Drill core quality and borehole depth measurement – the missing link.
Diamond drilling is designed to obtain a continuous rock sample from a straight bore – and it is the diamond driller’s responsibility to deliver this product.
After which the drill core is examined by geologists and geotechnical engineers to record different aspects of the rock retrieved, which logs are analyzed and correlated to model the subsurface. These data are then used by mining, metallurgical and civil engineers either to plan the safe extraction of ore, or to determine the ground reinforcement required to support a civil construction.
Therefore, the success of a mining venture or a civil project largely depends on: (i) how well the loggers record data for the engineers to use, and (ii) how representative the drill core is of the ground sampled.
Given 100% core recovery, the quality of the data supplied to the engineers to work their designs depends on the loggers’ observational and descriptive skills, their ability to analyze the data, as well as spatially reference the information. Thus, the risk of inadequate data collection is determined solely by evaluating the loggers’ proficiencies. This is the baseline risk – to which the driller makes no contribution as there is no core loss.
Where there is core loss, however, which is impossible to avoid, the increased level of risk depends on (i) the extent of the data gaps, (ii) how accurately each gap is mapped along the borehole path, and (iii) whether the reason for each gap is correctly identified.
Core loss due to poor drilling, mechanical failure, or other human error, must be minimized. The remaining gaps are attributable to changes in ground conditions, for instance where major structures or weak rock intervals are intersected, which are all zones of elevated risk that must be carefully evaluated.
Hence, all interventions taken to improve recoveries over these zones, such as changing the drill bit specification, or formulating a mud-mix to deal with swelling lithologies, must be meticulously recorded, to not only inform subsequent drilling campaigns but also because these data provide important clues as to the characteristics of the missing rock. Downhole geophysical logs are also good sources of information, so long as they can be properly depth correlated with the drill core logs. In critical cases, infill drilling specifically targeting these zones will also benefit from a knowledge of the measures previously applied to recover difficult rock intersections from the local terrane.
This raises the question as to how core loss should be managed?
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Most drilling contracts penalize poor core recoveries, typically set at a minimum of 95%, but this may not be an appropriate quality control measure since it can be circumvented, as discussed in the previous posts referenced below. It can also inadvertently, or otherwise, increase the incidence of ‘hidden’ core loss. Further, it encourages the metric of meters drilled per shift to be the dominant consideration used to assess drilling quality – for which drillers are motivated by production bonus payments, which incentive can be detrimental to core recovery, as well as increase the risk of borehole deviation.
The overriding factor for assessing drilling quality is simply whether the depth of the borehole is accurately tracked throughout the drilling process. It is only when the accuracy of this measurement is assured for each and every drill run (defined as A drill run occurs whenever the core barrel inner-tube is extracted after an advance, however short this may be) that the subsurface can be accurately mapped, including gaps due to missing sample.
It therefore remains to propose a contractual clause as well as an incentive reward that could facilitate effective QA/QC of drilling quality. For the former, there should not be a problem with penalizing anything less than 100% accuracy in borehole depth measurement. As regards the bonus, this could similarly be contingent on 100% accuracy, which if confirmed by independent audit, would be payable against the number of meters drilled.
? J.L. Orpen (Resource Exploration & Development Pty. Ltd.)
References:
1.??????The depth registration of drill core. https://www.dhirubhai.net/pulse/depth-registration-drill-core-john-orpen
2.??????Measuring core loss from diamond drilling - the benefits. https://www.dhirubhai.net/pulse/diamond-drilling-drill-core-measurements-2-loss-john-orpen
Driller Supervisor | DRILL CORP Sahara saudi
1 年interested to work my email [email protected]
Head of Project Initiation and Closure | Jefe de Operaciones en Explomin Perforaciones
3 年Importante la lectura sobre el producto final, la informacion que brindamos a nuestros clientes debe ser confiable el registro de los tramos perforados debe mostrarce tal cual; si no tenemos buena recuperacion podemos aplicar diferentes tecnicas para mejorar, medir la muestra antes de descragar del tubo interior te da una informacion real de lo perforado y recuperado. Saludos cordiales.
Diamond driller at Boart Longyear
3 年This is the product we are selling to the client and for me it is important to give them the best product I can, they spend huge dollars for drilling so we should strive for excellence
Diamond driller at Boart Longyear
3 年A good read John, as a driller I take great pride in my job, which is to produce core. My tape is always within reach and the stickup is measured after every run, we need to know where our bit is in the hole! I also do not write out my marking blocks in advance, they are done after each run and are to the centimetre. This is what we are paid to do!!
exploration surveyor at Digital Surveying
3 年Hi John, nice article . Hope you keeping well .