Method for Identification of Types and Amounts of Salts that may Precipitate due to Brine Dry Out and Application to UK Southern North Sea Candidate CO₂ Stores

Resident formation water vaporization in the near well zone may pose challenges for carbon dioxide (CO₂) storage operations. If dry CO₂ is injected into a reservoir, the brine in the very near well zone will evaporate into the CO₂ stream, leaving behind precipitated salts. This paper introduces a simple thermodynamic scale prediction approach to quickly identify salts that could precipitate at an injection site and subsequently lead to loss of injectivity and escalate the cost of capture operations. With this method, operators can forecast likely flow assurance related injectivity issues prior to injection of CO₂ and plan their injection schemes and mitigation strategies, if necessary. To conduct this study, formation water compositions were obtained from the literature for various formations worldwide, and compiled into a spreadsheet. The work of Talman et al. (2019) was used as a baseline for precipitation calculations as it clearly identified salt precipitation at an active CO₂ injection site – the Aquistore project in Saskatchewan, Canada – which has salinity greater than 300,000 mg/L. The analysis of the compiled data was divided into two parts. • Part 1 focused on demonstration, previous and operational carbon sequestration projects worldwide. • Part 2 focused on fields in the UK Southern North Sea. The existence of gas fields in the UK Southern North Sea near major regions of CO₂ emission and the presence of this mature gas province with many fields close to cessation of production makes it a desirable candidate for CO₂ storage. With some fields in this region suspected to be connected and communicating, attempt was made to infer possible connectivity/compartmentalization between fields by evaluating the available salinity of formation waters compiled from literature and annotating on the North Sea Transition Authority Offshore interactive map for further studies. In contrast to the literature that only addresses NaCl precipitation in formation waters having salinities of > 300,000 mg/L, this work shows that various other salts may also co-precipitate alongside halite, addressing brines with salinities greater than 100,000 mg/L. However, most of the salts that are likely to precipitate are highly soluble in water, so treatment with fresher brines will be sufficient to remove them, and scale dissolver chemicals should not be required. In the UK Southern North Sea fields, although NaCl remains the most dominant salt, MgCl2 and CaCl2 may also co-precipitate.