Delta Foams: Ultra Low Polymer Fracturing Fluids

By Phil Harris, Halliburton Energy Services, Inc.

Foam fluids make up about one-third of the fracture-stimulation treatments currently performed in the United States. Foam fluids were first used to stimulate low-pressure reservoirs because the gas in the foam helps recover the liquid portion of the fluid. However, foam fluids also demonstrate the following benefits:

  • The gas assist helps kick off production (usually without swabbing).
  • Because they are only about one-third liquid, foams cause minimal formation damage.
  • The liquid phase usually contains a gelling agent. Because of the reduced liquid loading, less polymer is pumped into the formation.
  • Halliburton has over 20 years experience with foam stimulation and has led the industry with dedicated foam-research facilities and a continuous effort to advance foam-fracturing technology. In fact, foam technology has progressed from simple water foams and linear-gel foams to crosslinked foams with nitrogen and/or carbon dioxide. With the recent development of Delta Frac fluid, a low polymer-concentration, borate-crosslinked fluid, foam technology has once again been advanced. Delta foams allow fracture-stimulation treatments with low polymer concentrations. Delta Frac fluids typically contain 15 to 25 lb/Mgal of guar polymer. These fluids can be used directly in foam. Foam structure also contributes to the overall viscosity of the fluid; therefore, Delta Foam fracturing treatments can be performed with 10 to 15 lb/Mgal less guar than conventional borate foam treatments or linear-gel foam treatments. In fact, Delta Foam can contain as little as 10 to 20 lb/Mgal of guar in the liquid phase.

    Foam Viscosity

    Fracture simulators are an essential part of fracturing technology. Simulators calculate volumes of fluid, schedule pumping rates, and estimate pressures required to generate fractures in subterranean formations. Fluid rheology determines many of the characteristics of fracture growth and must be accurately estimated to give reasonable predictions of fracture geometry. Therefore, when a fracture-stimulation treatment will be performed, fluid viscosity must be determined. The following three factors contribute to the viscosity development of crosslinked foams:

  • Foam viscosity increases as nitrogen (N2) quality increases.
  • Foam viscosity increases as polymer concentration increases.
  • Foam viscosity increases when the polymer is crosslinked.
  • Data gathered from a high-pressure, flow-loop viscometer shows that viscosity increases when gas is added at room temperature (Figure 1). In addition to foam quality, polymer concentration can increase foam viscosity (Figure 2).

    Figure 1: Foam Viscosity, Varying Gas Quality (40 lb/1,000 gal guar, 75°F)

    Figure 2: Foam Viscosity, Varying Polymer Concentration (75°F, 70% gas)

    Measuring Viscosity

    Noncrosslinked Foams

    To measure a foam fluid's viscosity at a specific temperature, we must generate the foam fluid and increase its temperature while monitoring viscosity. For linear-gel foam fluids, viscosity is maximum at 75°F and declines steadily as the temperature increases to 300°F (Figure 3). For nitrogen, carbon dioxide, or mixed-gas foams containing noncrosslinked guar polymers, viscosity can be calculated with the power-law parameters (n' and K') of the base gel fluid, the gas quality of the fluid, and the temperature of the fluid.1

    Figure 3: Foam Viscosity, Varying Temperature (40 lb/Mgal guar, 70% gas)

    Crosslinked Foams

    For linear-gel fluids, the straightforward viscosity calculation procedure is effective. However, predicting the viscosity of crosslinked foam fluids is more difficult because these fluids do not reach maximum viscosity at 75°F. The theory of foam viscosity states that the viscosity of the external phase (along with internal-phase quality) dominates the rheological character of the two-phase fluid. Flowing foam will attain an equilibrium texture that depends on the shear rate, the pressure, and the viscosity of the external phase.2 Equilibrium is dynamic, not static, so the foam lamellae are constantly changing and rearranging. In addition, the shear sensitivity of a borate gel in a fine-bubble structure may differ from the shear sensitivity of a borate gel in the bulk phase. If the structure of a borate gel is sensitive to the shear conditions within the structure of foam, then the viscosity of a borate-crosslinked foam may decline more rapidly than the viscosity of a linear foam.

    Nonfoamed Fluids

    Conventional Borate-Crosslinked Fluids

    Individual borate-crosslinked fluids show peak viscosity performance over a narrow temperature range. The viscosity of these fluids is strongly dependent on pH and borate ion concentration; no single borate fluid can represent the viscous behavior of the whole family of fluids over the entire temperature range.3 Figure 4 indicates that the viscosity of the particular nonfoamed, borate-crosslinked fluid under study was dependent on both temperature and pH. The peak viscosities with several fluids with different pH create a composite maximum-viscosity curve with negative temperature dependence (negative slope) for a pH above 8.5. The slope becomes more negative as temperature increases. In addition, the composite curve of minimum viscosities past the high-temperature peaks for the fluids of different pH describes a fluid that is no longer crosslinked but has reverted back to base-gel viscosity. The viscosity vs. temperature curve for a linear base gel would be identical to the minimum curve shown in Figure 4. At a given temperature, the ratio of the value of the maximum viscosity curve to that of the minimum viscosity curve indicates numerically how much benefit is derived from adding the crosslinker. If this ratio is known, crosslinked viscosity can be determined.

    Figure 4: Viscosity of Borate-crosslinked Gel (30 lb/Mgal guar, varied pH)

    Delta Frac Fluids

    Nonfoamed, borate-crosslinked Delta Frac fluids and conventional, nonfoamed borate fluids both have temperature and pH dependence.4 Figure 5 shows that viscosity performance can be improved at increasing temperatures if the crosslinker concentration is increased. The same principles of maximum and minimum composite viscosity that apply to conventional borate fluids also apply to nonfoamed Delta Frac fluids (Figure 4).

    Delta Frac fluids have been optimized for performance over a narrow temperature range. As a result, these fluids produce substantially higher viscosities than conventional borate fluids that were developed for a much broader temperature range. The maximum viscosity of optimized 25-lb/Mgal Delta fluids is about twice the maximum viscosity of 30-lb/Mgal conventional borate fluids.

    Foamed Fluids

    The viscosity of conventional borate-crosslinked gels can be demonstrated in foam fluids.5 Figure 6 shows individual foam fluids containing different pH borate gels as the liquid phase. As with the nonfoamed borate gels (Figure 5), viscosity performance differs over a range of temperatures. The maximum viscosity line of Figure 6 is similar to maximum viscosity lines for nonfoamed borate fluids; however, decreases in borate-crosslinked foam viscosity are not as dramatic as decreases in non-foamed fluid viscosity. The minimum viscosity line of Figure 6 can be used to represent noncrosslinked, linear-gel foam fluids.

    Figure 5: Viscosity of 25 lb Delta 140 Fluid (Varying Crosslinker)

    Figure 6: Borate-crosslinked Foam (25 lb guar, 50 quality N2)

    Figures 5 and 6 illustrate that maximum and minimum viscosity curves exist for all crosslinked foam fluid systems, including conventional borate systems as well as Delta Foam systems. However, Delta Foams have lower polymer loadings and higher viscosities than conventional borate foams. Figure 7 illustrates the high viscosities of individual Delta Foams for temperatures between 90° and 120°F.

    Figure 7: 70-Quality Delta Foam

    For a specific crosslinked foam at a specific temperature, the ratio between the maximum viscosity line and the minimum viscosity line represents the benefit of crosslinking. Because the minimum viscosity line for linear-gel foams can be calculated from Reference 1, we can apply the multiplying ratio to that specific viscosity and calculate the viscosity of the corresponding crosslinked foam fluid. Figure 8 is an example of the viscosity ratios of 50-quality N2 foams containing 15 to 35 lb of guar crosslinked with conventional borate.

    Figure 8: Viscosity Ratio Crosslinked to Linear-gel Foam (50-quality N2, borate guar)

    Figures 9 and 10 compare the viscosity performance of linear-gel foams, conventional borate fluids, and optimized Delta Foam fluids. Delta Foam systems replace noncrosslinked guar fluids with low-guar, borate-crosslinked fluids of equal viscosity. As Figure 11 illustrates, the viscosity of a 30-lb guar foam is similar to that of a 10- to 12.5-lb Delta Foam. These lower polymer loadings present less damage potential to the formation.

    Figure 9: Comparison of 70-Quality N2 Foams with 15 lb/Mgal guar

    Figure 10: Comparison of 70-Quality N2 Foams with 20 lb/Mgal guar

    Figure 11: Delta Foam Viscosity versus Linear-gel Foam Viscosity (100°F)

    Conclusion

    Gas quality, polymer concentration, and crosslinked character all influence the viscosity development of the crosslinked foam fluids. We used the ratio between linear-gel foams and crosslinked foams as a multiplying factor to predict the viscosity of crosslinked foams from the viscosity of linear foams. The results confirmed that crosslinked foams require less polymer in the external liquid phase. Reducing polymers reduces potential damage to the proppant pack and the formation. The new generation of Delta Foam fluids represents the lowest required polymer for high-viscosity fracturing fluids.

    References

    Refer to the Frac/Acid technical papers for these documents.

      1. Harris, P.C.: "A Comparison of Mixed-Gas Foams with N2 and CO2 Foam Fracturing Fluids on a Flow-Loop Viscometer," SPE Production & Facilities (Aug. 1995) 197-203. [SPE 20642]
      2. Harris, P.C: "Effects of Texture on Rheology of Foam Fracturing Fluids," SPE Production Engineering (Aug. 1989) 249-257. [SPE 14257]
      3. Harris, P.C.: "Chemistry and Rheology of Borate Crosslinked Fluids at Temperatures up to 300°F," JPT (March 1993) 264-269. [SPE 24339]
      4. Harris, P.C. and Pippen, Mike: "High Rate Foam Fracturing," paper SPE 39959 presented at the 1998 Low Permeability Reservoirs Symposium, Denver, CO, 5-8 April
      5. Harris, P.C. & Heath, S.J: "Rheology of Crosslinked Foams," SPE Production & Facilities (May 1996) 113-116. [SPE 28512]