Shear Sensitivity of Borate Fracturing Fluids

Abstract

Borate-crosslinked fracturing fluids have been used in the oil and gas industry for nearly 40 years. These fluids consist of three basic components (polymer, crosslinker, and pH buffer) which are considered relatively simple to optimize for a variety of field applications. Among the unique features of this fluid is the ability of the crosslink viscosity to "re-heal" or "recover" after exposure to high shear rates. Based on laboratory tests described in SPE 134266, it was determined that the "re-healing" time can be excessive for some borate-crosslinked fluids exposed to high shear, resulting in limited near-wellbore viscosity and screen-outs in the field. Using a laboratory-scale flow loop to simulate the wellbore shear environment, various borate-crosslinked fluids were exposed to a wide range of shear history conditions before loading into a high temperature, high pressure rheometer. In addition to the common viscosity versus time profile, the early-time viscosity development of each fluid was analyzed to quantify the effect of shear history on recovery time. This paper defines critical shear rates and exposure times that adversely impact early-time viscosity development of borate-crosslinked fracturing fluids. Also, the test results show that the impact of wellbore shear conditions on recovery time can be minimized by adjusting the concentration of the polymer, borate crosslinker, and/or pH buffer. The techniques and guidelines provided in this paper can be used to identify detrimental wellbore shear conditions that will lead to excessive recovery times. The paper also demonstrates optimizing borate-crosslinked fluids with common on-site tests can result in fluid compositions with increased shear sensitivity.

A review of fracturing fluid systems used for hydraulic fracturing of oil and gas wells

“…Aluminum is only effective within the pH range of 3–5. Zirconate can be applied at temperatures as high as 400°F, borate and titanate can be applied up to 325°F, and aluminum can only be applied at temperatures below 150°F. Borate can be applied at pH values of 7.5 and above depending on the temperature. Although borate crosslinks are not as susceptible to shear rate and shear history as other metal ions, some sensitivity to shear history especially during the early‐time viscosity development was reported by Bjornen et al…”

“…Zirconate can be applied at temperatures as high as 400 F, borate and titanate can be applied up to 325 F, and aluminum can only be applied at temperatures below 150 F. Borate can be applied at pH values of 7.5 and above depending on the temperature. Although borate crosslinks are not as susceptible to shear rate and shear history as other metal ions, some sensitivity to shear history especially during the early-time viscosity development was reported by Bjornen et al 16 Borate ions are the most commonly used crosslinking agents for guar polymer applications. Borax (sodium tetraboratedecahydrate) and boric acid (plus caustic soda) with crosslinking agent concentrations ranging from 0.024 to 0.09% w/w have been used as sources of borate ions to crosslink guar.…”

Water Management in Unconventional Oil and Gas Development—The Issues and Their Optimization

“…The high shear rates experienced by the fluid occur when the fluid is being pumped through the wellbore tubing. 12,16 Slickwater Fluids 12 Gelled fluids are known to exhibit shear-thinning properties, which mean that the viscosity of the fluid decreases with increasing shear stress.…”