Calcite deposition related to temperature and boiling in some Icelandic geothermal wells

Abstract

Calcite is a prominent secondary mineral in rocks. Calcite deposition is known to be troublesome for the exploitation of many geothermal fields in the world. This study is based on chemical data from selected wells in some Icelandic fields. The state of calcite saturation for a range of temperatures has been calculated for fluids from several geothermal wells in Iceland using the programme WATCH. Equilibrium calculations based on the WATCH computer programme have been used to estimate whether the fluid is saturated and whether supersaturation occurs upon boiling. Calculated and measured pressure and temperature profiles in several wells each with a single, dominant feedzone have been analyzed using the borehole simulator HOLA. In such cases HOLA is a useful tool to determine whether boiling takes place and if it does at what depth. Such simulations confirmed results for calculated supersaturation after boiling in wells. The borehole simulator was also used to study the effect of varying wellhead pressure on the depth of boiling in wells where calcite supersaturation was observed. Flashing depths which generally correspond to observed calcite deposition depths were found using the programme, and they were found to vary inversely with wellhead pressure. The two most important ways of combatting calcite deposition are drilling out and the addition of inhibitors. The cost of both increases with increased calcite deposition depth. The results of calculations on the variation of flashing depth with wellhead pressure are an aid in choosing the optimum wellhead pressure during production where the variation of flow with wellhead pressure shown by the characteristic curve must also be taken into account.