Improved Method for Estimating the Strength of Carbonate Rocks
Authors: Prasad Umesh , Curry David A. , Mohanty Bibhu , Nasseri Farzine 
Published: May 2016

Abstract
A substantial number of underground formations penetrated during oil and gas well drilling operations consist of carbonate rocks, limestones and dolomites. Knowledge of their strength can help drill bit and mud weight selection, drilling performance prediction, wellbore stability analysis, and even casing point selection.
Physical samples that allow direct destructive measurement of the strength of underground formations are frequently not available. Instead their strength is usually estimated from in-situ measurements of physical properties that are correlated with the rock's strength. The rock's unconfined compressive strength (UCS) is typically estimated from wireline or LWD (logging while drilling) measurements of the rock's acoustic travel time.
Systematic physical, mechanical, and mineralogical measurements were made on a number of carbonate rocks collected from outcrops and representing a wide range of compositions and properties. This paper presents these data and uses them to show that carbonate rocks' acoustic properties are not necessarily well correlated with their strength. It describes an improved method for estimating the strength of underground carbonate formations developed from the data and illustrates that method with wireline and core data. The new strength estimation method, which recognizes and accounts for the impact of grain size on carbonate rock strength, should prove particularly valuable for bit selection and drilling performance prediction.

Introduction
In many instances it is useful to know the strength of underground rock formations penetrated during oil and gas well drilling operations. This information can help drill bit and mud weight selection, drilling performance prediction, wellbore stability analysis, and even casing point selection. Very often physical samples, which would allow direct destructive measurement of the strength of underground formations, are not available. Instead their strength is usually estimated from insitu measurements of physical properties that are correlated with the rock's strength properties. The strength property most usually estimated in this way is the rock's unconfined compressive strength (UCS), which is the peak compressive stress the rock can support under uniaxial loading. Typically, UCS is estimated from wireline measurements of the rock's acoustic travel time, which is the reciprocal of the acoustic velocity.
A substantial number of underground formations contain carbonate rocks, limestones consisting primarily of calcite and aragonite (calcium carbonate), and dolomites consisting primarily of dolomite (calcium magnesium carbonate) and calcite. To evaluate methods for estimating the strength of carbonate rocks, we made systematic physical, mechanical, and mineralogical measurements on a number of carbonate rocks collected from outcrops and representing a wide range of compositions and properties. This paper presents these data and uses them to investigate the relationships between the UCS and other properties of carbonate rocks. The results are used to develop an improved method for estimating the strength of carbonate rocks, which recognizes and accounts for the impact of grain size. We show how the method can be modified to allow its use when grain size data are not available, and we illustrate its application using wireline and core data from a test well drilled in Oklahoma, USA.