It is important to understand what is being logged.

Transcription

1 1 Issue 6 July 2014 WIRELINE WORKSHOP A BIMONTHLY BULLETIN FOR WIRELINE LOGGERS AND GEOSCIENTISTS ENGAGED IN MINING AND MINERAL EXPLORATION What exactly is being logged anyway The Optical Televiewer sonde Logging iron ore 1. Overview If we ignore images for a moment and review the chemical and physical property measurements, it becomes apparent that there is often uncertainty about what is actually being measured by each sonde. A formation comprises the "matrix" (the solid mineral) and the porosity (waterfilled or air-filled voids). There is also the complication that clay minerals include water adsorbed within their lattice structure. So there is matrix, free water (in pores) and bound water (in clay). In igneous rocks, the water volume might be limited to open fractures. Some of these will be caused by the drilling process although drilling-induced fractures in igneous formations are usually "tight". It is important to understand what is being logged. Formations that are considered dry, those located above the ambient water table, might, in fact, be completely dry, damp, moist or partially saturated. If the driller touches the water table, he will circulate water throughout the dry section of the bore. Logs that are sensitive to the presence of water, such as inductive conductivity, will be difficult to interpret in nominally dry formations and it should be remembered that the volume of rock close to the borehole might have quite different characteristics, in this sense, than the volume beyond the partially invaded zone. Looking at submerged formations, which comprise the majority of logged metreage in exploration projects, we can consider the log responses of a couple of common measurements; resistivity and density. The resistivity log, which will be discussed in detail in a later issue of this bulletin, measures the extent to which the formation opposes the passage of electric current. The matrix is normally very resistive. The measured variables are the amount of water in the formation (free and bound water) and its conductivity. If the water conductivity is known

2 2 or can be estimated, the resistivity of the formation will be a function of porosity and/or clay fraction. In clean sandstones or limestones (confirmed as such by other measurements) the resistivity log, compensated for fluid conductivity, will be a measure of porosity. This is useful to oilfield loggers because the apparent porosity, which should overlay density-based porosity, will be reduced by the presence of (resistive) oil. The separation of resistivity-based porosity and density-based porosity in sandstone or limestone formations indicates that there is some oil in the pore spaces. In mineral logging, clay (which is conductive) and coal (resistive) dominate the resistivity log in sedimentary basins. A resistivity log (centre) compared to density and sonic The resistivity log has value because it is very sensitive to facies changes in the formation and offers a rather different description to density and sonic logs, for instance. One very useful attribute is its ability to describe burnt coal, which is conductive because the ash absorbs water, as opposed to most coal formations which are resistive. In most sedimentary rocks the resistivity tool logs the moisture not the matrix. In igneous rocks, where there is no primary porosity, the log will indicate very high resistivity, sometimes beyond its measurable range (the log plateaus out). In igneous rocks, water-filled fractured zones will be clearly visible as excursions to low resistivity. The matrix might play a part if it contains graphite or a conductive metal such as magnetite. In most cases, the resistivity log is describing the physical condition of the rock. It measures chemistry only when certain conductive minerals dominate the matrix. Similarly, the density measurement is considered as a porosity log in sedimentary environments. In most rocks, the matrix has a limited density range. Coal is the big exception (see issues 2 and 3). Density is a physical property log. In igneous rocks, where porosity is close to zero, the density measurement becomes a log of formation chemistry and is particularly sensitive to the presence of iron. Density logs run in ultra mafic rocks will, over large sections of the formation, mirror the magnetic susceptibility log, if scaled to do so. We can state, in summary (and very generally), that there are three major influences on the standard physical property logs (density, resistivity, neutron and sonic): The background matrix, silica/aluminium/magnesium Water in pore spaces and clay Iron within the matrix The natural gamma ray log is very useful because it can be considered as a log of chemistry only.

3 3 2. Measurement Focus A review of one wireline log measurement The Optical Televiewer It is common in exploration projects for the geologist to photograph the drill core. Sometimes this is an informal picture of a core box where depth markers are clearly visible. It is a prudent record of the rocks intersected by the borehole. Quite often the photography is more thorough, including ultra high resolution images of rotated and orientated core. The resolution is excellent, the orientation is usually uncertain and the cost is relatively high. A high resolution image from rotated core The ultra high resolution camera has the advantage that individual mineral grains may be identified on the image. Drill core photography allows control of image quality because core sticks can always be cleaned and wetted. Modern computers can cope with huge file sizes. Orientation and cost remain problematic. For image orientation, the acoustic televiewer, discussed in the last issue, provides some certainty (assuming its log is quality assured). It is an excellent geotechnical tool. A similar device, called the optical televiewer offers the geologist accurate orientation as well as a moderately high resolution image (about 1mm pixel size). Resolution of the optical image is certainly higher than that of the acoustic one and it is gratifying to see the geologist's face light up when we put aside the wiggly lines and show him an orientated optical image of the rock mass. It is an impressive record of formations intersected by a borehole. Unfortunately, unlike the acoustic version, optical image quality depends very much on borehole conditions. This is the caveat. Dirty water and mud-covered borehole walls seriously impact on optical televiewer image quality. Typically, the OTV senses reflected light in red, green and blue (RGB) colour scales via a conical mirror housed at the bottom of the sonde. The source of light, which illuminates the borehole wall, is a ring of light-emitting diodes positioned next to the sensor. Note that the single optical televiewer image is orientated and without perspective. There are limitations to the diameter of borehole that can be effectively illuminated. In water, 200mm is usually about the limit. In dry holes, the author has achieved excellent results to well over 300mm by attaching battery powered lamps to the tool's centralisers. The latest optical tools have more effective lighting due to advances in LED technology and some will work to 500mm borehole diameter. Image resolution is better than that achieved by the acoustic televiewer or the micro-scanner. An effective pixel size of better than 0.20mm of

4 4 borehole wall may be achieved at HQ diameter (depending on system design). A range of image enhancement tools is available. There are advantages and disadvantages when comparing the OTV with the ATV sonde. OTV offers higher resolution than ATV, as discussed above. OTV works in dry hole, ATV does not. OTV images describe geology more clearly than ATV images. Picking and classifying fractures is usually more difficult and time-consuming using the OTV image ATV offers a borehole cross section (360 degree caliper) with caving and breakout, OTV does not. OTV is adversely affected by opaque fluids and mud cake, ATV is mostly unaffected. If the OTV image is poor due to fluid conditions in the borehole, it is probably worth trying a water replacement exercise using clean (municipal) water. This seems obvious but, as a contingency, it is generally not considered or planned for in advance... perhaps it should be. Meta-sediments produce excellent OTV images. In this example the density sonde was run first...see the vertical scoring from its caliper arm. Lighting intensity can usually be adjusted from the surface. This is important as intense light in small diameter bores is not always optimum and some tools overheat if run on maximum power in dry hole conditions. As with the ATV sonde, a borehole magnetometerbased navigation log is generated from the OTV system. Effective centralisation improves log quality. From the logger's perspective, the optical televiewer log is not proper geophysics; it is a photograph of a rock. It is a geological tool rather than a geophysical one. Nevertheless, the logger is, of course, pleased to capture something of value to the geologist and the processing of the optical image results in wireline data that can be juxtaposed with other logs - resulting in a powerful link between geology and geophysics. Truncated OTV image with picked structures combined with lithology logs

5 5 As with the ATV, the OTV sonde will not navigate well in magnetic formations. The geologist should drill an angled borehole (>5 degrees from vertical). The logger will capture a log and process with respect to the high side of the bore rather than to magnetic north. A gyro navigation log or an estimate of borehole trajectory is then used to orientate the picked structures with respect to true north. Exactly as with core stick examination, the OTV image in dry hole is improved by wetting, if that is practical. Optical televiewer log resolution continues to improve and, while not yet equal to the core scanner in that respect, it offers a continuous log with reliable orientation. Clean borehole conditions and effective tool centralisation are essential to the capture of high quality data. The next issue will feature natural gamma and spectral gamma ray tools. 3. The Logger on Site - Capturing Good Iron Ore Logs Density is the key but it's tough to get right The key measurements, when logging in the iron provinces, are magnetic susceptibility and density. The first to determine the extent of oxidation (is it magnetite or haematite) and the second to establish thickness and ore grade. There is normally an empirical relationship between grade and density. Logs of natural gamma, density, mag susc and caliper describing an iron ore deposit The log above describes fresh and partially weathered ore of mid range density. Logging magnetic susceptibility is straightforward in all but the borehole diameter compensation, which is a significant correction. That will be discussed in a later issue. The difficult and hard to quality-assure measurement is density. Generally speaking, as with coal logging, the conversion from electron density to bulk density will always result in a slightly inaccurate but precise log. This chemistry correction is easier in iron ore than it is in coal seams because we do not have to overcome the issues associated with coal and its organic molecular structure. A straight line correction is valid.

6 6 As in coal logging, there is a need to compensate for borehole irregularities. This is more difficult in iron ore than it is in coal because the contrast between the water/air filled borehole column and the target mineral is large in terms of density. There is, as well, the complication that, due to low count rates in dense rock, there is usually only one detector used, the short spaced one. So no dual measurement compensation is possible. For a start, we need a special high-density calibrator Calibration must be very carefully performed because the count range at the dense end of the scale is relatively small due to the flattening out of the density/cps response curve, as in the example below. Response curve at 15cm using a Cs137 source. Note that the CPS range is relatively small at the dense end of the scale. The red dot is the aluminium block calibrator's nominal density (depends on jig design). The water point calibration is off scale at the top left. Caving anomalies would be magnified, in density terms, at over 4gm/cc. The water point, at 1gm/cc, is off scale. The curve is very steep at low densities making the log insensitive to minor caving in coal. The graph also tells us that the standard two-point calibration, water barrel and aluminium block, is not practical for densities over 3.5gm/cc. We need a high-density calibrator to describe the tail of the regression curve. Ideally, the calibrator would be a very large block of haematite buried in concrete and carefully cored with the relevant drill bit. The logger does not take a point measurement, because we cannot guarantee homogeneity. He logs the jig and compares the short log with core stick density measurements, taking the average CPS value over a fully resolved section of log. A logging contractor might not be able to afford such a jig so he either persuades his client to build one (near the operational area) or makes a dense jig from shot blast pellets combined with Ordinary Portland Cement mixed with waterproofing additive (Coprox). A PVC pipe is used to mould the conduit. The lateral depth of investigation is fairly small at iron ore density. Jig density should be about 4.6gm/cc, so, for BRD and HRD (15cm - 22cm) resolution, a very large jig is not necessary. Even so, this jig will not be portable. It will be a base jig.

7 7 The jig's volume can be calculated and a density ascribed by weighing the jig and deducting the weight of the container. Homogeneity is important so a lot of mixing is required. The mixture should not be so wet that the pellets settle and/or the dried jig is too porous. The jig can be logged wet or dry to suit borehole conditions. The density of the jig must be adjusted to equivalent haematite density. The iron atom has four extra neutrons so counting electrons will result in an understatement of true density. Here are some useful numbers. Parameter Comment Value Z/A is Atomic Number/Atomic Weight Weight proportion increases with extra neutrons Z/A of silicon (sandstone): Si, Z = 14, A = (same number of protons as neutrons) Z/A of oxygen O, Z = 8, A = (same number of protons as neutrons) Z/A of iron Fe, Z = 26, A = (4 extra neutrons) Z/A of magnetite Fe 3 O 4, Z/A of haematite Fe 2 O 3, Density of iron From literature Density of magnetite From literature (centre value of range) Density of haematite From literature (centre value of range) Note that haematite and magnetite Z/A corrections are very similar and the use of , when calculating the effective density of the jig, will work well enough for both. The logger determines the bulk density of the jig, calculates its electron density using the Z/A for iron, having calculated the iron proportion of the jig based on its bulk density (cement is 2gm/cc) then converts its electron density to its effective density - the density that the sonde would log if the jig were made of haematite. It's a tricky process but is only required to be done once. Compensation for borehole diameter is determined empirically using more than one conduit diameter. Alternatively, the conduit diameter should match the drill bit diameter. Standard Z/A correction If a water barrel, aluminium block and the haematite jig are used to describe the density response curve, the resulting log will be corrected for Z/A. The extent of the correction at a range of densities is described in the graph above. A density of 2.70 is used as an average value of limestone, dolomite, sandstone etc. General assumption: If a rock has a density of less than 2.7gm/cc, there will be water present in pore spaces. If it is more than 2.7gm/cc there will be some iron present. A typical Pilbara ore at 3.65gm/cc would be logged as electron density gm/cc. All the iron ore geologist needs to have confirmed is that a Z/A correction was applied at the dense end of the scale via calibration.

8 8 A well calibrated sonde, that is compensated for chemistry and borehole diameter, is a good start. Now we have to consider borehole wall conditions. In slimline cored boreholes accuracy should be high, as there are usually few caves. Logged density can be relied upon. There is one issue that can trip up the logger. He should be aware that intense magnetism can perturb his sonde's photo-multiplier tubes and render the density log incorrect. Sondes used in magnetite exploration should be fitted with Mu-metal screens covering their PM tubes. Just as logs of smooth core-drilled boreholes are usually correct, logs of boreholes with caved or uneven walls are invariably inaccurate and there is little that can be done about it. As an attempt to overcome this problem, cobalt 60 (Co60), as a source of higher energy radiation, is employed in some iron ore projects. Isotope Energy Half-life Minimum source-detector spacing Cs MeV years 15cm Co and 1.33 MeV 5.27 years 22cm Note that the cobalt source emits two gamma rays of different energies, both about double that of Cs137. The cobalt-based measurement has greater penetration and allows (and demands) a longer source-detector spacing while retaining acceptable count rates. It therefore logs a larger sample of the formation. It has a short half-life so regular calibration is necessary but loss of a source in-hole is less of a long term health risk. It has lower resolution than the Caesium version. It requires more lead shielding so the transport pot is very heavy. The Co60 source is used because it is perceived to be less affected by borehole irregularities. That is true, given the larger sample size, but it makes little difference in a slimline cored hole and does not sufficiently mitigate the caving problem in percussion boreholes. It is certainly an improvement but not the complete solution. That might be achieved if cobalt were employed in a fully compensated dual detector density tool. Single density works well enough in cored boreholes

9 9 In summary: The following processes need to be in place if a log or iron ore density is to be used quantitatively. The logger must measure plenty of counts so a short 15cm (Cs) - 22cm (Co) spacing is advised to minimise noise and natural gamma effects. Mu-metal screens should be fitted to PM tubes in order to avoid perturbation of count rates by magnetism. In very dense formations, where count rates will be low, regardless of detector spacing, the logger should run the tool slowly (<4m/minute). Standard tooling and software does not include high density calibration so the logger is required to describe the density response curve - preferably at more than one borehole diameter. A high density calibration jig should be made and given an effective (haematite) density value. Water, aluminium and ore jigs will allow description of a polynomial regression curve. The use of a haematite jig will automatically correct for Z/A at the dense end of the scale. Irregular borehole wall conditions will seriously affect log quality, causing an understatement of density. 4. Wireline data processing and analysis How to get the best from the logs Stacking wireline curves Individual coal seam densities are often remarkably similar across large areas of a prospect. As a QA exercise, it is well worth depth-matching the top or bottom boundary of a particular seam described by multiple logs. Bottom or top of seam depth matched for multiple logs The example above illustrates the QA benefit of stacking logs if no benchmark test well was employed (refer to issue 2). In this case, the contractor (and tool type) was changed during the project. We cannot assume that there will be no change in density across an entire coal field. However, these logs were captured in boreholes scattered randomly within the same relatively small area. We can certainly draw conclusions about the difference in measurement achieved by two logging tool designs. A correction can be made to achieve precision across the data set. A change in logging unit in November seems to have perturbed G-Wire's measurement, although, in that case, an investigation into the cause of the apparent shift would precede any adjustment. It is interesting that there are no outliers within each set of logs, which indicates that the tooling used is stable and/or coal seam densities are very consistent within the areas logged.

10 10 In the iron ore context, there is some scope for using stacking of curves in blast-hole logging. Blast-holes are percussion drilled, filled with air and of large diameter. The logger is faced with irregular borehole walls causing an understatement of density. This would be magnified by the fact that the holes are dry. In large holes, some caliper mechanisms are not strong enough to clamp the sonde hard to the sidewall and the measurement face either leaves the borehole wall or looks left and right slightly as the caliper snakes up the far side of the bore. The use of a double caliper arm or a double bow spring is indicated to stabilise sonde position in the hole. Even then, every log will be incorrect, always understating density to some degree. Stacking iron ore logs from multiple blast-holes in a small area of a bench The stacking example on the right is modified but illustrates the point. Multiple logs provide a visual analysis of outliers/consistency as well as a calculated average. The average can then be shifted by a precise value to the right (denser). This correction factor can be established by coring next to two or three reference boreholes. The corrected log, in this case called DENC, would most likely run along the right hand edge of the log cluster. The blocked density value is ascribed to the section of mining bench. The stacking method acknowledges the fact that the sonde is a machine and the boreholes are drilled by another machine. Both should behave in a predictable way and the logs will be inaccurate but precise. A stack of multiple logs will reflect the general lithological sequence qualitatively but will always understate density by, on average, a predictable amount. Precision is maintained. Stacking blast-holes in a precious metal mine The log on the right is an actual case, at lower densities, where caved boreholes yielded inaccurate logs that were stacked allowing an estimate of true density for the bench (after a correction factor was added) as well as the apparent depth of weathering. An empirically-based correction factor (which might be polynomial) will shift the black curve to the right (denser) edge of the cluster. This is a form of visual statistical analysis. Knowledge is gained by pattern recognition and precise adjustment. The process relies on consistency of calibration. These borehole were angled from vertical. The density mandrel always faces downwards (caliper upwards) which results in the best possible sidewall contact.

11 11 Leaving the coal basin Mineral logging tools are ideally suited to coal seam measurements but they also have applications in other areas of exploration and mining. In this issue we looked at the difference between logging the pore spaces in sedimentary rocks and logging the chemistry in hard rocks. The optical televiewer can be employed anywhere but clean borehole conditions are a prerequisite. Logging iron ore with the density tool provides a measure of thickness and grade but many QA boxes must be ticked if quantitative data are required. Multiple inaccurate logs are perfect for statistical analysis if they are precise. Visual analysis of stacked logs provides quality assurance and some ability to correct otherwise uncertain data. An OTV image of banded ironstone The next issue celebrates our first anniversary. There is a review of the preceding six Wireline Workshop bulletins and a focus on natural gamma logs. Marcus Chatfield July 2014 Copyrights apply (see wirelineworkshop.com) Editor/contact: Eugenie Joubert (wireline.workshop@telkomsa.net) For back copies, go to and click on "Previous Issues". If you want your colleagues to receive the bulletin, tell them to go to and click on "Signup".