Hydrothermal jet drilling is a non-contact drilling technology that has the potential to improve the development prospects of geothermal energy, by increasing the rate of penetration and decreasing overall drilling costs. In particular, this improvement would facilitate development of lower grade resources where deeper drilling is required. The mechanism involves impingement of a heated fluid jet onto the formation in the wellbore, where the high heat flux from the jet to the rock induces a steep temperature gradient and thermal stresses at the surface. If the resulting stress field is sufficient to induce microcrack growth, thermal spallation can occur, which is known to have high penetration rates in hard crystalline rocks such as granite and quartzite. Our research program is focused on improved understanding of the physical attributes and interaction of the hydrothermal jet and the rock surface, the heat flux, heat transfer coefficient, and dissolution phenomena. Barre granite has been selected as model hard rock relevant to deep EGS resources in conduction dominated environments. Experimental studies have examined pressure / jet temperature conditions of: 1) 250 bar / 389℃, 2) 225 bar / 370℃, and 3) 200 bar / 358℃. Heat flux delivered to the rock surface at these conditions ranged from 410-670 kW/m2 (41-67 W/cm2) with estimated surface temperatures of 369℃, 345℃, and 328℃, for conditions 1, 2, and 3, respectively. Additionally, hydrothermal jet drilling may be enhanced by application of chemically active solutions along with the hydrothermal jet to facilitate the dissolution of silicate minerals and weaken the structure of the rock. Results show that a 0.08 m NaOH solution injected at these elevated temperatures leads to a rock dissolution rate of 0.05-0.1 g/m2s, several orders of magnitude greater than dissolution in pure water at these temperature and pressure conditions.

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