Calcium transport during circulation in Hot Dry Rock system

Calcium transport was estimated in the Hijiori Hot Dry Rock geothermal system. During circulation test, we collected scale samples: calcite and aragonite at the pipeline and anhydrite at the deep production well. After a 3-month circulation test, we measured the thickness of calcite scale on the pipeline of HDR-2. The ∼50mm thickness means that about 2.7 tonnes of calcite (1.04 tonnes calcium) precipitated. Also we estimated anhydrite dissolution volume from the fluid chemistry at the injection and production wells. Anhydrite dissolved near the injection well by cool water but then Ca re-precipitated as anhydrite in the reservoir with temperature increase. After anhydrite precipitation, calcite precipitated in the surface pipeline and in the shallow well. After 3 months, 7.5 tonnes Ca was contained in the circulation fluid, and 2.7 tonnes Ca was contained in calcite scale in the surface pipeline and shallow production well.