Application of a Rigorous Scale Prediction Workflow to the Analysis of CaCO3 Scaling in an Extreme Acid Gas, High Temperature, Low Watercut Onshore Field in Southeast Asia

Abstract

Calcium carbonate is a pH dependent inorganic mineral scale that is influenced by CO2 and H2S partitioning. CaCO3 prediction must therefore include accurate modelling of the aqueous phase and all hydrocarbon phases present. pH dependent scale prediction challenges and the development of a rigorous procedure for generation of more accurate results were previously published. This procedure has now been applied to an onshore oilfield in Southeast Asia for assessment and management of CaCO3 scaling. A rigorous scale prediction workflow was applied to ‘at-risk’ field producers that showed CaCO3 scaling at and/or downstream of the wellhead choke valve (WHCV). By inputting relevant field data into an integrated PVT/scale prediction code and using the correct procedure, it was possible to evaluate scaling potentials. A series of sensitivity studies allowed well ranking based on the predicted severity of their scaling potentials. The approach validated mechanistic hypotheses for scale development in prolific low watercut, ultra-high CO2, sour, high temperature producers. Close matching of predictions with actual wellhead scaling events provided the basis for improved full-field scale management, and strategic targeting of onsite scale mitigation resources. Target field producers exhibited 0.2% to 25% watercut and presented different degrees of scale precipitation at and/or downstream of the WHCVs. Following well scaling potential assessment, each producer was subject to a series of sensitivity studies to identify (i) how scaling changed with time and (ii) provide focus on the key inputs that most impacted predictions. The initial findings, considering measurement errors (normal field variability), were surprising as key input parameters such as gas phase CO2 and produced water calcium ion concentration appeared to show minimal influence on the final scale prediction results for these wells; even more remarkable considering typical production featured very low salinity produced brine and ultra-high CO2 sour field gas. Focus was therefore shifted to field temperatures, pressure profiles and volumetric flow rates. Of importance is that the selection of ‘critical parameters’ is field specific and that the example presented here shows the variability in scale precipitation at different stages of well production, and how the scaling potential (SR and mg/L) must be evaluated together with the predicted daily theoretical mass of scale (kg/d). This is important in the study of wells with such variable water cut. The following paper demonstrates the value of a rigorous and systematic approach to the prediction of CaCO3 scale, which is often investigated using inappropriate or incomplete methodologies. In this work the authors demonstrate how the technique can address and explain important operational issues and provide solid foundations for implementing and indeed improving the field scale management program.