Fluids for conventional hydraulic fracturing have power-law rheology. Essentially, the apparent viscosity in a power-law fluid will reduce with exposure to shear (whilst pumped through equipment, flowlines and perforations). Cross-linked fluids are generating by adding a metallic cross-linker to a linear gel (usually a water base with added guar, or other viscosity generating polymer). The molecules in the cross-linker bind the polymers present in the linear gel, increasing the viscosity of the fluid. Common cross-linkers are:
- Borate – a pH dependent cross-linker which recovers its viscosity after exposure to shear. After shearing in the equipment, wellbore and perforations, the fluid will be able to suspend proppant control leak-off and keep the fracture open, and/or propagating
- Zirconate – a high-temperature cross-linker, where some viscosity will carry the proppant into the formation, but most of the viscosity development occurs in the formation on exposure to the insitu temperature, to suspend the proppant, control leak-off and keep the fracture open and/or propagating
- Titanate – a wide range pH cross-linker for use in acid fracturing cases, to be compatible with the low pH of acid and spent acid.
- Aluminium – a low pH and low temperature cross-linker for cross-linking “cleaner” linear gels.
Any of these cross-linking systems can also be delayed to viscosify at the optimum position in the wellbore, by varying concentrations of the cross-linker used, or using pH buffers for pH activated cross-linkers.