Case History of Coiled Tubing Wellbore Scale Removal and Acidizing in High Temperature Geothermal Steam Producer Well with Fast Temperature Build Up Using Modified Acid Pumping Technique

Abstract WELL X, a prolific steam producer operated by a geothermal company in Sumatra, Indonesia, is completed with a 13.375-in casing and a 10.75-in perforated liner across the feed zone. Over time, scale buildup inside the wellbore and behind the perforated liner led to a decline in steam production. In January 2022, a workover operation successfully removed the scale, restoring the well to its previous production levels. However, by 2023—only a year later—scale buildup reoccurred, necessitating another cleanup. Due to challenges in securing a workover rig, a rigless scale removal operation using coiled tubing (CT) was chosen. The operator was cautious about using acid through the CT, given prior experiences of parted CT during similar operations. WELL X exhibited a rapid temperature buildup, as observed during the PT survey conducted in September 2023, with a Bottomhole Static Temperature (BHST) of 566°F across the feed zone. No commercially available acid corrosion inhibitor could provide adequate corrosion protection for the acid required to remove the scale at such high temperatures. As a result, well quenching—cooling the wellbore by pumping cold fluid down the CT annulus—was necessary to safely pump the acid. The acid design factored in the reduced temperature from quenching and maintained sufficient acid strength to ensure effective scale removal. To maximize dissolution efficiency, quenching was halted during acid pumping on the CT backside, allowing the acid to react at its full strength within the feed zone The conventional approach to scale removal in geothermal wells using coiled tubing involves pumping acid down the coiled tubing while simultaneously quenching the well through the CT-casing annulus. This method helps maintain corrosion levels within acceptable limits for high-temperature wells during CT acid stimulation. However, the quenching process dilutes the acid, reducing its strength and effectiveness for scale dissolution. The method described in this paper is innovative and specifically designed to eliminate acid dilution while improving acid strength for maximum effectiveness in the feed zone. The approach involved a coiled tubing wellbore cleanup operation utilizing a 2.875-in rotating jetting nozzle conveyed by a 2.375-in CT. This was followed by acid stimulation to effectively remove scale buildup behind the 10.75-in perforated liner. Following the scale cleanup in the wellbore and acid stimulation to remove scale behind the perforated liner, the well was flow tested. The results showed a significant increase in steam production, with output reaching 2.5 times the level observed prior to the scale removal treatment. The coiled tubing operation was conducted safely, with no incidents were reported, addressing the operator's primary concern of potential coiled tubing failure based on past experiences. The acid operation was performed as per plan. The corrosion inhibitor package performed as designed and more importantly the production from the feed zone was observed to be significantly increased post acid treatment. The purpose of presenting this case history is to share insights and methodologies that can be applied to other geothermal wells. It highlights strategies to mitigate operational constraints, such as limited workover rig availability, by performing rigless scale cleanup operations to restore well production to its original levels.