Gelled fluids are routinely used in hydraulic fracturing to deliver fractures with higher propped width, due to the fluids' abilities to deliver higher proppant concentrations than their lower-viscosity analogues. The most common of these gels (borate-crosslinked polymers) have been used successfully in fracturing for decades, routinely exhibiting viscosity >100cP in conventional high-pressure, high temperature (HPHT) viscosity measurements. Recent studies have found that the viscosities of borate gels at actual downhole pressure conditions may be 80% less than that measured using standard HPHT rheometer measurement (which uses 400 psi top-pressure). A proposed mechanism for this phenomenon is a pressure-induced shifting of the crosslink/temperature stability near the "melt temperature" of borate-crosslinked gels, leading to a reversible thinning. A novel approach is posed in the current work to exploit the "pressure effect", by capturing the pressure-thinned fluid in a thin state and irreversibly breaking the gel viscosity.
As existing crosslinked-gels with oxidative breakers rarely achieve >50% retained pack-permeability, an improved break mechanism based on pressure could yield sizeable productivity-gain in propped fractures. A series of candidate breaker materials with potential affinity for the crosslinker was screened for gel-breaking performance in the current work. Preliminary screening of these additives led to one preferred alternative gel breaker material which demonstrated superior performance in pressure-induced breaking behavior. This study will illustrate advanced characterization of the breaker performance utilizing realistic sequences of temperature, pressure and shear rate that would be expected for a fracturing treatment. The advanced characterization also included fracture-conductivity measurements comparing a control gel with and without the alternative breaker.
Under these conditions, the fluids containing the candidate breaker material showed persistent viscosity-break, while the control fluids recovered their viscosities upon pressure-release. The preferred breaker material performance was further confirmed through a series of experimental conditions varying temperature, pressure (up to 10,000 psi), gel concentration, and breaker concentration. In fracture conductivity measurements, the gel which contained the preferred breaker also showed improved conductivity compared to control fluids without breaker.
The current study proposes a new method of achieving break of gelled fracturing fluid, which avoids some of the risks and disadvantages of oxidizer chemicals. These results will suggest the most preferred fluid conditions for applying the new breaker material in hydraulic fracturing.