Quantifying the Mechanical Specific Energy (MSE) with Differential Wellbore Pressure for Real-Time Well Automation

Abstract

Abstract Drilling efficiency is the ratio of unconfined compressive strength (UCS) to mechanical specific energy (MSE); this parameter is important for understanding the drilling process and essential for attempts at drilling automation. MSE is a measure of the energy required to remove a unit volume of rock and depends on many parameters, including the rate of penetration (ROP). However, the standard use of MSE is limited in optimizing drilling efficiency because it fails to account for the effect of wellbore differential pressure (wellbore pressure minus pore pressure) on ROP. UCS has an obvious influence on drilling efficiency, but it is difficult and expensive to measure UCS values under differential pressure. To eliminate the need for measurement of UCS, we applied a modified MSE (MMSE), which accounts for the effect of differential pressure. This is performed by multiplying the MSE by the ratio of pore pressure to bottom hole pressure. Bottom hole pressure is calculated from the mud weight and well depth and corrected for frictional losses. The pore pressure is calculated using the hydrostatic gradient. We overlaid resistivity logs from eight wells, which are sensitive to pore pressure, and compared the trends of MSE and MMSE. The MMSE shows better correlations with resistivity data, especially in over-pressured reservoirs, which results in a more accurate prediction of drilling efficiency. In normal pressured zones, the MMSE and MSE values show similar trends. As expected, they deviate from the typical compaction trend lines for the over-pressured zones. The depth and pore pressure data in the over-pressured zones are typically obtained from the drilling record. A higher correlation between MMSE and the resistivity log data occurs in overpressured intervals. The comparison between MSE and MMSE was also tested on historical well data. The calculated MMSE again closely correlates with the resistivity. Therefore, MMSE is expected to predict abnormal pore pressures (overpressure) using the degree of divergence of the MMSE from the MSE values. Also, during underbalanced conditions or indicating a potential kick, the value of the MMSE is less than one. This predicts a large rate of penetration (ROP) and potentially gives an improved method of predicting a kick. In general, the MMSE significantly improves the understanding of drilling rates and pore pressure prediction.