Research paperEmpirical strength prediction for preserved shales

- Shale strength correlates well with porosity and normalised cation exchange capacity.
- Velocity is not a good predictor of shale strength but a useful upper limit may be defined.
- Friction coefficient in shales correlates poorly with porosity and clay content.

Clay-rich shales form the overburden in many sedimentary basins and the seals to hydrocarbon accumulations in the subsurface. Their strength properties are not well understood and they are not often cored or carefully preserved to allow laboratory measurement of their properties. This paper presents the results of geomechanical tests carried out on a global suite of temporally and spatially diverse shales in terms of unconfined compressive strengths, cohesive strengths and friction coefficients. These shales were also extensively characterised in terms of their composition, porosity, physicochemical properties, grain size and rock physics response in order to derive empirical correlations to shale strength. The dataset was expanded using measurements published in the open literature along with some proprietary measurements. What might have been presumed to be good relationships such as strength with velocity or friction coefficient with clay content/porosity were found not to be the case. The best correlations were between strength and porosity or bulk cation exchange capacity. A correlation between friction coefficient and rigid grains was noted although the variables involved were not strictly independent. The relationship with cation exchange capacity may provide the opportunity of strength prediction from dielectric logs as dispersion of the dielectric constant is directly related to cation exchange capacity.