Hydraulic fracturing is the completion method of choice to maximize productivity and increase profits in unconventional resource plays. Standard laboratory protocols for measuring rock strength, Young's modulus and Poisson's ratio commonly do not account for moisture content in the rocks, yet these parameters are critical in fracture designs.
The process of water weakening is particularly complicated in shales due to the combination of Total Organic Carbon (TOC), swelling clays and reactive minerals like silica and calcite. A study was carried out to determine the effects of spontaneous fluid imbibition (brine and dodecane) on Young's modulus and hardness in shale. The measurements were made using a nanoindenter on shale samples from the Marcellus and the Woodford plays.
A key objective is to compare weakening effects of brine versus dodecane. It was found that irrespective of the shale wettability, brine led to a greater reduction in Young's modulus (45% reduction in Marcellus, and 25% in Woodford) than dodecane (25% reduction in Marcellus, and 17% in Woodford). Clay stabilizing solutions like KCl brine had limited success in reducing the water weakening effect. Lastly, Marcellus shale samples experienced a greater reduction in Young's modulus compared to Woodford shale samples possibly due to its larger pore throats, greater porosity, higher calcite content and strongly water wet character.
Water weakening effects have been studied on different rock types, namely sandstones, siltstones, andesite, granite and shales. The majority of the authors study the effects of spontaneous imbibition or partial saturation. Colback and Wiid (1965) were among the first to quantify the effects of moisture content on the reduction of uniaxial compressive strength (UCS) in quarzitic sandstones. They observed up to 50% reduction in UCS. Spencer (1981) studied the effects of polar and non-polar fluids on Young's modulus reduction in quartzose sandstone. He found that fluids like ethanol and n-decane also had a weakening effect, though it was not as severe as with water. He found that water caused the greatest weakening (50 % reduction) followed by ethanol (30 % reduction) and n-decane (10 % reduction). Clark et al. (1980) also showed that the attenuation increase and velocity reduction due to water saturation was much more than due to benzene saturation. They inferred that non-polar solvents like benzene had a much lower weakening effect.
