REAL-TIME DATA LOGGING WHILE DRILLING THROUGH MUD PULSE TELEMETRY

ABSTRACT

This study investigated the influence of mud channel characteristics on real-time data logging while drilling. Drill string bore filled with flowing mud as the data transmission medium in mud pulse telemetry can have significant changes in its properties as a result of mud type (water- based mud or oil-based mud) which selection depends on the formation being drilled and depth penetrated. Logging data are transmitted to the surface modulated onto pressure pulses which amplitudes are greatly attenuated as drilling depth increases. This attenuation in severe cases can make logging data in the pressure pulse not to be decodable by the transducers. MATLAB Simulink communication blockset was used to model the arrival probability of logging data based on the time of arrival at the surface while drilling using Poisson distribution function. The rate of penetration and data acquisition sample rate were simulated by varying the slope of the ramp function for depth and intensity of data read block for data capture. It was observed that the transmitted pressure pulses propagate to the surface unaffected by depth except in amplitude. Also, lower frequencies were observed to be better carriers of signal in mud pulse telemetry than higher frequencies because attenuation in high frequency components is relatively higher than the low frequency components. It was further discovered that influences of both drilling fluid viscosity and volume fraction of gas in drilling fluids on transmitted pulse amplitude were more notable than that of internal diameter of drillpipe.

CHAPTER ONE INTRODUCTION

1.0      BACKGROUND OF THE STUDY

Logging While Drilling – LWD refers to the technique of making downhole measurements of borehole geometry (inclination and azimuth), drilling system orientation (tool face), formation properties (resistivity, natural gamma ray, porosity etc), and mechanical properties of the drilling process (torque, weight on bit WOB, vibration, bit whirl and bounce etc) using sensors located in the bottom hole assembly (BHA) adjacent to the drill bit. That is, LWD is wireline-quality formation measurements made while drilling. This logging is made during drilling and can be recorded  downhole  and/or  transmitted  to  the  surface  (real-time  LWD)  through one  of the following telemetry systems;

    Mud pulse telemetry

    Electromagnetic wave telemetry

    Wired drill pipes, and

    Acoustic wave telemetry.

Traditionally, measurement while drilling (MWD) has fulfilled the role of providing wellbore inclination and  azimuth,  information  in  order  to  maintain  directional  control in  real  time. However, from 1980s to late 1990s, formation evaluation MWD (i.e logging while drilling- LWD) has paralleled and surpassed other aspects of drilling technology to the extent that it is now possible to replace very sophisticated wireline logs with real-time and memory stored logging while drilling [1].

Logging While Drilling (LWD) can be defined as the evaluation of physical properties of the formation being drilled, usually including pressure, temperature, and wellbore trajectory in three dimensional space while extending a wellbore depth. LWD is now a standard practice in drilling directional wells, where the tool cost is offset by rig time and wellbore stability considerations if other tools are used, especially in offshore locations. This helps to kick off the well and build the deviation until targeted angle is achieved without stopping the drilling process, thereby gaining time that would have been lost to tripping operations and running wireline logging tools.

1.1      FUNDAMENTAL OF LWD

As a system developed to perform drilling related logging downhole and transmit information obtained about the drilling process and formation extended to the surface in real-time while drilling a well; LWD is a full packaged rugged communication system. It is made up of the following:

    Data acquisition

    Data processing and

    Data transmission and reception.

LWD tools are conveyed downhole as part of bottom hole assembly (BHA). The tools are either contained inside drill collar (sonde/probe type) or built into the collars themselves (collar based type). Measurement and logging system can take several measurements, like natural gamma ray content of the formation, directional survey, tool orientation, borehole temperature and pressure, vibration, shocks, torque, weight on bit etc. The measurements are made downhole, stored in solid state memory for some time and later transmitted to the surface [5]. Data transmission methods vary from company to  company,  but  usually  involve  digitally encoding  data  and transmitting to the surface as pressure pulses in the mud column inside drill string. Some LWD systems have the ability to store the logging data for later retrieval with wireline or when the tools is tripped out of hole if the data transmission link fails. It is important to note that MWD tools which measure formation parameters (resistivity, neutron porosity, sonic velocity, gamma ray etc) are referred to as logging while drilling-LWD tools [5]. LWD tools use similar data storage and transmission systems. However, some have more solid state memory to provide higher resolution logs after the tool is tripped out than is possible with relatively low bandwidth mud pulse data transmission channel.

1.1.1   DATA ACQUISITION

The oil industry trend to deep formation exploration has increased technological challenges to drill [2]. Drilling of oil well is a complex operation which requires knowledge of adequate information about the drilling tool conditions downhole as well as the formation being drilled. This information is obtained at the surface either by logging after drilling a hole section or logging while drilling. Sensors located in the BHA near the drill bit downhole take the measurements. The sensors are tri-axial accelerometers, tri-axial magnetometers, gamma ray sensor, thermometers, neutron porosity and density sensors, etc.

Data acquisition for LWD can either be of two methods;

    Recorded

    Real – time

LWD recorded data transmission is obtained by sampling the downhole sensors, storing each data point in downhole memory, and retrieving the data when the tool string is tripped out of hole after each bit run [6]. Each data point is associated with a time from the downhole sensor clock. This is why synchronization of both surface and downhole clocks at the start of the bit run is critical. Depth monitoring versus time is performed on the surface during drilling while the time component of both depth and data files are matched to create sensor data versus depth information which is used to create logs.

Real-time LWD data is obtained by sampling the downhole sensors, encoding the data into binary formats, and transmitting the data through some telemetry medium to the surface whilst drilling [6]. The transmission is decoded at the surface, processed into a sensor data value and associated with depth as in recorded data acquisition mode to create real – time logs.

Directional surveying is the method of obtaining the measurements needed to calculate and plot in three dimensions (3D) the wellbore path. Measurement and Logging While Drilling tool uses primarily three accelerometers and three magnetometers to measure data. The environmental data measured by the accelerometers are total gravity field (TGF) and  magnetic dip angle (Mdip). The environmental data measured by the magnetometers are total magnetic field (TMF) and magnetic dip angle (Mdip). Therefore, both accelerometers and magnetometers are needed to obtain magnetic dip angle. The sensors, downhole, gather data and send to the microprocessor.

1.1.2   DATA PROCESSING

Since the data acquired downhole is used at the surface to monitor drilling processes, there has to be data processing system to enable credible transmission to the surface equipment.

Advances in downhole sensing for drilling optimization and formation evaluation have continually placed  heavy  demands  on  equipment  to  provide  higher  data  rate  with  reliable information detection from greater depths. Also generated data, downhole, are often large to be transmitted through a telemetry channel of limited bandwidth. The microprocessor in the LWD package processes, encodes, and sends the coded binary data to the transmitter.

1.1.3   DATA TRANSMISSION AND RECEPTION

The information gathered by the sensors at the BHA can be transmitted to the surface while drilling by several means [8]. These include mud pulses, wired drill pipe, acoustic waves and electromagnetic waves telemetry. Some of them are in use on commercial basis, while others are still under development. The most commonly used method is mud pulse telemetry [8]. Hence this research focuses on transmission challenges of mud pulse telemetry in order to proffer a reliable way of obtaining precise real-time LWD data transmission.

1.2      MUD PULSE TELEMETRY

The word “telemetry” basically amounts to accessing and transmitting data to and from remote locations. Mud pulse telemetry system is part of the LWD tool which is usually placed in the BHA of the drill string, as close as possible to the drill bit [1]. LWD mud pulse telemetry system is an electromechanical device which makes the measurements and transmits the data to the surface  using  positive,  negative  or  continuous wave  pulser  to  generate controlled  pressure fluctuations which carry the data to the surface within the drilling mud stream inside the drill string [1, 8]. The measured downhole data in binary formats are used to actuate the pulser. The

pulser will restrict, vent or continuously restrict and release the circulating drilling mud in drill string in a certain pattern depending on pulser design [8]. The pulses are as in figure 1.1 below.

(a)  Positive mud pulse  telemetry

(b)       Negative mud pulse telemetry

Fig 1.1 a, b, c Mud Pulse Telemetry Pulses

(C)      Continuous wave mud pulse telemetry

Subsequently, the pressure of the drilling mud column will vary accordingly and the coded pressure pulses will propagate through the drilling mud to the surface at roughly the speed of sound in mud (4000ft/s – 5000ft/s or 1200m/s – 1500m/s) where they are  intercepted and interpreted by stand pipe pressure transducers which continuously monitor the drilling fluid pressure. These data are sent to a sophisticated decoder that de-convolutes the encoded data from downhole. The entire process is virtually instantaneous, enabling real-time transmission for well drilling optimization. Mud pulse LWD uses positive pulse, negative pulse or carrier wave (mud siren) schemes to transmit measured parameters from downhole to surface in real-time to aid information evaluation, directional control, drilling efficiency, and drilling safety [1]. The downhole information measured by the sensors is sent to the microprocessor which processes it and  sends  it  to  the  surface  by  activating  the  transmitter.  Mud  pulse  telemetry  involves modulation of the flowing mud in the drill string by means of mechanical or rotary valve in the LWD tool. The data is decoded and depth correlated at the surface [1]. The true value of LWD is the provision of real-time dynamics and directional drilling data augmented by real-time information evaluation measurements, which are considered equivalent and often time more superior to the sophisticated wireline logs. Figure 1.2 below shows the schematics of the system.

Fig 1.2 Typical Mud Pulse Telemetry (redrawn and modified) (Tennent and Fitzgerald, 1997; Klotzb  et al, 2008; Mohammed Ali Namuq, 2013)

1.3      PROBLEM STATEMENT

Drilling of an oil well is a complex operation which requires adequate information about the drilling tools as well as the formation being drilled. This information is obtained either by logging after drilling a section of the well (wire-line logging) or by logging whilst drilling (real time logging). Real-time data logging while drilling of an oil well has a lot of challenges in practical field operations. Some of these challenges are difficult to represent because they vary from one rig location to another.   Inefficiencies of the mud pumps which provide the entire hydraulic power for the mud pulsing is an example of this kind of challenge. Realistically, the flowing mud-filled drill string which is the data transmission medium for mud pulse telemetry is an erasure channel with very limited feedback which erasure degree depends on its constituents. Generated data are often times large to be transmitted through a telemetry channel of limited bandwidth. Attenuation index of the surface pressure pulse amplitude increases as the well

deepens. This work verifies the effects the different constituents of the drilling mud channel have on signal transmission.

1.4      OBJECTIVE OF THE STUDY

This study is designed to determine the arrival probability of the transmitted mud pressure pulse based on the time of arrival on the surface site in spite of all the challenges highlighted in the problem statement. Specifically, it proposes the use of Poisson probability distribution function to model the arrival rate of real-time logging data transmission utilizing mud pulse telemetry. Numerical simulation of the model will be carried out with MATLAB software, also to verify the influences of the various constituents of the drilling fluid on signal transmission. The drilling fluid studied in this work is water-based mud (WBM).

1.5      PROJECT JUSTIFICATION

The logging while drilling provides real-time information about wellbore condition and surrounding formation properties. Transmitting this information to the surface through mud pulse telemetry is the most viable in the industry for now. The principal advantage of mud pulsing over systems employing electromagnetic, hard-wired drill pipe, and other communication methods is that  it  can  be  accomplished  in  cost  effective  manner  while  drilling  through  all  types  of formations, without interfering with other rig operations [2]. Knowledge of the conditions of the wellbore whilst drilling reduces the total drilling cost by cutting down the number of days a rig drills  a  particular  well.  It  equally  reduces  nonproductive  time  that  could  result  from unprecedented environmental accidents like uncontrolled kicks and blowouts. Logging data like well inclination and azimuth which do not change rapidly with time help the driller to steer the well to the target reservoir.

1.6      SCOPE OF THE STUDY

Data transmission through mud pulse telemetry for measurement and logging while drilling will be extensively reviewed. The transmission medium is flowing mud-filled drill string which is described as an erasure channel. This study presents a suitable numerical simulation and analysis of data transmission characteristics  in drilling  mud  channels.  It  is  limited  to  developing  a conceptual model with Poisson’s distribution function for determining the arrival probability of transmitted mud pressure wave, based on the time of arrival at the surface site from bottom-hole of the wellbore amidst all the drilling interferences using standard simulink communications blockset in MATLAB. Also, it will verify the influences of the mud channel parameters and signal carrier frequency on data transmission.

1.7      ORGANIZATION OF THE DISSERTATION

The chapter One of this study introduced real-time logging while drilling followed by literature review, methodology, simulation and results, and summarizing with conclusion and recommendations in chapters two, three, four, and five respectively.

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