For years, operators in iron-rich mineral basins, such as Woodford/Cana, Delaware and portions of the Eagleford have struggled with a rubbery-like substance adhering to surface treating equipment, accompanied with lost production. As a chemical company and a solutions provider, Innospec’s Oilfield Services Stimulation R&T group, with their knowledge and experience of FR chemistry, wanted to not only treat the issue, but to fundamentally understand the cause of the issue. The solution came about only by surmounting some major scientific obstacles—obstacles tackled by many, but all without long-term successful resolution of the issue.
In the fracturing industry, it is common to use synthetic friction reducers (FRs) to overcome the high friction pressures associated with pumping large water volumes at high rates. The physical form of the FR is either invert emulsion, suspension or dry powder. However, regardless of the FR form, the chemistry of the polymer is critical. The chemistry of most common frac FRs is either anionic or cationic polyacrylamide co-polymers (see Scheme 1).
These polymers are very high molecular weight, so even at the dilute concentrations used for friction reduction they still allow, under proper conditions, the formation of undesirable “goo” responsible for impaired fluid flow and production. Although an important part of the “goo,” the polymer itself must be subjected to downhole environmental conditions to cause the issue. Furthermore, the chemistry of the friction reducer polymer is a major factor in the formation of the “goo” and is highly dependent on the carboxylate content of the polymer.
Carboxylic acids are weak acids and partially dissociate in solution to form the negatively charged carboxylate anion (see Equation 1).
The pendent carboxylate, like all weak acids, possesses an acid dissociation constant, or pKa, and this value is a gauge of the acid strength. The definition of pKa is given in Equation 2 where Ka is the acid dissociation constant. The lower the pKa value, the stronger the acid or a larger percentage of the acid concentration is dissociated in water. The pKa of acrylic acid, a monomer in the acrylamide (AcAm) acrylate (AA) co-polymer, is 4.25. This means at pH 4.25, half the concentration of acid is dissociated and in the other half, the acid is protonated and having a neutral charge. Generally, any acid, such as HCl, with a pKa less than zero is completely dissociated and is considered a “strong” acid. For comparison of acrylic acid to other acids, see the following chart.
Acrylic acid, being a weak acid, and in its anionic form referred to as a carboxylate, produces charge repulsion between other carboxylate groups attached to the polymer that, in turn, acts to expand the volume of polymer when added to water. It’s this polymer chain expansion, of the high molecular weight polyacrylamide polymer, that provides the friction reducer with its efficiency and effectiveness in slick-water fracturing. In normal frac water with the pH ranging from 6.0 to 8.5, the dissociated form of acrylic acid (acrylate) is highly dominant, assuring high anionic character of the polymer for maximum chain expansion needed for friction reduction.
In the polymerization of acrylamide and sodium acrylate to produce the FR, the anionic acrylate monomer is randomly distributed along the polymer chain. Because the percentage of acrylate groups is typically about 30% (by mole), it is highly probable small blocks of acrylate exist within the polymer chain. These blocks resemble small islands of scale inhibitor moieties within the polymer chain and can interact with polyvalent ions in the water.
Multivalent Ion Impact on FR Agglomeration
In waters containing divalent and multivalent ion species (Ca+2, Mg+2, Fe+2,+3 , Al+3, etc.), there will be associations that form between the anionic acrylate ligands and the metal ion. For the purposes and remainder of this article, we will focus on the interactions between polymer FRs and iron, as this is related to the agglomeration.
In the case of Fe+2,+3, for example, these ion associations can generate intra- (within the same polymer chain) and inter- (two or more polymer chains) molecular crosslink junctions in the AcAm/AA FR co-polymer (see Scheme 2).
Couple the crosslinking, between iron and the weak acid component of the acrylate/acrylamide copolymer, with the known flocculation characteristics of these ultra-high molecular weight (15 – 18 x 106 Da) FR polymers, and you have unleashed the potential formation of an agglomeration nightmare. These same FR polymers are used in water treatment as flocculation aids to reduce total suspended solids (TSS). This agglomeration down-hole is “goo,” and can be comprised of a mixture of FR, metal ions (such as iron), formation fines, clay, oil etc., as shown in the drawing below.
So, if you have high iron (>20ppm) content in your connate or frac fluid brines with the potential to generate Fe+3 ions, and you are using a co-polymer of AcAm/AA, you have a high probability of generating a “goo” nightmare. Now, you may be thinking that if the AcAm/AA co-polymer has a cause/effect issue with iron-laden waters, you should merely use a cationic FR. This choice will give you positive returns initially, but because of a weak link between the cationic group and the polymer chain, the cationic groups will eventually hydrolyze, causing the cationic group to split off the polymer chain as a molecule similar to the KCl substitute chemical, choline chloride (see Scheme 3). The hydrolysis not only reduces the cationic character of the polymer, it also produces the identical carboxylate groups occurring in the acrylamide-sodium acrylate co-polymer, AcAm/AA, described above.
Changing to a cationic FR is normally not the right, long-term choice to prevent the formation of “goo” damage. If you are in iron-laden waters, the chemistry of the FR is critical and should be chosen with careful consideration of the environment the FR will be exposed.
So now that we know what not to use, what type of FR do we use? Well, there are various less efficient FRs such as guar gum or guar derivatives, but the loadings needed are slightly higher and they tend to produce proppant pack and formation damage, regardless of the water chemistry. In addition, they tend to be good food sources for bacteria.
Fortunately, there is now a proven solution, HiRate MAXX -3200G that provides effective, efficient friction reduction at low concentrations and is completely immune to water chemistry—both immediately after treatment as well as long-term.
In field trials, last year in the Woodford Cana field, a high-iron prone formation in Oklahoma, Innospec’s ground-breaking HiRate MAXX-3200G friction reducer was pumped at 0.3 gpt and was able to place 3.5 ppga 40/70 sand, as per treatment design. In fact, when experimenting with the loading during the early sand sub-stages, the minimum effective concentration in this formation and frac design was 0.15 gpt. Flow back samples collected after HiRate MAXX-3200G treatment were monitored for six months without any presence of “goo” and the operator has reported no “goo” related issues for 18 months up to the publication of this article. Afterward, multiple wells were treated in the same area HiRate MAXX 3200G —without the generation of any “goo.”
Innospec’s patent pending polymer is also anionic, but rather than relying on weak acid carboxylates, the new HiRate MAXX-3200G relies on a strong acid pendent group having very little affinity for cations in the water chemistry, especially iron. This inability to react with iron short circuits the goo formation while the polymer also provides effective and efficient friction reduction, allowing for low loadings of polymer in slick-water treatments. The friction reduction efficiency is based on the polymer’s anionicity due to the strong acid pendent group to assure a high degree of charge repulsion between anionic groups, maximizing polymer chain expansion in any water, regardless of TDS. This same polymer is used in slick-water treatments using produced water exceeding 200,000 ppm TDS.
For Innospec Oilfield Services, “chemistry matters” is more than a tagline. It is the guiding principle that drives our business partnerships. We are not just a chemical provider, we are a chemical company, providing affordable, fit-for-purpose solutions. In this case, our unique approach to our client’s agglomeration issues helped to determine that prevention by polymer selection is critical. Innospec’s HiRate MAXX 3200G was specifically designed to short circuit the polymer-iron interactions to prevent the “goo” seed from ever forming, therby preventing future production and surface facility issues.
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