In-Situ Generated Acid Stimulation for Productivity Enhancement – A North Sea Case Study

Abstract A new approach to acidizing gas wells where an inert dry chemical hermetically was sealed inside a metal carrier and deployed downhole via slick-e-line. Once in place downhole, an exothermic chemical reaction was initiated which produces hot acid vapour. This method permits target acid placement in a heated vapour form. The well was a depleted deviated gas well in a sandstone reservoir at approximately 15000 ft MD with 4 1/2" tubing. Three unique intervals were treated using 5 runs using a combination of HCl and Mud Acid (12:3 HCl/HF mixtures). Post-treatment, the production and operational/up-time increase of 30% was sustained for over a year. A process to determine the optimal tool quantity, placement and type was conducted by the operator and the service company. It was important to minimize total fluid pumped and to employ HF but also minimize the risk of handling live acid at surface. It was determined that one 2 ½" HCl tool, one 2 ¼" HCl/HF and three 2 ½" HCl/HF tools would adequately treat the interval. Another consideration was that a hold-up depth (HUD) was present half-way through the interval. It was decided to attempt to remove this deposit by spotting tools immediately above the HUD. The HUD was thought to comprise primarily of heavier solids originating from the formation combined with CaCO3. The first two runs used a 2 ½" HCl tool and a 2 ½" HCl/HF to remove scale and stimulate the near well bore. The third and fourth runs were ignited immediately above the HUD to remove some of the HUD and to squeeze and treat the interval below the HUD. The final run was done on an interval roughly 20 feet above the HUD. After each run, a build-up test was performed that showed a progressive improvement in the build-up time. Prior to treatment, the well had been consistently producing at 1.9 MMscf/day with a 32% uptime. Post-treatment, the well produced at 2.0 MMscf/day with a 42% uptime which was sustained for a year. Each run from run in hole to laying out the tool post-treatment was approximately 6.5 hours. The tools were deployed on slick-e-line and temperature and pressure was recorded downhole. All tools were deployed and operated successfully maximizing the amount of acid generated in-situ. Using this novel technique, the overall gas production and well up-time was markedly improved. The operation was conducted in an efficient manner with simplified logistics when compared to conventional acid treatments. The use of HF was identified in the selection process as being key to the success of the operation and the option to deploy in a safe manner that didn’t expose personnel to acid was a key determinant in employing this novel technique in the field.