A BIMONTHLY BULLETIN FOR WIRELINE LOGGERS AND GEOSCIENTISTS ENGAGED IN MINING AND MINERAL EXPLORATION

Transcription

1 1 WIRELINE WORKSHOP Issue 21 January 2017 A BIMONTHLY BULLETIN FOR WIRELINE LOGGERS AND GEOSCIENTISTS ENGAGED IN MINING AND MINERAL EXPLORATION Optical Televiewer issues Caliper logs and different tool types Coal density the final part of a series by Binzhong Zhou A Happy and Prosperous New Year to all our Readers! 1. Overview Optical Televiewer Logging Well it's a new year and things are picking up! There's no doubt about it, the frequency of queries received for contract logging services is increasing...at least, it is down here in the nether regions of Africa. Maybe it's a false dawn. Maybe it's got something to do with the coal price. Here's the five year graph.

2 2 During any recession in commodity prices and exploration activity, it's not a bad idea to look for lateral extension of the marketplace by offering new services or new ways of using existing technology. The writer remembers his own efforts to develop bridge pile investigation services. Steel tubes were wired to the reinforcing cage of each pile to act as conduits for a simple density sonde. After the concrete was poured and set, the pile could be logged and voids detected. Well, it worked quite nicely, proving on at least one occasion, the complete erosion of concrete from a section of pile due to underground water flows...the enemy of the piling contractor. The local boom in pile integrity testing lasted for about three years. Recently, this logging contractor has seen growing interest in the optical televiewer sonde, particularly in shallow boreholes, which are either dry or easily cleaned out with fresh water. It's mostly about formation integrity and fracture orientation in mines and civil engineering projects but also in groundwater casing inspection. Suppliers of optical televiewers have really upped their game in recent years. It has a lot to do with advances in light emitting diode (LED) technology which has rendered older designs obsolete...otvs in name only. You have to have one of the new ones. Image quality can be fantastic. The image is a map of the entireborehole wall at very high resolution and without perspective. It is normally orientated with the left (and right) edge aligned with magnetic north. So the log analyst can derive the dip and direction of bedding, layering, joints and faults as well as fracture condition and aperture. This is a major advantage if you are designing the foundations for a multi billion dollar power station or dam wall. In fact, it is surely difficult to argue a case for not running the sonde if site investigation boreholes are to be drilled on that type of project. The system offers an in situ, objective, orientated description of the ground that may be analysed, stored, transmitted and shared easily. The image may be viewed real time in the back of a logging truck. An OTV image showing bedding with washouts (Electromind SA). The user of televiewer data gets: The complete picture (no gaps) Orientation Aperture None of these benefits are guaranteed or consistently reliable from drill core. When the acoustic televiewer (as a mineral logging tool) was introduced in the mid nineties, it was well received by exploration geologists. It has all but replaced the resistivity dipmeter system. Geotechnical engineers use acoustic televiewer data routinely. The optical version has been used more in dry hole environments and as a geological rather than a geotechnical tool. The latest OTV designs, with their powerful LED lighting plants, offer advantages in very large boreholes with rugged walls where ATV images tend to be poorly defined.

3 3 There are limitations to the OTV system. Dirty water obscures the target. In the big diameter hole scenario, it would be necessary to clean out the opaque borehole fluid or the advantage of the lighting power is lost completely. It is remarkably difficult, even with very bright lights in small diameter holes, to describe the borehole wall through dirty water. OTV high side image in a slim borehole full of murky water much enhanced to show the roof of a cave This is illustrated by the log above where an OTV is run in a 60mm borehole with lights at full power. After maximum enhancement, in terms of image contrast, it is possible to pick the roof of a large cave (one object of the exercise) but, overall, the image is poor. There is a sinusoidal event at 2,032 metres that is just visible but little else is described. This 2,034 metre deep borehole contained just 5,750 litres of dirty water (the water table was very close to the surface), so an injection of 10,000 litres of tap water (via the drill rods) would have made a big difference to image clarity. Some experimentation in borehole fluid cleaning procedures is called for. Oily residue or water droplets often become attached to the lense cover. If the OTV is required to log the dry section of a borehole, the logger should avoid dipping the sonde into the water before logging upwards. Any oily liquid will float to the top of the water and beads of oily water damage the optical image. This usually has only an aesthetic effect on the data...to be avoided but not critical to the purpose of running the system. It is always annoying, though, to see the resulting continuous lines running down an otherwise perfect

4 4 image. It is more common in dry hole logging where the sonde has passed the fluid level. Sometimes it is worth pausing just above the fluid to allow water droplets time to dissipate. Vertical artefacts on OTV image caused by droplets of contaminated water on the lense cover. One can purchase water repellent sprays for use on camera equipment but the writer has not tried this approach yet so cannot recommend it. It is usually necessary to run a caliper tool to measure borehole diameter. This involves an extra run in the borehole but it is prudent, regardless of the need to measure diameter, to check borehole conditions before lowering a fragile and expensive piece of equipment. If a gamma sub is attached to the caliper tool it will provide a geological reference to plot with the OTV image (as shown in the log on page 1, gamma ray on the far left). Magnetic formations seriously affect image orientation. If a borehole intersects very magnetic formations and the borehole is close to vertical then neither acoustic nor optical images may be orientated. You get a good image and events on it may be identified, classified and counted but they cannot be orientated. There is no gyroscopic televiewer sonde. That would be an expensive device and, in fact, very few boreholes are drilled perfectly vertically through completely magnetic formations. Orientation of the borehole trajectory depends on the televiewer sonde's navigation sub. The logged trajectory, as well as having value as a navigation log, is used to rotate picked events on an orientated image to be with respect to true dip and azimuth.

5 5 The measurement of borehole tilt with respect to vertical, usually based on accelerometers, is unaffected by magnetism. The measurement of borehole azimuth, with respect to magnetic north and later corrected to be with respect to true north, will be damaged by magnetic formations. Orientation of the optical or acoustic images to magnetic north (left edge) will be damaged by magnetism confusing the magnetometer set in the navigation sub. The solution is to orientate the image with respect to the high side of the bore, as determined by the accelerometers. Events will be orientated and may be rotated by reference to a repaired magnetometer based navigation log, to a gyro based navigation log or, if necessary, to the estimated trajectory of the borehole. Images orientated wrt mag north and high side note, the high side orientation is the same as the borehole orientation In the example above, borehole tilt and azimuth are plotted on the far right. Tilt (red dotted curve) is just 2.5 degrees from vertical. Azimuth is close to north at about 18 degrees. Even at this tilt, there is a high side and the image on the right of two is orientated with respect to that. It is clear...not fuzzy, as happens when high side becomes hard to measure. Note that the two images look similarly orientated in this case because high side and magnetic north are very close to each other. One event picked from both images and corrected using the navigation log is shown as a sine wave to the right of the images. The two waves are indistinguishable. In magnetic formations the logger needs: A borehole of tilt that exceeds 1.5 degrees from vertical. A reliable navigation log, perhaps captured with a gyro sonde or one repaired from patchy data. Very often in these circumstances, the driller is required, by his client, to drill at an angle rather than nominally vertical. In most cases, where a borehole exceeds 50 metres in depth, the trajectory deviates from vertical anyway. The business end of an OTV sonde, with LEDs illuminated (ALT). If the borehole is perfectly vertical, the logger can use the magnetometer system for borehole orientation if the formation is magnetically benign or patchy (much can be done in processing). Serious problems of image orientation may be irreparable...so drill an angled borehole!

6 6 If a borehole is angled and the image aligned with its high side, either, the client must provide the logger with navigation data or the latter must generate his own log using a gyro sonde (an expensive but sometimes necessary option) or through processing of patchy borehole azimuth from the available navigation data. Patchy azimuth data manually enhanced The log above describes patchy navigation data captured by an optical televiewer sonde in the Northern Limb of the Bushveld Igneous Complex. The log in the middle, darker line, is manually corrected then filtered and shown on the right with the tilt log. The log is not perfect but, for the purposes of picked structure orientation and bearing in mind that the tilt log is unaffected by magnetism, this data set is acceptable because borehole azimuth variations are relatively small and will have little effect on a structure's orientation (tadpole plot). It is suggested that perfectly vertical borehole with magnetometer data that cannot be repaired will usually constitute a very small proportion of data captured in most logging campaigns. Questions to be addressed early in the programme: Which log is required, OTV, ATV or both? If OTV, can any dirty borehole fluid be replaced? If ATV, how deep is the fluid level (the sonde needs a water filled environment)? Is the formation expected to be magnetic? Should the driller purposely drill non vertical boreholes? Is the logger required to mobilise a gyro verticality system? OTV logging can be frustrating if the logger and his client fail to plan properly and make resources such as water tankers available on site.

7 7 2. Measurement Focus A review of one wireline log measurement Another Look at the Caliper Sonde (Refer to issue 5 from May 2014 pages 6 to 8) We often divide geophysical log measurements into three categories based on the type of energy involved: Electro magnetic energy Sonic energy Ionising radiation Caliper, like OTV, does not fall into one of these three groups and is not really geophysics at all...just a mechanical device used to assist in making geophysical measurements or to describe the borehole column. A single arm caliper as part of a density sonde being calibrated. There are several types of caliper mechanism and they all face challenges caused by strength limitations resulting from the need to deploy small diameter equipment. We can work through the range of equipment options for the geologist and consider their primary uses and limitations. There is no such thing in the equipment manufacturer's brochure as a single arm caliper tool. Single arms are limited to density logging tools like the one in the picture above. In this design, the caliper has two functions: To produce a borehole diameter log (also used as an aid to environmental compensation). To sidewall the sonde's collimated mandrel and radioactive source energy (looking at the formation). The density logging tool relies on tight contact with the borehole wall. This is compromised by a weak caliper mechanism or very large vertical borehole diameter. In very large hole, over 300mm diameter, a longer arm or arm extension may be fitted to the sonde but, due to the longer lever effect, the strength of the arm is reduced and help is required from a double bow spring. It makes sense to position the pulley on the side of a borehole as well. The writer has looked down big diameter boreholes during logging with centralised pulley wheel...the sonde is rhythmically pulled away from the wall in a series of jerks as the boom flexes. None of these problems exist in an angled borehole because the caliper always points upwards. It never attempts to support the weight of the sonde. The geologist should also note that the single caliper is not measuring full diameter in rugose borehole wall conditions. The sonde, at about three metres in length, dampens the log as one side of the bore cannot be measured in detail. If we remove the radioactive source from a density sonde, it becomes a single arm caliper tool, which is a very useful device for two reasons. Firstly, it can be used to check hole conditions if the logger's three arm tool is broken or has been forgotten (before risking a radioactive sonde).

8 8 Secondly, its arm never bears the weight of the sonde in angled boreholes where it offers a very precise (repeatable) measurement. Three arm logs (left) and one arm logs (right) repeated multiple times in the same large diameter angled borehole The three arm caliper sonde is a workhorse of the logging business. It can be extremely accurate if carefully calibrated before and after logging. It is routinely used to check borehole stability before running more expensive or radioactive sondes. Its diameter measurement is useful in processing televiewer data. It will describe open fractures and might be usefully combined with gamma ray and temperature devices as a first run tool. It can usually pick the casing joints for a CBL log if the CCL device fails. The three arm caliper tool measures borehole diameter based on one mechanism...its arms are not independent and it cannot log ovality. On one occasion, the author was asked to log the internal diameter of about 1,200 metres of raise bore pipe for a drilling contractor. What appeared to be a straightforward job, as usual, became anything but that. Routine work is easy. Special one off jobs usually cause problems...that's life. For a start, the logger did not have enough wireline on his winch and had no backup due to a recent loss on another job. It was decided to run a connection bullet to add 300 metres of capacity (fairly simple for single core cable). The borehole was dry and perfectly vertical as one might expect for a raise bore job. There would be a tricky moment or two as the depth wheel assembly was dismantled to allow the bullet to pass off and back onto the winch drum. Then there was the safety regime. If the loggers expected to performed any "work", they would be required to undergo a threeday safety induction course (the site was on mine property). It was necessary to do no more than computer work in the truck and allow the drill crew to rig up and deploy the sonde.

9 9 It was decided to use the drill pipe itself as a calibrator since the logger (thought) he knew the two internal diameters. That would avoid a possible confrontation with the Safety Officer. Well the logger's reference pipe turned out to be from a new batch of slightly different dimensions. It made no difference to the main goal, which was to determine the depths of nylon projections in the pipe, but the client spotted the error and the log had to be retrospectively corrected using the right calibration constants...embarrassing, certainly, but the strength of the wireline log is its precision and the fact that it can be reprocessed if necessary. Actually, the log was not perfectly precise. There was a slight drift of about 4mm over the 1,200 metres of caliper log. This was not caused by arm tip wear but possibly by the battering the tool received during its ascent through the pipe. There was a ten centimetre step every three metres. When the author sneaked onto the rig and put his ear above the pipe during the log, he could hear the clack, clack, clack as the arms negotiated each step...well over 300 times. Part of a raise bore pipe caliper log (much truncated) with anomalous nylon cable centraliser clearly visible. Then there was the heat. It was 40 degrees Celsius. With no air conditioning fitted (the unit does not normally operate in such warm climes) things started to become uncomfortable in the logging cabin. The logger raised his field PC onto a couple of screwdrivers to allow better air circulation but, as the winch was stopped at surface and before the logger closed the file, the PC switched itself off. It had to be placed in the refrigerator in the Operator's cabin for half an hour before it would restart. Thank goodness, the file had closed itself without being corrupted...all was well. A logging file should always close itself properly at loss of power. This had been a rushed job. No notice given and an eight hour drive to site. The logger could have predicted the heat and purchased a fan on the way to site. He could have calibrated at the motel the night before if he had known about the safety restriction. Let's face it, we will do anything to avoid another three day induction course. We are already experts on slope stability and risk assessment... maybe that's a good thing! Anyway, the point is that the three arm caliper is very good at measuring circular boreholes, casing and pipe. If properly calibrated, at relevant jig diameters, it can achieve an accuracy of better than +/ 2mm. It is used to measure wear on mineshaft backfill pipe for instance. It is not so accurate or precise if: The borehole has a large diameter The borehole is angled well away from vertical The caliper arms are long (over 30cm) The overriding problem is that, unlike the one arm tool, the three arm mechanism has to carry the weight of the sonde. When borehole angles get beyond 10 degrees or diameter/arm length exceeds 30cm, the tool weight sometimes becomes unbearable. Part of the variation on page 8 might be due to the three arm tool taking a different track (orientation of arms) on each ascent, whereas the one arm tool always takes the same track...caliper upwards. However, similar logs from the same project showed much more variation, even describing diameter as less than bit size. It is very helpful to plot BIT SIZE on any caliper log measurement where accurate volume is required.

10 10 In the project discussed here, where the client required a diamond assay based on carats per cubic metre, the bit size was 610 millimetres. A borehole volume log based on enhancement of mixed measurements (some caliper titles are hidden) After much experimentation with different sondes and centralisers, it was found that the one arm tool worked best deeper in the boreholes, where they were angled, and the three arm tool worked better in the more vertical upper zones. The one arm tool, with extended arm was unable to maintain firm contact when it was not lying tight to the low side. It is acknowledged that, in an oval borehole, the one arm device would overstate volume so some allowance must be made for that. The reader will ask why the mineral logger struggles to log the volume of a large borehole and what technology might improve log quality. Why not build a fatter, more powerful, multi arm caliper tool. Well, these jobs are not common but at least one equipment supplier has done just that. There are four arm caliper sondes that tend to be stronger than the single rack three arm device. They come in two broad categories; the dual axis (X Y) borehole geometry tool or BGT and the four arm caliper sonde whose arms are independent., both of which may be run with a verticality sonde attached in order to orientate stress dependent ovality or breakout. A four arm caliper sub with independent arms (right) and one axis under stated (angled hole, left). These devices are a big improvement on the three arm tool if the logger requires an orientated measurement or borehole volume. However, they both suffer (in extreme circumstances) from the same problems.

11 11 As with the three arm caliper, the four arm sonde must support the weight of the sonde. If long arms are required in an angled hole and particularly if a verticality sonde is attached, the sonde will be relatively heavy and will run eccentrically in the borehole. In the case of the independent arms, the upper arms is actually pushing downwards, adding to the load on the lower arm. The upper lower dimension is then measured perfectly but the lateral pair will not find the centre of the bore (see diagram, bottom left, page 10). Because the spring mechanisms in these four arm sondes are very strong, the problem normally resolves itself as soon as the sonde reaches a cave, allowing the arms to flex and find the centre of the borehole. Caliper logs X and Y converted to diameters and Y corrected manually below the lowest cave. Very often, the tool's motor and spring assembly will not lift the tool into the centre of the bore at TD The log above, from Angola, illustrates two things; the need for manual correction (shaded logs) of the Y diameter near TD and the great difference in borehole cross section. Clearly, the three arm sonde would not be the ideal tool for this borehole. Note how the bit size logs (dotted lines) allow us to make an educated correction of under stated diameter. It is very useful to have a choice of caliper arm length, regardless of which sonde type is used. A sonde will perform best if its arms are no longer than they need to be to measure the maximum expected borehole diameter. Most caliper logs are captured in slimline vertical boreholes with circular cross section. The three arm sonde is perfectly good at measuring the diameter of these boreholes, describing spalling (weak rock?), open fractures (aligned with fluid ingress?) and checking borehole conditions before an expensive or radioactive tool is lowered. If gamma and temperature subs are added, it becomes an excellent first run sonde in deep boreholes. Where borehole volume or the orientation of breakout is required, some form of four arm sonde is recommended.

12 12 3. Guest Article A serialisation over four issues Towards Quantitative Use of Density Logs for Coal (Binzhong Zhou) Part 4: Coal quality estimations from geophysical logs independent data set The previous examples (issue 20, November 2016) demonstrate the feasibility of the RBF based multi log approach for coal quality parameter estimation through a self controlled leave one out cross validation method. Correct estimation of coal quality parameters using geophysical logs through such a controlled training data set is a necessary condition for implementation of coal quality estimation. A stronger endorsement of the practical viability of such an approach is achieved if an independent data set, which does not belong to the control training data set, can be estimated correctly. To illustrate the application of the RBF method to a non control data set, the BMA data set is divided into two sub data sets: Sub set 1 from the boreholes in Area 1 and Sub set 2 from the borehole in Area 2 (Figure 5). The whole data set (Sub set 1 + Subset 2) is from the same data samples as those used in the previous controlled data set. We use Sub set 1 as the control training data set to derive the RBF internal computational coefficients and apply these coefficients to the corresponding geophysical logs to estimate the coal quality parameters. The figure below shows the estimated coal parameters RDs and ash percentages from geophysical logs GRDE, CODE, DENL, DEPO and ADEN for different data sets and the table on page 13 lists the statistics of the corresponding estimations. The resulting estimates for the whole data set and Sub set 1 are based on leave one out cross validation, while the estimates for Sub set 2 are results by applying the control Sub set 1 to Sub set 1. The cross plots and table clearly demonstrate that we can use multi geophysical logs to estimate coal quality parameters. Estimated coal parameters from geophysical logs GRDE, CODE, DENL, DEPO & ADEN for different data sets: (a) Estimated RDs for whole data set; (b) Estimated RDs for Sub set 1; (c) Estimated RDs for Sub set 2 using Sub set 1 as control data; (d) Estimated ash contents for whole data set; (e) Estimated ash contents for Sub set 1; (f) Estimated ash contents for sub set 2 using Sub set 1 as control data.

13 13 The statistics of estimated RD and ashes for different data sets. Concluding remarks Borehole logging densities are strongly influenced by the shoulder effect of coal/rock boundaries due to the intrinsic resolution of the density tool. This makes the logging densities from the samples around the boundaries less accurate and incompatible with the laboratory density measurements. There is no good way to suppress the shoulder effects currently. Therefore those samples from thin seam beds and those small samples around the boundaries should be excluded from data sets from analysis. This largely improves the correlation between the laboratory density measurements and borehole logging densities. Instead of the commonly used simple correlation with the densities from borehole logging, a multivariable data analysis approach based on Radial Basis Function (RBF) can be used for estimation of coal density and other coal quality parameter from multiple geophysical logs. This approach has a better chance of dealing with the complexity of coal quality parameters and hence improve the estimation accuracy of these parameters. The feasibility of the method was demonstrated using a data set from a mine in Central Queensland. The demonstrations were conducted on both self controlled training data sets and an independent data set. It is observed that the density logs play a key role in coal parameter estimations as they have strong correlations with coal quality parameters such as ash content, fixed carbon and specific energy. However, with the use of more geophysical logs, including logs with different resolutions such as short spaced density log DENB and long spaced density log DENL, the estimation accuracy is improved. Binzhong Zhou (CSIRO Energy) Binzhong.Zhou@csiro.au The author referred to: Fletcher, I.S., and Sanders, R.H., 2003, Estimation of in situ moisture of coal seams and product total moisture: Final Report for ACARP Project C10041.Preston, K.B., 2005, Estimating the in situ relative density of coal old favourites and new developments: The Proceeding of Bowen Basin Symposium 2005, Preston, K.B. and Sanders, R.H., 1993, Estimating the in situ relative density of coal: Australian Coal Geology, 9,

14 14 Zhou, B. and Esterle, J., 2008, Toward improved coal density estimation from geophysical logs: Exploration Geophysics, 39, Zhou, B., Fraser, S., Borsaru, M., Aizawa, T., Sliwa, R. and Hashimoto, T., 2005, New Approaches for Rock Strength Estimation from Geophysical Logs: Proceeding of Bowen Basin Symposium, October 2005, Yeppoon, Queensland, pp Thanks to Binzhong for his ongoing contribution to the mineral wireline logging industry. A full range of parameters (logged by Weatherford and processed in WellCAD) through a coal sequence in Southern Africa. Note the correspondence between the main log types, based on radiation (natural gamma GR, neutron NPOR and density BRD and LSD), electro magnetic (resistivity FE) and sonic (P wave travel time DT). Marcus Chatfield January 2017 Copyrights apply: Editor/contact: wilna@wirelineworkshop.com For back issues, go to: