dip log ppt

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Slide 88Dr Elena Pasternak

Dipmeter log and borehole imaging

Dipmeter» Multi-arm micro-resistivity log» Measures direction of dip of beds adjacent

to boreholeFormation MicroImager» Large numbers of micro-resistivity probes » Imaging through statistical analysis

(synthesises an image of lithology of a borehole face by using dipmeter log)


Dipmeter continued…

• Dipmeter is essentially a multi-arm microresistivity log. Three or four spring-loaded arms record separate microresistivity tracks, while within the sonde, a 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.

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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.


(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.

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Dipmeter

4 pad 4 track dipmeter

Locations of a, b, c, d –peaks on resistivitycurves give location of bedding plane (boundary between different rocks. Boundary does not conduct electricity well – high resistivity.)


Dipmeter continued…

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.

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Dipmeter continued…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.


Dipmeter tadpole plotFour 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

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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 dipmetertadpole 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.


Formation MicroImager (FMI)

Borehole image is produced in cylindrical and unrolled formats (software).

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Formation MicroImager

Unrolled format Cylindrical format


Dielectric logsDielectric logging - variation of the dielectric constant of the formation (Wharton, 1980) to measure more accurately porosity and water saturation.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.

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Dielectric logs continued…

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.


Porosity logs in combinationSonic (acoustic) log → porosityElectric logs → porosityRadioactivity logs → porosityDielectric 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)→porosityCombination» The three types of porosity measurements are differently

influenced by factors:– Lithology– Clay content– Presence of gas

» Combination increases accuracy

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Measurements and logging-while-drilling

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 measurements-while-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.


Measurements and logging-while-drilling 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. TheMWD system shows the direction of the bit. A steerable downholeassembly 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).

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Mud Logs

Drilling rate» Information about lithology» Qualitative indication of porosity

Investigation of cuttings lifted with mud» Traces of hydrocarbons

Gas detector


Mud logs

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Cost

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.


Summary (on common

wirelinelogs)

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Summary of the main types of wirelinelogs and their major applications


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dip log ppt

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