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Types of gels used for conformance improvement
- 1 Overview
- 2 Gel types
- 2.1 Chromium (III)-carboxylate/acrylamide-polymer gels
- 2.2 Chromium (VI) redox gels
- 2.3 Aluminum crosslinked gels
- 2.4 Gels crosslinked with an organic crosslinker
- 2.5 Biopolymer gels
- 2.6 Monomer gels
- 2.7 Polymer self-induced gels
- 2.8 Inorganic gels
- 2.9 Gels formulated with synthetic organic polymers
- 3 Classification of gel treatment types
- 4 R&D targets
- 5 References
- 6 Noteworthy papers in OnePetro
- 7 External links
- 8 See also
- 9 Category
Oilfield conformance improvement gels come in a wide range of forms and chemistries. Table 1 provides an overview of various conformance improvement gels.
Chromium (III)-carboxylate/acrylamide-polymer gels
Widely applied as sweep improvement treatments and as water and gas shutoff treatments, chromium (III)-carboxylate/acrylamide-polymer (CC/AP) gels are aqueous acrylamide-polymer gels in which the chemical crosslinking agent is a chromium (III) carboxylate complex. CC/AP gels have an exceptionally robust gel chemistry and are highly insensitive to oilfield and reservoir interferences and environments. They are also applicable over an exceptionally broad pH range. As a result, these gels, when properly formulated, are applicable to the acidic conditions associated with CO2 flooding for which most earlier oilfield polymer gels did not function well. The chromium (III), as used in the crosslinking agent of this gel technology, is relatively nontoxic, but it is highly regulated. This single-fluid gel technology provides a wide range of gel strengths and a wide range of controllable gelation-onset delay times. The gel technology is applicable over a broad range of reservoir temperatures and applicable to a broad range of conformance problems and reservoir mineralogies and lithologies. Chromic triacetate (CrAc3) is the often-preferred crosslinking agent used with the CC/AP gel technology. A chemical gelation-rate-acceleration additive package, involving chromic trichloride, has been developed for use with the CC/AP gel technology. Two chemical means are available to retard the rate of gelation of CC/AP gels that are applied to high-temperature reservoirs:
- Use of low or ultra-low hydrolysis polyacrylamide to capitalize on the slow formation of the required chemical crosslink sites on the polymer by means of autohydrolysis
- Addition of relatively strong carboxylate ligands, such as lactate or malonate, to the gelant solution.
Chromium (VI) redox gels
One of the early conformance improvement gel technologies involved acrylamide polymers that were chemically crosslinked together using a chromium (VI) redox system. This oilfield gel system has largely fallen from favor because of issues relating to the use of a crosslinking agent that contains toxic and carcinogenic chromium (VI) and because the crosslinking chemistry is rather complicated and subject to a number of oilfield interferences.
Although other aluminum crosslinking agents have been developed and used in conformance improvement gels, aluminum citrate crosslinked gels have historically been the most widely applied. The early widespread application of the aluminum citrate gel technology was conducted in the sequential-injection mode, involving the repeated sequential injection of aqueous slugs containing, respectively, the polymer and the aluminum citrated crosslinking agent. The application of conformance gel treatments, involving the sequential injection of aqueous slugs containing the different chemicals that are required to form the gel in the reservoir, have largely fallen from favor.
More recently, aluminum-citrate acrylamide-polymer gels, which are formulated with low concentrations (200 to 1,200 ppm) of polymer and which are referred to as colloidal dispersion gels (CDGs), have been somewhat widely used as large-volume treatments applied through injection wells to "matrix rock" for improvement of waterflood sweep efficiency. CDGs are a form of discontinuous microgel particles. The mechanism and means by which this particular gel technology generates incremental oil production is not fully understood.
Published laboratory studies have reported the following regarding aluminum-citrate CDGs.
- CDGs of acrylamide polymers crosslinked with aluminum citrate are not readily injectable into, and propagatable through, matrix rock of normal permeabilities (e.g., sandstone of < 1,000 md). Seright discusses an experimental flooding study where an aluminum-citrate CDG was observed to not be readily propagatable, after two hours of aging, through matrix rock during a flooding experiment involving the injection of an aluminum-citrate colloidal dispersion gelant solution, containing 300 ppm high-MW HPAM, into a 700 md Berea sandstone core plug at 105°F and at a superficial velocity through the sandstone of 16 ft/d.
- Aluminum does not readily propagate through reservoir matrix rock.
- Aluminum crosslinking of the polymer of CDGs normally occurs within several hours.
- Aluminum-citrate CDGs do not preferentially enter high permeability zones any more selectively than is dictated by Darcy’s law.
- Aluminum-citrate CDGs do not viscosify water more than do the gel’s polymer without the addition of the crosslinking chemical.
If aluminum-citrate CDGs are not readily injectable into and propagatable through matrix reservoir rock of normal permeabilities (sub-Darcy permeabilities), there are two possible explanations for the apparent success of a number of large volume aluminum-citrate CDG treatments in terms of generating conformance improvement and generating incremental oil production when treating "matrix-rock conformance problems."
- The successfully treated "matrix rock" reservoirs may actually have been at least somewhat naturally or otherwise fractured.
- The offending high-permeability strata within the successfully treated matrix-rock reservoirs may have possessed multi-Darcy permeabilities.
There has been a long-standing desire within the oil industry to develop effective conformance improvement polymer gel technologies using benign organic chemical crosslinking agents that would impart carbon-carbon-bond chemical crosslinks between the gel polymer molecules. This would avoid the use of metal crosslinking agents and would result in exceptionally strong and stable polymer gels. However, with the possible exception of the polyethyleneimine-crosslinked gel technology briefly discussed later in this section, no such gel technology has been developed and reduced to commercial practice.
The majority of organically crosslinked polymer-gel technologies developed to date have been based on phenol formaldehyde chemistries. These gels either use phenol and formaldehyde as the chemical crosslinking agent or use derivatives or precursors to phenol and formaldehyde. There have been attempts to identify and use less toxic and more environmentally friendly derivatives of phenol and formaldehyde as the crosslinking agents.
Some of the most thermally stable polymer gels for use in high temperature conformance improvement treatments have been formulated with acrylamide polymers that are chemically crosslinked with organic crosslinking agents that are based on phenol-formaldehyde type chemistry.
At this writing, an organically crosslinked gel technology that does not involve a phenol-formaldehyde type crosslinking chemistry was somewhat recently developed. The gel technology involves the use of a specially manufactured and derivatized acrylamide polymer and the use of polyethyleneimine as the crosslinking agent. This gel technology is most readily applicable to reservoirs with temperatures exceeding approximately 180°F, and reports regarding the field application of this conformance polymer-gel technology have been favorable overall.
Gels based on the crosslinking of biopolymers with organic or inorganic crosslinking agents have been pursued. A popular conformance biopolymer-gel technology in the 1970s and early 1980s was based on gels of xanthan polymer crosslinked with an inorganic chromium (III) crosslinking agent.
Conformance-improvement gels based on the in-situ polymerization of organic monomers to form polymers with and without the inclusion of crosslinking monomers have been developed and successfully applied. Early monomer-gel treatments were often based on the in-situ polymerization of the acrylamide monomer; however, this is seldom practiced currently because of toxicity and environmental concerns. Most modern monomer-gel technologies for oilfield application are based on the in-situ polymerization of less toxic acrylate monomers.
An advantage of monomer gel technologies is the low water-like viscosity of the gelant solution. Disadvantages include cost and the sensitivity of the polymerization reaction to oilfield interference and environments. Care also needs to be taken to carefully distinguish between "linear" (uncrosslinked) and crosslinked oilfield monomer gels.
An older monomer gel technology involved gels formed from the reaction of formaldehyde with phenol. A modern day concern with this gel technology is the toxicity and environmental issues associated with the use and handling in the field of formaldehyde, phenol, and/or their chemical derivatives.
Polymer self-induced gels
A conformance improvement biopolymer gel system has been reported that involves the injection of the polymer into the treated reservoir volume in the form of an alkaline high-pH solution. Once emplaced in the reservoir rock, the pH of the polymer solution is reduced by spontaneous or induced means. Following the reduction in the polymer solution’s pH, the polymer solution spontaneously forms a gel.
A variety of inorganic gel technologies have been developed and applied over the years. These include gels based on, respectively, silicate or aluminum ion chemistries, along with gels of hydroxides of iron and magnesium.
Gels based on silicate chemistries were some of the earliest gel technologies applied for conformance improvement. A silicate gel is formed when a relatively high-pH aqueous solution, containing a sufficient concentration orthosilicate monomers or oligomers of orthosilicate, has its pH reduced or is exposed to hardness cations. Aqueous-based conformance-improvement silicate gels can be formed in a petroleum reservoir by either "internally catalyzed" or "externally catalyzed" means. Internally catalyzed silicate gels are formed by including in the aqueous gelant solution an acid-generating chemical that will spontaneously decrease the pH of the gelant solution when it is placed in the reservoir. Externally catalyzed silicate gels are usually formed by contacting the orthosilicate monomer or oligomer solution with a brine (e.g., reservoir brine) that contains a high level of harness cations (e.g., Ca++). Internally catalyzed silicate gels are often favored for use in conformance improvement treatments during oil recovery operations. Externally catalyzed silicate gels are often used during drilling operations for lost circulation applications. Several potential concerns should be noted. Solution-aging, filterability, and quality-control issues can be a concern for silicate gelant solutions that are to be injected into matrix-rock reservoirs. There can be safety and environmental issues associated with the acid-generating chemical used in internally catalyzed silicate gel systems.
Silicate-based gels of conformance-improvement treatments have been applied successfully in Hungary. A large-volume sodium-silicate gel treatment was reported to have been applied to an offshore Norwegian oil production well. A large number of silicate-based conformance improvement gel treatments have been applied worldwide.
The main advantage of inorganic gels is their environmentally friendly nature. Disadvantages historically have been that many of the inorganic gels were relatively weak gels, and a number of the inorganic gels do not provide good long-term fluid-shutoff performance. The latter is especially true for inorganic hydroxide "gels" that tend to convert over time to solutions containing ineffective oxide solids.
Gels formulated with synthetic organic polymers
Although a number of the early conformance-improvement gel technologies were based on inorganic gels and biopolymer gels, the recent trend has been toward the application of oilfield polymer gels based on the use and crosslinking of synthetic organic polymers, primarily acrylamide polymers.
Classification of gel treatment types
Conformance-improvement gels can be classified in several manners:
- Conformance-improvement gels can be classified as to whether they are intended to treat matrix-rock conformance problems, involving permeabilities less than roughly two Darcies, or treat reservoir high-permeability anomalies (usually fractures), involving permeabilities greater than roughly two Darcies. A subcategory of this classification is whether the gel treatments for treating matrix-rock conformance problems are to be placed in the near-wellbore environment (radial penetration of less than ~15 ft) or to be placed deeply in the far-wellbore environment (radial penetration of greater than ~15 ft). Although this classification scheme was originally developed for the CC/AP polymer gel technology, the classification scheme is also generally applicable to all other conformance improvement polymer gel technologies.Gels used to treat high-permeability-anomaly conformance problems are often treating fluid-flow problems involving linear flow, such as that occurring into fractures. Near-wellbore gel treatments placed in unfractured matrix rock are usually used to block radial fluid flow. Near-wellbore gel treatments that are placed in matrix rock are of relatively small volume, are often total-fluid-shutoff gel treatments, and are often relatively simple and straightforward to apply. These gel treatments usually have a relatively low risk factor if they can be placed properly and if the conformance problem is correctly diagnosed. In addition, these gel treatments can have high payout-to-cost ratios. Gels placed deep in matrix rock are large volume treatments that can be relatively costly and technically complex. In addition, these gel treatments can have a relatively high risk factor associated with them and tend to render a relatively low payout-to-cost ratio. The application of low-concentration aluminum citrate colloidal dispersion gel (CDG) treatments that are applied via injection wells has been cited as a possible exception to this trend.
- The widely applied form of conformance improvement gel treatments, namely crosslinked organic-polymer bulk gel treatments, can be classified as to whether when being injected from the wellbore into the reservoir, the gel fluid is a gelant solution or is a preformed or partially preformed gel fluid. In most instances, such as with the CC/AP gel technology, the polymer-gel fluid must be in its gelant form (i.e., before any initial microgels form) for the gel fluid to be readily injectable into matrix reservoir rock (such as sandstone having a permeability less than 1,000 md). However, for polymer gels to be selectively placed in high-permeability anomalies such as fractures, the gel should be designed so at least some initial gelation has occurred when the gel leaves the wellbore to assure that the gel will not substantially leak off from the high-permeability anomaly (e.g., a fracture) into the adjacent matrix reservoir rock. This is the key to properly formulated polymer gel being able to selectively treat fracture conformance problems, without substantially damaging the adjacent matrix reservoir rock.
- Conformance improvement organic polymer gels are classified as to whether they are the more widely applied bulk gels possessing relatively high polymer concentration with a continuous crosslinked polymer-gel structure on the macro scale or they are microgels, alternately know as colloidal dispersion gels. Microgels have been purportedly used to treat deeply in matrix reservoir rock. Microgel solutions contain low polymer concentrations (usually < 1,300 ppm) and do not have a continuous crosslinked polymer gel structure on the macro scale. Examples of conformance-improvement microgels are low concentrations of acrylamide polymer crosslinked with aluminum citrate and low concentrations of acrylamide polymer crosslinked with zirconium lactate. As mentioned previously, the conformance-improvement mechanism by which microgels function when treating “matrix-rock” reservoirs is not fully understood.
- The fourth classification relates to the injection mode of the gel chemicals. Early oilfield conformance gel technologies tended to be based on the sequential injection of fluids containing, respectively, two of the reactive chemicals of the gel’s chemical make up. For example, the polymer would be injected in one aqueous fluid followed by injection of the crosslinking agent in a second fluid. This was done at that time because the gelation-delay onset time could not then be controlled and/or delayed sufficiently to permit the gel to be injected into the reservoir as desired and required. This strategy is flawed for two reasons. First, operational constraints almost always require a substantial inert fluid spacer be placed between the two reactive gel-forming solutions to prevent mixing of the reactive chemicals in the injection tubing. This practice essentially precludes formation of a gel in the near-wellbore region. Second, when the second reactive fluid begins to diffuse and/or finger into the first reactive fluid in the reservoir, gel will begin to form and tend to divert the second reactive fluid from further mixing. In general, this outcome leads to the highly inefficient use of injected chemicals. Most current oilfield gel technologies do not involve the sequential injection mode. Essentially all current technologies involve the injection of a single gelant solution that contains all of the gel chemical constituents. State-of-the-art single-fluid gel technologies have sufficient chemical gelation-onset-delay-time chemistries to allow proper placement.
A popular early polymer-gel technology for improving conformance within matrix rock reservoirs was the repeated sequential injection of aqueous slugs containing, respectively, HPAM and aluminum-citrate crosslinking agent. The mechanism by which the sequential-injection aluminum-citrate gel treatments were originally purported to operate was the so-called “layering mechanism.” This mechanism envisioned that repeated sequential injection of the gel-forming aqueous slugs would result in alternate adsorbed layers of crosslinking agent and polymer that would crosslink to form gel that would build out from the pore walls of matrix reservoir rock. The layering mechanism was later recanted by its original proponents. Today, most polymer-gel treatments involving acrylamide polymers crosslinked with aluminum citrate are injected as a single-fluid.
Foamed gels provide the possibility of reducing the unit-volume cost of a given oilfield gel by replacing the bulk of the volume of the relatively expensive liquid phase of a gel with a relatively inexpensive gas phase. Foamed gels, in principal, would combine desirable features of foam-blocking agents and classical gels for use in conformance-improvement treatments, especially for use in the far-wellbore environment in which differential pressures are relatively low. The low density of foamed gels provides a driving force during placement in the reservoir for foamed gels to seek out different and, at times, more favorable flow paths than denser fluids, such as conventional aqueous gel fluids. This would be especially true when foamed gels are placed in highly conductive vertical fractures. Such selective placement could be particularly effective in reducing gas override, as occurs during CO2 flooding in naturally fractured reservoirs of the US Permian Basin and the Rocky Mountain region. Two countervailing issues relating to conformance foamed gels are that foamed gels are relatively more complex chemically and operationally compared with conventional gels, and their low density requires more pump horsepower to be expended during injection than conventional aqueous-based gelants of the same viscosity. Foamed gel has been applied as a conformance-improvement technology at the Rangely field CO2 flooding project.
One of the drawbacks of many gels is their low compressive strengths that prevent effective use when encountering large differential pressures in large-aperture reservoir fluid-flow conduits, such as centimeter aperture fractures or solution channels. When solids are added to polymer gels, the compressive strength of the gel can be greatly increased and increased up to that of Portland cement when the gel is fully loaded with an appropriate solid.
There are numerous near-wellbore conformance problems that are best treated with gel. A small minority of these wells randomly and “unpredictably” require a plugging material with more compressive strength than the gel alone can provide. Previously when such unexpected well and conformance problems were encountered and detected during a gel treatment, a separate cement job had to be called out after, or during, the gel job. When an unexpected well and conformance problem is now encountered, appropriate solids can be added to the gel fluid as it is being pumped and only needs to be added near the end of gel fluid injection to be able to cost effectively plug the flow conduits that have unexpectedly wide apertures. This can be done cost effectively on the fly without the need to subsequently conduct an additional cement squeeze treatment. One of the keys to the effective addition of solids to conformance improvement gels is to know how to precisely control the screen out of such solids addition during the placement of the gel treatment.
Gels functional only in the presence of water
Attempts have been made in the past to develop conformance improvement gel technologies that are functional and active only in the presence of water, but inactive or inactivated in the presence of oil. There has been revived interest in developing such a gel technology. One of the technical challenges that has to be overcome is that even when there is 100% oil flow in a matrix-rock, connate water also exists.
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- 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
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