Historically, stuck-pipe events have been shown to cost the industry several hundred millions of dollars per year, and over 30% non-productive time (Muqeem et al. 2012). In all cases, remedial operations to salvage the subject well are expensive; and success is not guaranteed. This paper describes a case history demonstrating how a differentially stuck string was freed in a relatively short time using an innovative enzyme/weak acid recipe.

The string was a 4-1/2 in. lower completions bottom hole assembly (BHA) comprising of swell packers, and inflow control devices. The string was deployed in a carbonate reservoir. After deploying the string in the first 1,000 ft of open hole, the crew stopped to fill up the string to confirm circulation. Upon resumption of operations, the completions string was found to be stuck with no string movement possible in the upward or downward direction. Full circulation was still achieved. The fluid in hole was a water-based fluid.

The string was diagnosed to be differentially stuck. It was established that the stuck point was across the high permeable zone. Efforts to work the string free with glycol pills proved unsuccessful. An attempt to place 20%; HCl acid induced up to 30 - 40 barrels per hour (BPH) dynamic losses at the shoe. Luckily, the losses healed relatively quickly, but the string remained stuck. After 29 hours of several unsuccessful attempts, a recipe of an enhanced combination pill of enzymes, and a weak acid accelerator was proposed, pumped across the stuck point, and allowed to soak. After a relatively shorter period of soaking, the string became free. The action of the enzyme/acid pill worked by destroying the polymer base of the filter cake created by the water-based drilling fluid. The homogeneous placement and action of the enzyme/weak acid pill across the stuck area ensured that it was more effective than the conventional HCL acid pill. Hence, the application is being presented as a potentially safe, efficient, and cost-effective option to free a differentially stuck BHA in water-based drilling fluid.

Success in freeing a stuck-pipe usually depends on two critical factors - an accurate diagnosis of the mode of stuck-pipe event (i.e. differentially or mechanically); and the ability to implement the remedial operations quickly, and effectively. By implementing the above described technique, significant non- productive time (NPT) was averted. The technique has since been implemented in subsequent differential stuck-pipe events in water-based drilling fluids with a 100%; success rate.

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