GG 450
May 1, 2008
BOREHOLE LOGGING
Drilling Technology Basics
Drilling is an art that has progressed a great deal since the
first wells were drilled about 150 years ago. At that time,
the main product was kerosene to be burned in lamps.
Gasoline didn't come in until the automobile was on its way
around 1900. Drillers didn't trouble themselves with
environmental concerns, and the process was dirty and
wasteful.
BOP Stack: Wells now all have a blow-out-prevention stack
(BOP stack) at the ground level which can cut and seal the
pipe and/or any casing strings quickly if pressure in the hole
indicates that a highly overpressured zone has been
encountered.
Drill pipe is HEAVY. The Joides Resolution string weighs
about 600 tons. The drillers must be sure that the pipe is
always in tension. The only part of the pipe in compression
is at the bit itself. Heavy weighted pipes (drill collar) just
above the bit keep the bottom section in tension.
The pipe is kept in rotation as much as possible with fluid
pumped down through the pipe through the bit. The fluid
keeps bits of rock (cuttings) from falling back down and
clogging the pipe.
The pipe can only rotate in one direction. If it were to rotate
backwards, all the screw joints between pipe stands would
come apart.
Drilling mud is used to bring
heavy cuttings back to the
surface. If water were
used, these cuttings would
tend to sink back down the
hole because of the density
difference.
What can be measured in a
borehole is limited by the
characteristics of the hole.
While drilling, holes are
CASED with steel pipe to
prevent the hole from
collapsing in around the drill
pipe.
As the hole is deepened, more casing – with smaller diameter is inserted
in the hole. The diameter of the outermost casing might be 16”, and the
smallest only 5”.
Most drill holes are not cored. Drillers depend on cuttings
brought up with the drilling mud to tell what rock they are in,
and on WELL LOGGING.
The pressure at the cutting head is kept high to prevent blowouts. Water penetrates the formations and can confuse the
picture obtained from logging.
HOLE
Riser
drillings in
marine
drilling
allows
deeper
penetration
because
mud can be
circulated
up to the
drill ship.
Down-hole logging methods
In all wireline methods, a string
of probes is lowered down into
the well. Different properties are
measured by the probe and
plotted as a function of depth.
Logging is usually done as the
probe is coming back up the
well. Probes are stacked
together in strings to obtain as
much information in as little time
as possible. A string might be 30
m long.
Advantages of Logging
Provides objective and quantitative data for existing
wells.
Requires only one or two people.
Test can be repeated.
Non-destructive.
Fast and economical.
Much cheaper than coring.
Disadvantages of Logging Methods
Equipment is expensive.
Will not define all important contacts.
Requires trained people for data collection and
interpretation.
Geophysics in
well logging:
COMMON T O OLS
tool
method
uses
seismic (sonic)
ref r action
bor e hol e
teleview e r
reflection
spontaneous
potential (SP)
measures
natural
voltages
resistivity
(laterolog)
induction
electrical
ve r y hig h resolution
velocities
vs. depth, porosity
estimates,
shea r modulus,
fracture
porosity
high resolution ho
le
charact e rist ics
perme a bility - g o od
differ e ntiation
b etween sh ales
(highly conductive)
a nd other
rocks
formation
resistivity,
ch a nges in
lithology
resistivity in
d r y holes or high
resistivity for
m ations
apparent
d ensity of f ormations
density
porosity
Natural gam
caliper
ma
induced
electrical
gamma r a y
scattering
neutron
ener gy
changes
by
hyd r ogen
natural
rad iation
porosity of for
minera
logy
hole d iamet e r
m ation
Array Seismic Imager
The Array Seismic Imager
(ASI) consists of an array of
five seismic sensors. Each
sensor package contains
orthogonal geophones. This
tool is used in Vertical
Seismic Profiling, where s
seismic source at the top is
recorded by the sensors in
the hole, providing excellent
estimates of interval
velocities.
Dipole Sonic Imager (DSI)
The sonde provides the best method available today for
obtaining borehole compressional, shear and Stoneley
slownesses. (Slowness is the reciprocal of velocity and
corresponds to the interval transit time measured by standard
sonic tools.) Dipole technology allows borehole shear
measurements to be made in "soft" rock as well as "hard" rock
formations. The DSI tool is a multireceiver tool with a linear array
of eight receiver stations, a monopole transmitter and two dipole
transmitters. The receiver array provides more spatial samples of
the propagating wavefield for full waveform analysis. Frequency
determines the wavelength that drives the depth of investigation
of the measurement. Low frequency penetrates deeper into the
formation and helps read beyond altered zones.
Dipole Sonic
This seismic tool provides
excellent estimates of interval
velocity that can be used in
conjunction with reflection
processing.
The BHTV provides an acoustic reflection image
of the borehole, Particularly useful for detection of
dipping layers and faulting.
Dual Induction Tool (DITE)
The Phasor Dual Induction - Spherically Focused
Resistivity tool (DITE-SFR) provides measurements of
spontaneous potential (SP) and three different resistivity
values; deep induction, medium induction, and shallow
resistivity. Since the solid constituents are orders of
magnitude more resistive than pore fluids in most rocks,
resistivity is controlled mainly by the conductivity of the pore
fluids and by the amount and connectivity of the pore space.
Applications: Porosity estimate: In sediments which do
not contain clay or other conductive minerals, the
relationship between resistivity and porosity has been
quantified by Archie's Law. Archie's Law relates the
resistivity to the inverse power of porosity.
Dual Laterolog (DLL)
The Dual Laterolog (DLL) provides two resistivity
measurements with different depths of investigation into the
formation: deep and shallow. A current beam 2 ft-thick is forced
horizontally into the formation by using focusing currents. For the
deep measurement both measure and focusing currents return to
a remote electrode on the surface; thus the depth of investigation
is greatly improved, and the effect of borehole conductivity and of
adjacent formations is reduced. In the shallow laterolog the
return electrodes are located on the sonde.
Applications: Porosity estimate: Because of the inverse
relationship between resistivity and porosity, the dual laterolog
can be used to compute the porosity of the rock from Archie's
equation.
Geologic High-Resolution Magnetic Tool (GHMT)
The Geological High-Resolution Magnetic Tool (GHMT) provides
magnetic susceptibility and total magnetic induction
measurements. The main use of the GHMT is to provide a
magnetic reversal sequence in sediment.The GHMT consists of
two sondes. The Susceptibility Measurement Sonde (SUMS)
makes an induction-type measurement to record a signal related
to formation susceptibility. Its depth of investigation and vertical
resolution are about 80 cm and 40 cm, respectively. The Nuclear
Resonance Magnetometer Sonde (NMRS) accurately measures
the total magnetic induction in the borehole. Its depth of
investigation is theoretically infinite (most of the Earth's field is
generated in the Earth's core) and its vertical resolution is about
45 cm.
Example of
magnetostratigraphy (ODP
Leg 165)
The figure shows the
derivation of
magnetostratigraphy from the
susceptibility and total
induction measurements.
Correlation indicates normal
magnetic polarity zones, anticorrelation indicates reverse
magnetic polarity zones. The
interpretation column is then
compared to a standard
geomagnetic polarity time
scale.
Formation MicroScanner (FMS)
The Formation MicroScanner sonde (FMS) is a highresolution resistivity-measuring device consisting of four
orthogonal imaging pads each containing 16
microelectrodes in direct contact with the borehole wall.
The button current intensity is sampled every 0.1 in (2.5
mm). The tool works by emitting a focused current from the
4 pads into the formation measued by potential electrodes.
The current intensity measurements reflect microresistivity
variations of the formation.
FMS
Applications･Mapping of bedding
planes, fractures, faults, foliations,
and other formation structures and
dip determination.･Detailed
correlation of coring and logging
depths.･Precise positioning of core
sections where core recovery is less
than 100%.･Analysis of depositional
environments.
Multi-sensor Gamma Tool (MGT)
and the Natural Gamma Tool (NGT)
measure the natural gamma ray
radiation of the formation. Different
isotopes (mainly potassium, thorium,
and uranium) emit gamma radiation
at different energies. By counting
gamma rays at different energy
levels, by measuring the gamma-ray
spectrum, one can estimate the
concentration of the three different
elements.
Natural Gamma
Some applications are: A.
Clay typing: Potassium
and thorium are the primary
radioactive elements
present in clays; B.
Mineralogy: Carbonates
usually display a low
gamma ray signature; C.
Ash layer detection:
Thorium is frequently
found in ash layers. The
ratio of Th/U can also help
detect these ash layers.
In the log to the left,
depth is the vertical
axis, and the gamma
data are plotted.
An experienced log
analyst can interpret
such records to
provide the geologist
with likely lithology of
the rocks surrounding
the well.
Accelerator Porosity Sonde (APS)
The Accelerator Porosity Sonde (APS) uses a powerful
neutron source to detect hydrogen. Five detectors provide
information for porosity evaluation, gas detection, shale
evaluation, and borehole corrections.
Applications: Porosity: Reservoir engineering; in the study
of the volcanic rocks that make up the upper oceanic crust, a
good in-situ porosity measurement is important. Other
properties such as density, seismic velocity, and permeability,
depend on porosity. Lithologic determination: The porosity
curve, often combined with the density log, can be used to
detect shaly intervals, or minerals such as gypsum, anhydrite
and salt layers.
Logs from nearby drill holes
can be correlated to identify
variations in structures and
lithologies. In the oil
industry, they provide data
on how quickly a reservoir is
being depleted and how
quickly water is encroaching
on producing regions.
The logs to the left (red
areas) show the location of
the Cretaceous impact
deposits in the Yucatan
wells.
The logs on the next slide
show an area of producing
wells.
In the Ocean
Drilling Program,
logs fill gaps
where core
recovery was
poor and provide
“regional” data
away from the
hole.
Correlation of various
logs can provide
considerable
information to the
geologist.
Modern data and
methods allow
correlations
between well logs
to include hole
geometry and 3-D
representations.