In conventional hydraulic fracturing operations, specially designed fluids are pumped downhole at high rates and pressures, which fracture the reservoir rock, and create a new narrow opening. As fluid is continually placed in the reservoir, sand or synthetic proppant (ceramic) is added to the fluid in increasing concentrations, to hold the fracture open once the fluid pumping and high pressure maintenance has ceased.
A viscous fluid is used to initially fracture the reservoir, generate the width of the opening, and to control the leak-off of fluid into the formation. Several systems are available for increasing the viscosity of the fracturing fluid system, the most widely used being guar. Varying concentrations of guar are added to a based fluid, usually water, to create a linear gel. This is called hydrating the polymer, and higher guar concentrations give higher viscosities. On addition of a cross-linker, the linear fluid then develops further, complex viscosity, and becomes a cross-linked system, with power-law rheology.
There are many types of guar available for creating a linear gel:
- Guar – the most commonly used and available. Within the Guar category, there are also varying levels of “cleanliness” of guar, that is, some will leave less residual damage in the formation after a clean-up operation. Hydroxypropyl Guar (HPG) is said to be cleaner than standard Guar, Carboxy-Methyl Hydroxypropyl Guar (CMHPG) is thought to be even cleaner still
- Cellulose – Hydroxyethyl Cellulose (HEC), and Carboxy-Methyl Hydroxyethyl cellulose (CMHEC), commonly used in gravel packing
- Xanthan – less common in a fracturing fluid system, however thermally stable and therefore still has applications when treating high-temperature reservoirs.
In unconventional hydraulic fracturing treatments in shales, linear gel additives are used as the only viscosity generating additives, as the leak-off occurs at a much lower rate, and lower viscosity fluids help to generate the longer and narrower fractures more suitable for the unconventional environment.