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MinEx CRC is developing a downhole geochemical tool utilising Laser-induced Breakdown Spectroscopy (LIBS). The tool incorporates a high-powered, variable-focus laser and optics, and spectrometers capable of detecting all elements on the periodic table to part per million levels. The prototype tool fits within NQ diameter boreholes and can be deployed by wireline.
Laser-induced Breakdown Spectroscopy utilises a rapid pulsed laser focussed on the analyte to generate a high-temperature plasma containing ionised material from the analyte. As the plasma cools all elements within it emit light at discrete, measurable wavelengths between ~200 and 900 nm (similar to the visible range for the human eye). The resulting spectral signature is a quantifiable fingerprint of the analyte composition.
LIBS analysers comprise a laser responsible for generating the plasma, optics for capturing and delivering the light signal, spectrometers for detecting the spectra, and back-end processing (e.g. deconvolution, multivariate analyses, calibration) for elemental identification and quantification.
The LIBS technique has had wide application in materials classification and quantification including in deep sea and extra-terrestrial settings. In the last decade handheld LIBS analysers have been commercially available to the mineral exploration community and more recently 2D LIBS scanning tools for detailed chemical and textural mapping have entered the market.
LIBS has a number of advantages over other potential in-field, real-time analytical techniques including:
MinEx CRC are developing a first-of-a-kind downhole LIBS tool intended to leverage the benefits of the LIBS technique to solve the challenge of obtaining real-time, in-hole geochemistry during a drilling campaign.
The all-in-one prototype tool has been fabricated and successfully trialled in simulated borehole conditions in the laboratory. Field trials are scheduled in 2024.
The downhole LIBS tool does not contain a permanent source of harmful radiation.
The in-built laser is switchable and can be configured to operate only when deployed in the borehole.
With a length of ~1.7 m and weight of <5 kgs the LIBS prototype tool is lightweight and easy to transport.
The prototype tool has outer diameter of 75 mm, designed to fit within NQ boreholes. Further miniaturisation will be explored in future versions of the tool.
The tool will be driller deployable by wireline and geochemical logging can be conducted in parallel with complimentary survey and geophysical logging techniques with little to no additional time penalty.
At logging rates of 2 to 10 m/min and typical sampling rates of 10 Hz the LIBS tool will deliver 60 to 300 individual analyses per meter.
Each LIBS analysis represents an area of <100 μm – at mineral grain-scale for most geological materials. As a result, the output from a LIBS traverse of the borehole wall is highly scalable. It can be broken down into individual mineral (and mixed mineral) observations or bulked out over hand-specimen (decimeter) or mining-scales (meter) depending on geological or engineering parameters.
Preliminary results from the prototype MinEx CRC LIBS downhole tool indicate that analytical precision and detection limits will be comparable to commercially available hand-held LIBS analysers. Detection limits are expected to be at part per million levels for most elements.
Standardised calibration procedures and analytical standards will be provided to users in order to ensure accurate and precise analyses.