Experimental Measurement of the Persistence of Permeability Reduction in Porous Media Treated With Xanthan/Cr(III) Gel Systems
- R.W. Eggert Jr. (U. of Kansas) | G.P. Willhite (U. of Kansas) | D.W. Green (U. of Kansas)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- February 1992
- Document Type
- Journal Paper
- 29 - 35
- 1992. Society of Petroleum Engineers
- 5.3.4 Reduction of Residual Oil Saturation, 5.4.10 Microbial Methods, 1.14.3 Cement Formulation (Chemistry, Properties), 1.14 Casing and Cementing, 2.4.3 Sand/Solids Control, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 5.7.2 Recovery Factors, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex)
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A series of long-term experiments was conducted to determine the persistenceof flow resistance in unconsolidated sandpacks treated with persistence of flowresistance in unconsolidated sandpacks treated with xanthan/Cr(III) gels. Gelswere prepared with 2,000 ppm xanthan and Cr(III) concentrations from 25 to 200ppm. These systems exhibit various degrees of swelling or syneresis. Theexperiments were conducted by first allowing a gel to set up in a sandpack andthen continuously flushing the sandpack with brine for up to 4 months at aconstant pressure drop of 13.3 psi/ft. Flow rate was monitored to calculatepermeability changes with time. Flow experiments indicated that the measurementof swelling and syneresis in bulk-gel tests is not a good predictor of gelresponse in unconsolidated sandpacks. Excellent permeability reduction wasobtained in sandpacks when gels that exhibited 35% to 60% reduction in volumein bulk tests owing to syneresis were used. Gels were most effective atretaining flow resistance in the range of 35 to 75 ppm Cr(III), where thesandpacks regained less than 0.1 % of their original 4,000 md permeabilityduring the experiments. These observations were supported by experiments inwhich the pH of the injected brine was varied between 3.0 and 6.5. The swellingtests on the bulk gel indicated that permeability would decrease as pHincreased and would increase as pH decreased. This occurred; however, themagnitude of the permeability change was less than inferred from bulk-gelmeasurements. Other experiments indicated that residual oil does not affectpermeability modification in sandpacks. The amount of Cr(III) removed from thesandpack during brine flushing ranged from 38% for the 25-ppm-Cr(III) gel to70% for the 200-ppm-Cr(III) gel. In all systems, effluent Cr(III) concentrationdeclined exponentially as flushing continued, suggesting that the Cr(III)recovered never reacted with the polymer.
Many reservoirs have large variations in permeability with depth. Some ofthese variations result from the depositional environment that existed when thereservoir framework was laid down. In other cases postdepositional changes,such as diagenesis and fractures, may be responsible. Fractures may also beintroduced during primary operations. When secondary recovery processes, suchas primary operations. When secondary recovery processes, such aswaterflooding, are implemented in such heterogeneous systems, injected fluidsflow in the direction of least resistance, bypassing a portion of theoil-bearing layers. These "thief zones" reduce the portion of theoil-bearing layers. These "thief zones" reduce the volumetric sweepefficiency of the oil displacement process. One method to improve waterfloodvolumetric sweep efficiency is to inject a substance into a well to change theinjectivity profile and the distribution of water flowing in the reservoir.profile and the distribution of water flowing in the reservoir. Permeabilitymodification, conformance control, permeability Permeability modification,conformance control, permeability adjustment, profile modification, and fluiddiversion are terms often used to describe this process. Several methods ofchanging the permeability in the vicinity of an injection well have beenproposed, including injection of cement slurries, particulate solids, silicagels, monomers that polymerize in situ, water-soluble polymers, and gels orcolloidal structures formed by metal ions and polymers. Of these methods,in-situ gelation appears to be applicable to a wide range of reservoirconditions and has been successful in several field tests. Permeabilitymodification by in-situ gelation consists of the injection of ahigh-molecular-weight polymer, such as polyacrylamide or xanthan gum, and ametallic ion crosslinker, usually chromium or aluminum, into an injection wellwhere volumetric sweep is suspected to be a problem. The gel solution flowsinto the fractures or high-permeability streaks and forms a gel there within afew days. After treatment, it is hypothesized that the injected water willspread more evenly over the net pay interval. The effectiveness of apermeability-modification project depends on the persistence of the long-termpermeability modification. The gel must withstand a continuous high pressuredrop near the injection well without degrading in the high-permeability streaksor fractures. Consequently, gel formulations are extensively evaluated forstability in bottle tests. Many polymer/metal ion compositions are not stablein bottle tests. Gels containing high concentrations of metal ions oftenundergo syneresis (shrink) with time by expelling solvent. Thus, personsevaluating gel systems have assumed that a gel was satisfactory forpermeability-modification treatments if the gel was stable in bottlepermeability-modification treatments if the gel was stable in bottle tests forlong periods. Compositions that exhibited syneresis were discarded from furthertesting because it was thought that syneresis in the reservoir would lead tothe formation of water channels in the gels and destroy the intendedpermeability-reducing properties of the gels. This paper describes results of astudy of the persistence of permeability reduction in unconsolidated sandpackstreated with a permeability reduction in unconsolidated sandpacks treated witha Cr(III)/xanthan gelling system.
Gel-Swelling Theory. Gels formed by the crosslinking of xanthan with Cr(III)exhibit two unique properties when studied in bottle tests: swelling andsyneresis. The swelling property is observed when a gel has the capability toabsorb solvent and to increase the gel volume when contacted with excesssolvent. Syneresis describes the decrease in the volume of a gel caused byexpulsion of a portion of the solvent upon aging of the gel. Syneresis is oftencaused by continued crosslinking, which leads to a higher crosslink density, acontraction of the polymer network, and expulsion of solvent. Syneresis isusually observed sometime after the initial gel forms. Swelling and syneresisare thermodynamic properties of cross-linked polymer systems that can bedescribed theoretically from Flory and polymer systems that can be describedtheoretically from Flory and Herman's polymer-network theory. According to thistheory, the equilibrium swelling state of a gel is a "balance between twopotentials; a mixing potential that favors the dispersion of the polymernetwork into the potential that favors the dispersion of the polymer networkinto the solvent, and an elastic potential that acts to resist any furtherdeformation of the network from its most probable configuration. Swelling andsyneresis represent predictable changes in the volume of a gel as it movestoward a thermodynamically stable state. When a gel undergoes syneresis, thecontraction of the gel network leads to separation of the gel and solventphases. when exposed to excess solvent, a gel swells to reach a stable statecorresponding to the properties of the solvent and the gel. Xanthan/Cr(III)gels exhibit properties of the solvent and the gel. Xanthan/Cr(III) gelsexhibit swelling and syneresis under a wide variety of conditions.
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