Root Cause Analysis of Calcite Scale Deposition in Gas Wells Using Rigorous Scale Predictions and Kinetics of Scale Formation

Abstract Severe calcite scale deposition has been found downstream of wellhead chokes in multiple wells of an Algerian gas field. Preliminary scale predictions showed no risk for CaCO3 precipitation at this location, so a more in-depth root cause analysis was required to determine the cause of scale deposition and to put in place effective mitigation strategies. An initial review of all relevant field data was conducted to investigate key operational parameters and identify representative fluid chemistries for scale modelling. State-of-the-art scale prediction software was used to gain insights into the mechanisms behind these scaling episodes and to run multiple sensitivity studies for salinity, produced water rate, temperature and pressure and show their impact on the calcite scale risk. Kinetics of calcite precipitation in the form of "scaling induction time" were calculated to improve the understanding of scale formation and the likelihood of scale deposition downhole and topside. This wide range of results provided sufficient information to paint a clear picture of the calcite scale problem affecting this field. Calcite scale risk was found to be very severe downhole with particularly fast induction time, caused by water evaporation as a consequence of depressurization in the wellbore. However, it was also shown that an increase in water production significantly reduces this scaling problem as evaporation plays a lesser role and CO2 repartitioning becomes the main driver for calcite precipitation. This study confirmed that there is no risk of calcite precipitation downstream of the wellhead and that operating conditions are far from fluid saturation. Hence, the main cause of calcite deposition in the choke is solid transport from downhole. When comparing flowline and wellhead pressure trends it was also clear that there were fewer scaling events at the choke than initially believed by the operator and that the main area of focus should be downhole. This thorough approach provided a robust framework for predicting and mitigating scaling issues in these gas-condensate wells. This paper presents a comprehensive field study of calcite risk in gas condensate wells and provides the basis for a more effective mitigation strategy. A new kinetic model for the precipitation of calcite scale was applied to provide a better understanding of the operational issues encountered in this field.