New pH-Buffered Low-Polymer Borate-Crosslinked Fluids
- Kenneth H. Nimerick (Schlumberger Dowell) | Harry L. Temple (Schlumberger Dowell) | Roger J. Card (Schlumberger Dowell)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- June 1997
- Document Type
- Journal Paper
- 150 - 156
- 1997. Society of Petroleum Engineers
- 2.5.2 Fracturing Materials (Fluids, Proppant), 1.8 Formation Damage, 5.4.10 Microbial Methods, 4.1.5 Processing Equipment, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 4.1.2 Separation and Treating, 4.3.1 Hydrates, 2 Well Completion, 2.4.3 Sand/Solids Control
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The utilization of low concentrations (15 to 20 lbm/1000 gal) of borate-crosslinked guar has not been successful in the past because of the inability to effectively control the pH and B(OH)-4 concentration of the fluid. Today's new pH buffer technology allows the use of low concentrations of guar at temperatures up to 210 F. These new pH-buffered borate-crosslinked guar fluids can be effectively utilized at temperatures up to 240 F by increasing the polymer loading. Due to the lower achievable polymer loadings, these new pH-buffered borate fluids possess excellent cleanup and retained proppant conductivity. In field documented cases, well cleanup times have been reduced by more than half. Laboratory conductivity tests show that these pH-buffered fluids, utilizing reduced polymer loadings, produce retained conductivity values of 10 to 25% (absolute) higher than alternative conventional borate-crosslinked fluids incorporating higher polymer loadings.
Since the 1960s, borate-crosslinked guar has been used to enhance the transport of proppant during fracture stimulation of oil and gas reservoirs. These earlier, as well as many current, borate-crosslinked fracturing fluids utilize a water-soluble boron source (boric acid and other boron-containing compounds) and sodium hydroxide as the alkaline activator to increase the pH to 10.0 to 10.5 or higher. A pH of 9.5 or higher at the fluid use temperature is required in order to convert the majority of boric acid to borate anions. These anions are the effective crosslinking species for borate crosslinking of guar and related polysaccharide based fluids.
Ingri calculated a series of equilibrium constants involving the species of B(OH)3 and B(OH)-4 as a function of pH. From these data the percent of B(OH)3 existing as B(OH)-4 as a function of pH is shown in Figure 1. This information is useful for determining the amount of B(OH)3 needed to effectively borate crosslink guar especially at low-polymer loading. Additional information on borate equilibrium chemistry can be found in the literature.
The next and equally important aspect of crosslinking guar with borate is the effect of temperature on pH. Generally, NaOH is added to the gel containing B(OH)3 to produce a pH of 10 to 10.5 at surface temperatures to initiate an effective borate-crosslinked guar fluid. At these pHs, 60 to 85% of the B(OH)3 used is converted to the crosslinking species of B(OH)-4 (Fig. 1). As the temperature increases, the pH decreases (Fig. 2) with a subsequent reduction in the B(OH)-4 concentration (Fig. 1) which decrosslinks the fluid. Under these 10 to 10.5 pH conditions and using 40-lbm guar and 1.2-lbm B(OH)3/1000 gal gel, we have an effective borate-crosslinked guar at temperatures up to about 180 F. Figure 3 shows the effect of decrosslinking this fluid on viscosity with increasing temperature. The net effect of increasing temperature is to lower pH and reduce the B(OH)-4 concentration which results in decrosslinking and reduced fluid stability.
The use of alkaline activators which resist pH change as a function of temperature (pH stable activators) were found to be much more effective than NaOH for pH control when crosslinking low-polymer loadings. Alkaline stabilizers that produce a buffered pH of about 9.8 at ambient temperatures were chosen. The pH decline with increasing temperature is shown in Fig. 2 for borate-crosslinked guar using a NaOH activator and for a pH-buffered activator. The slope of the pH-buffered fluid is significantly lower than for the NaOH-activated fluid which relates to better high-temperature stability of the fluid. Utilizing a pH-buffered activator provides consistent reproducible surface crosslinks as well as predictable downhole rheology even when using a variety of mix waters to prepare the fluid. Controlling the fluid pH and B(OH)-4 concentration is required for producing reliable 15- and 20-lbm borate-crosslinked guar fluids. These pH-buffered fluids are capable of sustaining a pH above 9.35 (Fig. 2) at temperatures above 200 F.
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