Application of specific energy for lithology identification

Abstract

The previous applications of specific energy to drilling operations have focused mainly on drilling optimization and identification of inefficient drilling conditions. Recent advances in specific energy extend its applications to overpressure detection and pore pressure prediction. In this paper, an attempt is made to further extend the application of specific energy to real-time identification of subsurface lithology. The concept is based on the principle that the total energy required to break and remove a unit volume of rock is a function of lithology. The proposed methodology is tested using a recently drilled exploratory gas well in the tertiary deltaic system of the Niger Delta basin. In general, an excellent agreement is observed in trend between the traditional lithology identifiers (gamma ray and sonic velocity ratio) and the total energy consumed in breaking and removing the penetrated rocks. Unlike the logging while drilling (LWD) technique commonly employed in the industry (including the application of near bit sensors placed few feet behind the bit), the proposed methodology can provide a reliable means of picking formation tops and identifying subsurface lithology at the bit with no extra cost since drilling parameters are routinely recorded at the wellsite during the drilling of a well. The proposed methodology will assist the drilling engineers and geologists in determining the casing setting depths and coring points without having to drill too deep into the formation of interest.