Most conventional and unconventional reservoirs use viscous fluid systems for hydraulic fracturing for various reasons, such as good proppant transport, less fluid loss, less pumping friction pressure, etc. Salt is often introduced as part of the fluid system for clay protection as well as to increase hydrostatic pressure, resulting in reduced surface pressure. This paper investigates the effect of salt concentration on reduced viscosity for different types of polymers used in fracturing fluids.
Different polymers, such as guar, hydroxypropyl guar (HPG), carboxymethyl hydroxypropyl guar (CMHPG), carboxymethyl cellulose (CMC) and other biopolymers, are used for preparation of fracturing fluids. Aqueous solutions of these polymers become viscous gel on hydration. KCl salts at different concentrations were mixed with the different polymer gels at room temperature and the viscosity change was observed at a shear rate of 511sec-1. The effect of salt on the hydration rate was also observed at room temperature for the different polymers. A hydration time of up to 60 min was maintained. Plots with different polymers were compared at fixed salt concentrations to identify which polymers experienced more severe effects. It was observed that the CMC category fluids experienced a high reduction to viscosity with the salt content than other fluids. A chemical explanation is given to understand salt effects on different polymer structures.
Many offshore wells use seawater for preparation of fracturing fluids, which is known to contain high salt concentrations. Even drilling fluids use different polymers at various salt concentrations to increase cutting stability as well as mud weight. Formation water can also contain high salt content. When drilling or stimulation fluids mix with formation brine, a high reduction to viscosity can occur, resulting in poor proppant or cuttings transport. This study should aid the decision-making process when choosing polymers and adjusting viscosities considering salt effects.