Development of an Acid-Soluble Residue-Free Drill-In Fluid: Eliminating Reservoir Damage with Waxy Corn Starch

Summary Completely removing and dissolving the mudcake formed by drill-in fluid (DIF) without any residue is a challenging and complex task during the well completion process, as mudcake removal involves polymer degradation and dissolution of filtration reducer. As a traditional technique, a combination of gel breakers and acids is applied to remove mudcake; this method can degrade polymers and dissolve acid-soluble materials, such as calcium carbonate, but it cannot dissolve filtration reducers, such as starch. This drawback allows filtration reducers to easily enter reservoir pores, eventually causing particle blockage damage that is difficult to be removed by inorganic or organic acids. In this study, based on the acid hydrolysis rates (AHRs) of waxy corn starch (WCS) and normal corn starch (NCS), the differences in turbidity of the resulting solutions, and the characteristic of the acid-hydrolyzed WCS without residue, an acid-soluble residue-free DIF (RF-DIF) was developed using WCS as a filtration reducer. Acid residue tests of the mudcakes formed by the DIF on the discs were conducted, and the microscopic pores within the discs were analyzed. Moreover, the reservoir protection performance of the DIF was evaluated, and the pore distribution and connectivity of the treated cores were reconstructed. The experimental results showed that the outer and inner mudcakes formed by DIF containing WCS (Waxy-DIF), which appeared on the surface and inside of the ceramic filter discs, respectively, could be completely dissolved by acid, resulting in the acid solution with a turbidity of less than 30 nephelometric turbidity units (NTU). By comparison, the mudcakes formed by the DIF with NCS (Normal-DIF) could not be completely dissolved by acid, leading to high turbidity. In addition, under the same conditions, the permeability recovery rates of the cores treated with Waxy-DIF and Normal-DIF were 100% and 83.17%, respectively, indicating that Waxy-DIF was more effective in protecting the reservoir, which can meet the needs of oil and gas production.