Downhole geophysics – the glue for geological modelling.
Any geological model created solely from drill core data is necessarily deficient – irrespective of the core recovery percentage, either by length or by volume.
No driller can be expected to deliver an unbroken train of solid rock samples and wherever core is lost due to changes in the rock properties, as opposed to poor drilling, the risk is high for the geological modeller.
To elaborate: It is not unknown for an unmapped major structure to catastrophically impact a project, which outcome could have been avoided had the intervals of core loss been investigated to establish whether they were due to bad ground, or bad drilling at least.
Equally problematic for investors is the independent discovery of ore in ground that was previously held. In one case the deposit was overlooked because soft matrix drill bits were used so that weak intersections of mineralized rock were inadequately sampled. In addition the mud-mix was wrong so that cuttings were not flushed efficiently from the borehole, further abrading the weak intervals, reducing their volume and adversely affecting the assay values.
Consequently, logs of the geology intersected in a borehole are incomplete if the only source of information is the drill core. The resulting model, lacking any data from the sections of core loss, is not representative and could dangerously underperform for the engineers who need it to plan for safe ground development.
The procedure for acquiring the missing detail to present as coherent a picture as possible of ground conditions for the engineers to analyze and design their works is relatively simple. The five stages are:
1.??????Identify and map the intervals of core loss along the borehole path. This task is split into two steps since the size of the gaps and their position along the borehole path can only be correctly depth referenced using stacked depth registration, which as explained in the first post of this series can only happen after completion of the bore. Hence, until that has occurred the sections are temporarily depth referenced using standard depth registration (Fig. 1).
2.??????Classify the core breaks at the end of each segment. Most fractures lock together showing no core loss. At the opposite end of the spectrum is a discontinuity with sides that don’t match indicating a cavity, especially if it is a geode. Some subterranean voids intersected are old mine workings that the driller will help identify. In between are rubble zones and mechanical grinds. The latter must be minimized and the former have to be inspected to decide how much of the disintegration is natural due to changes in the rock, and how much is due to mechanical shattering. Natural fragmentation, usually somewhat weathered with friable edges, is recorded as a core loss zone, whilst sharp-edged, fresh rubble indicates a drilling problem that must be fixed, but the core loss zone has no modelling relevance. Note, it is precisely because of the importance of properly evaluating rubble intervals that triple tubes are used for geotechnical drilling.
3.??????Run an appropriate selection of downhole probes to profile the geophysical properties of the rock drilled. Deciding on a suite of sondes suitable for characterizing the terrane being drilled is a site-specific exercise for which a selection matrix can be helpful (Fig. 2). Then, before surveys are run it is necessary to field-test all probes to confirm their calibration. In addition, since all instruments carry onboard sensors that continuously record temperature, pressure, roll rate and centralization – for which manufacturers specify limits that should not be exceeded – it is important that these records are delivered to the customer immediately on completion of each survey to confirm none of the operational limits were exceeded.
4.??????Depth match the drill core and downhole wireline logs. Precisely matched datasets from core and bore reinforce the credibility of the geological model developed from the combined data package – however, the deeper the bore the less likely the logs will match so that the following is required:
4.1.???Confirm borehole depth. There are two methods:
4.1.1. Method one. Independently measure the hole depth by: (i) observing the driller advance to recover a short length of core so as to confirm the drill string is on bottom; (ii) measuring the stick-up; (iii) count the rods as extracted; (iv) measure the total length of the core barrel outer tube; (v) calculate the drill string length – multiply the number of rods by their standard length and add the core barrel length; (vi) find the borehole depth by subtracting the stick-up from the drill string length.
4.1.2. Method two. Projects that incorporate an acoustic scan of the borehole wall for structural analysis are at an advantage if the tool is onsite whilst drilling is ongoing. The sonde can be run in-rod and the threaded rod-joins show up on the scan enabling a rapid, definitive and auditable rod count. As in method one: (i) the borehole is advanced to confirm bottom; (ii) before the inner tube is pulled the scan is run in-rod until the wireline cable goes slack indicating the tool is resting on the inner tube head; (iii) the ultrasonic tool is set to survey at normal running speed on the out run; (iv) the rod joins are counted on the scan to calculate the drill string length, including the known length of the outer tube; (v) the stick up is subtracted to measure the borehole depth.
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4.2.???Confirm wireline depth measurement accuracy. Once a sonde is deployed in-hole the only method of keeping track of its location in the borehole is by measuring the cable length during the out and in runs. Depth encoders perform this measurement as the cable is spooled past the sensors mounted on the winch. The issues and solutions are:
4.2.1. Depth encoder accuracies with no tension applied to the cable. The normal field test is to pull a mark on the wireline cable past two survey pegs fixed at least 50 m apart, measure the variance and compare with the manufacturer’s specifications.
4.2.2. Depth encoder accuracies with tension applied to the cable. This is the crucial test, and the most difficult to conduct – unless an acoustic scan of the drill string is done (see Method two above). Now the constant spacing between the threaded rod joints can be compared with the ATV depths marked on the scan to directly graph cable stretch downhole with increasing weight and temperature – and use these data to adjust scan depths.
4.2.3. Run a combo tool string. Downhole sondes are usually run in open holes, so the first tool to be deployed is a three, or four-arm caliper used to assess borehole wall irregularities, enabling the wireline engineer to assess the safety of lowering other, expensive sondes beyond potential traps. Since most combo tool strings incorporate a caliper measurement instrument the traces of the borehole wall irregularities can be compared and any distinct patterns used to adjust scan depths accordingly. Note. Several sondes also carry natural gamma sensors, specifically so the traces can be used for depth matching, however this technique only works if the radiation sensor is correctly calibrated on each tool.
5.??????Highlight the geophysical properties of the core loss zones. Once the depth registration of the drill core and wireline logs is aligned, it remains to examine the geophysical data obtained over the delineated core loss sections in detail. The resulting interpretation of the rock properties of these zones is invaluable for cementing any geological model into the most reasonable representation of the subsurface explored in the drilling campaign – using all available data to greatest advantage.
Figure 1. The Standard and Stacked methods of measuring depth along drill core. (Orpen, J. and Orpen, D. 2020). Note: Stacked core loss gaps can be confidently distributed to breaks identified as being due to natural, or mechanical causes – impossible with Standard depth registration where gaps are amplified by apparent core loss.
Figure 2. A selection matrix for choosing a suitable suite of slimline sondes depending on the project requirements. (Orpen, 1999).
? J.L. Orpen (Resource Exploration & Development Pty. Ltd.)
References:
Owner, Alan Guest Technical consulting cc
3 年I’ve enjoyed all these chapters, now just to be compiled into a single book????