Mixing Viscosified Pills

Introduction

Mixing a viscosified pill is a common task. The hydration and yield time for hydroxyethylcellulose (HEC) polymer varies in completion fluids that have different salt compositions and different densities. Factors which affect HEC hydration are time, temperature, shearing, fluid composition, pH, and HEC formulation.

Determining Product Type

Single Salt Fluids. TETRAVis can be used in any low density clear brine fluid when sufficient agitation, shearing, and time are available. TETRAVis L, an economical prehydrated polymer, is well suited for use in all single salt fluids. Its advantages are a shorter hydration time than that of a dry product, a decrease in the possibility of fish eyes, and greater ease of handling.

Multisalt Fluids. TETRAVis L Plus is the preferred viscosified pill for multisalt and higher density fluids. It is double the strength of TETRAVis L.

Brine Type

• All low density (<12.0 lb/gal) single salt fluids can be viscosified readily with TETRAVis or TETRAVis L. Excessive yield times can often be shortened by heating the fluid to a temperature higher than 130°F or by using TETRAVis L, as it is prehydrated.

• High density two salt fluids, such as NaCl/NaBr and CaCl₂/CaBr₂, may be viscosified with TETRAVis L or TETRAVis L Plus. Hydration times in these fluids will vary as a function of the salt and water concentrations of each particular fluid blend. Prior to the actual blending of any pill, a fluids specialist should run a pilot test.

• ZnBr₂/CaBr₂/CaCl₂ fluids can be viscosified with TETRAVis L Plus, as long as the ZnBr₂ concentration is greater than 28-30% by weight. When the ZnBr₂ concentration in the working fluid is less than this range, cut a 19.2 lb/gal ZnBr₂ spike fluid back with water to the desired density to make your pill. (This process is illustrated in Example J.)

• Based on standard composition, 19.2 lb/gal ZnBr₂ can be cut back with fresh water to a density of 12.1 lb/gal and still result in a fluid containing 30% ZnBr₂, which is sufficient to ensure viscosification.

Example J shows the calculation used to determine the fraction of both 19.2 lb/gal ZnBr2 fluid and fresh water required to prepare 1 bbl of 15.9 lb/gal final working fluid. The process in the following example is identical to the earlier example of mixing two known fluids to achieve a fluid of a certain target density. It uses Equation 8 and Equation 9. Table 12, which follows the example, provides cutback values for 19.2 lb/gal zinc bromide (ZnBr₂).

EXAMPLE J. Determining the Fraction of 19.2 lb/gal ZnBr₂ Fresh Water Required to Prepare one bbl of 15.9 lb/gal Fluid

For a 15.9 lb/gal working fluid, your pill will have the proportions of 0.696 bbl 19.2 lb/gal spike fluid and 0.304 bbl fresh water.

TABLE 12. 19.2 lb/gal Zinc Bromide (ZnBr₂) Cutback Table

Densities are approximate.

TETRAVis L Plus Curves

TETRAVis L Plus is TETRA's proprietary blend of prehydrated hydroxyethyl cellulose (HEC). The optimum amount of HEC polymer required in completion fluids varies with differences in salt composition and density. The curves in Figure 7 through Figure 15 have been provided as a useful reference. For each brine group, the first illustration shows viscosity in Marsh Funnel seconds at different concentrations of TETRAVis L Plus in gal/bbl. The second illustration for each brine group shows yield point at different concentrations of TETRAVis L Plus in gal/bbl. The final illustration for each brine group shows plastic viscosity at different concentrations of TETRAVis L Plus in gal/bbl.

FIGURE 7. Marsh Funnel Viscosity of Low Density CBFs

^{(Measured at 75°F)}

FIGURE 8. Yield Point of Low Density CBFs

^{(Measured at 75°F)}

FIGURE 9. Plastic Viscosity of Low Density CBFs

^{(Measured at 75°F)}

FIGURE 10. Marsh Funnel Viscosity of Calcium Brines

^{(Measured at 75°F)}

FIGURE 11. Yield Point of Calcium Brines

^{(Measured at 75°F)}

FIGURE 12. Plastic Viscosity of Calcium Brines

^{(Measured at 75°F)}

FIGURE 13. Marsh Funnel Viscosity of Zinc/Calcium Bromide Brines

^{(Measured at 75°F)}

FIGURE 14. Yield Point of Zinc/Calcium Bromide Brines

^{(Measured at 75°F)}

FIGURE 15. Plastic Viscosity of Zinc/Calcium Bromide Brines

^{(Measured at 75°F)}

Blending Equipment

In viscosifying any CBF, vigorous agitation is essential for efficient polymer hydration. This is especially critical for the dry product. You have a number of options when it comes to suitable blending equipment:

1. Usually the best available equipment for blending TETRAVis pills on location is a sand control blender.
2. Cement unit displacement tanks can be used for blending TETRAVis pills on location.
3. The rig slugging pit can also be used for blending TETRAVis pills at the rig site; however, due to their low energy shearing action, they are less cost effective than other options.