Chapter 5 Measurement While Drilling

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UNIVERSITI TEKNOLOGI PETRONAS

PETROLEUM ENGINEERING DEPARTMENT

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1. INTRODUCTION

2. MWD SYSTEMS

2.1 Power Sources

3. MWD - DIRECTIONAL TOOLS

3.1 Calculations for Inclination, Toolface and Azimuth

3.2 Normal Surveying Routine

3.3. Accuracy of MWD Surveys

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4. MWD - GAMMA RAY TOOLS

5. TRANSMISSION AND CONTROL SYSTEMS

6. SURFACE SYSTEM

7. CONFIGURATION OF MWD SYSTEMS

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MWD systems allow the driller to gather and transmit information from the b

hole back to the surface without interrupting normal drilling operations.

This information can include

Directional deviation data,

Petrophysical properties

drilling data, such as WOB and torque.

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The information is gathered and transmitted to surface by the releva

transmission equipment which is housed in a non-magnetic drill collar in

assembly (Figure 1).

This tool is known as a Measurement While Drilling Tool - MWD Tool.

The data is transmitted through the mud column in the drill string, to surface

At surface the signal is decoded and presented to the driller in an appropria

The transmission system is known as mud pulse telemetry and does not

line operations.

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Figure - 1

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Commercial MWD systems were first introduced in the North Sea in 1978 a

effective method of taking directional surveys.

To take a directional survey using conventional wireline methods may take 1

Using an MWD system a survey takes less than 4 minutes.

Although MWD operations are more expensive than wireline surveying

But an operating company can save valuable rig time, which is usually more

terms of cost.

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Recent MWD tool are more complicated tools which will

provide not only directional information and drilling parameters (e.g. torq

but also geological data (e.g. gamma ray, resistivity logs).

The latter tools are generally referred to as Logging While Drilling - LWD To

As more sensors are added the transmission system must be improved

So MWD tools are becoming more sophisticated.

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The main difference between the All MWD systems currently available

The method by which the information is transmitted to surface.

All three systems encode the data to be transmitted into a binary code

Transmitting this data as a series of pressure pulses up the inside of the dri

The process of coding and decoding the data will be described below.

The only difference between the systems is the way in which the pressure p

generated (Figure 2).

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(1) Negative Mud Pulse Telemetry

(2) Positive Mud Pulse

(3) Frequency Modulation (Mud siren)

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Figure – 2.1

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Figure – 2.2

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Figure – 2.3

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2.1 Power Sources

No wireline connection to surface

All the power required generated down hole.

Either a battery pack or

a turbine-alternator must be installed as part of the MWD tool.

The turbine has been the standard method of power generation in the p

and frequency modulation tools.

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2.1 Power Sources

Since less power is required in the negative pulse system batteries hav

However, with more sensors being added and higher data rates require

being replaced with turbines in negative pulse systems also.

Turbines have several advantages over batteries (Table 2) but turbines

to mechanical failure.

Filter screens are used to prevent debris in the mud from damaging the

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All MWD systems use basically the same directional sensors for calculating

Inclination,

Azimuth

Tool Face.

The sensor package consists of

3 orthogonal accelerometers and

3 orthogonal magnetometers (Figure 3).

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Accelerometer

An accelerometer measure the component of the earth’s gravitational fie

axis in which it is oriented.

It works on the “force-balance” principle.

A test mass is suspended from a quartz hinge which restricts any moveone axis only (Figure 4).

As the mass tends to move due to gravity acting along that axis, its cen

maintained by an opposing electromagnetic force.

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Accelerometer (cont.)

The larger the gravitational force, the larger the pick-up current required

The voltage drop over a resistor in the pick up circuit is measured

This is directly related to the gravitational component

Depending on the orientation of the BHA the reading on each accelerom

different.

From these 3 components the angle of inclination and tool face can be

(Equations 1 and 2).

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Figure - 3

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Figure -4


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3.1 Calculations for Inclination, Tool face and Azimuth

In the following equations a, b, c, x, y, z refer to the accelerometer and

readings with axes as shown in Figure 3.

- the angle between C accelerometer and vertical.

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Magnetometer


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Magnetometer

Figure - 5


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3.2 Normal Surveying Routine


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3.2 Accuracy of MWD Survey


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MWD Gamma Ray Tools


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MWD Gamma Ray Tools


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MWD Gamma Ray Tools


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Figure - 6


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U S O OG O S


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Reference

Harriot Watt Material


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Chapter 5 Measurement While Drilling