Well voltages that develop at the contacts between shale

Well logging is a technique to record continuously physical
properties of rock as a function of depth. It is recorded by moving a down hole
logging probe and its sensor output at the surface through an electrical cable.
Different well logging methods are discussed below.


Logging Techniques

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The caliper tool records the borehole size variations. It consists
of one or more spring-loaded arms, which are pressed against the borehole wall
as the tool is raised inside the borehole. Motion in and out from the borehole
wall is recorded electrically and transmitted to surface recording equipment.


It indicates the presence of mud cakes which confirms the presence
of porous and permeable layers and helps to estimate volume of cement. It
measures the diameter of borehole and its volume and is useful in correcting
some of the other open hole logs which are very sensitive to hole size



Potential Log

The SP log records the change in
naturally occurring potentials as a function of depth in the borehole. It
arises due to salinity contrast between formation water in permeable bed and
the salinity of mud filtrate. No
current is sent into the formation.

 The SP log is recorded by
measuring the potential difference in milli-volts between an electrode in the
borehole and a grounded electrode at the surface. The spontaneous potential log
is a record of potential difference between the potentials or voltages that
develop at the contacts between shale or clay beds and a sandy aquifer, where
they are penetrated by a drill hole. 



b)      Resistivity Logs

By definition, resistivity is a function of the
dimensions of the material being measured; therefore, it is an intrinsic
property of that material. The current is passed between electrodes A and
B, and the voltage drop is measured between potential electrodes M and N. 
The current is maintained constant, so that the higher the resistivity between
M and N, the greater the voltage drop will be.

. Using porosity derived from
a neutron, gamma ? gamma, or acoustic velocity log, and Ro, the resistivity of
a rock 100% saturated with water, from a deep investigating resistivity log,
one can determine the resistivity of the formation water in granular sediments.


c)      Induction Logging

The basic induction logging system is shown
below. Induction logs measure conductivity. Induction devices provide
resistivity measurements regardless of whether the fluid in the well is air,
mud, or water. The measurement of conductivity usually is inverted to provide
curves of both resistivity and conductivity. The unit of measurement for
conductivity is usually milliSiemens per meter (mS/m), but milli-mhos per meter
and micro-mhos per centimeter are also used. 


Nuclear Logging

Nuclear logs are unique because the penetrating capability
of the particles and photons permits their detection through casing and annular
materials, and they can be used regardless of the type of fluid in the
borehole. The detection of radiation is based on ionization that is directly or
indirectly produced in the medium through which it passes. Three types of
detectors presently are used for nuclear logging: Scintillation Tube,
Geiger-Mueller Tubes, and Proportional Counters.


Gamma logging

Gamma logs (gamma ray logs/natural-gamma
logs) provide a record of total gamma radiation detected in a borehole.
American Petroleum Institute (API) gamma

ray unit as the standard for scales on
gamma logs is defined as 1/200 of the difference in deflection of a gamma log
between an interval of very low activity in the calibration pit and the
interval that contains the same relative concentrations of radioisotopes as
average shale, but approximately twice the total activity. Under most
conditions, 90% of the gamma radiation detected probably originates from
material within 150 to 300 mm of the borehole wall.

In rocks that are not contaminated by
artificial radioisotopes, the most significant naturally occurring
gamma-emitting radioisotopes are K40 and daughter products of the
uranium and thorium-decay series. Only gamma spectral logging can provide the
identification and relative concentrations of the natural and man-made
radioisotopes that produce the total radioactivity measured by a gamma
log.  Borehole-gamma spectrometry can provide more diagnostic information
on lithology, particularly the identification of clay minerals.  Uranium
and thorium are concentrated in clay by the processes of adsorption and ion
exchange. Fine-grained detrital sediments that contain abundant clay tend to be
more radioactive than quartz sands and carbonates.

Gamma-Gamma Logging

logs, also called density logs, are records of the radiation from a gamma
source in the probe after it is attenuated and backscattered in the borehole
and surrounding rocks. Gamma-gamma logging is based on the principle that the
attenuation of gamma radiation, as it passes through the borehole and
surrounding rocks, is related to the electron density of those rocks. Gamma-gamma
logs may be used to distinguish lithologic units and to determine well
construction, in addition to determining bulk density, porosity, and
water-content. The chief use of gamma-gamma logs has been for determining bulk
density that can be converted to porosity.  Gamma-gamma logs
conventionally are recorded with bulk density increasing to the right, which
means that porosity increases to the left. At many sites,
gamma-gamma logs provide more accurate porosity data than neutron and acoustic
velocity logs.


Neutron logs are made with a source of neutrons in the probe
and detectors, which provide a record of the interactions occurring in the
vicinity of the borehole. Most of these neutron interactions are related
to the amount of hydrogen present. Neutron probes contain a
source that emits high-energy neutrons. Neutron source used in porosity logging
tools is americium-beryllium.

In many rocks, the hydrogen content is
related to the amount of water in the pore spaces. This relation is affected by
the chemical composition of the water, hydrogen in some minerals and bound
water in shales.  Neutron logs are most suitable for detecting small
changes in porosity at low porosities; gamma-gamma logs are more sensitive to
small changes at high porosities. Although the interpretation of neutron logs
for porosity and moisture content are stressed as primary applications, much
use has been made of the logs for determining lithology.  Like gamma logs,
they can be used for lithology and stratigraphic correlation over a wide range
of borehole conditions. 

Modern Logging Techniques

Nuclear Magnetic Resonance

Many (though not all) atomic nuclei can be thought
of as very small bar magnets, with a north pole and a south pole. The nuclei
spin at a constant rate, with the spin axis exactly coinciding with the line
between the north and south poles. This is the principle applied in NMR.In a rock, NMR relaxation depends on the size of the pores: the
larger the pores, the longer the NMR relaxation time.

The sensitivity of NMR to pore size has two
applications. The first is permeability. The permeability is proportional to
the square of the diameter of the pores, so one expects the permeability to be
proportional to the square of the NMR relaxation time. The second application
of NMR data is to determine a distribution of pore sizes. Since pores within a
single rock can vary greatly in size, the distributions are very broad. The
pore size distribution tells geologists a lot about a rock.



Dipmeter, FMI, FMS and Acoustic Tele-viewer

Dipmeter logs determine the orientations of
sandstone and shale beds in the well, as well as the orientations of faults and
fractures in these rocks. The original dipmeters did this by measuring the
resistivity of rocks on at least four sides of the well hole. Modern dipmeters
actually make a detailed image of the rocks on all sides of the well hole.
Borehole scanners do this with sonic (sound) waves, whereas FMS (formation
microscanner) and FMI (formation micro-imager) logs do this by measuring the
resistivity. Three dimensional bore hole imaging can also be done by sonic
tools like Acoustic Tele-Viewer (ATV).