Permeability Reduction by Treatment With KUSP1 Biopolymer
- C.S. McCool (U. of Kansas) | A.K. Shaw (Best Systems Inc.) | Singh Amitabh (Schlumberger Oilfield Services) | Saibal Bhattacharya (Kansas Geological Survey) | D.W. Green (U. of Kansas) | G.P. Willhite (U. of Kansas)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 2000
- Document Type
- Journal Paper
- 371 - 376
- 2000. Society of Petroleum Engineers
- 5.6.5 Tracers, 5.4.10 Microbial Methods, 5.8.7 Carbonate Reservoir, 5.4 Enhanced Recovery, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 2.4.3 Sand/Solids Control, 4.1.2 Separation and Treating
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The application of KUSP1 biopolymer for use in permeability reduction treatments of oil reservoirs was investigated. KUSP1 is a nontoxic polysaccharide that is produced by a bacterium. Alkaline solutions of KUSP1 have low viscosities and can be gelled by different methods. Three methods were examined to gel KUSP1 in porous media. Two of the methods are based on the reduction of pH of alkaline KUSP1 solutions. KUSP1 is soluble in alkali but forms a gel when the pH is lowered to values below 10.8.
The first method employed the hydrolysis of an ester to reduce pH and trigger gelation. Gel times on the order of days were observed for KUSP1 solutions that contained the monoethylphthalate (MEP) ester. The performance of the KUSP1-MEP system in treatments of sandpacks, Berea cores, and carbonate field plugs was studied at 25°C. Brine permeabilities were reduced by factors that ranged from 75 to 4,500. Permeability reduction by treatments with the KUSP1-MEP system was stable to brine flow for periods of up to six months.
The second method to gel alkaline KUSP1 solutions was based on the chemical interactions with rock material. It was found that interactions with Berea sandstone, Baker dolomite, silica sandpacks, and field core plugs were insufficient to reduce the pH of alkaline KUSP1 solutions to values required for gelation.
Alkaline KUSP1 solutions were gelled by the addition of boric acid in the third method. Bulk gelation studies conducted at 25, 45, and 65°C showed that gel times could be regulated up to several days by selection of pH and boric acid concentrations. The gels were more rigid than those produced by pH-reduction methods, indicating a different gelation mechanism. Syneresis was observed and was more pronounced at lower pH values and higher temperatures. The KUSP1-boric acid system reduced brine permeabilities in sandpacks and Berea cores (25, 45°C) by factors that ranged from 50 to 3,300.
KUSP1 is a name given to the biopolymer that is extracted from a medium after the growth of the bacterium Cellulomonas flavigena.1 The extracted biopolymer has several characteristics that make it a candidate for use in permeability reduction treatments. KUSP1 is a nontoxic polysaccharide that has environmental advantages as compared to other gelled polymer systems. Alkaline solutions of KUSP1 exhibit viscosities on the order of 2 to 3 cp at 25°C. Low-viscosity gelants are preferred in many situations where permeability reduction treatments are indicated.2 Previous studies have shown that KUSP1 solutions can gel and reduce permeability by several methods.3,4 The characteristics of KUSP1 provided the incentive to study its performance in permeability reduction treatments.
A unique property that distinguishes KUSP1 from many other polymers is that alkaline solutions of KUSP1 form a hydrogel when the solution is neutralized.3 The transition between solution and hydrogel is reversible. A permeability reduction treatment is implemented by injecting an alkaline solution of KUSP1 and relying on some method to reduce the pH of the solution during or after the placement of the KUSP1 solution in the desired zone of the reservoir. Several methods to reduce the pH have been proposed. The first method is to incorporate a chemical in the gelant that would react to form acid and reduce the pH to values where gelation occurs. We tested several chemicals for their solubility and ability to react at alkaline conditions. Two esters were identified that would produce acid at rates that would give gel times on the order of hours or days. Both agents, ethylbenzoate-2-sulfonic acid (EBSA) and monoethylphthalate (MEP), were not commercially available but were easily prepared.
A second method to reduce the pH of KUSP1 gelants during placement in the reservoir relied on the interactions between the alkaline KUSP1 solution and rock material. The pH of the injected solution is reduced by an ion exchange reaction and by silica dissolution. Sodium ions in the injected solution are exchanged with hydrogen ions present on the rock surface. Kinetically controlled dissolution of silica-rich minerals reduces the pH of resident fluid. Experiments were conducted to determine if interactions between representative reservoir rock would reduce the pH of injected fluids to a level required for in-situ gelation.
KUSP1 solutions also gel when mixed with boric acid.4 Gels are formed over time periods that are much longer than for borate-crosslinked gel systems that are used for hydraulic fracturing.5 This was the third method investigated. The physical appearance of the gels when KUSP1 is gelled by boric acid indicates that the gelation mechanism is different than when the solution pH is reduced.
The objective of this work was to determine the efficacy of using KUSP1 for permeability reduction treatments. Experiments were conducted to characterize the behavior of KUSP1 systems. Ester hydrolysis, addition of boric acid, and fluid-rock interactions were studied as methods to gel KUSP1 solutions in situ. This paper summarizes work reported in Refs. 6 through 12.
A standard procedure was used to prepare KUSP1 stock.13 Cellulomonas flavigena was grown for 72 hours in a simple medium containing excess glucose during which a polysaccharide is deposited as a capsule surrounding the bacterium. The polymer was isolated by dissolving the cells in 1 M NaOH and removing the cell bodies by centrifugation. A filtration procedure for the KUSP1 stock was developed that utilized carbon black, filter aid, and a 5-µm filter. Injectivity of unfiltered solutions was poor.
The EBSA and MEP esters were prepared by dissolving their respective anhydrides in ethanol and heating the mixture to 50°C for a day. The mole ratio of 2-sulfobenzoic acid cyclic anhydride to ethanol was 1:2. The mole ratio of phthalic anhydride to ethanol was 1:3. It was assumed that all of the anhydride was converted to ester.
In-situ gelation experiments were conducted in sandpacks, Berea cores, and field core plugs. Sandpacks were prepared with acrylic holders (1.5-in. i.d., 1-ft long) and Wedron silica sand. Berea cores (2-in. diameter or 2-in. square, 1-ft long) were fitted with endplates and coated with epoxy. Ports at 2-in. intervals allowed for the measurement of pressure drops along the length of the sandpacks and Berea cores. Field core plugs were fitted with endplates and coated with epoxy. At least one intermediate pressure port was installed. Lengths of the field core plugs ranged between 2 and 3 in.
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