Wireline Bulletin May 2015
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Transcript of Wireline Bulletin May 2015
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2
Hydrology is all about porosity, permeability, confinement and pressure. Taking a simple example of a vertical borehole drilled into horizontal beds with only minor confinement, there is no pressure imbalance. We can cause one by pumping out some water. The water level in the borehole and surrounding rock falls, resulting in a local "drawdown" and "cone of depression"...a pressure reduction that draws water into the borehole through permeable formations. The deeper the drawdown, the greater the pressure imbalance. This is known as the hydraulic "head" and is measured in metres or feet.
Drawdown and water ingress
In this scenario, without the pump, there is no pressure imbalance and no water flow up and down the borehole. In order to detect any inflow points, whether for groundwater or geotechnical studies, it is necessary to pump water into or out of the borehole. The pressure applied to the borehole column depends on the change in water level with respect to the ambient water table. If, for instance, the water table was located at 50 metres depth, the water level in the borehole could be raised to 0 metres by topping up and holding the level at the surface. 50 metres of water pressure (approximately and practically referred to as 50 decibars or 5 bars) is applied to the borehole column below the water table. The rate of reverse (positive) flow could be measured at the surface and recorded in litres per hour at 5 bars pressure. Conversely, if water is pumped out, the pressure imbalance is determined by measuring the depth of drawdown below the natural water table.
Where there is groundwater movement up and down a borehole column, there has to be a pressure imbalance, natural or induced. Natural flows within a borehole column are common and are the subject of study by hydrologists. These flows result, again, from a pressure imbalance caused by varying connected aquifer elevations. Natural is perhaps the
wrong word, because the flows would not exist if the borehole were not there. It is the pressure imbalance (the hydraulic head), in this case, that is natural, rather than induced.
Where the hydraulic head is not great enough to bring borehole water to the surface, there could still be small flows up and down the borehole between discrete aquifer intersections. Movements of fluid can be logged using some type of flowmeter.
Subtle flows might require a very sensitive flowmeter device to measure them. They are often not detected by the mechanical impeller flowmeter. In this scenario, some form of stationary measurement tool will be the best option.
The stationary or point measurement systems include, for flows exceeding 2 centimetres per minute, heat‐pulse and electro‐magnetic sondes. The most popular is the heat‐pulse flowmeter. This device is designed to log natural flows between
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ingress and egress points in the borehole. For very slow flows, where these tools cannot operate effectively, some form of tracer method must be tried. Regardless, if a fluid flow exists, it can usually be measured.
An artesian basin with hydraulic head caused by topographic elevation
Flow rates may be restricted by limited permeability of the rock mass, the type of fracturing or by the vacuum effect caused by an aquitard. Major flooding will require more than one intersection of a confined aquifer, natural or drilled. If the aquitard in the diagram on the right were pierced a second time half way up the hill, the rate of flow into the basin will increase.
Measuring aquifers is a complex business.
The volume of the aquifer depends on porosity, the fraction (0 to 1) or percentage (0 to 100%) of air or water‐filled pore spaces in a volume of rock. It is denoted by the symbol Φ and is usually measured, in mineral logging, as a percentage of the total volume of a particular rock (for instance SST%). Refer to the January 2015 bulletin.
The flow potential of an aquifer depends on its hydraulic conductivity, denoted as K. It is the effective
permeability of the formation. This is the intrinsic permeability, a constant measured in darcys and millidarcys, (mostly in the millidarcys, sand has a permeability of about 1 darcy) modified by the density and viscosity of the fluid in the pores. It is length per time, feet per day, for instance, under a hydraulic gradient of 1 ft per 1ft.
This can be confusing because, although it is referred to as feet per day, it is not just length, it is also a volume...feet of fluid per square foot per day. It is feet per day of a cross sectional area of aquifer measuring 1 ft
by 1 ft. Note that a cubic foot is equivalent to 7.48 US gallons or 28.3 litres of water, approximately.
K = κγ/μ where κ is the intrinsic permeability of the formation, γ is the density of the water and μ is its viscosity. The modification is often small because fresh water has a density of 1 (gm/cc) and, at 20 °C, has a viscosity of 1.002 (mPa.second).
The estimated rate of flow of an aquifer is called transmissivity. It represents the volume of water flowing through a cross sectional area of an aquifer. It is denoted as T and equals hydraulic conductivity multiplied by the aquifer thickness, B. The thickness is the aquifer's vertical thickness. So ... T = KB.
For example, using the metric SI system, if the
horizontal hydraulic conductivity of an aquifer (K) was 10 metres per day (through a cross sectional area of 1 square metre) and its thickness (B) was 20 metres, the transmissivity of the aquifer would be 200 metres per day or 10 x 1 x 20 = 200 cubic metres or 200,000 litres of water per day.
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This is interesting but not much to do with the daily work of the wireline logger. It is included to allow the logger to converse in a meaningful way with his hydrological client. Wireline loggers can assist the hydrologist by logging the borehole caliper (varying cross‐sectional area), the natural gamma ray count (lithology and clay fraction of the formation), the porosity (from neutron, density and sonic logs) and the rate of flow at a known pressure head. The logger cannot measure permeability directly. His tools include:
Three or four arm caliper sonde
Fluid temperature and conductivity sonde (usually combined)
Fluid sampler
Water quality (including Ph, oxygen, nitrate, pressure, redox)
Flowmeter (spinner, heat‐pulse or electromagnetic)
Acoustic televiewer (fracture orientation and aperture)
The most popular stationary (point) measuring device for flow up and down a borehole is the heat‐pulse flowmeter. The reference, in this case, is a pulse of warmed borehole fluid created by an element in the sonde then detected and timed as it passes upwards or downwards, from the source of heat, past a thermistor. The sonde must be stationary during measurement.
A heat‐pulse flowmeter (RG)
It is very sensitive, being able to measure flows of as little as 2cm per minute. At slower flows, the heat pulse dissipates and warms to ambient temperature. Logging a borehole takes a lot of time and, in very slow flows, ambiguous results are common and require multiple recordings to assure veracity of the data. This is, however, a proven and effective tool for groundwater study.
The electromagnetic flowmeter provides both sensitivity and the ability to move, albeit slowly, along the borehole column while capturing data. It operates according to Faraday's law of induction, which states that the
voltage induced across an electrical conductor moving at right angles through a magnetic field is directly proportional to the velocity of the conductor. The electromagnet creates a strong magnetic field across the flow passage. As water (the conductor) flows through the magnetic field, a voltage is generated which is proportional to the average water velocity across the magnetic field. The induced voltage across the electrodes is measured by the tool's electronics. The polarity of the generated voltage is dependent on direction of flow (this description was modified from the USGS website).
For very sensitive (ultra slow flow rate) logging applications, tracer flowmeter techniques may be employed. These are chemical (usually brine), optical (food dye) or radioactive (short half‐life) tracers that are injected into the borehole and detected later after being moved upwards or downwards by water flow. The chemical tracer may
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be detected using a fluid conductivity sonde and the food dye by a borehole camera or by the (rather more costly) optical televiewer. Radioactive tracer depths will be logged by a simple gamma sonde. These methods detect very small rates of flow. Fluid flow may be monitored after several hours, or even days.
The radioactive tracer, usually an injected isotope of iodine with a half‐life of 8.1 days, is very sensitive (it can be detected through casing or pipe) but is not used in groundwater studies. These methods are expensive and not ideally suited to routine logging.
For faster flows of fluid, gas and even air, the impeller flowmeter or “spinner” is recommended. It is the most common logging probe used for measuring vertical fluid movement in boreholes and is a relatively inexpensive and reliable instrument.
Where flow is induced for groundwater production or geotechnical analyses, the spinner is essential because it may be trolled quickly along the borehole, thus identifying the exact water ingress or egress depth.
In recent times, the use of the acoustic or optical televiewer for groundwater studies has become very popular, particularly where open fractures are the main conduit for water flow into the borehole.
22.. MMeeaassuurreemmeenntt FFooccuuss
A review of one wireline log measurement
The Spinner Flowmeter Log
The spinner is a mechanical device designed to measure flow up or down the borehole column while stationary or, more commonly, trolling along the bore at about ten metres per minute.. It may be employed effectively where flow rates exceed 50 centimetres per minute.
A spinner flowmeter (Geovista)
The impeller will not kick off at such a small flow rate but a change in flow of that magnitude should be noticed by a good tool design if the impeller is already spinning (due to the sonde's movement along the bore). It is not always necessary but centralisation is a good idea. In some circumstances, the centralisation can be fitted to a sinker bar to make sure the tool runs downwards smoothly through a borehole against strong upward flow. If flow is too fast for the impeller to cope (count rate plateaued out), the tool should be run in the same direction as flow at high speed. True flow per minute can then be calculated after allowance for tool movement and direction. If that fails, and it sometimes will in gas flows or when the logger only has a large impeller, the practical option, if no smaller cage/impeller size is available, is to restrict the cage aperture with plastic tape.
Unlike the more sensitive point‐measurement systems, the spinner gives us what we really like to have; a wiggly line...a continuous, objective, high resolution measurement plotted against depth which can be juxtaposed with other log curves or images.
The spinner flowmeter comprises a three or four blade helical impeller mounted on a shaft held by two low friction and very hard jewelled bearings. The bearing assembly, which might include a tensioning spring, can be adjusted by the logger. The friction of the shaft with lightweight plastic impeller mounted on these bearings and the effect of the tensioning spring must be overcome before any movement may be noticed by the sonde's
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electronics. Optical and magnetic counting systems are employed although the latter, based on Hall voltage detection, is the most common option.
The logger should hold the sonde and shake it gently. If the shaft rattles, it is not tensioned sufficiently. He should tighten the adjustment screw just until the shaft stops rattling, no more. Correct tensioning makes a big difference to sonde performance.
Tool designs vary so sensitivity, minimum detectable flow rate, counts per revolution and maximum recordable rotation rate depends on the tool employed. The stated minimum flow of 50cm per minute is probably the best that can be achieved with a spinner.
A conservative flow threshold, for operational planning purposes, is better set at 1 metre (3 feet) per minute for continuous (trolling) measurements.
The spinner flowmeter may be used to measure gas flows but, in mineral logging, it is most often employed to log groundwater. In this scenario, the minimum data set would include flowmeter (revs or CPS), winch speed (metres or feet per minute) and borehole caliper (centimetres or inches diameter). Applications include:
Identifying ingress points for artesian flow
Identifying permeable zones and measuring flow rates in production wells during pumping
Measuring flow rates from perforated casing sections during pumping (long term monitoring)
Identifying far field open fractures with flow potential for geotechnical studies
In groundwater production, it is important to locate the ingress points in a borehole. In major, large diameter, groundwater wells, steel or PVC casing must be set in place with perforated or screened sections aligned with
the main aquifer inflows. The wireline logger can assist in this by running a spinner flowmeter sonde downwards during pumping.
The process is complicated by the need to lower the delicate flowmeter first followed by the submersible pump. The pump is then activated, making sure that it is deep enough to avoid drying out but not so deep that it limits the logged metreage excessively. Good judgement here is important.
Pumping out and running in
Flow direction is upwards (negative flow) so the greatest measurement sensitivity is achieved by running the sonde downwards. Inflow points are described by a drop in flow rate as the sonde descends.
What we need to measure is flow rate, in litres per minute or gallons per minute, passing the sonde. If we can measure that, we can deduct the effect of the sonde's movement up the borehole column, determine rate of flow in the borehole and quantify each ingress flow rate.
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Measurement calibration is very important and, in practical terms, very easy to perform. Impeller revolution rate (the count rate) versus winch speed is normally very close to a linear relationship.
In the field, only one flow direction is required to be calibrated, relative (to the sonde) upward flow or relative downward flow. In the diagram on the previous page, the sonde is running in against induced flow so relative upward flow is essential. It is better, however, to calibrate both directions and run downwards then upwards.
For the upward flow calibration: before pumping commences, the logger would lower his sonde to 30 metres above the bottom of the borehole or a place where he expects no natural flows (in the casing). He would then log downwards, starting at just 1 metre per minute and gradually increasing to 30 metres per minute. Assuming no significant natural flows are present (confirmed by a stationary check) the logger has recorded a calibration.
Tool count rate versus winch speed (for cross‐plotting)
Movement of the sonde downwards will induce rotation of the impeller which is proportional to winch speed. We need to convert tool CPS to flow speed and deduct the winch‐induced flow speed from the total flow speed.
Flowmeter speed versus count rate
Some systems convert CPS to REVS first but there is little need to know revolutions per minute. The log below describes the process from left to right. Winch speed (SPED) is plotted with the flowmeter log converted to total flow speed (SPEF) and three‐arm caliper log (CALI). SPED is deducted from SPEF to leave actual borehole flow speed (FLOR). A log of cross‐sectional area in square centimetres (AREA) is created from CALI. Borehole flow speed is converted to flow volume per hour (FLOW) by multiplying SPEF*100 with AREA and converting to litres per hour. The differential of FLOW is calculated for presentation purposes (far right).
Winch speed, flow rate and derived logs
This example describes water flows through open fractures rather than primary permeability. The total flow rate (the fastest flow near the top of the log) should equate to the pumping out rate. Flow rates are dependent on pressure imbalance (head) so draw down depth, below the water table, should be measured if possible.
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Increases in borehole diameter will slow the actual borehole flow (FLOR) but the log of volume per time (FLOW) will straighten out this effect.
The flowmeter works well in groundwater logging if flows are sufficient to activate the impeller. If this is the case, both down and up‐logs should be calibrated, run and plotted together. Some interesting flow patterns can result.
Natural upward flow
If the effect of the movement of the sonde is removed from both logs and assuming flow rates do not exceed that deduction, the logs will mirror each other.
In the log example below a group of inflows at around 140 metres are "stolen" from the upward flow by a permeable layer at 80 metres. Log responses like these are often very subtle and shading between the curves enhances the presentation.
Ingress low down then a thief zone identified at 80 metres
For geotechnical logging the same procedure is applied in reverse. Geotechnical boreholes, even mine shaft pilot holes, tend to be cored at small diameter (HQ or smaller). A submersible pump does not fit easily into these boreholes and the draw down is usually so deep that much of the borehole cannot be logged. In order to overcome this problem, water is injected into the borehole at surface.
The first and simplest option is to top up and hold the water level at zero depth while logging upwards from TD.
If the water table is at 50 metres then approximately 5 bars of pressure will be applied to the column below 50 metres. At 40 metres, only 4 bars will be applied, at 30 metres, 3 bars and so on. The top 50 metres will not be logged under same pressure as the zone below 50 metres, which could be a disadvantage in shallow boreholes. The unconsolidated layer represents a problem.
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The unconsolidated layer must be cased off and the casing sealed at its base (ground in tight) as well as possible. For important jobs, the casing should be grouted in place.
Loss of water at the base of the casing is a big problem because water supply is usually limited, the site can be flooded and there will be a loss of pressure if a pump is used. Any manageable water loss at the base of casing will be measured by the sonde.
If the water table is high up in the borehole or if a constant high pressure is required, it is necessary to seal the borehole column and inject water, under pressure.
Pressured flow profile logging (Weatherford)
The wireline is run in via a stuffing box. Even at small borehole diameters, a significant upward force will result from the pressurisation. For instance, 10 bar pressure in an NQ hole results in an upward force equivalent to about 480 kilograms. At HQ diameter the same pressure would need to be contained by a weight of about 720 kilograms. The picture above shows weights (500Kg) attached to a framework which is, in turn, bolted to the riser.
Operational safety warning: It is essential to calculate the forces involved, especially in large diameter boreholes where pressure should be reduced. Any pumped pressure can be added to the depth of water table in order to establish the effective pressure imbalance or head that is applied to the borehole column below that depth. The sonde is positioned near to TD first, to save water usage. The pump is started and, after operating pressure is reached, the logger should wait for ten minutes in order to activate the reverse flow regime before logging upwards.
Slowly speeding up over the bottom 10 metres (extra calibration) to a constant logging speed of ten metres per minute is normally about right. The object is to provide the geotechnical engineer with a quantitative measure of fracture flow potential for the whole borehole.
The log is plotted with other hydrological data and, if available, the acoustic televiewer image.
One type of caged impeller flowmeter (Mount Sopris
Instruments)
The log below (page 10) is a typical presentation from a hard rock environment where fractures are the main conduit for water inflow (outflow in this case). In the author's experience, this reverse flow method has previously described hydraulically conductive events at depths down to 1200 metres. The effect of friction from the borehole wall does not seem to compromise the measurements.
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The flow profile log is different to the standard wireline logs because, in a sense, the measurement looks well beyond the borehole's normal frame of reference.
A typical presentation of flowmeter data plotted with other relevant logs
Overall, the challenges in the reverse flowmeter method lie in the pumping and sealing of the borehole (as well as the irritation of having to remake a cable head in order to work through the stuffing box).
Some careful planning might be required to avoid losing water pressure near the surface. A 10,000 litre water bowser is normally sufficient to log hard rock holes where fractures are the target. Multiple tanker trips might be needed for boreholes drilled through very permeable formations.
Running the flowmeter sonde is fairly straightforward if water conditions are good. Oily deposits in the borehole column are always problematic and will seriously affect the smooth running of the impeller.
As a general rule, it is best to calibrate in the borehole rather than in some surface jig. Borehole fluid viscosity might vary which will impact on tool response. Calibration in the actual borehole, in terms of viscosity and diameter is likely to be more accurate than in a jig.
Calibration near to the bottom of the borehole is sensible, as this zone does not normally exhibit natural flows. However, the logger must always be careful not to go too close to the mud lying at TD as this will jam and possibly damage his delicate impeller mechanism.
33.. TThhee llooggggeerr oonn ssiittee The geologist trusts the logger's depth...don't let him down
Take a tape measure
It should always be remembered that the wireline log is a graph with two axes. The Y axis is depth and depth is important. The logger can spend hours calibrating his sonde and running it carefully only to neglect to check the Y axis. Geologists tend to question the X axis, the log measurement, but, for some reason, believe implicitly in the depth measurement. Any failure, then, to calibrate one's depth system would be a breach of trust.
Depth measurement tolerance is usually +/‐ 1 metre per 1,000 metres.
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That's 1mm per metre or 0.1%, which sounds a bit tight but equates to 3 cm per 30 metres...a practical length to use for calibration purposes. It is best performed in a borehole, not horizontally on the ground. A 50 metre tape measure should be part of the logger's standard equipment and his client is advised to establish early on what depth tolerance applies to his particular logging system.
Checking depth on site is normally straightforward. The logging technician attaches the tape securely to his sonde (with the logger's friend ‐ electrical insulation tape) while it is suspended in the casing and, with the help of a colleague in the truck, lines up a depth with a surface reference, the top of the casing, for instance.
A 50 metre tape measure should be kept in the truck
The man in the truck sets the same depth on his control panel. The sonde is run into the casing while the tape runs between the fingers of the logger who keeps it tight. 30 metres down the hole the winch is stopped and depths are compared.
This method has been tested using different loggers and different tape measures and results were always consistent, well within 1 centimetre in 30 metres.
In the BC era (before computers), depth accuracy depended mainly on the rate of depth wheel wear. The
calibrated wheel was case hardened but continuous use would eventually wear through the hardened surface and it would lose circumference; the system would then overstate depth. Worn out wheels were thrown away and replaced by new ones ‐ about once every five years on average.
The advent of computers has introduced the possibility of setting wheel circumference and pulses per revolution in the logging program. This saves throwing away the wheel but introduces a source of human error. A miss‐key or use of the wrong PC‐ winch combination will cause significant error, perhaps over several jobs. This commentator is not sure whether that represents progress or not. Proper procedures are important.
The best way to quality assure depth measurement is against a borehole standard. The test well should be logged two or three times, with a density tool for instance, very carefully with calibrated depth systems, if possible from different trucks. The average of these runs can be used as "true depth".
Test well depth check
It is important to use a well defined near‐surface reference on the log, not ground level. Logs are depth shifted to overlay perfectly at this upper reference. A comparison of the distance between two reference points, roughly 100 metres apart if possible, provides a very good check on depth wheel wear over time.
If the logger changes the logging program settings, he should relog the test well for confirmation and keep the file on record. On intensive logging contracts, this QA check should be performed on a monthly basis or whenever the winch/computer combination is changed.
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How to get the best from the logs
Blocking coal seams
The highest resolution density measurement for a Caesium‐based logging sonde is 15 centimetres. The log is referred to as BRD (bed resolution density). Higher resolution can be achieved, about 8 centimetres, if an Americium source is used, but this arrangement often suffers excessively from borehole wall effects.
The BRD log below (centre) has depth column ticks at 15 centimetre intervals. A high resolution televiewer image is used to reliably block thin coal seams and the BRD curve is depth matched for comparison.
Overstatement of thin beds by a density log
In this case, coal beds with a thickness of less than 15 centimetres are overstated. The problem is not too serious because coal beds of less than 15cm in thickness are not normally of economic significance.
Overstatement of coal partings by a density log 1
The same effect occurs at coal seam partings where overstatement does represent a problem. In some sections of the log above, the shoulder effect, rather than fully resolved coal or parting, dominates the density log. This limitation of the log is not always important, it depends on the thickness of coal beds. Note the verticality orientated drilling‐induced fractures intersecting the coal on the amplitude image AMPM.
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Explanation of thin bed thickness overstatement
Where bed or parting thickness is less than resolution (source‐detector spacing), its thickness is overstated. In the example above, a 2cm thick parting has an apparent thickness of 12cm when described by an SSD log with 22cm resolution. It is important to under‐pick this type of event.
Overstatement of coal partings by a density log 2
The log above shows good definition of thicker partings but a disproportionate effect from smaller dense partings within, what appears to be (from the ATV image), quite homogenous coal.
If the thin partings are carefully under‐picked (estimated based on experience) or can be defined more accurately using a televiewer image or micro‐resistance log, the use of blocking and block averaging, with appropriate tool resolution, works well (see issue 2, November 2013, page 6).
There is no doubt that juxtaposition of televiewer and density data adds value to the process and that the two logs are complementary for detailed coal seam analysis.
Acoustic and optical televiewer images look quite different to each other in coal measures.
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The ATV image is not infallible. Some partings in coal exhibit very similar acoustic properties to the coal itself. In the example below, an acoustic image (middle‐left) and an optical one (far right), captured in the same coal seam, illustrate, in this particular case, the inability of the ATV log to effectively define a parting.
ATV versus OTV images in a coal seam
The optical televiewer image defines the parting particularly well but offers less contrast at the roof boundary. It looks, from the optical image, like all the small drilling‐induced fractures are packed with drilling fines.
It is worth noting the effect of small lenses of higher density formation, visible in the ATV image, on the density log. They are reflected in the SSD density measurement just above the main parting, at 243.4 metres and lower down at 244.80 metres.
Log analysis is often more complex than expected and it can take a lot of effort to get it just right.
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Navigation logs and the dipmeter
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