Thermal cooling to improve hydraulic fracturing efficiency and hydrocarbon production in shales

Abstract

Unconventional hydrocarbon reserves contained in oil and gas shale formations are proving themselves to be abundant sources of current and future energy supply, unlocked through the technologies of horizontal drilling and hydraulic fracturing. Despite the various technology improvements that have buoyed the ”shale revolution” in the last decade, there remain very significant opportunities to further improve hydrocarbon recovery from shales by making hydraulic fracturing more efficient. In this paper, we look into the possibility of stimulating a rock matrix to a higher degree with hydraulic fractures by deliberately cooling down the rock. Cooling reduces in-situ thermal stress, which lowers initiation and propagation pressures of hydraulic fractures. Moreover, when a laterally confined solid undergoes temperature reduction induced by cooling, a thermal stress gradient is developed in the solid body. We perform sensitivity analyses to show that in an in-homogenous shale, this thermal stress gradient can lead to differential contraction of its various mineralogical constituents, which in turn may create thermal cracks. The opening of such cracks increases shale permeability and provides additional pathways for the flow of hydrocarbons, thereby enhancing productivity. Here, we solve the coupled equations of stress, heat transfer and flow using finite element techniques for hydraulic stimulation amplified by cooling. It is shown that thermal cracks in tight formations induced by thermal cooling have the potential to improve the productivity of horizontal wellbores placed in shale by an estimated 16% for the case of methane gas flow through thermally stimulated shale of micro-darcy permeability.