CCS Well Control Impact of CO2 on Drilling Fluid Performance

Abstract

By the implementation of carbon capture and storage (CCS) as an important climate action, infill drilling in CO2-rich storage sites will be expected. This implies that actions for handling an influx of CO2 into the drilling fluid must be prepared for. This study aims to close some of the many knowledge gaps when drilling for CO2 storage wells. To detect and handle CO2 well control incidents, it is necessary to develop software tools that take properly into account the properties of CO2 and drilling fluid and how the mixture of the two performs. To support the development of a such tool, experimental characterization of density, rheology, phase envelope, hydrate formation and drilling fluid stability are performed on drilling fluids mixed with CO2 for relevant pressures and temperatures. Based on these experiments and verified thermodynamic model descriptions, local models for properties of drilling fluid-CO2 mixtures are developed and integrated in an existing drilling software suite. Considerations on similarities and differences with natural gas kicks are made. Results from the experimental campaigns are presented and analyzed with respect to how the drilling fluid properties are affected by the CO2, e.g.: How does the phase envelope of the drilling fluid-CO2 mixture change with different amounts of CO2, i.e., to which degree is CO2 dissolved in the drilling fluid under different conditions, and when is it a free gas or liquid? What is the impact of CO2 on the drilling fluid-CO2 mixture density? Under which conditions will the constitution of the drilling fluid break down, i.e., loss of rheological properties and risk of weight material sedimentation or drilling fluid component separation? Furthermore, we consider how these effects can be captured in local constitutive models and integrated in a hydraulic model that can be used for simulating well control incidents. This study provides new insights into drilling fluid-CO2 interaction that are necessary in order to handle risks related to well control incidents when drilling into existing CO2 storage sites. The outcome of this study will help build experimentally verified model algorithms. Implemented in software tools this will enable more accurate simulations of such events than what is available now, and will help in evaluating risks, precautions and operational procedures associated with handling such events.