Comparative impact of cold water and thermochemical cooling methods on breakdown pressure for improved stimulation in unconventional formations

High breakdown pressure poses a significant challenges in terms of pumping pressure and associated costs during stimulation operations in unconventional reservoirs. These reservoirs are characterized with low porosity, low permeability, and high in-situ temperature due to greater depths. To address this challenge, the current study investigates the effectiveness of various cooling strategies and their comparative impact on the breakdown pressure and strength of unconventional rocks (Kentucky sandstone and Eagleford shale). These rocks were heated up to 150 °C and cooled down by following three different strategies: ⅰ) spontaneous cooling i.e. without any external aid ⅱ) cooling with cold water and ⅲ) cooling with endothermic chemicals involving NH₄Cl and NaOH. With endothermic cooling, the temperature of Kentucky sandstone decreases from 150 °C to 56.7 °C in 14 s, while Eagleford shale cools from 150 °C to 43.8 °C in 10 s. The endothermic cooling was followed by the Cold water and spontaneous cooling which showed slower and less pronounced temperature drops in both rocks as compared to the endothermic cooling. The endothermic cooling also leads to highest reduction in rock strength and breakdown pressure. The strength shows a reduction of 21.9 % in Kentucky sandstone and 25.4 % in Eagleford shale while the breakdown pressure reduces by 38.6 % and 37.3 % for the Kentucky sandstone and Eagleford shale respectively. The study also shows the structural changes in the rocks, particularly rock morphology and pore volume. FIB-SEM analysis shows the development of multiple micro-cracks in the rocks which plays an important role in reducing the breakdown pressure. The outcomes of this study indicate that pre-fracturing cooling treatment using endothermic fluids can enhance the effectiveness of hydraulic fracturing operations by reducing the formation breakdown pressure.