polymer Placement Technique

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Abstract
A new technique for altering injection-well profiles uses a stiff-gel, short-setting-time polymer to reprogram water distribution. This paper explains the technique and presents case histories in a San Andres reservoir where 16 producing wells, surrounding three polymer-squeezed (and reacidized) injection wells, have responded with substantial oil-production increases. Introduction Newly drilled water-injection wells in the Shell-operated Jordan University Unit in the Jordan (San Andres) field of West Texas exhibited thin intervals of high permeability (and poor profiles) soon after injectivity began (Fig. 1). The wells had been selectively acidized (straddle packers were used to isolate each perforation while acidizing) with surface pressures of not more than 500 psi. Since fiberglass casing was cemented across the entire injection interval, the use of cement to squeeze off selected perforations (before reacidizing perforations not taking water) was not feasible perforations not taking water) was not feasible because of its high density and hard set. A plugging material with a low viscosity and a specific gravity approaching that of fresh water was needed to plug the rock matrix near the wellbore. A cross-linked polymer was used because its specific gravity and viscosity approaches that of fresh water, and it is a nonparticulate. Therefore, the polymer could be used without hydraulically fracturing the rock or "plating out" on the wellbore face. The use of polymers in injection-well profile control has been well documented. A different method of application of a cross-linked, stiff-gel polymer (American Cyanamid Co. AM-9 Chemical Grout) was successfully used to alter injection profiles in the three San Andres dolomite water-injection wells. Proven fluid design techniques were successfully used to premix a gel solution and a catalyst solution at the surface (on the fly) for a downhole (3,800 ft) setting time of about 20 minutes at 90F. The resulting solution had low viscosity pumping characteristics, yet rapidly increased in viscosity at the desired setting time. Consequently, the solution pumping characteristics were also similar to those of fresh water. In addition, an underdisplacement gel placement technique was used to assure that uncontaminated polymer was gelled in the pore network (within a radius of 6 to 8 ft) and back into pore network (within a radius of 6 to 8 ft) and back into the wellbore. The underdisplacement technique and reacidizing of old, tight perforations were the keys to injection-well profile change in the Jordan University Unit. Production-History data from wells surrounding the three injectors reflect an increase of about 550 BOPD over an 18-month period. The increase in oil production is a direct result of injection-well profile production is a direct result of injection-well profile improvement. See Table 1 for an economic analysis of the cost of the three worked-over injection wells and of the production results associated with the 16 surrounding producing wells. Need for New Sealing Method Many types of formation sealing materials have been used for treating thief zones in waterflood injection wells. The objective is to cause highly permeable, "water-drinking" sections to be sealed off and the injection water to enter other unswept zones. Portland cement, plastering agents, emulsions, resins, etc., all have been tried with some degree of success. The limited success probably resulted from a lack of effectively vertical permeability between the thief zone and the adjacent unswept sections. JPT p. 749

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