ICAM 2011 presentation on rock typing of PDC drill cuttings

FEI Australia Center of Excellence for Natural Resources in collaboration with CO2CRC is going to present a third talk at the 10th International Congress for Applied Mineralogy (ICAM 2011) in Trondheim, 1-5 August 2011. Here is a preview of our presentation on a novel lithotyping approach performed on Polycrystalline Diamond Compact (PDC) drill cuttings. Micron-scale compositional mapping, using FEI's QEMSCAN^® automated mineralogy solution, and cutting-by-cutting classification by the iDiscover™ software package are demonstrated to provide detailed reservoir rock properties beyond chemical assays and modal mineralogy reports.

Petrological reconstruction of the subsurface based on PDC drill cuttings: an advanced rock typing approach"
by David Haberlah, Pieter W.S.K. Botha, Nicole Dobrzinski, Alan R. Butcher and John G. Kaldi

Polycrystalline Diamond Compact (PDC) drill bits are increasingly used in conjunction with motors and turbines as a fast and cost-effective way to drill wells. However, produced rock cuttings, in particular from clastic reservoir rocks, are often considered too fine and problematic for conventional petrological analysis. Recent advances in automated scanning electron microscopy and energy-dispersive x-ray spectroscopy (SEM-EDS) have transformed the petrological analysis of drill cuttings by replacing conventional qualitative descriptions of handpicked samples, with ultra-fast, quantitative and repeatable petrological analysis. Further developments include moving from chemical and mineralogical whole-rock analysis towards textural characterisation performed on a cutting-by-cutting and mineral grain-by-grain basis. Automated SEM-EDS compositional mapping allows for the definition of rule sets that classify individual cuttings into categories based on parameters such as mineral associations, grain sizes and shapes. As a result, drill cuttings can be classified into lithotypes representing subsurface rock types.

This study demonstrates that accurate and detailed reservoir characterisation can be based on SEM-EDS compositional maps of PDC drill cuttings. Lithofacies associations are reported from rock cuttings at continuous 5 m depth intervals from the CO2CRC’s CRC-1 well for a continuous stratigraphic interval in the Late Cretaceous Skull Creek Fm and Lower Paaratte Fm of the Otway Basin, Victoria, Australia. The CO2CRC Otway Project is the world’s largest research and geological storage demonstration project of the deep geological storage of carbon dioxide (CO2). For each cutting interval, a chemical assay, modal mineralogy report, and corresponding compositional maps were reported. A key advantage of automated SEM-EDS solutions such as QEMSCAN^® is that particles can be categorised on the basis of mineralogy, grain size and texture. Here, the drill cuttings were first classified into three general lithotypes, corresponding with sandstone, shale and cemented clasts, and subsequently divided into more specific lithotypes. These provide detailed information on the depositional environment and diagenetic history of the rock formations, and highlight intervals of cementation and intra-formational seals within the reservoir. The lithotyping results were plotted against wireline log data and show a strong correlation with the gamma-ray log.

This study demonstrates that combining automated SEM-EDS measurements of PDC cuttings with advanced digital image analysis and processing, can significantly contribute to the petrological reconstruction of the subsurface. This reinforces drill cuttings are a valuable source of geological and engineering information, potentially reducing the requirements for routine coring and wireline logging.