Laboratory tests, statistical analysis and correlations for regained permeability and breakthrough time in unconsolidated sands for improved drill-in fluid cleanup practices

Abstract

Empirical models for estimating the breakthrough time and regained permeability for selected nondamaging drill-in fluids (DIF's) give a clear indication of formation damage and proper cleanup treatments for reservoir conditions analyzed in this study. We determined values of breakthrough time and regained permeability for common polymer-carbonate and sized-salt/saturated brine DIF's for a range at reservoir properties including temperature, drill solids content, and percent of acid in the cleanup treatment. We chose these DIF's because they form tight, thin filtercakes that control fluid leakoff and afford more complete wellbore cleanup properties than standard drilling muds, and we chose reservoir properties that could be varied and measured. Beginning with a large database of 101 tests with 8 independent variables such as type of drill-in fluid, temperature, screen type, presence of gravel pack, formation type, type of drill solids, concentration of drill solids, and cleanup treatments, we analyzed the importance of each variable. After that, we identified the independent variables we were taking into account during this research. Those variables were temperature, drill solids content, and concentration of hydrochloric acid in the cleanup treatment. Then we generated a matrix for each set of experiments that allowed us to organize and measure the conditions we were looking for, regained permeability and breakthrough time. In measuring the regained permeability, we used a linear-flow cell apparatus. In measuring the breakthrough time that particular cleaning procedures take to flow across the filter cake, we used a ceramic disc cell apparatus. We used statistical software to select properties, formation, and diagnostics of the models and to develop relationships among the properties of the DIF's. We developed four new empirical models for estimating the breakthrough time and regained permeability in polymer carbonate and sized salt. High correlations resulted with R² values between 0.851 and 0.986 corroborated by close values of adjusted R-square and low P-values give validity to the correlations found. This technique gives a broad overview of the formation damage as well as the proper cleanup treatment for similar conditions presented in the field.