High-Performance Brine Viscosifiers for High Temperatures

Fig. 1—Generalized illustration of the association of a polycation with an anionic surfactant. In the case of the brine viscosifier described here, the blue structures are Polymer A and the yellow structures are the surfactant.

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Many viscosifiers currently used in high-density solids-free reservoir drilling fluids (RDFs) and completion fluids (CFs) are incompatible with high-density brines or require the use of prohibitively expensive brines to achieve target densities. There is substantial commercial benefit to developing a brine viscosifier with a higher temperature tolerance than is currently offered. A supramolecular viscosifier package has been developed that uses noncovalent associations between additives to enhance the thermal resilience of divalent brine fluids. Static aging-test data indicate that the supramolecular viscosifier outperforms high-performance, commercially available starch/xanthan brine viscosifier. 

Background

The exploitation of oil and gas reservoirs found at increasing depths and the development of horizontal and slimhole drilling have increased the demand for high-density solids-free RDFs, CFs, and workover fluids (WFs). The use of divalent brine-based drilling fluids and CFs in well sections with temperatures exceeding 260°F is limited because of degradation of the polymers used to viscosify the fluid and suspend drill cuttings.  

Effective drilling fluids have numerous functions in well construction. They must have the physical properties required to carry cuttings from beneath the drill bit up the annulus of the well for separation at the surface. They must also provide cooling for the bit and be able to reduce friction between the drillstring and the formation being drilled. This must be accomplished without damaging the formation. The inflow of fluids or solids from the drilling mud can cause formation damage, which can reduce the rate of penetration during drilling and can also decrease the rate of hydrocarbon production from a producing well.  

RDFs, CFs, and WFs must be density-adjusted to provide the hydrostatic head to preserve the integrity of the wellbore walls and to prevent blowouts. Although drilling through much of the well can be accomplished with oil- or water-based fluids weighted by solid particles of such minerals as barite, the reservoir sections offer a different challenge because of the concern that these solids could plug pores in the formation and reduce hydrocarbon-flow rates. It is for this reason that operators look to brines to provide density in RDFs. Recently, research in nanomaterials has revealed the potential of low-solids nanofluids for the prevention of formation damage. 

Low-solids brines are also typically sought in CFs and WFs. CFs are those fluids used to flush potentially formation-damaging materials from the wellbore after drilling and before perforation. These materials include drilling-fluid additives such as fluid-loss agents that may adhere to the formation face, formation cuttings and clays entrained in the drilling fluid and deposited on the face of a formation, and filter cake on the formation left from the drilling fluid. The filter cake typically contains solid materials from drilling-fluid-additive residue from the drilling fluid. CFs control well pressure, prevent the collapse of tubing from overpressure, and provide fluid-loss control. Fluid-loss-control agents can be added to the bulk CF or supplied as a pill. Typical fluid-loss pills include oil-soluble resins, calcium carbonate (CaCO3), and ground salt.  

WFs are typically used in cleaning and repairing old wells to increase production. CFs, WFs, and kill fluids are typically designed to prevent fluid from the formation from intruding into the wellbore while preventing wellbore-fluid leakoff, which is known to cause formation damage. Brines are used in WFs just as they are in RDFs and CFs to mitigate the formation damage imparted through the use of fluids with insoluble particulates such as barite.  

Material costs, as well as the required brine density, typically drive the selection of the brine. The brines typically used—with corresponding density ranges—include the following:

  • potassium formate, 8.4–13.1 lbm/gal
  • potassium chloride, 8.3–9.7 lbm/gal
  • sodium chloride, 8.3–10.0 lbm/gal
  • sodium bromide, 8.4–12.5 lbm/gal 
  • calcium chloride (CaCl2), 8.4–11.8 lbm/gal
  • calcium bromide (CaBr2), 8.4–14.2 lbm/gal
  • CaCl2/CaBr2 mix, 11.8–15.2 lbm/gal
  • CaCl2/CaBr2/zinc bromide (ZnBr2) mix, 14.2–19.2 lbm/gal
  • ZnBr2, 14.2–19.2 lbm/gal
  • cesium formate, 15.0–19.2 lbm/gal  

The Need for Viscosifiers

Brines require viscosification to suspend cuttings and to help limit fluid loss to the formation. Typical polymers currently used to viscosify brines are carboxymethyl cellulose, hydroxyethyl cellulose, xanthan gum, guar gum, and hydroxypropyl guar. Fluid loss is an important issue for brines. For this reason, polymers are typically used in tandem with bridging agents to regulate fluid-loss control in brines. The polymers used for fluid-loss control are usually crosslinked starch, while the bridging agents are sized particles of CaCO3. The CaCO3 is available in sizes from several millimeters to several microns. These particles are readily solubilized in acid. In order to allow the fluids to maintain their viscosities and yield points at higher temperature, pH buffers, antioxidants, and oxygen scavengers are often added. Monoethanol amine is commonly used as an antioxidant to protect the viscosifier by inhibiting hydroxyl radicals from degrading the polymer structure. Magnesium oxide (MgO) is often added as a buffer to the pH and to inhibit hydrolysis of the polymers in the brine.  

The commercially available fluids reported in the complete paper contain a dual-functional viscosifier and fluid-loss additive (xanthan and crosslinked starch), a high-surface-area grade of MgO for a pH buffer, a silicone defoamer, monoethanolamine, and sodium thiosulfate as an oxygen scavenger. In addition to these additives are the bridging agents (CaCO3), with an average particle size from 2 to 12 μm.

The viscosifier described in the complete paper can be applied as a drop-in replacement for xanthan and crosslinked starch in a standard brine-formulation package used in RDFs, CFs, and WFs. The resulting fluids with this viscosifier display temperature tolerance superior to that of the RDF currently used in divalent brines. The product is described as a supramolecular viscosifier package using noncovalent associations between additives, enhancing the thermal resilience of divalent brine fluids. The product performance of an RDF formulated with the supramolecular viscosifier package is compared with that of a standard starch-based RDF.

This thermally resilient synthetic viscosifier is proposed as a direct replacement for the xanthan/crosslinked-starch system. This viscosifier results from the combination of polymer and surfactant. Polymer A is known to form tight associations with certain surfactants. The proposed structure formed from this association is a “necklace-and-bead”-type structure, as depicted in Fig. 1 above.  

The supramolecular viscosifier outperforms the high-performance, commercially available starch/xanthan brine viscosifier. The addition of this new polymer blend increases the brine suspendability both before and after aging at high temperature. The resulting fluid displays enhanced performance over the leading commercially available solids-free fluids. This viscosifier package involves synergies of additives within the brine that greatly enhance the thermal stability of the polymers in the brine. The enhanced performance is highlighted through dynamic and static aging tests from 300 to 325°F.  

Much of the complete paper is devoted to a detailed discussion of experiments describing the use of some polymeric and surfactant additives that may serve as “drop-in” replacements for xanthan and starch in completion brines and RDFs.  

Conclusions

A brine viscosifier has been developed from multiple molecular building blocks. The viscosifier was tested in comparison with the industry-standard crosslinked-starch/xanthan. Where the standard fails to perform (above 260°F), the supramolecular viscosifier maintains substantial structure and thermal resilience such that a brine formulated with this viscosifier could suspend drill cuttings as its intended purpose. The ability to suspend cuttings is evidenced in post-thermal-aging yield points greater than 2 lbf/100 ft2. It is proposed that the combination of the materials and the molecular interactions of the additives with one another enhance the thermal resilience. 

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 183964, “High-Performance Brine Viscosifiers for High Temperatures,” by Peter J. Boul, Suhaib Abdulquddos, and Carl J. Thaemlitz, Saudi Aramco, prepared for the 2017 SPE Middle East Oil and Gas Show and Conference, Manama, Bahrain, 6–9 March. The paper has not been peer reviewed.

High-Performance Brine Viscosifiers for High Temperatures

01 November 2017

Volume: 69 | Issue: 11

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