Abstract:

Enhanced geothermal system (EGS) has been used to extract heat from deep hot dry rock with low permeability by creating an artificial geothermal reservoir. Hydraulic fracturing, along with chemical stimulation, was usually adopted to improve the permeability of a target EGS reservoir. In this study, we collected granodiorite samples from the Gonghe basin of Qinghai Province and subjected them to a systematic chemical stimulation test by use of three chemical agents (sodium hydroxide, hydrochloric acid and mud acid) at three different injection rates. The results show that the permeability of the rock samples increased upon injection of hydrochloric acid or mud acid, with the latter bringing larger increases; whereas the application of sodium hydroxide solution reduced the permeability. Among the three chemical agents, mud acid exhibited the strongest ability of dissolving feldspar minerals in granodiorite; in contrast, quartz was eroded most severely by sodium hydroxide solution. Nevertheless, during the chemical stimulation test using sodium hydroxide solution, we saw amorphous silica or aluminosilicates precipitation due to excess dissolution of the primary minerals on the fracture surface; and precipitation resulted in microfracture filling which definitely had a negative effect on improving permeability. Thus, in general, mud acid is the best chemical agent for the target hot dry rock in this study. At a moderate injection rate (3 mL·min-1), mud acid can most effectively erode granodiorite samples and remarkably enhance sample permeability. Lowering injection rates, however, could cause secondary mineral precipitation and fracture filling therefore lowering sample permeability.

Key words:  hot dry rock, enhanced geothermal system, chemical stimulation, mud acid, permeability