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When an aqueous gel is contacted under appropriate conditions, chemical breakers can degrade the gel back to a low-viscosity and watery solution. This article briefly describes the use of gel breakers and the types of gel breakers used in the industry.
Use of gel breakers
Two possible reasons to use a breaker after a conformance-improvement gel treatment are:
- To remove gel from the wellbore or perforations
- To undo a gel treatment in the near-wellbore region if it was determined after its placement that the emplaced gel was not beneficial
Problems in using chemical breakers for gels in fractured reservoir
There are several reasons why a chemical breaker cannot be used to successfully and fully degrade a gel that has been placed deeply in either a matrix-rock or a fractured reservoir. First, successfully delivering the chemically reactive gel-breaker solution deeply in an oil reservoir is a daunting task. Second, and more fundamentally problematic, even if a chemical breaker solution could be 100% effective in the reservoir during its entire gel-breaking life, once injected into the reservoir, the gel-breaker solution would tend to wormhole through the emplaced gel. Thus, the chemical breaker would only be able to regain a small fraction of the pregel-treatment fluid-flow capacity within the gel-treated reservoir volume.
Types of gel breakers
Strong acid solution
Many biopolymer gels and freshly placed inorganic gels can be chemically broken and reversed by contacting them with a strong acid solution. However, acids are usually ineffective at chemically breaking down metal-crosslinked synthetic-organic-polymer gels, such as metal-crosslinked acrylamide-polymer gels. Acrylamide-polymer gels can be chemically degraded back to a watery solution by contacting them with a free-radical chemical breaker, such as hydrogen peroxide, sodium hypochlorite of bleach, and ammonium peroxide. Free radicals chemically degrade polymer gels by a polymer backbone scission mechanism.
Hydrogen peroxide is, in many instances, the most chemically powerful of the gel chemical breakers commercially available. However, its decomposition is catalyzed by tubular rust and many other oilfield substances, such that the injected hydrogen peroxide can be rendered essentially spent before it can be delivered to the downhole gel. The use of hydrogen peroxide may be favored when plastic-coated well tubulars have been used. Hydrogen peroxide is an extremely reactive chemical. It is advised to not inject concentrations exceeding 10% hydrogen peroxide; however, a concentration of less than 5% is ill advised, because ineffectively low concentration of hydrogen peroxide will often result downhole. Hydrogen peroxide decomposes to water and free oxygen during the gel-degradation process. The creation of oxygen in the wellbore and/or the reservoir after the hydrogen peroxide is injected raises safety issues that need to be addressed as part of the hydrogen peroxide selection process.
Bleach (containing sodium hypochlorite) is probably the most widely used material to chemically breakdown acrylamide-polymer gels. It is more chemically robust downhole than hydrogen peroxide. A note of caution: when hydrogen peroxide or bleach is used to break gels crosslinked with a chromium (III)-containing crosslinking agent, some of the chromium (III) will be converted, at least temporarily, to chromium (VI). Because any Cr (VI) that might be formed in the chemically reducing reservoir environment is rapidly converted back to relatively nontoxic Cr (III), field experience has shown that this is often only a theoretical concern. Most oil reservoirs are characterized as having a chemically reducing environment. It should be noted that certain types of metal-crosslinked polymer gels, under certain conditions, can be degelled when contacted with an aqueous solution containing a high concentration of either a caustic chemical (e.g., sodium hydroxide) or a strong ligand (e.g., oxalate).
If an effective delayed and single-fluid reversible-gel technology, especially a reversible-polymer-gel technology, were to be developed in which the gel chemical breaker or gel-breaking mechanism were chemically built directly into the gel structure itself, there would be numerous oilfield applications for such reversible gels. The delayed gel reversal/degelation time would need to be controllable.
Water-soluble chemical breaker
If a water-soluble chemical breaker (breaker not built into the gel structure itself) were incorporated into an aqueous gel formula and the gel were placed downhole under a differential pressure (as usually is the case), then as the chemical breaker begins to break down the gel, the differential pressure would begin to squeeze water, including the dissolved breaker, out of the gel. Unfortunately, increasing breaker concentration is required as the concentration of the polymer increases in the gel. When a water-soluble breaker is being squeezed out of the partially broken gel, the opposite trend is occurring. Thus, the use of water-soluble chemical breakers incorporated into a single-fluid aqueous gel under differential pressure always results in an incomplete gel break and always leaves a significant gel residue. To date, the addition of a water-soluble chemical breaker into a single-fluid aqueous gel formula has not proven effective in fully degrading a gel when the gel is broken under differential pressure.
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