Dipmeter log and borehole imaging z
Dipmeter » Multi-arm Multi-arm micro-resistiv micro-resistivity ity log » Measures direction direction of dip of beds adjacent to borehole
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Formation MicroImager » Large numbers of micro-resist micro-resistivity ivity probes probes » Imaging through through statistical statistical analysis analysis (synthesises (synthesises an image of of lithology lithology of a borehole face by using dipmeter dipmeter log)
Dr Elena Pasternak
Slide 88
Dipmet Dipmeter er contin continued ued… … • Dipmeter is essentially essentially a multi-arm microresistivi microresistivity ty log. Three or four spring-loaded arms record separate microresistivity microresistivity tracks, while within within the sonde, a magnetic magnetic compass records the orientation of the tool as it is drawn up the hole. A software is used to correlate deviations (kicks) on the logs and calculate the amount and direction of bedding dip. Resolution of the dips depends on the averaging scale, could be both small scale (few cm’s only) and large scale. • As a result structural dip is determined. determined .
Dr Elena Pasternak
Slide 89
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Making a dipmeter measurement •As the dipmeter is brought up the well, the electrodes on each arm are in contact with the rock layers. •If the rock layer is dipping, different arms will contact the layers at different depths. •The sequence of contacts between individual arms and each layer is used to compute the dip of the layer. •If the layer is horizontal, all arms of the dipmeter contact the layer at the same time. Dr Elena Pasternak
Slide 90
(A) Three-arm dipmeter sonde. (B) sidewall core gun. This device fires cylindrical steel bullets, which are attached to the gun by short cables, into the side of a borehole. Small samples of rock may thus be collcted from known depths. Dr Elena Pasternak
Slide 91
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Dipmeter Locations of a, b, c, d – peaks on resistivity curves give location of bedding plane (boundary between different rocks. Boundary does not conduct electricity well – high resistivity.)
4 pad 4 track dipmeter Dr Elena Pasternak
Slide 92
Dipmeter continued… z
The first dipmeter tool had three arms 120º apart (need three points to derive the eqn of a plane in 3D). This was replaced by the four-arm dipmeter. Originally having only 4 micrologs, the number was eventually increased to 8. There was then a major jump to increase the number of tracks to 25, and to 200.
Dr Elena Pasternak
Slide 93
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Dipmeter continued… z
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Two ways to present dipmeter data are tadpoles and stick plots. A stick plot uses lines (sticks) to sho the dip measurements. Depth is recorded on the vertical axis with the well represented by a vertical line. The angle on the stick is the dip measurement.
Dr Elena Pasternak
Slide 94
Dipmeter tadpole plot z
Four basic types of motif are commonly identifiable » Uniformly low dips (referred to as green patterns) are generally seen in shales and indicate the structural dip of the formation » Upward declining dip sequences - as we move up towards the surface the dip angle decreases (referred to as red patterns), may be caused by the drape of shales over reefs or sandbars; by the infilling of sandstones within channels; or by the occurrence of folds, faults, or unconformities » Upward increasing dip sequences - as we move up towards the surface the dip angle increases (referred to as blue patterns), may be caused by sedimentary progrades in reefs, submarine fans, or delta lobes. They may also be caused by folds, faults, or unconformities » Random (bag o’nails) motifs can reflect poor hole conditions or they might be geologically significant, indicating fractures, slumps, conglomerates, or grainflows
Dr Elena Pasternak
Slide 95
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•Dip is plotted on the horizontal axis with zero dip on the left. Depth in the well is the vertical axis. •Conventional dipmeter tadpole plot showing the four common dip motifs. •Each motif can be produced by several quite different geological phenomena. •The head of the tadpole shows the amount of dip. The tail of the tadpole points in the direction of dip. Dr Elena Pasternak
Slide 96
Formation MicroImager (FMI) z
Borehole image is produced in cylindrical and unrolled formats (software).
Dr Elena Pasternak
Slide 97
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Formation MicroImager
Unrolled format
Cylindrical format
Dr Elena Pasternak
Slide 98
Dielectric logs z
Dielectric logging - variation of the dielectric constant of the formation (Wharton, 1980) to measure more accurately porosity and water saturation.
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Idea: The dielectric constant is a factor that controls electromagnetic wave propagation through the medium. (Electromagnetic waves do not propagate where there is a current, because all energy goes into the current.) » Water has a dielectric constant that is much higher than for other fluids or the rocks. It ranges from ≈50 for freshwater, to 80 for saline water. » Oil has a dielectric constant of about 2.2, air and gas 1.0. » Sedimentary rocks have values of between 4 and 10.
Dr Elena Pasternak
Slide 99
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Dielectric logs continued… z
Dielectric logs respond to water, whether it is connate water, mud filtrate, or water bound to mineral grains. If the depth is low, it may record high readings where mud filtrate has invaded permeable HC-bearing zones. This problem may be overcome, as with resistivity logging, by running shallow and deep dielectric logs together.
Dr Elena Pasternak
Slide 100
Porosity logs in combination z z z z
Sonic (acoustic) log porosity Electric logs porosity Radioactivity logs porosity Dielectric logs (electromagnetic wave propagation, salty water – bad dielectric, dielectric constant in salty water > than in fresh water > HC; cf. resistivity of salty water is low, higher in fresh water and HC) porosity Combination →
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» The three types of porosity measurements are differently influenced by factors: – Lithology – Clay content – Presence of gas
» Combination increases accuracy Dr Elena Pasternak
Slide 101
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Measurements and loggingwhile-drilling z z
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Wireline well logs are run after the well has been drilled. In the 1980s, sensors for the bottom of the drillstring and a data transmitting process were developed to give a real time log as the well is being drilled called measurementswhile-drilling (MWD) and logging-while-drilling (LWD). MWD measures well properties such as azimuth and deviation. LWD measures rock and fluid properties such as short and long normal resistivity, natural gamma-ray, formation density, and neutron porosity.
Dr Elena Pasternak
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Slide 102
Measurements and logging-whiledrilling continued…
The sensors are located just above the drill bit on the drillstring. The power to the sensors is supplied either by a turbine driven by the circulating drilling mud or electrical batteries. The data can be transmitted to the surface by fluid pulse telemetry. The data are coded digitally in pressure pulses that are sent up the well through the drilling mud. They are recorded on a pressure transducer on the surface where they are decoded by a software. MWD is very useful in drilling deviation and horizontal wells. It records a directional log that shows the orientation of the drill bit, the direction in which the well is being drilled (in real time). The measurement is made with a magnetometer in the downhole tool that measures the direction of the Earth’s magnetic field. Geosteering is the drilling of a horizontal well while continuously adjusting the direction of the bit to keep well within the target formation. A LWD system is used to sense the target formation top or bottom. The MWD system shows the direction of the bit. A steerable downhole assembly is used to adjust the direction the well is being drilled to keep the well within a target formation which can be quite thin (eg, 2m).
Dr Elena Pasternak
Slide 103
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Mud Logs z
Drilling rate » Information about lithology » Qualitative indication of porosity
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Investigation of cuttings lifted with mud » Traces of hydrocarbons
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Gas detector
Dr Elena Pasternak
Slide 104
Mud logs
Dr Elena Pasternak
Slide 105
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Cost z
Depending on the boreholes (exploration or development) and their location (onshore/offshore), the cost of well logging can generally be estimated at about 5 to 15% of the total cost of the borehole.
Dr Elena Pasternak
Slide 106
Summary (on common wireline logs)
Dr Elena Pasternak
Slide 107
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Summary of the main types of wireline logs and their major applications
Dr Elena Pasternak
Slide 108
Dr Elena Pasternak
Slide 109
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Dr Elena Pasternak
Slide 110
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