Hot brine injector Nag-67 in the Tiwi geothermal field (Philippines) had been in operation for over 10 years when injectivity decline indicated a workover was required in 2000. The operation consisted of drilling-out wellbore scale followed by acid dissolution of scale formed in the near-wellbore formation. The workover increased the injection capacity of the well to near its initial-use capacity. Scale-volume estimates from brine chemistry, and from stoichiometric amounts of silica dissolved during the acidizing, suggested that the decrease in injectivity was largely due to scale deposition in the near-well formation. Reactive transport modeling was used to simulate mineral deposition and injectivity loss. A porosity-permeability relationship was calibrated using observed injection indexes to reproduce the loss of injectivity. The relationship captured very well the steep loss of injectivity, and the simulated amounts of precipitated amorphous silica were consistent with the estimated amounts from field data. Significant precipitation of amorphous silica, and reductions in porosity and permeability, were predicted to occur mainly within a 10m radius from the well. Injectivity recovery by acid injection was also simulated, and the predicted amount of amorphous silica dissolved by acid was consistent with the estimated amount. (C) 2004 CNR. Published by Elsevier Ltd. All rights reserved.