After numerous conventional acid treatments failed in a mature well with limited access resulting from localized CaCO₃ scale buildup, which had also caused discontinuation of water injection, the operator turned to SLB for help. A bullheaded foamed deployment of CAL-Acid™ scale dissolver enabled controlled scale dissolution and greater chemical penetration into the reservoir for maximum results.

When production-induced deposition of CaCO₃ in the near-wellbore area caused discontinuation of water injection in a mature well in the central North Sea, the operator conducted numerous conventional acid treatments. However, the localized scale buildup also limited treatment success by preventing access to the entire perforated section. Mechanical diversion was considered too high of an operational risk, so the operator asked SLB for a chemical treatment that would be nondamaging to the reservoir. The concern was the presence of both water- and acid-sensitive clay minerals—abundant illite and expandable illite/smectite along with minor chlorite and kaolinite—which would be prone to experiencing damage during acid treatments at the reservoir’s high temperature (135 degC).

SLB conducted tests to ensure that an organic acid would not damage the reservoir. CAL-Acid scale dissolver was selected because it reacts at a lower rate than the equivalent carbonate reaction with mineral acid. The rapid yet controlled high scale dissolution would provide greater chemical penetration into the reservoir. CAL-Acid dissolver also has an inherent sequestering capacity that prevents reprecipitation of undesired species, such as iron, that could potentially damage the formation.

Treatment was conducted by bullheading from surface. The initial diversion used foam with high N₂ content and the F100 formulation of EZEFLO™ surfactant. This was followed with foam-energized CAL-Acid scale dissolver targeting both the upper and lower perforated intervals. The increase that occurred in tubing head pressure (THP) during the foam displacement (Step 7 in the figure) suggested diversion into nonproductive intervals. High THP during the KCl “bump” (Step 9) after shut-in indicated that the foam did not collapse prematurely, supporting effective contact for scale dissolution.

Operational issues limited sampling and volume control during flowback; thus, the mass balance could not be accurately calculated to determine the total amount of scale dissolved. However, the operator confirmed a strong response to the foamed CAL-Acid dissolver treatment that restored the productivity index to almost the virgin value in the well. In addition, the peak values of aluminum and silica in the effluent analysis aligned with data from coreflood testing, indicating that only minimal clay dissolution had occurred, with little detrimental effect on well productivity.

On the basis of the success of this approach, the operator planned to repeat the treatment across the platform’s wells.