Post on 07-Feb-2018
Geosteering Resistivity Tool
L o g g i n g t o o l s f o r o i l e x p l o r a t i o n
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Enhanced Resistivity Measurement
Most MWD Resistivity tools are of the “Wave Propagation” type, and can
accurately measure the average rock-formation resistivity. However, this
type of tool may be difficult to use in horizontal drilling because it requires
complex interpretation of phase and attenuation logs to see an adjacent
bed-boundary. It is easy to mistake curve separations due to other effects
such as anisotropy or invasion, resulting in drilling through a bed-boundary,
and then having to backtrack. CBG Corporation has invented and patented
(US patent #6,064,210) and UK patent 2,353,596, a method for measuring
resistivity as a vector quantity. For example, up to one meter before
mistakenly drilling out of a reservoir, advance indication of the distance and
direction to a nearby shale bed is available so that corrective action can
be taken. The addition of directional information to the resistivity reading
provides a new enabling technology for geosteering, at a price that is
affordable for directional-drilling companies
The GRT is an advanced imaging laterolog device, incorporating four
independent receiving electrodes to create an azimuthally-sensitive
resistivity measurement. This capability allows the GRT to detect a
nearby bed, determine whether it is conductive or resistive, and ascertain
its direction and distance. In the diagram at top right (from a computer
model), the electric current density is higher in the upper conductive rock
region (e.g. a shale), so the upper electrode receives a bigger current.
Unlike competing tools, it is not necessary to rotate the GRT to obtain
directionally-sensitive resistivity measurements.
Directional resistivity data from the GRT can be graphically presented on
a surface display (patents pending) in an enhanced version of the familiar
compass rose drillers display typically employed in directional drilling. A
continuously-updated borehole image showing the distance and direction
to conductive/resistive anomalies enables the operator to intelligently direct
the drilling process, without having to study resistivity logs.
Geosteering Resistivity Tool
Sample resistivity display on compass rose
Modeled GRT current density
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Advanced Cost-Effective Design
State-of-the-art techniques were applied in creating the GRT. For example,
FEMLAB 3D Finite-Element-Analysis modeling was used to optimize
mechanical and electrical parameters. A graph (at right) from the model of
the GRT illustrates how the ratio of current received by a pair of electrodes
can be used to distinguish a parallel conductive bed from a resistive bed,
and determine the distance to the boundary.
Benefiting from its innovative electronic and mechanical design, the GRT
provides accurate and reliable operation, combined with low cost and
serviceability. Sophisticated self-calibrating measurement circuitry ensures
that stability and precision are maintained across the full range of operating
conditions. Should it be necessary to service or repair the GRT, its rugged,
probe-based construction makes disassembly and reassembly quick and
easy. The GRT does not employ any third-party proprietary technology so
there are no licensing fees required.
GRT receiver electrodes
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Logging While Drilling
Of course, in addition to its application in directional drilling, the GRT
performs exceptionally as a conventional LWD resistivity tool. In the graph
at right, the 4.75 inch diameter tool is shown capable of very accurate
measurements in a 6.5 inch diameter hole whatever the mud resistivity.
Even in severe washouts up to 12 inches in diameter, the tool retains
acceptable accuracy. A complete log is saved in the GRT internal memory
and can be subsequently downloaded through a high-speed link to a PC
when the GRT is retrieved from the well. It is not necessary to remove the
tool from the BHA to download data. Furthermore, the GRT can include
an internal Gamma-Ray tool (standard or directional), providing directional
capability that is unique in the industry.
The GRT can be provided as a stand-alone tool, or part of a complete
LWD resistivity system, including a surface display, display software, and
data communications/control electronics. A mud Pulser can be mounted
above or below due to the full 10-wire bus passing through the tool. The
GRT is available compatible with Tensor-type systems, and can be readily
integrated into other industry-standard tool strings. A built-in Muleshoe
helix at the top end of the tool ensures that the Directional Sensor package
is always correctly oriented with the GRT sensors.
Since the GRT is a laterolog-type tool, it will not give accurate results in
oil-based muds if there is a significant amount of emulsion breakdown.
However, in high resistivity formations operation at reduced accuracy is
still possible.
Laterolog tools excel in very high-resistivity formations, where it may be
necessary to distinguish between oil and fresh water. An example is the
heavy-oil Orinoco Basin of Venezuela, where the wave-propagation tools
have been found unusable. Another example is in fresh-water injection
well situations, such as in Saudi Arabia. Wave-propagation tools are not
accurate in resistivities above about 200 ohm.meters.
The GRT has another advantage over other directional types of resistivity
tools. As the graph (at left) indicates, the detection distance for an adjacent
contrasting rock formation is dependant only on resistivity contrast, and not
on the actual resistivities. Detection distance is constant for a wide range
of resistivities, particularly for the most important high values typical of oil
reservoirs.
GRT Maximum Detection Distancemeasurement accuracy +/- 2.5%
form
atio
n r
esis
tivi
ty
bed resistivity
distance(inches)
20000 -
2000 -
200 -
20 -
2 -
0.2 -
0.2 2 20 200 2000 20000
- - - - - -
8 16 24 32 40 48
- - - - - -
Geosteering Resistivity Tool
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Geosteering Resistivity Tool
GRT Calibrator/Verifier Product Description
The Calibrator/Verifier is a dual purpose accessory for CBG’s Geosteering Resistivity Tool. It can be used to test the GRT over its full measurement range and confirm that it is operating properly. It is also capable of downloading the entire resistivity log from the internal memory of the GRT, and can subsequently transfer the log to a PC for analysis. Since the GRT electrodes are utilized for data communications, the log dump does not require time-consuming disassembly of the tool. A 3-hour log can be transferred from the GRT to the Calibrator/Verifier in roughly one minute, and over 400 hours of log data can be stored.
The Calibrator/Verifier is battery-operated and completely portable, and is housed in a rugged weather-resistant enclosure for use in the field. The Calibrator/Verifier has a simple menu-based user interface with an LCD display and 3-button keypad. It is quickly and easily attached to and removed from the GRT without the use of special tools or hardware.
Calibrator/Verifier specifications:Battery Complement = six rechargeable cellsOperating Time = 6 hours Recharge Time = 1 hourComm. link to GRT = proprietary serial interface, 57.6KbaudComm. link to PC = RS-232 serial link, 57.6Kbaud (USB soon to be available)
GRT field test log
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2601 McHale CourtSuite 145
Austin, Texas 78758
Tel: 512-491-7541Fax: 512-491-7561
w w w . c b g c o r p . c o m
Operational Specifications
Power Source External Lithium Battery Pack (14.4V to 28.8V nominal)
Battery Life 600 hours with one 28.8 volt Battery Pack (10% duty cycle)
Types of Measurement Real-time Resistivity Vector (magnitude + angle), LWD Average Resistivity Log
Average Resistivity Range/Accuracy 0.2 to 20,000 Ω-m., +/-10% (worst-case), +/-1.5% (if borehole is on-gauge)
Vertical ResolutionThin-Bed Resolution 17 cm / 6.75 inches
Maximum Operating Temperature 150 °C / 300 °F
Maximum Pressure 1400 bar / 20,000 PSI
Detection Distance for Parallel Rock Formation > 1 meter / 40 inches with >10:1 resistivity contrast
Depth of Investigation (Invasion Tolerance) 25 inches diameter @ 50% point on PGF curve
Internal Memory 16 MBytes, up to 1600 hours data log (1 sample / 10 seconds) with high-speed port, 400 hours (1 sample / 4 seconds)
Mechanical Specifications
Tool Length 3.5 meters / 11.5 feet
Nominal O.D. / Wear Band O.D. 4.75 inches / 5 inches
Tool Connections A.P.I. 3.5 inch IF, box upper/pin lower
Maximum Mud Flow Volume 300 GPM @ 50 feet/minute flow rate
Maximum Dogleg Severity Rotating 15°/100 feet, Sliding 20°/100 feet
Electrical Connections 10-pin Rotary, male upper/female lower
Toolface / Directional Package Orientation Muleshoe Helix built into upper box connection
G R T S p e c i f i c a t i o n s
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