Oil & Gas Measurement While Drilling (MWD tool) systems allow the driller to gather and transmit information from the bottom of the hole back to the surface without interrupting normal drilling operations. This information can include directional deviation data, data related to the petrophysical properties of the formations, and drilling data, such as WOB and torque (check also: torque & drag).
The information is gathered and transmitted to the surface by the relevant sensors and transmission equipment which is housed in a non-magnetic drill collar in the bottom hole assembly (check also: types of Bottom hole assembly). 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 the surface. At the surface, the signal is decoded and presented to the driller (check also: Driller Job Description | Knowledge & Responsibilities) in an appropriate format. The transmission system is known as mud pulse telemetry and does not involve any wireline operations.
To take a directional survey using conventional wireline methods may take 1-2 hours. Using an MWD system a survey takes less than 4 minutes. Although MWD operations are more expensive than wireline surveying an operating company can save valuable rig time, which is usually more significant in terms of cost.
More recently MWD companies have developed more complicated tools which will provide not only directional information and drilling parameters (e.g. torque, WOB) but also geological data (e.g. gamma-ray, resistivity logs). The latter tools are generally referred to as Logging While Drilling – LWD Tools.
As more sensors are added the transmission system must be improved and so MWD tools are becoming more sophisticated. Great improvements have been made over the past few years and MWD tools are now becoming a standard tool for drilling operations.
Measurement While Drilling (MWD) System Components In Oil & Gas Wells
All MWD systems have certain basic similarities which are as follows:
- A downhole system which consists of a power source, sensors, transmitter and control system.
- A telemetry channel (mud column) through which pulses are sent to surface.
- A surface system which detects pulses, decodes the signal and presents results (numerical display, geological log, etc.).
Since there is no wireline connection to the surface all the power required to operate the MWD tool must be generated downhole. This means that either a battery pack or a turbine-alternator must be installed as part of the measurement while drilling MWD tool. The turbine has been the standard method of power generation in the positive pulse and frequency modulation tools. Since less power is required in the negative pulse system batteries have been used.
However, with more sensors being added and higher data rates required, batteries are being replaced with turbines in negative pulse systems also. Turbines have several advantages over batteries but turbines are more prone to mechanical failure. Filter screens are used to prevent debris in the mud from damaging the turbine
The main difference between the 3 MWD systems currently available is the method by which the information is transmitted to the surface. All three systems encode the data to be transmitted into a binary code and transmitting this data as a series of pressure pulses up the inside of the drill string. 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 pulses are generated.
Negative Mud Pulse Telemetry
In all systems, the fluid must be circulating through the drill string. In the negative mud pulse system, a valve inside the MWD tool opens and allows a small volume of mud to escape from the drill string into the annulus. The opening and closing of this valve create a small drop in standpipe pressure (50 – 100 psi) which can be detected by a transducer on the surface.
Positive Mud Pulse
In the positive mud pulse system, a valve inside the measurement while drilling MWD tool partially closes, creating a temporary increase in standpipe pressure.
Frequency Modulation (Mud siren)
In the frequency modulation system, a standing wave is set up in the mud column by a rotating slotted disc. The phase of this continuous wave can be reversed. The data is transmitted as a series of phase shifts. Many tools also include the ability to record downhole data for later retrieval at the surface. Although this undermines the principle of access to real-time data it allows the operator to gather large volumes of data (typical petrophysical data) and therefore eliminate expensive electric wireline logging operations.
MWD Sensors & Directional Surveys
All MWD systems use basically the same directional sensors for calculating inclination, azimuth, and tool face. The sensor package consists of 3 orthogonal accelerometers and 3 orthogonal magnetometers.
An accelerometer will measure the component of the earth’s gravitational field along the axis in which it is oriented. It works on the “force-balance” principle. Atest mass is suspended from a quartz hinge which restricts any movement to along one axis only. As the mass tends to move due to gravity acting along that axis, its central position is maintained by an opposing electromagnetic force.
The larger the gravitational force, the larger the pick-up current required to oppose it. The voltage drop over a resistor in the pickup circuit is measured, and this is directly related to the gravitational component. Depending on the orientation of the BHA the reading on each accelerometer will be different. From these 3 components, the angle of inclination and tool face can be calculated.
Measurement while drilling (MWD) Magnetometer
A magnetometer will measure the component of the earth’s magnetic field along 1 axis. If a wire is wrapped around a soft iron core and then placed in a magnetic field, the current induced in the pick-up circuit will vary depending on the angle at which the toroid is placed. Therefore the size of current is related to the direction of the coil with respect to the direction of magnetic field. As with the accelerometer the voltage is measured across a resistor in the pick-up circuit of the magnetometer. The voltages read at each magnetometer can then be used to calculate azimuth.
Sensors Surveying & Calculations
In the following equations a, b, c, x, y, z refer to the accelerometer and magnetometer readings with axes as shown.
Inclination – the angle between C accelerometer and vertical. Looking at a verticalion cross-section
Toolface – the angle between the high side and B accelerometer. Looking down the tool along the C axis:
Azimuth – the angle between the Z-axis and magnetic North, when projected onto the horizontal plane. Looking in the horizontal plane we define 2 vectors V1 and V2 where V1 lies along the tool axis.
Notice that accelerometer readings are also used in the calculation of azimuth. If there is any mistake in the accelerometer readings, therefore, inclination, toolface, and azimuth will all be wrong. Since we are relying on the magnetometers responding only to the earth’s magnetic field any local magnetic effects from the drill string must be isolated. There must be enough non-magnetic drill collars above and below the sensors to stop any such interference. As a result of this, the sensors will be operating 40′ – 80′ behind the bit (the exact distance must be known before the tool is run).
MWD (Measurement While Driling) Normal Surveying Routine
The usual practice in taking a survey is to drill to the end of string and make the connection. Run in the hole and tag bottom. Pick up 5′-10′ and keep pipe steady for 2 minutes (this allows survey data to be stored). Re-start drilling and survey data is transmitted to the surface. In some tools the transmission is initiated by rotation, in others, it senses pump pressure. During steering (Check also Rotary steerable drilling system) run, where a drilling mud motor is being used an update of toolface, is usually transmitted every minute. This is of great value to the directional driller as he monitors the progress of the well.
Accuracy of MWD Surveys
MWD companies quote slightly different figures for accuracy but generally within the following limits:
These figures compare favorably with Magnetic single shot Survey accuracies and MWD offers the advantage of being able to repeat surveys at the same depth with little loss in rig time
Measurement While Drilling MWD Surface System In oil and gas wells
All MWD systems have a pressure transducer connected to the standpipe manifold. This transducer must be sensitive enough to detect small pressure changes (50-100 psi) occurring for only ±/≤ sec.
The series of pulses must then be decoded and processed to give the required information. The simplest surface system is that used by Teleco (positive pulse). This has a microprocessor included in the downhole tool so that only numerical values of azimuth inclination and tool face need to be transmitted to the surface.
A simple binary code is used whereby a pulse detected within a certain time period = 1, no pulse detected = 0. The binary number is then converted to a decimal number for the final result.
The equipment necessary to do this can easily be installed in the driller’s dog house. In other systems, only the raw data is sent to the surface, in which case more sophisticated equipment is needed (electronic filters, computers, etc.).
This equipment is usually housed in a special cabin or in the mud logging unit. Since this cabin may be located some distance away, the survey results are relayed to a rig floor display unit where the directional driller can see them. Formation evaluation logs require plotting facilities which are also housed in the cabin.
TRANSMISSION AND CONTROL SYSTEMS
There is a wide variation in the design of these electronic packages, and they are being continually upgraded. The voltages at each sensor must be read and stored in the memory until the tool is ready to transmit.
The measurement while drilling MWD control system must coordinate the acquisition, storage, and transmission of this data. Since there is no electrical on/off switch controlling the system from the surface the tool must react to some physical change (e.g. detecting an increase in pump pressure).
Once the transmission is initiated the data is sent to the surface via the mud column as a series of pulses. In some systems, it is the presence or absence of a pulse that carries the information, in others, it is the time interval between pulses. Although these pulses travel at around 4000 ft/sec several pulses may be necessary to transmit one number. With more sensors and more data to transmit the control system becomes a critical factor (e.g. valuable GR signals may be lost while the tool is sending directional data).
There is also the problem of collecting vast amounts of data, but not being able to transmit it quickly enough. Transmission speeds of up to 0.8 bits per second are available. Survey data words typically consist of 10 bits, and formation data words consist of 11 bits.
GR Log & Measurement While Drilling
The GR log is a long-established part of the formation evaluation. Gamma rays in the formation are emitted mainly by radioactive isotopes of Potassium, Thorium, and Uranium. These elements occur primarily in shales, and so the GR log is a good shale indicator.
Engineering Applications of the GR log In MWD
There has been a big increase in the use of GR logs run in combination with the MWD directional tool.
Since any change in lithology must be known as quickly as possible the GR sensor should be placed as near the bit as possible, below the directional sensors. Running an MWD GR log has the added problems of rigging up a depth tracking system.
The type of sensors used to detect gamma rays must be both robust and efficient. The most robust sensor is the Geiger-Muller tube, but unfortunately, it will only detect a small percentage of the rays being emitted by the formation. A more sensitive but less rugged sensor is the Scintillation counter. Both types are used in MWD GR tools but the scintillation counter is the more popular.
Comparisson Between Measurement while drilling MWD & WL Logs:
It is interesting to compare GR and Resistivity logs from an MWD tool with those obtained from wireline logging after the oil or gas well has been drilled. Several points must be borne in mind when making these comparisons:
(i) The logging speeds are very different (wireline @ 1800 ft/h MWD @ 10 -100 ft/h). The resolution of the two logs will therefore be affected.
(ii) Wellbore conditions may be different since the MWD log was made, e.g. cavings
(iii) Measurement while drilling MWD log is made through a drill collar, so the attenuation of gamma rays will be greater.
(iv) Central position of the sensors may be different, especially in high angled holes.
Directional sensors and GR sensors are well established for MWD use. More sensors are being developed and the term LWD – Logging Whilst Drilling is now used to describe these tools.