16.5.8.3 Uncertainties of Dipmeter Logs
There are many uncertainties involved in interpreting
dipmeter logs. The quality of the dipmeter data varies
with the hole conditions, temperature and depth, type
of mud used, rock type and tools. Generally FMS data
are of better quality than SHDT data due to the
increased button density and borehole coverage. Similarly, SHDT gives better results than HDT due to
improvements of the tool and the algorithms used in
processing. Data from the OBDT are generally of
poorer quality than any of the resistivity tools (related
to the use of oil-based mud), and results must accordingly be treated with care. Also it is not always possible to find dipmeter patterns that are compatible with
sedimentological models of cross-bedded strata.
16.5.9 Image Logs
16.5.9.1 Generalities and Basic Principles
Borehole imaging has been one of the most rapidly
advancing technologies in wireline well logging. The
term “borehole imaging” refers to those logging and
data-processing methods that are used to produce
centimetre-scale images of the borehole wall and the
rocks that make it up. The context is, therefore, that of
open hole, but some of the tools are closely related to
their cased-hole equivalents.
Borehole imaging can be performed by optical
imaging, electrical imaging and acoustic imaging. A
wide range of imaging tools is available, among them
CBIL (Circumferential Borehole Imaging Log), UBI
(Ultasonic Borehole Imaging) and CAST (Circumferential Acoustic Scanning Tool), with LWD imaging
tools becoming increasingly important. Optical imaging tools were the first borehole imaging devices.
Today they furnish a true high-resolution colour
image of the wellbore. Video cameras may record
the lamination and bedding and because they are oriented, palaeocurrent direction may be inferred. This
method may also be very useful for recording fractures
and large vuggy pores.
Electrical (microresistivity) imaging devices were
developed as an advancement on dipmeter technology.
Traditionally, they have required a conductive borehole fluid, but this requirement has since been obviated
by oil-based mud imaging tools. In the microresistivity
imaging tool, the pads and flaps contain an array of
button electrodes at constant potential (Fig. 16.36).
An applied voltage causes an alternating current to
flow from each electrode into the formation and then
to be received at a return electrode on the upper part of
the tool. The microelectrodes respond to current density, which is related to localised formation resistivity.
The tool, therefore, has a high-resolution capability in
measuring variations from button to button.
Acoustic borehole-imaging devices are known as
“borehole televiewers”. They provide 100% coverage
of the borehole wall. Modern acoustic borehole imaging tools contain a magnetometer to provide azimuthal
information. The borehole televiewer operates with
pulsed acoustic energy so that it can image the borehole wall in the presence of opaque drilling muds.
Short bursts of acoustic energy are emitted by a rotating transducer in pulse-echo mode. These travel
through the drilling mud and undergo partial reflection
at the borehole wall. Reflected pulses are received by
the transducer. The amplitudes of the reflected pulses
form the basis of the acoustic image of the borehole
wall. Data are usually presented as depth plots of
enhanced images of amplitude and borehole radius
(Fig. 16.36). To some extent, the acoustic and electrical images are complementary because the ultrasonic
measurements are influenced more by rock properties,
whereas the electrical measurements respond primarily to fluid properties.
16.5.9.2 Uses of Image Logs
Borehole image logs provide high-resolution directional data sets and are powerful tools in subsurface
reservoir characterisation. They span the scale gap
between core and seismic observations and although
they do not replace cores, they provide key sedimentological and sub-seismic structural information,
allowing quantification of subsurface fracture
networks and providing inputs to geomechanical and
petrophysical studies. The applications of image logs
range from detailed reservoir description through reservoir performance to enhanced hydrocarbon recovery. Specific applications are fracture identification,
analysis of small-scale sedimentological features,
evaluation of net pay in thinly bedded formations,
and the identification of breakouts (irregularities in
the borehole wall that are aligned with the minimum
horizontal stress and appear where stresses around the
wellbore exceed the compressive strength of the rock).
It is important to detect open fractures and also partly
healed fractures which cannot be closed by increased
422
N.H. Mondol
There are many uncertainties involved in interpreting
dipmeter logs. The quality of the dipmeter data varies
with the hole conditions, temperature and depth, type
of mud used, rock type and tools. Generally FMS data
are of better quality than SHDT data due to the
increased button density and borehole coverage. Similarly, SHDT gives better results than HDT due to
improvements of the tool and the algorithms used in
processing. Data from the OBDT are generally of
poorer quality than any of the resistivity tools (related
to the use of oil-based mud), and results must accordingly be treated with care. Also it is not always possible to find dipmeter patterns that are compatible with
sedimentological models of cross-bedded strata.
16.5.9 Image Logs
16.5.9.1 Generalities and Basic Principles
Borehole imaging has been one of the most rapidly
advancing technologies in wireline well logging. The
term “borehole imaging” refers to those logging and
data-processing methods that are used to produce
centimetre-scale images of the borehole wall and the
rocks that make it up. The context is, therefore, that of
open hole, but some of the tools are closely related to
their cased-hole equivalents.
Borehole imaging can be performed by optical
imaging, electrical imaging and acoustic imaging. A
wide range of imaging tools is available, among them
CBIL (Circumferential Borehole Imaging Log), UBI
(Ultasonic Borehole Imaging) and CAST (Circumferential Acoustic Scanning Tool), with LWD imaging
tools becoming increasingly important. Optical imaging tools were the first borehole imaging devices.
Today they furnish a true high-resolution colour
image of the wellbore. Video cameras may record
the lamination and bedding and because they are oriented, palaeocurrent direction may be inferred. This
method may also be very useful for recording fractures
and large vuggy pores.
Electrical (microresistivity) imaging devices were
developed as an advancement on dipmeter technology.
Traditionally, they have required a conductive borehole fluid, but this requirement has since been obviated
by oil-based mud imaging tools. In the microresistivity
imaging tool, the pads and flaps contain an array of
button electrodes at constant potential (Fig. 16.36).
An applied voltage causes an alternating current to
flow from each electrode into the formation and then
to be received at a return electrode on the upper part of
the tool. The microelectrodes respond to current density, which is related to localised formation resistivity.
The tool, therefore, has a high-resolution capability in
measuring variations from button to button.
Acoustic borehole-imaging devices are known as
“borehole televiewers”. They provide 100% coverage
of the borehole wall. Modern acoustic borehole imaging tools contain a magnetometer to provide azimuthal
information. The borehole televiewer operates with
pulsed acoustic energy so that it can image the borehole wall in the presence of opaque drilling muds.
Short bursts of acoustic energy are emitted by a rotating transducer in pulse-echo mode. These travel
through the drilling mud and undergo partial reflection
at the borehole wall. Reflected pulses are received by
the transducer. The amplitudes of the reflected pulses
form the basis of the acoustic image of the borehole
wall. Data are usually presented as depth plots of
enhanced images of amplitude and borehole radius
(Fig. 16.36). To some extent, the acoustic and electrical images are complementary because the ultrasonic
measurements are influenced more by rock properties,
whereas the electrical measurements respond primarily to fluid properties.
16.5.9.2 Uses of Image Logs
Borehole image logs provide high-resolution directional data sets and are powerful tools in subsurface
reservoir characterisation. They span the scale gap
between core and seismic observations and although
they do not replace cores, they provide key sedimentological and sub-seismic structural information,
allowing quantification of subsurface fracture
networks and providing inputs to geomechanical and
petrophysical studies. The applications of image logs
range from detailed reservoir description through reservoir performance to enhanced hydrocarbon recovery. Specific applications are fracture identification,
analysis of small-scale sedimentological features,
evaluation of net pay in thinly bedded formations,
and the identification of breakouts (irregularities in
the borehole wall that are aligned with the minimum
horizontal stress and appear where stresses around the
wellbore exceed the compressive strength of the rock).
It is important to detect open fractures and also partly
healed fractures which cannot be closed by increased
422
N.H. Mondol