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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