Seal Capacity and Petrophysical Evaluation of Shales Using Destructive and Non-Destructive Methods

Ben Clennell and Dave Dewhurst
CSIRO Petroleum, Perth, Australia

Compaction and diagenesis alter seal capacity and strength of mudrocks. Characterization of mudrocks for determining seal capacity and mechanical behaviour usually involves slow, destructive methods such as mercury injection, triaxial testing and gas adsorption. While these methods remain useful, they can be supplemented and extended using non-destructive methods.

Tools and methods for NMR analysis have been developed for reservoir rocks with relatively large pores. Shales are often designated as background or baseline in log analysis and have hardly been considered in NMR petrophysics. However, NMR total porosity and clay bound water index (BVI) are useful for shale characterization. Data from NMR tools and lab spectrometers at short echo spacings (200-300 microseconds) allow detailed interpretations with predictive value for seal and geomechanical characterization. In most mudrocks, clay bound water can be subdivided into adsorbed/interlayer water with very short relaxation times (a few ms or less) and microcapillary water with relaxation times in the 10 ms to 100 ms range. Consolidation and swelling of shales can be monitored over time using NMR to track the positions of these peaks.

X-ray tomograms are indispensable for shale characterization, revealing fractures, layering and bioturbation that can control seal quality. Dielectric properties also correlate closely with both water content and swelling-clay content of shales, as well as shale strength. Velocity trends correlate with failure and strength characteristics and together with instantaneous attributes, they can be excellent predictors of mudrock seal integrity and overpressure risk.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005