• Siggins, Anthony
• Dewhurst, David
• Clennell, Ben
• Raven, Mark
• Pawel, Lis Et Al.
• Sarout, Joel

Abstract

Knowledge of mechanical, physical and petrophysical properties of shales has slowly increased in recent times partly through investigations of the problems they cause for drillers (wellbore stability, overpressure), further through investigations of top seal capacity and integrity (capillary and mechanical properties) and finally through the advent of shales as reservoirs for prospecting for unconventional sources of gas (flow, diffusion, strength, fracturing). In particular, understanding factors controlling the strength of 'conventional' shales (e.g. overburden shales, top seals etc which are fully saturated and clay-rich) are important for predicting wellbore stability and trap integrity, issues which if not well understood, can cost billions of dollars a year. In this paper, we look at laboratory methods of measuring the strength of shales and then try to relate them empirically to other more easily measured physical and petrophysical properties. While regularly used as a proxy for rock strength, velocity was found to be a poor indicator of absolute shale strength, although useful for determining upper bounds. Porosity and cation exchange capacity give good empirical correlations to strength on a global suite of shales. However, partial saturation complicates this picture as water content affects rock strength in both fully and partially saturated clay-bearing shales. Strength and static mechanical stiffnesses can increase as water saturation decreases in low porosity, low clay, hard, stiff shales, as do dynamic moduli such as Young’s and Shear moduli calculated from velocity. This has implications for proper preservation of clay-bearing shales for laboratory testing for mechanical, physical and petrophysical properties, especially those now considered as reservoirs as well as for predicting gas shale properties from seismic data or wireline logs under partially saturated conditions.

Topics

• strength
• porosity