Calcite Scale Inhibition: The Case of Mahanagdong Wells in Leyte Geothermal Production Field, Philippines

Abstract

The formation of a calcite scale blockage inside the wellbore of a production well has been one of the problems encountered in Mahanagdong. The presence of a calcite blockage was confirmed in at least six (6) wells, which is mainly attributed to high calcite saturation indices in their fluids. These blockages constrict the flow of geothermal fluids in these wells thus significantly reducing their output. Based on production historical data, decline in field steam availability in Mahanagdong-A sector is mainly attributed to this problem.

To meet the steam requirement of the MG-A power plant, output of the wells with calcite blockage should be recovered. Thus, mechanical clearing using a drilling rig was conducted in each of the affected wells to remove the blockage. However, due to cost and risk involved in conducting periodic mechanical clearing, the use of a chemical inhibitor in preventing recurrence of calcite blockage deposition inside the wellbore was considered.
To date, a calcite inhibition system was already installed in two (2) of the affected wells in Mahanagdong. The calcite inhibition system basically consists of surface injection facility for the preparation and injection of chemical solution and a downhole injection facility to allow injection of chemical solution inside the wellbore of a producing well below the flash point depth.

Based on initial results, decline rate in both wells with installed calcite inhibition system has been reduced significantly from an average of 4.0 kg/s-month to less than 0.5 kg/s-month in terms of total massflow.