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1 1 WIRELINE WORKSHOP Issue 3 January 2014 A BIMONTHLY BULLETIN FOR WIRELINE LOGGERS AND GEOSCIENTISTS ENGAGED IN MINING AND MINERAL EXPLORATION A closer look at the density log what is being measured what about chemistry? Boreholee compensation in less than perfect conditions. Overview 1. The oilfield logger is interested in formation porosity and whether it is filled with brine, oil or gas. Techniques for determining this were established decades ago. Since then, it s all been about taking very precise and accurate measurements from extremely hostile borehole environments. The mineral logger has a wide range of targets but estimating the density of coal is the big challenge. He must, at least, make a precise measurement. The quality assurance techniques described in previous bulletins are helpful but, like the oil logger, the mineral logger must run his carefully calibrated sonde in a borehole whose environment is not always conducive to the capture of reliable density logs. Irregular borehole wall conditions impact on precision. The term precision can be extended, for practical purposes, to mean the ability of a density log of a particular coal seam to give the same density readings regardless of how the borehole was drilled. Can the combined drilling/ /logging process generate precise measurements of coal seam density? The answer is yes, if borehole conditions are not too adverse and if the logger s sonde is designed with an effective borehole compensationn system. Time to take a break in this thought process and consider the fundamental principles of density measurement. 2. Measurement Focus A review of one wirelinee log measurement The (gamma gamma) formation density log The density sonde employs a radioactive source which bombards the formation with gamma rays. A scintillation crystal detector measures the abundance of gamma rays which decreases (after a certain threshold) with distance from the source and with density. At a fixed distance from the source, the gamma count will be proportional to formation density.

2 2 The dominant phenomenon is called Compton scattering. Gamma rays collide with electrons multiple times, gradually losing energy, until eventually they are absorbed. In denser rocks, counts are lower because most gamma rays are absorbed before they get to the detector. This process logs electron density and that is all. Whereas the geologist will measure density by establishing the weight and volume of a sample, the logger estimates the concentration of nucleons (protons and neutrons) via the electron count (one electron per nucleon pair). Denser rocks have more nucleons per volume. The relationship between electron density and bulk density is fairly constant for the major rock forming elements like silicon, aluminium and magnesium. The Z/A ratio, electrons (and so protons)/total nucleons is about 0.5. So the conversion from electron density to bulk density is, theoretically, straightforward. There is one problem; hydrogen. Rock formations, particularly sediments, contain significant amounts of water and so hydrogen (H has one proton and no neutron; Z/A=1). Water (H 2 O) has a Z/A ratio of There is a requirement then to apply a sliding correction between 100% water (1gm/cc) and no water (tight sandstone at 2.65gm/cc). A competent logging contractor will perform this correction as standard procedure and it is automatically achieved if he employs water as his low end calibrator. There is another problem; this sliding correction does not work for coal. Coal is not a mineral, it is a vegetable (most recently described as a mineral of organic origin). Its molecular structure is much lighter than the crystalline structures of minerals. So, at a typical coal density, there will be an over estimation of water fraction and a corresponding over correction for H. Since the chemistry of coal is not fixed in nature, a perfect correction is impossible. The density log is never perfectly accurate in coal. A mineral sonde can, however, be made to measure coal density within an acceptable tolerance. A practical solution was introduced by BPB Instruments (later Reeves then Weatherford) in the early 1980s. For electron densities lower than 1.752gm/cc, the straight line correction is replaced by a fixed value of gm/cc which is the average correction required by coals of different ranks within that density range. Density correction (addition) versus electron density using water and limestone electron densities In a perfect borehole (tube like) of known diameter, the logger should be able to capture a precise log of electron density. Accuracy will depend on the quality of his calibration system and equipment characterisation. In less than perfect borehole conditions, as in oilfield logging, the challenge is to compensate for variables such as fluid type, natural gamma effects, borehole diameter and borehole wall irregularities (skin damage). The variables of formation chemistry, described above, relate to accuracy and are compensated for at the end of the process. They rely on a precise and accurate log of electron density.

3 3 The density log borehole compensation To achieve measurement precision in a range of borehole conditions, the logger must employ a logging system that provides reliable borehole compensation. That is the crux of the issue, the difference between an average density sonde and a good one. Compensation includes: Detector dead time Natural gamma effects Fluid density Fluid level Borehole diameter Caving and mud cake Z/A effects (chemistry) Mathematically correct for data loss at high count rates Natural gamma added to induced gamma, noticeable at LSD, will lower density Heavy mud used instead of water will raise apparent density Dry hole instead of water filled conditions will lower apparent density Larger volume of water or air around the sonde will lower apparent density Water/air filled voids or mud cake standoff will lower apparent density Correction for proportions of H or Fe in the formation Most mineral logging is performed in water filled boreholes of varying diameter. The natural gamma effect is minimal if a source of sufficient activity is employed. All the corrections are valid but, in most cases, borehole diameter, skin damage (minor caving) and Z/A correction are the main factors. Luckily, a very large proportion of mineral boreholes are cored using a diamond drill bit. These tend to be tubelike in nature but skin damage is a factor in coal measures (very often resulting from collapse around drillinginduced fractures or cleats). Rugosity (corrugations caused by the drilling process) will also result in an understatement of density. Rugosity on a core sample The dual density sonde is sidewalled (by the caliper) and collimated (the mandrel is increased in diameter with lead or tungsten shielding behind the detectors) to ensure that just one side of the borehole is measured. The volume of investigation includes water within the corrugations. Effect of rugosity on measurement volume A density sonde includes two or three detectors within the collimated mandrel. The long spaced density (LSD, +40cm spacing) has a large volume of measurement and so is insensitive to minor caving. However, it lacks resolution so usually one or two shorter spaced logs, short spaced density (SSD, +20cm) or bed resolution density (BRD, 15cm) are included. We should not overestimate the effect of rugosity. LSD measures a large volume at coal density. A combination of the logs is used to mathematically compensate for rugosity, skin damage and mud cake effects. If the SSD log is filtered sufficiently it will overlay the LSD log in undamaged borehole. The two logs will then have the same resolution but different depths of investigation. Rugosity or caving will have a proportional effect on the two logs that will separate them. This proportionality must be known. If, for instance, a rugose

4 4 zone reduced LSD by 10% and SSD by 30%, the LSD could be increased by half of the difference between the two logs. This correction would be applied throughout and a new compensated LSD log generated. That log could be used to correct the SSD log and generate a second, higher resolution, compensated log. This is the basis of dual density compensation and it must be applied before the log is corrected for the Z/A variable. Same resolution, different measurement volume 3. Overview continued The key benefit of a wireline log is that it provides QA in terms of depth and thickness control and a check on the driller. It provides a good basis for the calculation of the core recovery percentage. Coal resources are quoted on an in situ density basis. That is what the logger measures. The geologist will only develop confidence in the wireline density log over time if inconsistencies are eliminated. The logger should concentrate on calibration and test well log precision. If his sonde performs well, LSD and a filtered version of SSD should overlay in good borehole. If they do not overlay in coal, the best advice is to believe LSD and shift SSD to best fit. Then remove the filter. Working against LSD SSD log overlay in good borehole cross section are tooling issues such as: Old/weak caesium source, causing low CPS and resulting natural gamma influence on LSD Weak caliper arm, causing inconsistent contact with the borehole wall LSD spacing too long, causing low CPS and resulting natural gamma effect Worn mandrel face (can be calibrated out, up to a point) A bent sonde (not as unusual as one might think) Uranium or its daughters in coal measures (in this case, adjust LSD at separation to fit SSD) Lack of adequate log response characterisation for borehole compensation The test well provides some measure of precision and precise data are sufficient for the coal geologist to satisfy most of his requirements. In that case, though, correction to true density has to be empirically based. However, if the boreholes are caving in the coal measures, the test well is not sufficient alone. It cannot adequately represent hole to hole variables, especially in non cored boreholes. Effective compensation (dual measurement) is required. Non cored (percussion/rc) boreholes will not yield precise density data without effective compensation for caving. It should, by now, be clear to the reader that density logging is (or should be) a complex business. The writer makes no apology for spending the first three issues of Wireline Workshop labouring the point. In summary, a good density log depends on careful calibration of well designed tooling and a test well bench mark for the first goal precision.

5 5 Precision is compromised in less than perfect borehole conditions so mathematical compensation based on tool response characterisation is required. Accuracy depends on calibration, borehole compensation and the conversion of a precise log of electron density to true bulk density via a correction for an estimated Z/A ratio. Further empirical correction using site specific laboratory based relationships (for test well core) might still be required to satisfy the geologist. The logger guarantees the precision of his measurement. Compensated density in a coal seam with LSD log showing corrections (shaded) 4. The logger on site Radioactive source security is a big issue The thorny topic of radiation and its use in mineral logging will be addressed in some detail in following instalments of the Wireline Workshop bulletin. Fundamentally, the big issue is risk...the risk of losing a source down a deep borehole. It s worth considering a change of paradigm. Source shields on site with warning labels During a drilling campaign in the DRC, the logger wakes up one day to find that his truck has been broken into and the tool box and a 200mCi radioactive source are stolen. The chief geologist is informed. He and his Company had never considered the scenario of a radioactive source being lost in the local community. This source is not really secure He now has to report the incident to his superiors (it s a boardroom level headache) as well as the DRC government and the police. In bed that night, the geologist imagines what the unsophisticated locals might be doing with the source (after selling the lead shield)...cutting it open, placing it on the cupboard as an ornament, throwing it down a well or just storing it under the kiddies bed. He can t sleep. This can go on for decades.

6 6 Suddenly an angel appears and offers the geologist a choice. The source can remain lost in the community or, instead, the day s events can be reconfigured so that the source, attached to a density sonde, is stuck in a 200 metre deep borehole, cable still attached and unbroken. It s an easy choice, of course. Deep down in the ground, the source is not harming anyone. There is a (statistically proven) 95% probability that the sonde/source will be fished out of the borehole. Whatever happens, the geologist at least knows where the sonde is. The incident can be managed. While not diminishing the seriousness of a stuck sonde with radioactive source attached, this story provides some perspective. Burying a source is not the worst thing that can happen. Both the source holder and shield must have a warning clearly marked on them. The source must be transported in a locked cage (or chained) within a locked vehicle. The vehicle must carry external warning signs on two sides and the rear door. There must be warning signs within the vehicle next to the source s location. The geologist should provide an alternative source storage bunker or pit in a fenced off area. The source bunker should have no other purpose, such as a store of paint or wheel barrows. Internal and external signage should be in place as well as signs on the fence and the locked gate. Graphic signage, supplementing the radiation logo, which might be understood by local people, can be added as an extra deterrent. Carefully designed education, on the risks associated with the radioactive source, should form part of the standard safety training for local recruits. It is important to recognise that the biggest risk associated with the use of radiation in mineral exploration is loss of a source on the surface, either through temporary misplacement in the logging truck, loss in transit or theft. We need to know where the source is. 5. Wireline data processing and analysis How to get the best from the logs Geotechnical derivatives Having captured quality assured density and sonic logs, the log analyst can derive rock strength parameters such as the dynamic moduli of elasticity. Contraction (stress) of a volume of rock in one direction, results in expansion (strain) along the axis perpendicular to it. Similarly, if stretching (stress), is applied to a material it will contract (strain) along the axis perpendicular to the axis of pull. The ratio of perpendicular contraction to expansion is Poisson s Ratio. This strain ratio has no units and, in rock mechanics, is valued between 0 and 0.5. Note: for practicality, mathematical syntax is replaced by straight line formulae which generate log curves with unique mnemonics. PVEL is P velocity and PWAV is P slowness etc. Mnemonics in the formulae are enclosed {}.

7 7 The important relationship is the ratio of compression wave to shear wave logs (PSR). For velocity logs: PSR = {PVEL}^2/{SVEL}^2 or ({PVEL}/{SVEL})^2 For transit time (slowness) logs: PSR = {SWAV}^2/{PWAV}^2 or ({SWAV}/{PWAV})^2 This is the basis of the calculation for Poisson s ratio, which is independent of density. POIS = ((0.5*{PSR}) 1) / ({PSR} 1) The velocity of the shear wave depends on the shear modulus (µ or GMOD), which may be calculated from logs of shear wave velocity and density (RHOB). Units are MPa (megapascals). GMOD = ({RHOB}*({SVEL}^2))/10^3 With GMOD and Poisson s ratio, we can calculate Young s modulus (YMOD) in MPa. YMOD = 2*{GMOD}*(1+{POIS}) The bulk modulus (BMOD) is calculated as follows. Units are MPa. BMOD = ({RHOB}*({PVEL}^2 (4/3)*{SVEL}^2))/10^3 The standard logging industry units for density and velocity are grams/cubic centimetre and metres/second respectively. The standard SI units for density and velocity are Kg/cubic metre and metres/second with results (in this context) in Pascals. Using, instead, standard wireline log units the formulae will result in a log in megapascals if a division by 10 3 is applied (shown). If the logger prefers to use slowness rather than velocity, the following formulae apply. GMOD = ({RHOB}/{SWAV}^2)*10^9 YMOD = 2*{GMOD}*(1+{POIS}) BMOD = ({RHOB}*(1/{PWAV}^2 4/(3*{SWAV}^2)))*10^9 Again, for practical purposes, standard log units are used and results are in MPa. The dynamic moduli, based on wireline loggers using sonic vibration, do not correspond perfectly with the static moduli, based on laboratory tests using applied mechanical pressure. They are not normally employed directly in geotechnical analyses. Dynamically based values are generally higher than those derived from static tests are. Nevertheless, the measurements are continuous, objective and in situ. So, if an empirical relationship can be described between any of the dynamic moduli and the static moduli (there will normally be a strong relationship) the logs add value. The formulae above are listed for reference purposes and may be pasted directly into a parser. More on geotechnical logs in the next issue. We will also look at televiewer logs.

8 8 Developing a theme A precise and accurate log of formation density is difficult to capture, especially in caved zones. Sometimes, for empirical analyses, a precise log of density is sufficient. The sonic log is normally accurate and describes intact rock velocity regardless of most borehole conditions. Quality assured density and sonic logs have many uses in mining and mineral exploration. They are the most popular of the various physical property logs. Both logs have an application in the important geotechnical field. Next issue we discuss one empirically derived parameter, UCS (uniaxial or unconfined compressive strength). It is a measure of intact rock strength. But what about fractures in the rock? We will look at geotechnical logging with sonic and density tools but also fractures and breakout orientation from the acoustic televiewer sonde. Marcus Chatfield January 2014 Copyrights apply (as listed in wirelineworkshop.com) Editor/contact: Eugenie Joubert (wireline.workshop@telkomsa.net) For back copies, go to and click on "Previous Issues". If you have not yet subscribed, go to click on "Signup" and complete the simple form.