Pages 527–538
Abstract
Research summary supplied by the authors
It is well established that the productivity of a horizontal well can be significantly impaired by residual filter cake following drilling and completion operations. Mitigation of drilling-induced formation damage remains an active area of research, with substantial developments over the past decade. The objective of this study is to develop a non-corrosive wellbore clean-up fluid comprising an enzyme system developed through site-directed mutagenesis and an organic acid, formulated both as an aqueous solution and in a buffered state, to remove external filter cake and reduce skin damage to near zero prior to well completion. Biopolymer-specific genetically engineered enzymes targeting cellulose, xanthan, starch, and guar derivatives were developed to ensure compatibility with weak acid and buffer formulations used in the cleanup fluid. The dual-action mechanism of depolymerization and dissolution was designed to penetrate deeply into CaCO3–biopolymer-based nanodamaging drilling fluid filter cake in a controlled and systematic manner, rather than through aggressive, localized reactions typical of conventional acid or oxidizer treatments. This approach resulted in complete dislodgement and removal of the filter cake from the wellbore. Weak organic acids such as acetic acid and corresponding buffer systems were formulated to dissolve carbonate particles while maintaining corrosion levels below the maximum acceptable limits. Compatibility tests between the weak acid–buffer systems and the protein-engineered enzymes were conducted to validate the formulation stability. The effectiveness of each organic acid and buffer system was evaluated by estimating filter cake removal efficiency under HPHT conditions, while corrosion rates were measured to assess the metallurgical advantages of the proposed system over conventional corrosive treatment fluids (e.g.: Hydrochloric acid and Hydrofluoric acid). The formulated clean-up fluid was successfully tested on ceramic disks in HP HT system. The polymer-based drill-in fluid produced a very low-permeability filter cake (<0.1 mD), which significantly minimized internal filtrate invasion. Controlled deposition of this lowpermeability filter cake on carbonate core plugs resulted in reduced damaging invasion. Several ester hydrolysis reaction kinetics were evaluated, and a suitable acid precursor was selected based on its ability to generate the required acid concentration within three hours under downhole conditions. The selected precursor generated approximately 3% acid after three hours and maintained low acid concentrations for up to 12 hours. Measured corrosion rates were significantly below industry-accepted limits, eliminating the need for acid corrosion inhibitors. Return permeability of muddamaged carbonate core plugs reached up to 96% following exposure to the clean-up solution. A new drill-in fluid was developed, and its associated filter cake was completely removed using the recombinant enzyme–buffered acid. Unlike conventional wellbore clean-up fluids, which are highly acidic and require corrosion inhibitors, the proposed formulation effectively minimized corrosion of downhole tools without the use of corrosion inhibitors.