Step Changed Towards Initial Wellbore Clean Up for New Wells, With Cost Competitiveness

Abstract This paper presents case study of single-step post-drilling wellbore cleanup jobs in presence of sensitive downhole tools Measurement While Drilling (MWD) & Logging While Drilling (LWD). Wellbore cleanup with MWD/LWD presence was not possible earlier when oxidizers/strong acids were used as mud filter-cake cleanup agents. With the development of recombinant enzyme through Site-Directed Mutagenesis, a non-corrosive, highly compatible and polymer-specific cleanup fluid recipe was designed, which met these tools specifications and saved more cost and rig-time in field application. After developing starch-xanthan-biopolymer specific enzymes stable up to 120°C, and testing substrate hydrolysis efficiency, acid concentrations were optimized for different completion plans and formation temperatures. Filter cake removal efficiency was studied in high temperature high pressure (HT-HP) environments, while corrosion levels for each system were measured at temperatures up to 120 °C. Compatibility tests with different elastomers were conducted to evaluate compatibility with MWD/LWD tools. After a full-scale risk analysis and gaining high confidence level, the new cleanup fluid was pumped through Pressure-Balanced Liner (PBL), (circulation by pass sub) around LWD tools, outside bottom hole assembly (BHA), immediately after the completion of drilling without pulling out the drill string. The cleanup fluids were seen to be stable up to 120°C at formation salinity and in the presence of the required acid and buffer dose. Studies on enzymatic kinetics by substrate hydrolysis and reducing sugar estimation show the complete dissolution of the polymers. HT-HP American Petroleum Institute (API) filter showed more than 95% removal of mud cake. Corrosion tests with carbon steel coupons resulted in a corrosion level of 2.6 to 4.3 mil per year (MPY), significantly below the recommended limit of 4.6 MPY. When evaluated on elastomers, namely Nitrile Butadiene Rubber (NBR), Hydrogenated Nitrile Butadiene Rubber (H-NBR), Silicone & Ethylene Propylene Diene Monomer Rubber (EPDM), the cleanup fluid system caused no significant changes in the size (2%), weight and microstructure of the elastomers. After a thorough risk and Quality Assurance& Quality Control (QA/QC) analysis exercise, the system was trialed in one well through one-step treatment, replacing the traditional two-step treatment, while the BHA was in place. The pumping was done through PBL-sub, placed ahead of the MWD-LWD tools. This initiative saved rig time by eliminating a dedicated trip for clean out. Encouraged by this result, more than 50 well-cleanup jobs have been conducted successfully. Additionally, 15-20% production enhancement is achieved in all these wells with no failure reported. This project provides a fully compatible cleanup system with single-step treatment design even in presence of required downhole tools. It replaced the traditional process with a step change through proper assessment and evaluation and resulted in significantly reducing well-completion cost. The post-job QA/QC reports reveal that the system is within acceptance criteria. Disseminating this knowledge may be helpful to reduce cost and save rig time by replicating it in larger petroleum community.