Evaluating the Effect of Friction Reducers in Slickwater on Shearing Properties Between Fracture Surfaces of Shale Rocks

Abstract Hydraulic fracturing operations involve complex interactions among fracturing fluids, proppants, and rock surfaces, where tribological factors like friction, lubrication and wear play an important role in fracture propagation and proppant transport. This study focuses on examining the effect of friction reducers on the coefficient of friction (COF) during interaction of two shale rock surfaces. A calcareous shale block is cut orthogonal to the bedding plane to extract 5.5mm D × 15mm L pins and 37.5mm D × 15mm L core samples. Both the surfaces are polished to achieve a smooth uniform finish. A pin-on-disk tribometer is used to measure the coefficient of friction (COF) and study the frictional behavior of shales at a constant load of 15 N, with varying sliding speeds and rock/fluid combinations (dry, water environment, and slickwater with friction reducer). Pre- and post-test surfaces were analyzed using SEM, μXRF mapping, and surface roughness measurements to assess changes in the rock surface. Preliminary results for dry shale samples show that the COF ranged between 0.75 and 0.90, with the COF decreasing with an increase in the sliding speed. This is potentially due to the frictional heat generated during the process caused by dehydration of hydrous clay minerals in the shale, releasing water that formed a lubricating layer over the contact area. Additionally, localized frictional heating may have led to the formation of a frictional melt on the shale surface, further decreasing COF during sliding. In tests with shale saturated in DI water, the COF initially ranged from 0.2 to 0.4, indicating the lubricating effect of water, which prevented direct rock-to-rock contact. Over time, as the water evaporated, the COF increases to the values comparable to the dry state, reflecting the transition back to dry conditions. In tests conducted with slickwater containing friction reducer, the COF remained relatively low and stable across all sliding speeds, converging near 0.68–0.70. This consistent performance is attributed to the formation of a persistent boundary film by the friction reducer, which effectively minimized interfacial shear and reduced sensitivity to sliding velocity. In this work, we replicate the sliding behavior of rock surfaces during the hydraulic fracturing process using the pins and rock from the same shale samples in a pin-on-disk tribometer. The study provides novel insights about the effect of slickwater fracturing fluid on rock shearing in shale.