As a result of severe formation damage in producing wells in a mature reservoir, an operator in the North Sea needed a chemically diverted mineral scale dissolver treatment that could be bullheaded from the surface to enable successful acid/scale contact across the entire reservoir section. Although numerous conventional acid treatments had been performed, they had achieved limited success in restoring liquid production rates because of the inability to access the entire perforated section as a result of localized calcium carbonate scale buildup. The operator considered mechanical diversion to be too high of an operational risk.
SLB was asked to design and execute an effective and repeatable treatment for Well A, which had upper and lower nonproducing intervals as a result of scale. The treatment was required to be nondamaging to the reservoir using a diversion technology that was free of solids. In addition, considering the reservoir contained relatively abundant illite and expandable interlayered illite and smectite, with small quantities of chlorite and kaolinite, the operator wanted to limit the damage to clays that can occur during acid treatments at high reservoir temperatures (135 degC).
SLB performed extensive laboratory qualification to ensure that the organic acid selection would be nondamaging to the reservoir, which contained both water- and acid-sensitive clay minerals, while maintaining a high calcium carbonate dissolution capacity. CAL-Acid scale dissolver, which provides a viable, fast-acting, low-corrosion alternative to conventional mineral acid treatments of carbonate scales, was selected. In addition, CAL-Acid dissolver possesses an inherent sequestering capacity that prevents the reprecipitation of undesired species, such as iron liberated through scale dissolution, which could potentially damage the formation.
The treatment was deployed as a proof-of-concept design via bullheading from the surface. The design was based on diversion using foam with a high nitrogen content combined with F100 EZEFLO™ surfactant, with the subsequent foam-energized CAL-Acid scale dissolver targeted to the upper and lower perforated intervals.
During foamed displacement, tubinghead pressure increased, suggesting diversion into the nonproducing intervals. High tubinghead pressure during the potassium chloride “bump” following the initial shut-in indicated that the foam did not collapse prematurely. Effluent ion analysis during flowback detected elevated calcium levels, indicating that at least part of the dissolver penetrated the scaled portions of the well. However, because operational issues led to minimal flowback sampling and volume control during flowback, mass balance could not be accurately calculated to determine the total quantity of scale dissolved.
The returning pH of the CAL-Acid dissolver increased to neutral, demonstrating a high degree of acid spend. Peaks of aluminum and silicon were observed, and the values were in line with those observed during coreflood testing, indicating that some clay dissolution had occurred but likely had minimal impact on well productivity.
The operator reported that as a result of the foamed acid treatment for Well A, the well productivity index appeared to have increased to very close to new well conditions.
