Dip meter mud log

7/24/2019 Dip meter mud log

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Slide 88Dr Elena Pasternak

Dipmeter log and boreholeimaging

Dipmeter

Multi-arm micro-resistivity log

Measures direction of dip of beds adjacentto borehole

Formation MicroImager

Large numbers of micro-resistivity probes

Imaging through statistical analysis

(synthesises an image of lithology of aborehole 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 cms only)

and large scale.

As a result structural dip is determined.


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Making a dipmeter measurementAs 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

resistivity

curves give

location of

bedding

plane

(boundary

between

different

rocks.

Boundary

does not

conduct

electricity

well highresistivity.)


Dipmeter continued

The first dipmeter tool had three arms 120 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 presentdipmeter data aretadpoles and stickplots.

A stick plot useslines (sticks) to shothe dipmeasurements.Depth is recorded onthe vertical axis withthe well representedby a vertical line.The angle on thestick is the dipmeasurement.


Dipmeter tadpole plot Four basic types of motifare commonly identifiable

Uniformly low dips (referred to as green patterns) aregenerally seen in shales and indicate the structural dip ofthe formation

Upward declining dip sequences - as we move up towardsthe surface the dip angle decreases (referred to as redpatterns), may be caused by the drape of shales over reefsor 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 towardsthe 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 byfolds, faults, or unconformities

Random (bag onails) motifs can reflect poor holeconditions 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 inthe 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.


Formation MicroImager (FMI)

Borehole image is produced in cylindrical and unrolled

formats (software).


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Formation MicroImager

Unrolled format Cylindrical format


Dielectric logs Dielectric 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.)

Waterhas a dielectric constant that is much higher than for other

fluids or the rocks. It ranges from

50 for freshwater, to 80 forsaline 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 combination Sonic (acoustic) log porosity

Electric logs porosity

Radioactivity logs porosity

Dielectric logs (electromagnetic wave propagation,salty water bad dielectric, dielectric constant in saltywater > than in fresh water > HC; cf. resistivity ofsalty water is low, higher in fresh water and HC)porosity

Combination 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 adata transmitting process were developed to give a realtime log as the well is being drilled called measurements-while-drilling (MWD) andlogging-while-drilling (LWD).

MWD measures well properties such as azimuth anddeviation.

LWD measures rock and fluid properties such as short andlong normal resistivity, natural gamma-ray, formationdensity, 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 thecirculating drilling mud or electrical batteries. The data can be transmittedto the surface by fluid pulse telemetry. The data are coded digitally in

pressure pulses that are sent up the well through the drilling mud. Theyare recorded on a pressure transducer on the surface where they aredecoded by a software.

MWD is very useful in drilling deviation and horizontal wells. It records adirectional log that shows the orientation of the drill bit, the direction inwhich the well is being drilled (in real time). The measurement is madewith a magnetometer in the downhole tool that measures the direction ofthe Earths magnetic field.

Geosteering is the drilling of a horizontal well while continuouslyadjusting 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 keepthe 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 meter mud log