Automated photo-microscope supports rock analysis

Paul Boughton

Conwy Valley Systems has become a leader in the supply of computerised photo-microscopy systems for inspection, statistical analysis and classification of rock samples to assist oil and gas exploration.

Known generically as a digital petrography tool and marketed under the trade name, PETROG, the latest system is powered by a Nikon Eclipse 50iPOL binocular microscope equipped with the Japanese firm's DS-series digital camera.

The advance in this branch of technology has been so profound that it has become almost essential for petrophysicists and remote sensing geophysicists to use it for calibrating and ground-truthing their otherwise largely relative, as opposed to absolute, measurements of rock structures.

The automated point-counting system is a much faster and more accurate alternative to visual microscopy accompanied by manual tick-box recording of results. The user has a much better understanding of the rock sample, its constituent minerals, oil-bearing capacity and extraction potential. PETROG displays results almost immediately on the screen and has the added advantage of storing all photographic images for future re-analysis, if required.

Another alternative way of testing rock is crushing analysis, which simply calculates the porosity of the sample and hence the amount of oil it could contain, but it gives no indication of how the holes were interconnected and thus how well oil would flow.

The uniqueness of PETROG derives from the invention of a stepping stage which allows the polarising microscope stage to rotate automatically. Unlike a conventional microscope on which the slide and stage are fixed, apparatus for looking at rock has to view light at different angles through a polarising filter. Only in this way it is possible to distinguish between different constituents such as feldspar, quartz and clay, hence the need to index the stage in known increments. The system is equally suited to studying samples illuminated episcopically (by reflected light) or diascopically (by transmitted light).

A further application in the energy sector is the analysis of coal samples to assess the quality of a seam. It enables a mine to determine how to blend its output and where it should be sold. For instance, a steel mill is able to use information on the coal's microstructure to calculate how long it will burn at blast furnace temperature, allowing the steelmaking process to be controlled and optimised.

The construction industry also benefits from PETROG, since a sample of man-made concrete can be analysed as easily as natural rock. It is an important new target market for Conwy Valley Systems. A recent success was the purchase by CEDEX, a Spanish government research agency in Madrid, of the digital petrography tool for monitoring the integrity of civil engineering structures in Spain's built environment, including critical structures such as dams, looking for early warning signs of failure in the concrete structure.

The first application in this sector dates back to 2004, when a PETROG system was installed in Cornwall, UK, to support mortgage providers by checking for a specific impurity in locally made concrete that can potentially cause cracking. The so-called mundic tests are needed because, before 1950, houses in the South West of England were often built with concrete block containing aggregate consisting of copper or lead mine waste which in turn contains sulphides that can oxidise and accelerate degradation. The PETROG installation, at Petrolab in Redruth, was an early example of a system fitted with a digital camera (model DXM1200) from Nikon Metrology. Other manufacturers' digital cameras and microscopes can be similarly utilised, according to customer preference.

The Nikon Eclipse 50iPOL polarising microscope is a compact desktop model with up to five centerable objectives ranging from 4x to100x mounted to the nosepiece. The bright, 30 Watt halogen light source consumes little power and so generates minimal heat, reducing the chance of temperature-induced focus drift.

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