Enhancement of Pit Depth Rank Statistical Analysis for Estimating Remaining Casing Life

Abstract

Pit depth rank (PDR) chart is a statistical tool used to analyze multifinger caliper data. The method of analysis with PDR charts, which is based on the Extreme Value Theory is used to predict the probability of penetration depths greater than what was recorded exist on the sampled casings. Predictions are inferred from a regression line fitted to the pit depth values against pit depth rank plot. Modifications to the PDR charts were applied to improve the fit of this regression line. In this study, the Gumbel distribution was used as the working equation of the pit depth rank analysis. Caliper survey data were selected and categorized per sample size. In addition to this, plotting positions were used in the calculation. Performance indicators that measured the goodness of fit of the regression line were used to determine the plotting position with the best fit. Fits better than those from Normal PDR were achieved through the aforementioned method; furthermore, a correlation between the sample size and the improvement of regression fit was observed.