Mineral scales were formed in wells and surface installations during a 2-year circulation test performed at the Hijiori Hot Dry Rock (HDR) site, Yamagata, Japan. Anhydrite deposited in the deeper parts of the production wells, while silica and calcium carbonate precipitated at the surface, downstream of the wellhead, in amounts that depended on the temperature and chemical composition of the produced fluid. In well HDR-2a, closer to the injector, most of the scale was calcium carbonate; in HDR-3, further away from the injection well, there was slight precipitation of amorphous silica. As fluid circulation progressed and temperature decreased, scaling in the flow line of well HDR-2a changed from amorphous silica to calcium carbonate. (C) 2007 Elsevier Ltd. All rights reserved.