SYSTEMS THEORY: MEASUREMENT-WHILE-DRILLING TOOLS (MWD) AND LOGGING-WHILE-DRILLING (LWD)

SYSTEMS THEORY: MEASUREMENT-WHILE-DRILLING TOOLS (MWD) AND LOGGING-WHILE-DRILLING (LWD)

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Systems Theory: Measurement-While-Drilling Tools (MWD) And Logging-While-Drilling (LWD)

Executive Summary

This paper provides an evaluation of the role and functionality of Measurement-While-Drilling Tools (MWD) And Logging-While-Drilling (LWD) tools, which are utilized in the conduct of surveys on the nature of oil wells. These are critical tools given that they provide sensitive information on issues such as safety, quality, chemical and well composition and presence of adequate commodities for subsequent and related decision-making. The paper will provide a description of the functionality and processes utilized in collection of samples. It provides a detailed view of the components and how they function together to collect samples from oil wells and delivery of such to the surface for subsequent decision-making.

 

 

 

 

 

 

 

 

 

 

 

 

 

Systems Theory: Measurement-While-Drilling Tools (MWD) And Logging-While-Drilling (LWD)

Introduction

The proposed survey system to be utilized in oil drilling system functions in a similar manner to all other rigs, which have identical components. The difference is primary based on the location (either marine or land) systems which are used to deliver oil onto the surface for subsequent processing. The rigs are divided into either land or marine rigs. The land rigs are further divided into helicopter portable rigs, light land rigs and heavy land rigs. Marine rigs are divided into either floating or bottom supported rigs. The floating rigs are further classified as semi-submersible, drill ship and drilling barge, and whereas the bottom supported rigs are divided into platform, submersible, and jack up rigs (National Research Council (U.S.), 2011, 18).

With growing demand and consumption of oil and natural gas around the world, there is need to ensure that oil and gas production systems are reliable and safe to ensure optimized capacities. In addition, environmental conditions of a field usually influence engineering works and facility design. The proposed measurement while drilling (MWD) system provides a means of collection and transmission of down-hole information to the surface without any interruptions to normal operations in the drilling processes.

Conceptual System Design

Offshore and deep-sea projects are usually complex and marked by numerous challenges and risks. The high level of risk-involved ranges from issues such public relations, procurement construction and political risks inherent in similar natured projects (National Research Council (U.S.), 2011, 23). The rotary drilling method is usually utilised in the development of deep wells. It performs functions identified as follows:

  1. Penetration operations: the drilling system usually breaks down the rocks located at the bottom hole through rotation and weight. The rotation force of the rotary table is subsequently transmitted via a drill stem and onto the bit. A section of the weight of the drill collars is exerted on the bit as the bit weight is utilised to induce movement against the rock.
  2. Hoisting operations: the drill stem that has a bit is moved by the hoisting system. The casing present is also moved by the hoisting system.
  • Conditioning and circulation of the existing drilling fluids through a circulation system
  1. Prevention of the formation fluids from access the well bore and controlling their movements

Drilling fluids assume a critical role in the functioning of a drilling system. They are conventionally referred to as muds and play a critical role in rotary drilling. Their functions are outlined as follows:

  1. Removal of the cuttings from the hole to the surface. Cuttings are usually separated from the mud within the shale shaker.
  2. Control of the existing hydraulic pressure within the hole through adjustments of the density of the mud to ensure prevention of possible collapse of the borehole wall.
  • Cooling and lubrication of the drill stem and bit.

The drilling fluids are made up of base fluids, chemicals, clay minerals and inert solids. In conventional rotary drilling systems, the rotary table rotates the drill stem (American Society of Mechanical Engineers, 2005, 14). on the other hand the down hole motor  and top drive device are utilised in the rotation of bit in horizontal and direction well drilling as well as undertaking improvements in operations within vertical well drilling. In addition, technological advancements have brought about new techniques such as measurement-while-drilling (MWD) tools as well as logging-while-drilling (LWD) tools, which have been effective in provision of real-time acquisition of down-hole information (National Research Council (U.S.), 2011, 29).

The measurement-while-drilling (MWD) system provides a variety of information such as drilling parameters, directional information and geological data such as resistivity logs and gamma arrays. Modern day tools are described as Logging While Drilling (LWD) tools. This is because of the addition of new sensors to the transmission systems to enhance collection of information in increasingly competitive environments. Data is transmitted through binary code and as a series of pressure pulses within the drill string. This process of decoding and coding of data is classified as:

  1. Negative Mud Pulse Telemetry

In all drilling systems, fluid movement is necessary within the drill string. Within this system, a valve tool opens and provides for movement of small volume of mud to escape into the annulus from the drill string.

  1. Positive Mud Pulse

Within the positive mud pulse system, the valve inside the tool closes partially, provide a temporary growth in the pressure within the standpipe.

  • Frequency Modulation (Mud siren)

A standing wave is established in the mud column through the rotation of a slotted disc. This process can be revered, whereas the data is moved thorough a serious of phase shifts. Numerous tools are used and possess capabilities of recoding down-hole information for subsequent retrieval and evaluation at the surface. However, this may undermine the basic principal of accessing data in real-time. It provides the operators with means of gathering significant volumes of data, which is typical to petrophysical data. This is effective as it eliminates expensive electric wireline logging operations.

Preliminary System Design

The following is an outline of the MWD system provided in Figure 1.

Figure 1: MWD System

Figure 2: Mud Pulse Telemetry Systems

Tool sizes 1.75″ OD – 9.5″ OD
Maximum Temperature ~200°C
Maximum Pressure 20,000 psi
Power Sources Lithium batteries(up to 800hrs op. time) /Turbine
Telemetry Type Positive pulse/Negative pulse/Siren/EM/Down-hole recording
Sensors Directional (MTF/GTF)

Petrophysical (Gamma ray/Resistivity/Neutron)

Drilling (Vibration/DWOB/Torque/Temp./Ann. pressure)

Power Sources

All power necessary for operation of the MWD is provided down-hole given that no wire lines connection to the surface for power is viable. This demands that a turbine-alternator or battery pack is installed as a component of the MWD tool. The use of turbine power is a standard method that is utilized in the frequency modulation and positive pulse tools. Given that less power is demanded in the negative pulse system, batteries can be utilized. On the other hand, the addition of new sensors and higher data demands, batteries can be replaced using turbines within the negative pulse systems.

Turbines possess numerous benefits when compared with batteries. However they are prone to mechanical failures. In addition, filter screens are utilized to prevent the entry of debris within the mud from causing any damage to the turbine.

Advantages and Disadvantages of turbines and Battery Packs

Power Source Advantages Disadvantages
Batteries (lithium) Compact Temperature limits and time limits
Turbine Higher power output than batteries , unlimited operating time Demands the need for filters

Figure 3: Advantages and Disadvantages of Power Sources

Detail Design and Development

MWD-Directional Tools

MWD systems utilize similar directional sensors in the calculation of azimuth, inclination, and tool face. A sensor package is composed of 3 orthogonal magnetometers and 3 orthogonal accelerometers. The accelerometer measures components relative to the earth’s gravitational field along in its axis. It utilizes the basic force-balance principle. An identified test mast is hanged on quartz hinge that serves to restrict any form of movement within the axis. The mass tends to move because of gravity acting within the axis. The central position is usually sustained by the presence of an opposing electromagnetic force. The mass usually moves because of gravity acting within the axis. A high gravitational force demands the need for significant pickup current to oppose the force. Decline in voltage in the resistor within the pickup circuit is established which is related to the gravitational force. Relative to the orientation of the BHA, readings within the accelerometer vary. Using the three components, the tool face and angles of inclination can be determined. The magnetometer is utilized to provide measure of the inherent component of the magnetic field of the earth along its axis.

Normal Surveying Routine

Common practice in conducting surveys is to drill with the aim of kellying down and establishment of a connection.

Accuracy in MWD Surveys

MWD entities usually provide differing figures that have slight variations for accuracy. They are usually within the following identified limits:

Inclination Azimuth Tool-Face
+/- 0.25° +/- 1.50° +/- 3.00°

MWD is preferable because it provides avenues for repeat surveys at similar depths with minimal loss of rig time.

Gamma Ray Tools

Gamma Ray (GR) logs are a critical and long running section of formation evaluation. The rays are usually emitted by radioactive isotopes of elements such as uranium, thorium, and potassium. The identified elements usually take place in shales, which makes GR an effective indicator of shales. The GR has critical engineering applications. This has promoted increase in the utilization of GR logs together with MWD directional tools.

Generic Application Specific Application
Geological
  1. provides immediate indications of changes in lithology composition
  2. picking formation tops
  3. coring points
  4. Vclay (shaliness) indicator
  5. differentiation between cavings and cuttings
  6. Correlation between wells
Engineering
  1. Selection of casing points
  2. Identification of troublesome formations
  3. Identification of drilling problems and challenges
  4. Conducting checks before EWL

Figure 4: Applications of MWD

Conclusion

As a result of growing demand and consumption of oil and natural gas around the world, it is the imperative of oil and gas production entities to ensure reliable and safe processes. In addition, environmental conditions of a field usually influence engineering works and facility design. The proposed measurement while drilling system provides a means of collection and transmission of down-hole information to the surface without any interruptions to normal operations in the drilling processes. The emergence of such technologies provides drillers with avenues for establishing viability, reliability, and collection of critical information for subsequent decision-making.

 

 

Bibliography

American Society of Mechanical Engineers, 2005, Drilling fluids processing handbook, Elsevier, Amsterdam.

Chin, W. C., 2014, Measurement while drilling (MWD): Signal analysis, optimization, and design, Scrivener Publishing/Wiley, New Jersey.

Guo, B., & Liu, G., 2011, Applied drilling circulation systems: Hydraulics, calculations, and models, Gulf Professional Pub, Burlington.

Gulf Publishing Company & American Association of Drilling Engineers, 1999, Shale shakers and drilling fluid systems: Techniques and technology for improving solids control management, Butterworth-Heinemann, Woburn.

Leonov, E. G., & Isaev, V. I., 2010, Applied hydroaeromechanics in oil and gas drilling, Wiley Hoboken.

National Research Council (U.S.), 2011, Scientific ocean drilling: Accomplishments and challenges, National Academies Press, Washington.

National Energy Technology Laboratory (U.S.), David, G., Robert, R., Robert, S., John, F., Technology International Incorporated, 2008, Advanced Seismic While Drilling System. United States Dept. of Energy, Washington.

Rehm, B., 2008, Managed pressure drilling, Gulf, Pub Houston.

Rehm, B., 2012, Underbalanced drilling: Limits and extremes, Gulf Publishing Company, Houston.

Sandia National Laboratories., United States., United States., Williams, C. V., Lockwood, G. J., Normann, R. A., Bishop, L. B., Floran, R. J. 1995, Environmental Measurement-While-Drilling system for real-time field screening of contaminants,  United States Dept. of Energy, Washington.

 

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