Technical Limit Specific Energy—An Index to Facilitate Drilling Performance Evaluation

This paper describes a method developed to represent the inherent drilling difficulty posed by the combination of rock properties, depths and pressures that make up a field's drilling environment, thereby facilitating comparisons of drilling performance from different fields that take account of varying rock "drillability". The method uses the concept of specific energy - the lower the specific energy in a given environment, the more efficient the drilling process. An algorithm has been developed to estimate the technical limit specific energy from wireline sonic, lithology and pressure data. This represents the lowest specific energy that can reasonably be expected for that combination of rock properties and pressures. Data were analyzed from 10 wells, drilled in varying on and offshore locations, which were selected as good benchmarks for the performance achievable in their drilling environments. Three measures were used to represent drilling performance in these wells – total dry hole (excluding coring, logging and completion activities) days per 1,000 m, normal drilling days per 1,000 m, and rotating days per 1,000 m. These measures showed wide variations across the 10 wells, reflecting the range in drilling difficulty posed by the different geological environments. However, the rotating days per 1,000 m showed encouraging correlation when plotted against the wells' technical limit specific energies. The other performance measures also showed clear trends when plotted against technical limit specific energy. The existence of these correlations demonstrates that technical limit specific energy is a valid measure of whole-well drillability, and provides a benchmark against which other wells' drilling performance can be compared. The performance measures for the new well are compared with the "benchmark performance" trends at the new well's technical limit specific energy, revealing whether the drilling performance departs substantially from the expectation for a well of similar drilling difficulty.