Solubilization of wellbore filtercakes formed from drill-in fluids

Abstract

Research was performed to study the degradation of filtercakes formed by water-based drill-in fluids (DIF), primarily sized-salt (SS) and sized-calcium carbonate (SCC) DIFs. The experiments to degrade DIF filtercakes varied temperature (43⁰C to 71⁰C), drilling solids concentration (2% to 6%), and acid concentration (1.6% to 7.4%). The objective of these tests was to predict with fundamental kinetic relationships the time necessary to degrade the filtercakes. Water-based DIF filtercakes have shown promising results in determining degradation and reaction rates when exposed to hydrochloric acid treatments. Using a ceramic disk apparatus, static experiments were performed to determine the time needed for acid breakthrough and to measure polymer degradation. Further polymer degradation testing involved a well-mixed reactor. Reaction rates were estimated by using a modified chromotropic acid assay (CTA) to measure biological polymer content in conjunction with first-order kinetic equations. Experiments showed that polymer content is critical for acid cleanup fluid to penetrate filtercakes. At least 75% of polymer content in filtercakes must be degraded for breakthrough to occur. For faster acid breakthrough times, high temperature and acid concentration in combination with low drilling solids content are necessary. Comparing static systems and well-mixed systems demonstrate a considerable difference in reaction rates. The variance is caused by diffusional limitations inherent in the static system. Pulsing of acid fluid is recommended for treatment purposes so that diffusion is not a limiting factor in filtercake degradation. Using the calculated activation energy and preexponential factors, charts were created for prediction of polymer degradation in well-mixed systems. Acetone was studied as a solvent for synthetic oil-based DIFs. Concentrations of over 88% acetone in water solubilize the organic phase of synthetic oil-based DIFs. However, current bans on synthetic oil-based DIFs ceased research and directed experiments toward water-based-DIFs.