Empirical estimation of uniaxial compressive strength of shale formations

The uniaxial compressive strength of rock (σc) is an important parameter for petroleum engineers, drilling operations, and all related activities from exploration through to production and abandonment. A thorough understanding of the parameters affecting σc is a basic prerequisite for accurate geomechanical modeling of the reservoir and overburden properties. Uniaxial compressive strength plays a significant role in mud weight determination while drilling, especially for a troublesome lithology such as shale. However, standard geomechanical practice requires well-preserved core samples for measurement of σc in the lab. Because core samples are not often available, there is a need for alternative methods to obtain fit-for-purpose values of σc, based on other related rock parameters. Our primary objective was to identify a minimum set of related rock properties that could be used to predict σc. From a review of existing data in the literature, supplemented by laboratory measurements on Iranian samples, we established a database and accomplished extensive statistic analysis. Also, a normality test was executed to make sure a statistically acceptable set of data was collected. We suggested that two parameters of Young’s modulus (E) and porosity (ϕ), which might be estimated from geophysical log data, were sufficient for a reliable prediction of σc in shale formations, and the overall contribution of E was more than ϕ. We obtained a prediction equation with improved accuracy compared to previous investigations. Furthermore, we determined that the relative sensitivity of shale strength to porosity and Young’s modulus very much depended on the range of porosity.