| Abstract |
Three production wells in Mindanao Geothermal Production Field, APO-1D, APO-3D, and SP-4D, have been plagued by calcite scale deposition. Drilled from the same pad, these wells are mainly drawing ~240-250░C neutral brine from the same aquifer at ~1100 m. Discharge tests showed their high potential for calcite scaling. Even before start of commercial utilization in March 1997, massive calcite scales already developed at ~1140-1170 m in APO-1D. These scales were likely formed when cooler (~220░C) brine near well bottom became heated as it rose inside the well during bleed conditions. In 1998, APO-1D was cement-plugged from ~1195 m down to bottom. During commercial utilization, flashing of brine caused deposition of calcite scales at 660-710 m in APO-1D, at 780-815 m in APO-3D and 610-700 m in SP-4D. After only four months of utilization, small and subhedral calcite scales already formed total blockages in APO-1D and in SP-4D requiring two mechanical work-over jobs for each well between February 1998 and May 1999. In APO-3D, large and well-developed calcite prisms formed after thirteen months of utilization. So far, these scales have not been detrimental to APO-3D's output. Deposition of calcite scales during utilization can be detected by monitoring declining trends in mass flow correlative with decreasing calcite saturation index (CSI) values. Fluid chemistry changes due to calcite scaling have been masked by inflow of reinjection waters into the production sector. |