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Gels for conformance improvement
Gels are a fluid-based system to which some solid-like structural properties have been imparted. In other words, gels are a fluid-based system within which the base fluid has acquired at least some 3D solid-like structural properties. These structural properties are often elastic in nature. This article discusses the use of gels in conformance improvement treatments.
All of the conformance improvement gels discussed are aqueous-based materials. The term “gel” as used in this page (unless specifically noted otherwise) refers to classical, continuous, bulk, and “relatively strong” gel material and does not refer to discontinuous, dispersed, “relatively weak,” microgel particles in an aqueous solution. Gels discussed in this page, when formed in a beaker for example, constitute a single and continuous gel mass throughout its entire volume within the beaker. The term “gelant” refers to a gel fluid before any appreciable crosslinking of the gel’s chemical building blocks has occurred. The term gel refers to a gel fluid that has attained either partial or full chemical-crosslinking maturation. Types of gels used for conformance improvement discusses polymer gels, as well as inorganic gels and monomer gels. Resin treatment for conformance improvement briefly discusses a conformance-improvement plugging material (i.e., a resin) that involves an organic-fluid-based gel.
An older definition of gel is “a jelly-like substance formed by the coagulation of a colloidal solution into a semisolid phase.” In modern oilfield and technical literature, the term gel includes the elastic and semisolid material that results from chemically crosslinking together water-soluble polymers in an aqueous solution. Crosslinked-polymer gels can possess rigidity up to, and exceeding that of, Buna rubber. They contain polymer concentrations in the 150 to 100,000 ppm range (but more commonly 2,000 to 50,000 ppm and most commonly 3,000 to 12,000 ppm). Gels are often formulated with relatively inexpensive commodity polymers.
Gels have found broad application as oilfield fluid-flow blocking agents because gels are often an exceptionally cost-effective plugging and/or permeability-reducing agent for use in a number of different conformance improvement treatments. Conformance improvement gels are, for the most part, essentially nothing but water (often produced brine) with the remainder of the gel constituents incorporated as low concentrations of relatively inexpensive polymers and chemical crosslinking agents.
Gel conformance improvement treatments
Gels operate, for the most part, either by diverting fluid flow from high permeability, low oil saturation reservoir flow paths to low permeability, high oil saturation flow paths, and thereby promoting improved flood sweep efficiency and incremental oil production, or by reducing oil-production operating costs by reducing excessive, deleterious, and competing coproduction of water or gas.
Oilfield gel conformance treatments can be applied in a number of forms including sweep improvement treatments, water shutoff treatments, gas shutoff treatments, zone abandonment treatments, squeeze and recompletion treatments, and water and gas coning treatments involving fractures and other linear-flow high permeability reservoir anomalies. Gels are particularly effective for treating oil production coning problems when the coning is occurring via linear flow in “vertical” fractures.
When there is a good match between a given conformance problem and a particular gel technology, relatively large volumes of incremental oil production and/or substantial reductions in oil production operating costs can be achieved profitably, by means of the shutting off of excessive, deleterious, and competing co-production of water or gas. Gel treatments are usually applied in the course of normal and ongoing primary, secondary, or tertiary oil recovery operations, just as an operator would apply an acid or scale inhibition treatment. Gel treatments are an emerging oilfield technology that can help extend the life of maturing oil reservoirs that are approaching their economic limit.
Although conformance improvement gel treatments have existed for a number of decades, their widespread use has only begun to emerge. Early oilfield gels tended to be stable and function well during testing and evaluation in the laboratory, but failed to be stable and to function downhole as intended because they lacked robust chemistries. Also, because of a lack of modern technology, many reservoir and flooding conformance problems were not understood, correctly depicted, or properly diagnosed. In addition, numerous individuals and organizations tended to make excessive claims about what early oilfield gel technologies could and would do. The success rate of these gel treatments was low and conducting such treatments was considered high risk. As a result, conformance improvement gel technologies developed a somewhat bad reputation in the industry. Only recently has this reputation begun to improve. The information presented in this article can help petroleum engineers evaluate oilfield conformance gels and their field application on the basis of well-founded scientific, sound engineering, and field performance merits.
How gel treatments function
Gels used in conformance improvement treatments reduce permeability in the reservoir treatment volume in which the gels are ultimately placed. Conformance gels do not function as viscosity-enhancing agents during oil recovery flooding operations.
Although microgels, which are colloidal-sized aggregates of suspended and noncontinuous gel particles, have, at times, been claimed to be viscosity-enhancing agents during their “propagation” through reservoir flow paths possessing high permeabilities, there is no substantiation in the petroleum engineering literature of any microgels substantially enhancing the viscosity of an aqueous oil recovery drive fluid beyond the viscosity of the gel’s polymer solution alone. In fact, microgel gel-containing aqueous solutions tend to have lower viscosities than the polymer solutions from which they were derived. Permeability reduction is the dominant mechanism by which microgels impart conformance improvement. Oilfield microgels are also referred to, at times, as colloidal dispersion gels.
The more widely applied conformance improvement gels are characterized as being bulk gels that function as permeability reducing agents. Bulk gels have a continuous chemically crosslinked polymer network structure throughout the entire macroscopic scale of the gel. These gels are blocking and plugging agents to fluid flow in the reservoir volume in which they have been placed. For example, gels used in near-wellbore water-shutoff treatments of producing wells are selectively placed in a high permeability strata (that are not in fluid or pressure communication with the other reservoir strata) to function purely as a plugging and blocking agent to fluid flow.
If a gel is placed some significant distance into the fractures of a naturally fractured reservoir surrounding an injection well, the gel not only functions as a blocking agent, but also as a diverting agent. These gels divert injected oil-recovery drive fluid from predominantly flowing through the high permeability, low oil content fractures to predominantly flowing through the relatively low permeability, high oil saturation matrix reservoir rock adjacent to the gel-filled and gel-treated fractures.
What benefits can be expected
For good gel-treatment well and well-pattern candidates, the following is a partial list of benefits that can be achieved.
- Generate incremental oil production through conformance improvement, and possibly generate large volumes of incremental oil production per unit cost of chemical expended
- Substantially reduce oil-production operating expenses per unit cost of chemical expended
- Reduce competing water production that can be unproductive, costly, and environmentally unfriendly
- Reduce competing gas production that can be unproductive, excessive, and economically detrimental
- Improve the performance of an ongoing oil-recovery operation
- Reduce certain environmental liabilities by reducing the amount of excessive and unnecessary co-production of environmentally unfriendly fluids, such as highly saline and hard reservoir brines
- Extend the economic lives of marginal wells, well patterns, and oil fields
Properties of an ideal gel system
An ideal conformance improvement gel technology should be applicable as injection and production well treatments, as sweep improvement treatments, and as water and gas shutoff treatments. It should also be applicable to all reservoir mineralogies and lithologies and to a wide variety of reservoir and flooding-operation conformance problems. The ideal gel technology should be a single-fluid system and should possess a robust gel chemistry, which requires that it be
- Insensitive to oilfield and reservoir environments and chemical interferences (especially H2S and CO2)
- Insensitive to all reservoir minerals and fluids
- Applicable over a broad pH range.
An ideal conformance improvement gel technology should also
- Involve a simple and straightforward gel-forming chemical system
- Be applicable over a broad range of reservoir temperatures
- Be stable over the long term
- Provide for a broad range of gel strengths, including rigid gels
- Provide highly controllable and predictable gelation-delay onset times.
For matrix-rock reservoir treatments, an ideal gel technology must include gelant solutions that are readily injectable into matrix reservoir rock. An ideal gel must be environmentally acceptable and friendly, be formulated with low concentrations of relatively inexpensive chemicals, and be formulated with readily available (preferably commodity) chemicals. Finally, it should reduce the permeability to water flow in matrix rock more than the permeability to oil and gas flow.
Temperature considerations and limits
All gels, especially polymer gels, have a finite upper temperature limit above which the gels are not stable or functional. Significant progress has been made over the past two decades in increasing the upper temperature limit for the successful application of conformance-improvement gels. A continuation of this trend can be reasonably expected.
For gel technology, the reservoir temperature limit for applying matrix-rock, near-wellbore total-shutoff treatments is reported to be 300°F. Sydansk reported that the upper temperature limit for treating high-permeability-anomaly conformance problems (e.g., fractures) with this gel technology can be up to approximately 270°F. Sydansk also discusses laboratory testing at 300°F that demonstrate how appropriately formulated chromium(III)-carboxylate/acrylamide-polymer (CC/AP) gels can essentially totally block flow to water in sandstone at high differential pressure conditions (1,000 psi per 3 in.) for an extended period of time (testing conducted for 23 days). Fig. 1 is a photo of a CC/AP gel that remained stable, rigid, and clear after aging for 2.5 years at 300°F. This CC/AP gel is used for near wellbore, total-fluid-shutoff purposes in high-temperature matrix-rock reservoirs. For most polymer-gel technologies, such as the CC/AP gel technology, it is necessary to increase the polymer concentration within a given formula as temperature increases to maintain gel stability, performance, and strength similar to that of the gel formula at lower temperatures.
Conformance improvement gels, consisting of an acrylamide polymer crosslinked with a set of organic crosslinking agents, have been reported to form strong and stable gels up to 350°F. This is an interesting observation because, in both this and the CC/AP conformance gel technology, it is believed that it is not the crosslinking chemistry that is limiting high-temperature gel stability, but limitations in the thermal stability of the gel’s organic polymer. Thus, one interpretation of these observations is that the acrylamide polymer used in the organic-crosslinked acrylamide-polymer gels was of a purer and more stable form.
The upper reservoir temperature limit, at which a given gel technology is stable and functional, is an interrelated function of polymer concentration and chemistry used in the gel, hardness divalent-ion concentrations within the gel’s makeup water, polymer purity, and, if used, the chemical stabilizer package. For organic polymer gels, stability is, in part, a function of the level of free-radical and free-radical-precursor chemical impurities in the polymer material itself. Free radicals cause organic-polymer backbone scission and associated polymer-gel degelation.
It is imperative to truly know what the upper reservoir temperature limit is for a conformance gel technology that is to be applied to a high-temperature reservoir and to not apply such a treatment at a temperature exceeding the temperature rating of the gel being used.
- Sydansk, R.D. and Southwell, G.P. 2000. More Than 12 Years of Experience with a Successful Conformance-Control Polymer Gel Technology. SPE Prod & Fac. 15 (4): 270. SPE-66558-PA. http://dx.doi.org/10.2118/66558-PA
- Seright, R.S., Lane, R.H., and Sydansk, R.D. 2001. A Strategy for Attacking Excess Water Production. Presented at the SPE Permian Basin Oil and Gas Recovery Conference, Midland, Texas, 15-17 May 2001. SPE-70067-MS. http://dx.doi.org/10.2118/70067-MS
- Dovan, H.T., Hutchins, R.D., and Sandiford, B.B. 1997. Delaying Gelation of Aqueous Polymers at Elevated Temperatures Using Novel Organic Crosslinkers. Presented at the SPE International Symposium on Oilfield Chemistry, Houston, Texas, 18–21 February. SPE-37246-MS. http://dx.doi.org/10.2118/37246-MS
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