Incursion of subterranean tar into the wellbore during and/or after drilling has the potential to result in a variety of problems. If the tar is soft and deformable, it may ‘refill’ the wellbore and require additional drilling time and casing operations. An additional problem is adhesion or accretion of the tar directly onto the drillstring or bottom-hole-assembly (BHA). Such fouling of the drillstring/BHA often requires unplanned tripping to physically remove the adhered tar. It is also conceivable that the tar-related problems may ultimately result in the hole being plugged and abandoned. For these reasons, an approach has been developed to (1) emulate the reported wellbore accretion, and (2) develop lab-based approaches to help minimize/reverse the tar adhesion.

Investigation of tar accretion onto steel rods/tubes with a Gulf of Mexico tar was studied in a synthetic-based mud, to evaluate possible avenues of remediation. In addition to evaluating the conditions necessary to induce tar accretion in the laboratory, further studies were performed to affect reversal of the intentional accretion and also to help prevent initial accretion. Included are the results from the induced accretion and various additive-based attempts to mitigate the tar adhesion.

In order to induce lab-based accretion, it proved necessary to develop a testing method to emulate the scope of tar accretion observed in the field. This methodology is herein described along with the optimized fluid system. Of specific interest in this study is the ability to use this novel fluid either as a sweep, spot or additive as a means to help reduce the inherent ‘tackiness’ of the tar and thereby help reduce/prevent the adhesion to the emulated drill string. The fluid treatment developed is environmentally acceptable and uses a novel chemical composition which proved successful in reversing and preventing tar accretion under the laboratory experimental parameters.

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