21. Measurement While Drilling (MWD) and its Application in Directional Drilling

MWD (Measurement While Drilling) is a system developed to perform drilling related measurements downhole and transmit information to the surface while drilling a well.

The Measurement While Drilling (MWD) Tool came in to application in early 1970’s.

Before the introduction of MWD, all survey data were obtained by stopping the drilling process for wireline logging. For wireline logging, we had to stop the drilling process, put the drill pipe on slip, break out Kelly, lower the wireline tool, retrieve the tool, read the survey and plan the further action. This used to increase the non productive time (NPT).

The MWD tool transmitted the survey reading to surface through the mud stream in the drill pipe. The drilling process was stopped for few minutes and survey readings were obtained in pump off condition. This saved times to greater extent compared to wireline logging.

The transmission of survey data though mud stream was one of the means. Other means of transmission were electromagnetic and acaustic which were developed in later stages.

Thus MWD was considered a better option for survey data transmission compared to wireline procedure.

Initially the system delivered three basic information: Inclination, Azimuth and Toolface. These three parameters helped the directional driller to position the well correctly to the desired target.

Later, MWD was equipped with Gamma Ray sensor to detect the natural radioactivity and characterize shale presence, gauge to measure annular pressure which are useful in slim hole to determine ECD, Strain gauge to measure WOB and torque on bit.

MWD tools can also provide information about the conditions at the drill bit. This may include:

• Rotational speed of the drillstring

• Smoothness of that rotation

• Type and severity of any vibration downhole

• Downhole temperature

• Torque and Weight on Bit, measured near the drill bit

• Mud flow volume

Fig 21.1 Mud Pulse Telemetry Components

Fig 21.2 Positive Mud Pulse Telemetry

Fig 21.3 Positive Mud Pulse Telemetry

Fig 21.4 Negative Mud Pulse Telemetry

Fig 21.5 Negative Mud Pulse Telemetry

Fig 21.6 Continuous Wave Telemetry

Fig 21.7 Continuous Wave Telemetry

Fig 21.9 Turbine Alternators

Fig 21.10 Mud Screen

Fig 21.10 Accelerometers and Magnetometors in Directional Sensor

Fig 21.13 Geiger-Muller Tube

Fig 21.14 Scintillation Counter

Fig 21.15 Turbine RPM Sensor

Fig 21.17 Rig Floor Display Unit Installed at Rig Floor

Fig 21.18 Radio Modem

Fig 21.22 MWD Components

Fig 21.23 MWD Tool Sequence

Fig 21.24 MWD Tool Sequence

Here I'll emphasize my discussion on MWD based on Positive Mud Pulse Telemetry.

COMPONENTS OF MWD SYSTEM

Telemetry Channel

Transmission System

Power Source

MWD Sensors

Surface Systems

TELEMETRY CHANNEL

Telemetry Channels are the channels or medium via which the downhole data is transmitted to the surface.

Following are the transmission channels used for data transmission :

Hard Wire Method

Electromagnetic Method

Acaustic Method

Mud Pulse Telemetry

MUD PULSE TELEMETRY

Most of the MWD systems commercially available are based on some form of mud pulse telemetry.

The major components of a mud pulse telemetry system are shown in figure below: 

The downhole components are all housed in a nonmagnetic drill collar (NMDC).

The major components are:
(a) a power source to operate the tool : it operates the tool when we plan to take a survey

(b) sensors to measure the required information;
(c) a transmitter to send the data to surface in the form of a code;
(d) a microprocessor or control system to coordinate the various functions of the tool : it powers up the sensors, stores the information that has been measured and then activates the transmitter to send the data in the form of a coded message.

The surface equipment consists of:
(a) a standpipe pressure transducer to detect variations in pressure and convert these to electrical signals;
(b) an electronic filtering device to reduce or eliminate any interference from rig pumps or downhole motors that may also cause pressure variations;
(c) a surface computer to interpret the results;
(d) a rig-floor display to communicate the results to the driller, or plotting devices to produce continuous logs

TRANSMISSION SYSTEM

Here in transmission system I shall concentrate on the transmission via drilling fluid, i.e., the mud pulse telemetry.

The Mud Pulse Telemetry can be further categorized into :

Positive Mud Pulse Telemetry (Maximum use in industry)

Negative Mud Pulse Telemetry

Continuous Wave Telemetry

Positive Mud Pulse Telemetry :

Positive mud pulse telemetry (MPT) uses a hydraulic poppet valve to momentarily restrict the flow of mud through an orifice in the tool to generate an increase in pressure in the form of a positive pulse or pressure wave which travels back to the surface and is detected at the standpipe.

To transmit data to surface, this valve is operated several times, creating a series of pulses that are detected by the transducer, and decoded by the surface computer. 

The surface computer initially recognizes a set of reference pulses, which are
followed by the data pulses. The message is decoded by detecting the presence or absence of a pulse within a particular time-frame. This binary code can then be translated into a decimal result. A chart recorder is used to monitor the sequence of pulses.

Negative Mud Pulse Telemetry

Negative MPT uses a controlled valve to vent mud momentarily from the interior of the tool into the annulus. This process generates a decrease in pressure in the form of a negative pulse or pressure wave which travels back to the surface and is detected at the standpipe.

The rapid opening and closing of this valve therefore creates a drop in standpipe  that can be detected by the pressure transducer.




Continuous Wave Telemetry

Continuous wave telemetry uses a rotary valve or “mud siren” with a slotted rotor and stator which restricts the mud flow in such a way as to generate a modulating positive pressure wave which travels to the surface and is detected at the standpipe.

One of the discs is stationary while the other is driven by a motor.
The constant speed of the motor creates a regular and continuous variation in pressure that is essentially a standing wave. This wave is used as a carrier to transmit the data to surface. When information is to be transmitted the speed of the motor is reduced so that the phase of the carrier wave is altered (i.e., reversed).

The carrier wave is therefore modulated to represent the data required.
The surface equipment detects these phase shifts in the pressure signal and translates this into a binary code.

This is a more sophisticated telemetry system and offers a higher data rate than the previous two mud pulse methods.

POWER SOURCES

The MWD Tool works in two situations :
(a) When the circulation is ON
(b) When there is no circulation, i.e. while tripping when the pump is in OFF condition.

Also, once lowered, the MWD tool is not retrieved back to surface unless there is some kind of problem with tool.

Thus to continuously provide power to the tool, we require a power source.


The power source can be :
(a) Batteries
(b) Turbine Alternators

Batteries : (Lithium Battery, usually 24 V)

They are compact and reliable since they contain no moving parts.
They have a finite operational life and are temperature-dependent.
batteries.
Since it has no relation with drilling fluid motion, this enables the tool to operate while tripping and also enables operation independent of mud flow hydraulics.
They have been successfully used for applications in which only directional data are required. As they provide a limited power output, they are not preferred with multisensor tool.

 Turbine Alternators :

With the trend of using multisensor tools for downhole survey, turbines are becoming more widely used to provide power to the MWD tool.

The flow of mud through the tool is harnessed by the turbine blades, which rotate a shaft connected to an alternator, hence generating electricity.

The electrical power generated must be controlled by a voltage regulator. Although this system provides more power and longer operating life than a battery pack, power failures can occur if the turbine is damaged.

To prevent this damage a screen can be installed upstream of the turbine to filter out any debris in the mud.
The screen may be positioned at the top of the drill string for ease of access if it requires to be emptied or removed to allow passage of wireline tools.

MWD SENSORS

An MWD tool is equipped with the combination of following sensors depending upon the requirement :

Directional Sensor
Gamma ray Sensor
Temperature Sensor
Downhole WOB/ Torque Sensor
Turbine RPM Sensor

Directional Sensor :

The directional sensors currently being used in MWD tools uses triaxial magnetometers and accelerometers.

These sensors measure the required angles of inclination, azimuth and toolface.

Since the magnetometers measure azimuth relative to Magnetic North, the correct magnetic declination must be applied to the results.

The C axis is aligned with the axis of the tool, and the B axis defines the reference for measuring toolface angle.

The angular offset between the B axis and the scribe line of the bent sub must be measured before running in the hole.

Both magnetometers and accelerometers give voltage outputs that have to be corrected by applying calibration coefficients. The corrected voltages can then be used to calculate the required directional angles.

Some kind of signal (like when drill string rotation stops or when the pumps are shut off) are sent from surface to the MWD control system.

The control system after receiving such signals power up the sensors.

A transducer or motion sensor within the downhole tool recognizes this signal and initiates the survey.

During the time when the sensors are actually taking the measurements the drill string must remain stationary for accurate results to be obtained.  This period is generally less than 2 min., after which normal drilling can resume.

The driller resumes the normal drilling process once the MWD rig display unit displays the updated survey.

The measurements of inclination azimuth and toolface are sent in a predetermined order. It generally takes 2-4 min. for transmission of a complete directional survey.

Accuracy of the survey :

± 0.25° for inclination, ± 2.0° for Azimuth and ± 3.0° for Toolface, which may vary from one tool manufacturer to other.

Gamma Ray Sensor :

All of the earth's rock formations exhibit varying degrees of radioactivity.

The gamma ray log is a measurement of the natural radioactivity of the formations.

Gamma rays are emitted by radioactive elements such as isotopes of potassium, thorium and uranium.

These elements are found more commonly in shales than in other rocks.

Thus by measuring the gamma-ray emission from a sequence of rocks it is therefore possible to identify shale zones.

To be most effective in detecting changes of lithology, the gamma ray sensor should be positioned as close to the bit as possible, so that only a few feet of a new formation are drilled before the tool responds.

For practical reasons, the distance between the bit and the gamma-ray sensor is about 6 ft.

Two basic types of detectors are used by MWD companies to measure gamma rays:
(a) Geiger-Muller tube
(b) Scintillation counter

Geiger-Muller tube :

It consists of a cylinder that contains an inert gas at a fairly low pressure.

A high-voltage electrode (± 1000 V) runs through the centre of the chamber.

As gamma-rays enter the chamber they cause ionization of the gas, creating a flow of fast-moving electrons towards the central electrode as shown in figure below.



The current of electrons can therefore be used to measure the amount of gamma-rays emitted from the formation.

Scintillation counter :

It uses a crystal a crystal of thallium-doped sodium iodide.

The natural gamma-rays emitted by the formation passes through the sodium iodide crystal.

The radiation excites the crystal, which produces a flash of light or scintillations when the gamma ray interacts with the crystal.

The light emitted by the crystal strikes the photocathode and releases electrons.

The electrons travel through a series of anodes, causing the emission of more electrons.

This generates a voltage pulse which is proportional to the original flash of light.

The amount of radiation entering the sensor can therefore be measured by counting the number of pulses over a given time period.

NOTE :

The Geiger-Muller tube is not as accurate as the scintillation counter, since it can only detect a much smaller percentage of the total rays emitted.

It does have the advantage, however, of being more rugged and reliable and being cheaper than the scintillation counter.

In addition to providing lithologic discrimination, the gamma ray sensor provides:

Formation bed boundary and thickness determination.
Well to well structural correlation of beds.
Depth control and casing seat selection.
Estimation of shale fraction in reservoir rocks.
A primary log for sedimentological studies.
Monitoring of injected radioactive materials.

Temperature Sensor

The temperature sensor is usually mounted on the outside wall of the drill collar, and therefore monitors the annulus mud temperature.
The sensing element may be a strip of metal (e.g. platinum) whose electrical resistance changes with temperature.
The sensor can be calibrated to measure temperatures ranging from 50 to 350°F.

Downhole WOB/Torque Sensor

These measurements are made by a system of sensitive strain gauges mounted on a special sub placed close to the bit.
The strain gauges will detect axial forces for WOB and torsional forces for torque.
By placing pairs of gauges on opposite sides of the sub, any stresses due to bending can be eliminated.

Turbine RPM Sensor

When drilling with a downhole turbine, the actual speed at which the bit is turning is not known at surface.

The only effective way of monitoring the rpm is to use a turbine tachometer linked to an MWD system to provide real time data.

The downhole sensor consists of a 2-in. diameter probe that is placed very close to the top of the rotating turbine shaft.

On top of the shaft are mounted two magnets 180° apart.

As the shaft rotates, an electric coil within the probe picks up voltage pulses due to the magnets (shown in figure below).




By counting the number of pulses over a certain interval, the turbine speed in rpm can be calculated.

This information is encoded as a series of mud pulses that are transmitted at intervals to surface to let the driller know how the rpm is changing.

SURFACE SYSTEMS

Standpipe Pressure Transducer
Rig Floor Display Unit
Radio Modem

Standpipe Pressure Transducer :

The standpipe manifold has a number of pressure taps where gauges may be installed.

The transducer can be installed at a convenient point by removing one of these gauges.

Inside the transducer is a sensitive diaphragm that detects variation in pressure and converts these hydraulic pulses to electrical voltage pulses.

The voltage output is relayed to the rest of the surface equipment by means of an electric cable.

Rig Floor Display Unit :

Rig Floor Display Unit is a display panel installed at the rig floor where the directional survey result (azimuth, tool face, inclination) are displayed for the convenience of directional driller.
The rig floor display unit is powered via rig power.
It generally requires either 120V or 240V power supply.

The display on Rig Floor Display unit looks as shown below :


Radio Modem :

The radio modem is used to communicate with the rig floor RT via the workstation (Laptop).
All information and data sent between the two components is encrypted for security purposes.

ALL COMPONENTS OF MWD

1. (UPS) Uninturupted Powe Supply

2. Barrel Wrench

3. Pressure Transducer

4. Rig Floor Display Unit

5. Pick-Up Plate

6. Digital Multimeter

7. Remote Terminal Case

8. Spanner Wrench

9. Small Dart Float, Large Flapper Float

10. Vibration Switch

1. Drill Pipe Screen

2. Muleshoe Crawn Wrench

3. Ring Bar

4. Orienting Bar

5. Short Sinker Bar

6. J - Wrench

7. Over shot Bell

8. Over Shot

9. Long Sinker Bar

10. Spang Jars

1. Orifice

2. Mule Shoe (Landing Sleeve)

3. Mule Shoe (Crown)

4. Poppet

5. Stinger

6. Piston Shaft

7. Stinger Barrel

8. Lower Piston Cap

9. Upper Piston cap

10. Stinger Spring

POST ENDS