Application of Rock Strength in Drilling Evaluation

Abstract

Abstract Different sources can be used to develop rock strength information along the wellbore. Such strength information is important when assessing the stability of the wellbore, selecting mud weights and designing casing programs. However, there are other areas, especially in drilling, where rock strength information is applicable, but still underutilized. A methodology is developed to estimate drilling time and bit wear before drilling if rock strength is known. To estimate drilling time and bit wear, effects of other parameters like drilling parameters, well bore size and drilling bit design has to be normalized. This methodology has been used to estimate drilling time and estimate bit wear and further evaluate drilling performance while drilling. After drilling the additional information has been used to conduct a post analysis and transfer knowledge from well to well. The advantage of this methodology is it eliminates the effect of geological variability when comparing performance between wells and fields. Introduction Various methods can be used to develop rock strength profiles along the wellbore. Such strength information is crucial when analyzing the safe mud weight window for assessing the stability of the wellbore, selecting mud weights and designing casing programs. Strength information is also used for completion and hydraulic stimulation design. However, there are other areas, especially in drilling, where rock strength information is applicable, but still underutilized. To obtain the rock strength along the well bore, logs, rock mechanical tests or even drilling data can be used. In this paper we address how to obtain this rock strength and some areas where rock strength has been provided to give valuable information for drilling purposes. Rock strength calculations The rock mechanical parameter that is most important when conducting drilling analysis is unconfined compressive rock strength (UCS)1. The UCS can be determined from Mohr Coulomb failure criteria. The Mohr-Coulomb failure criterion in terms of peak loads is given as: Equation (1) Where S'v is vertical effective stress, S'h is horizontal effective stress, and a is failure angle. Effective stresses are defined the difference between total stresses and pore pressure. Equation (2) Sv is the total stress, pp is the pore pressure. There are several methods to obtain UCS along the well bore. In most cases, the availability of data determines which methods to choose. Different methods for obtaining UCS are described below.