Amanda Barlow


Blog Post created by Amanda Barlow on Jan 31, 2017

Part 8


The third party service providers make up about half of the workforce on an offshore rig. With so many hi-tech and specialized operations being performed at all stages of the drilling operations it’s imperative that experts in their field perform these tasks. The first group of people we’ll look at are the “mudloggers” who’s job it is to monitor the drilling operations from the time they “spud” the well to the time they secure the well after drilling and testing has been completed.




“Mudlogger” is the generic term used to describe the field specialists who monitor the well and also collect samples for the geologist. The career progression for a “mudlogger” is to generally start as a sample catcher while they learn about the drilling operations, then progress to a mudlogger and with further experience, become a data engineer.


Sample Catchers

Dedicated sample catchers aren’t always part of the team but they often get “thrown in” as a complimentary part of the mudlogging services. They don’t need to have any prior experience in working offshore or as a mudlogger, so it’s a very good entry level job and is generally the starting position for graduate geologists who wish to work offshore. Although you don’t need to be a geologist to be a sample catcher, most of them will be and will go on to get trained as a mudlogger.


Sample catching is without a doubt the least glamorous and lowest paid of all jobs on the rig…but you have to start somewhere! The role of a sample catcher is to provide the most basic geological data acquisition on the rig and to assist with all general activities when possible. The main duties of the sample catcher are:

  •  Ensure that representative geologic samples are caught throughout the drilling or reaming phases of the well program. This is done by collecting cuttings (drilled rock) samples, from the proper “lagged” (explained below) depths and at the proper intervals as required for evaluation. These samples are collected off the shale shakers, screened and washed, divided into correct portions, and packed into sets for the Client, partners and government agencies. They may also have to assist in core recovery and packaging as required.
  •  Prepare a clean “cuttings” sample on a sample tray for the wellsite geologist and mudlogger, who will then examine it under the microscope and describe the lithology of the drilled formation.
  •  Assist mudloggers and data engineers to perform regular and frequent calibration checks of instruments, perform normal routine maintenance of sensors and other equipment and also assist logging crew with rig-up/rig-down procedures.


shale shaker


The sample catcher reports directly to the mudlogging crew who will ensure his duties are performed correctly. This may include on-the-job training as required. They work out of the mudlogging unit, which is always close to the shale shakers and these are generally one or two levels below the drillfloor. The shale shakers are vibrating screens that separate the drilling fluid from the drilled rock cuttings. The “shaker house” is a very noisy place and double hearing protection must always be worn. There will be multiple shakers to accommodate the large volume of cuttings that can be produced when the drilling rate of penetration is high (i.e. they are drilling fast!). It’s a very “dirty” job and multiple layers of personal protective equipment need to be worn to prevent skin contact with the drilling mud, which can cause serious skin inflammation.


Mudloggers and Data Engineers (DE)

Mudloggers and data engineers are responsible for gathering, processing and monitoring information pertaining to drilling operations. They don’t only collect data using specialist data acquisition techniques – they also collect oil samples and detect gases using state-of-the-art equipment.

The information amassed by these guys is analyzed, logged and then communicated to the team that is responsible for the physical drilling of the well. Without the help of the mudlogger, the drilling operations would be less efficient, less cost-effective and much more dangerous. The mudlogger is vital for preventing hazardous situations, such as well blowouts. They also provide vital assistance to wellsite geologists and write detailed reports based on the data that is collected. Being an entry-level position, employees will be given a mixture of ‘on-the-job’ training and expert in-house training courses, which cover different aspects of drilling operations. A major part of the training will focus on the use of specialist computer software.


Typically, you will need a degree in geology to start a career as a mudlogger. However, candidates with degrees in physics, geochemistry, chemistry, environmental geoscience, maths or engineering may also be accepted.


Along with the sample catchers and data engineers, the mudloggers work out of the muddlogging unit, which is a pressurized sea container-type of office, which is positioned close to the drillfloor and shaker house. The unit will have an air-lock compartment when you first enter it so as to maintain the positive pressure within the unit whenever somebody leaves or enters the unit. This is the main control room for monitoring the drilling operations and is full of sophisticated and delicate equipment and computer systems. Positive pressure needs to be maintained to ensure the air pressure inside the container is higher than that of the outside area to prevent contamination of sensitive monitoring equipment – and also to ensure the safety of the crew working inside the unit should the outside air become contaminated through uncontrolled releases of hydrocarbons from the well.


mudlogging unit


One of the most important tasks of the mudlogger is to oversee the collection of not only geological samples but also mud and gas samples from the well during drilling operations. To be able to do this accurately they have to know the exact “lag time” (or “bottoms-up time”) that it will take for the drilled cuttings or mud and gas to arrive at the surface after being drilled and circulated up the outside of the drillhole (annulus) while suspended in the drilling mud. The lag time may be a few minutes in a shallow hole or as much as several hours in deep wells with low mud flow rates. To be able to work this time out accurately there are many factors that have to be taken into consideration. The lag time depends on:

  • the annular volume fluid
  • flow rate, which in turn require knowledge of:
  • dimensions (internal diameter (ID) and outside diameter (OD)) of surface equipment, drill string tubulars and casing and riser.
  • mud pump output per stroke, pumping rate and efficiency.


While the computer’s software will work this out automatically, the calculated value may be incorrect however, if the operator has entered erroneous or incomplete values for the pipe or hole dimensions, or if the hole is badly washed out. This has to be monitored very carefully to avoid catching mud, gas and cuttings samples at incorrect depths.



The mudloggers and DE’s monitor the drilling operations via a series of sensors that are placed at various locations around the drillfloor, pit room and shaker house. The main drilling and mud parameters that are recorded are: hook movement, weight on hook, standpipe pressure, wellhead pressure, rotary torque, pump strokes, RPM, mud pit levels, mud density, mud temperature, mud resistivity and mud flow. These parameters are monitored in real time and any deviances from the expected normal values must be immediately reported to the driller. The DE will view and monitor all the drilling parameters on a screen as shown below.


mud logger drilling screen


The five most important monitoring tasks that the mudlogger and DE must watch out for are:

  • Rate of penetration increase, which could indicate they have drilled into a reservoir formation
  • Mud pit volume gain or loss, which could indicate the well is taking a kick, or losing fluid into the formation
  • Mud flow rate change
  • Mud density variation
  • Indication of oil or gas.


The mudlogging unit is a very confined workplace and there may be up to several people working in there at any one time, especially if it’s a “combo” unit, which houses the mudloggers, MWD engineers and possibly also the directional driller. Generally the same service provider company performs all of these roles so it is quite common for data engineers to progress into a role as an LWD/MWD engineer. Other common career progressions for mudloggers/data engineers are as a wellsite geologist or drilling fluids engineer (mud engineer).


crew change in ML unit


The complete list of responsibilities of the mudloggers is too exhaustive to detail in this article but the above-mentioned roles are the main ones. Like most jobs on the rig, daily reports are a big part of the data engineer’s responsibilities. The mudloggers report directly to the wellsite geologist, who is generally working in the mudlogging unit alongside them. Because the mudloggers are required to monitor the drilling operations from the commencement of drilling they will always be employed on a permanent rotating roster, which is generally 4-weeks on, 4-weeks off.



MWD / LWD Engineers and Directional Drillers


The terms Measurement While Drilling (MWD), and Logging While Drilling (LWD) are not used consistently throughout the industry. Although, these terms are related, the term MWD refers to directional-drilling measurements, while LWD refers to measurements concerning the geological formation made while drilling (also referred to as Formation Evaluation While Drilling (FEWD)).


Measurement While Drilling (MWD)

MWD typically concerns measurement taken of the wellbore inclination from vertical, and also magnetic direction from north. Using basic trigonometry, a three-dimensional plot of the path of the well can be produced. Essentially, an MWD engineer measures the trajectory of the hole as it is drilled (for example, data updates arrive and are processed every few seconds or faster). This information is then used to drill in a pre-planned direction into the formation, which contains the oil, gas, water or condensate.


An MWD downhole tool is also "high-sided" with the bottom hole drilling assembly, enabling the wellbore to be steered in a chosen direction in 3D space known as directional drilling. Directional drillers rely on receiving accurate, quality tested data from the MWD engineer to allow them to keep the well safely on the planned trajectory. MWD tools are generally capable of taking directional surveys in real time. The tool uses accelerometers and magnetometers to measure the inclination and azimuth of the wellbore at that location, and they then transmit that information to the surface. With a series of surveys, measurements of inclination, azimuth, and tool face, at appropriate intervals (commonly every 30ft or 10m), the location of the wellbore can be calculated.


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


Use of this information can allow the operator to drill the well more efficiently, and to ensure that the MWD tool and any other downhole tools, such as a mud motor, rotary steerable systems, and LWD tools, are operated within their technical specifications to prevent tool failure. This information is also valuable to geologists responsible for the well information about the formation that is being drilled.


Logging While Drilling (LWD) tools and Formation Evaluation

The measurement of formation properties during the drilling of the hole through the use of tools integrated into the “bottom hole assembly” (BHA) can be expensive but has the advantage of measuring properties of a formation before drilling fluids invade deeply. Further, many wellbores prove to be difficult or even impossible to measure with conventional wireline tools, especially highly deviated wells. In these situations, the LWD measurement ensures that some measurement of the subsurface is captured in the event that wireline operations are not possible. Below is an example of an LWD/MWD bottom hole assembly:




LWD tools take measurements of formation properties. At the surface, these measurements are assembled into a pictorial data log for fast and instant interpretation of the formation. LWD tools are able to measure a suite of geological characteristics including density, porosity, resistivity, acoustic-caliper, inclination at the drill bit (NBI), magnetic resonance and formation pressure. The MWD tool allows these measurements to be taken and evaluated while the well is being drilled. This makes it possible to perform geosteering or directional drilling based on measured formation properties, rather than simply drilling into a preset target. Image logs are also possible, and there is an increase in demand for formation pressure tests and collection of fluid samples that can be obtained by increasingly sophisticated LWD tools. Until recent years, pressure and fluid sampling could only be done when drilling was completed and wireline logs were run, but with the advances in LWD technology it is now becoming more routine to perform these tests while drilling the well so important drilling decisions can be made on the fly.


There are many different LWD tools available and every logging company has their own proprietary hardware and software. Tool mnemonics (acronyms used to explain the type of tool) feature heavily in formation evaluation programs as most logging tools, individual logging sensor measurements and log curves are known by their individual signature acronyms.


The three must-have curves you need for a basic well log analysis are: gamma ray, porosity and resistivity. These three curves give an excellent quick-look log analysis of reservoir formations and can give the wellsite geologist and shore-based petrophysicists an almost real-time preliminary interpretation of the zones of interest. This LWD tool data takes only seconds to get to surface and decoded into the data that is shown on the screens. The time it takes from when the rock is drilled to when the data arrives at surface is dependent on how far behind the bit the individual LWD tools are positioned. For “Near Bit” tools, such as Gamma Ray and Resistivity, this can be less than a metre so the information is received very soon after drilling. Compare this data with how long it takes to get the actual “cuttings” to surface – which can be anywhere from 30 minutes to a couple of hours – and you can appreciate the benefits of having LWD tools in the BHA. In an operation where time equals money, you want your important decision-making data as soon as possible.


WL reservoir logs


Data Acquisition

Downhole MWD hardware consists of sensors built into a drill collar positioned near the bit. Electrical energy for the system is provided by a battery pack or generated by a small turbine. In a battery pack MWD system, information is recorded and stored downhole in the microprocessor. The data are retrieved when the MWD collar is brought to the surface and are transferred to the computer in the logging unit for additional processing.


In a typical turbine-powered “real time” MWD system, data are sent directly to the surface by mud telemetry, which utilizes the column of fluid inside the drill pipe as a transmission line for digital acoustic signals. Downhole measurements recorded by the sensors are transmitted through the mud as positive or negative pressure pulses or as a continuous, fixed-frequency pressure wave.

The mud telemetry signals are detected with pressure transducers in the standpipe at surface. A computer then records the digital signals. Data are converted to engineering units and processed to generate depth- or time-based output.


As you can see, it is almost essential for LWD engineers to have a degree in electrical, mechanical, chemical, petroleum or civil engineering although many also progress into it from mudlogging/data engineering positions within the same service provider company. Because of the sometimes extreme physical environment the tools are subjected to downhole (extremes of temperatures and pressures), tool wear and breakdowns are an all-too-common occurrence in LWD operations which places extreme pressure on the LWD operators to perform their job. You have to be able to work under pressure and be thick skinned to be able to handle not only the troubleshooting operations but also the barrage of verbal abuse the LWD engineers are likely to face when their tools fail and “non productive time” is logged on the daily reports. If you think that character of the grumpy company man that John Malcovich portrayed in the “Deepwater Horizon” movie was exaggerated, think again…there really are people like that working on offshore rigs!



Wireline Logging - Formation Evaluation after Drilling


Wireline logs are recorded when the drilling tools are no longer in the hole and are made using highly specialized equipment entirely separate from that used while drilling. To run wireline logs, the hole is cleaned and stabilized and the drilling equipment pulled out of the hole. There is usually several wireline “runs” with different tools being used for different types of petrophysical data collection and formation sampling in each run.


After the well has been prepared for logging operations, the first logging tool is attached to the logging cable (wireline) and lowered into the hole to its maximum drilled depth. Most logs are run while pulling the tool up from the bottom of the hole, although just to be sure of having a record, measurements are recorded on the way down as well. The cable attached to the tool is both a support for the tool and a canal for data transmission and is wound around a motorized drum during the logging.


There is an instantaneous display as a log is acquired both on the rig and, if requested, by satellite link at the client’s or operators shore-based office. Data is also stored electronically for future processing and editing.


Because rig time is expensive and holes must be logged immediately, modern logging tools are multi-function and multi-modular. Despite the use of combined tools, the recording of a full set of wireline logs still requires several different tool descents. While a quick shallow logging job may only take 3 – 4 hours, a deep-hole, full set may take 2 – 3 days or longer. Formation pressure testing and sampling runs can take up to 12 hours to perform each run.


The wireline operations are performed from the wireline unit, which is placed within close proximity to the drillfloor. The tools are lowered down the hole via a series of pulleys (sheaves) that direct the wireline cable from the drum at the unit to the open hole on the drillfloor. The wireline technicians will assist the wireline engineers with the pre-run tool checks, rigging up and rigging down of the tools and general maintenance of equipment while the engineers sit in the unit and monitor the data acquisition and processing during and after the run.



The wireline unit is extremely small and during wireline operations there will be at least three people working in the unit: the wireline engineer, the wireline technician operating the cable drum and the wellsite geologist. The running of the logs is a very intense operation and constant monitoring by the wellsite geologist and wireline engineer is essential. It is quite often the case that with many of the logs (e.g. formation pressure testing and side wall core operations) both the wellsite geologist and wireline engineer don’t get to leave the unit for their entire 12-hour shift – except for emergency bathroom breaks! Other crew members bring meals into them so they can eat while they continue to work.


inside WL unit


Wireline engineers are sourced from the same educational backgrounds as MWD/LWD engineers. Electrical engineering is the most ideal base to be starting from but not the only route to get there. The wireline technicians don’t necessarily need any formal qualifications as all their training is done on the job.


With technological advances in LWD tools and practices, wireline logging is slowly losing dominance as the main source of formation evaluation data. More and more services are being provided by LWD tools that mean many wireline contingent runs are no longer required. Despite this, wireline logging is, and will remain to be, a critical and necessary part of the offshore drilling operations.


When it comes to offshore mudlogging, MWD/LWD and wireline operations, the three major logging companies that service the industry are: Schlumberger, Halliburton and Baker Hughes. These three companies all have their own divisions of the individual services and some of them still operate under a name of a previous company that has been bought out by one of the big three mentioned (e.g. Geoservices mudlogging services). These three companies all have extensive shore-based support teams that work alongside the client petrophysicists and drilling team to provide timely and reliable data.


Part 9 of Offshore Oil and Gas PEOPLE will continue to explore the many different roles that are performed by third-party service providers on the rig.





Don’t forget to supply any feedback or questions you may have about anything in this article or the previous ones. I’m attempting to give you a general overview in these articles of the typical offshore rig environment. I hope it helps give you a better understanding of what it’s like to work offshore.



If you have only just tuned into “Offshore Oil and Gas PEOPLE” then you can find the rest of the series of articles here: Part 1, Part 2, Part 3, Part 4, Part 5, Part 6 and Part 7.




Amanda Barlow is a wellsite geologist in the offshore oil and gas industry with a field-based geology career spanning over three decades. As well as being a recreational marathoner who has run over 40 marathons in 16 different countries she is also a published author of two books: “Call of the Jungle – How a Camping-Hating City-Slicker Mum Survived an Ultra Endurance Marathon through the Amazon Jungle” and also “An Inconvenient Life – My Unconventional Career as a Wellsite Geologist”. You can connect with her through the Pink Petro community, LinkedIn: or through her Facebook page: