What Is Mud Logging & Why Do I Need It?

There are many definitions and ideas of what a Mud Log is and why it developed over the years. Very early in the history of oil well drilling, wells were drilled where oil seeps were observed. Geologists would often live at the drill site and using a magnifying glass, would look at the rock-cutting samples that were produced by the drill bit and circulated to the surface. They would also study any rainbow sheen observed on the return pits to determine oil content.

That geologist would keep notes about the rocks observed and any ancillary sheen information noticed. These notes became the 'Log'. Over the years, various symbols were used along with the rock descriptions to quickly determine various lithology mixes, the percentage of each rock observed, and a record based on the depth of the rock strata drilled through. By the 1920s, a 'hot wire' device was developed using the concept of a Wheatstone Bridge to measure combustible gas. The burning of the gas over the Wheatstone Bridge changed the conductivity of the system, indicating higher or lower quantities of gas.

In 1940, JT Hayward wrote the definitive paper "Continuous Logging at Rotary-Drilling Wells". After over 75 years, the basic concepts he described have not changed.

The 'technique' of mud logging is relatively simple. As the bit rotates and crushes rock, the drilling fluid circulates those rock cuttings to the surface. The logger calculates the time/volume necessary to move the cuttings from the bottom of the hole to the surface…the lag. The cuttings are collected, washed, and analyzed.

Many tests are utilized, from eyeball description of logging geologists’ interpretation of the mineralogy, the texture, the color, the hardness (matrix or cementation), ancillary minerals, and percentages of various minerals) and the description is written on the ‘log’. Other tests may be using a black light (UV) to determine fluorescence, cut (solvent applied and determine if liquid hydrocarbons wash out), calcemetry, etc.?Today, digital photographs of the samples are typically captured and included with the log.

Other data collected at the well site has changed over the years. Today, unlike in the past, all surface measurements of drilling parameters are collected and distributed by an EDR (Electronic Data Recorder) company. Typically, the only data collection done nearly exclusively by land mud loggers is the gas information. As measuring devices became more sophisticated, systems became more complex. The systems include:

• Gas extractor: A device used to agitate the drilling fluid to break out the gas from the fluid
• Gas conditioner: A device (or series) used to de-humidify and assure clean gas sample for the analyzer
• Gas Analyzer(s)

Gas analyzers have become the most complex, today bringing what was once highly sophisticated laboratory measuring devices to the well site. Let's look at these systems from simplest to most complex

Hot Wire

As described above, the original from the 1920's (and still used today) is the Hot Wire. –Utilizing a Wheatstone bridge, a filament is heated and the gas/air mixture is burned, changing the resistance of the bridge. The deflection indicates the amount of combustible gas.

?Advantage: inexpensive, easy to operate

?Disadvantage: easily contaminated, need oxygen, lower concentrations gas.

Infrared Detectors

Utilizes a source of IR light, an optical filter for proper wavelength, and an optical infrared receiver

As gas flows between the transmitter and receiver, the hydrocarbon molecules absorb some of the IR energy. The system detects the drop in IR energy which is related to the quantity of gas.

Flame Ionization Detector (FID)

The FID can be used as a total gas system, or if a column of packed material is placed in line, the various individual gas molecules can be seperated to be analyzed one component at a time.

Flame ionization detectors work on the principle of ions liberated in the combustion of the sample species. Sample compounds will ionize inside of a ?ame, whereas the carrier gas will not. A permanent ?ame (usually fueled by hydrogen gas which produces negligible ions in combustion) will ionize any gas molecules exiting in the chromatograph column that are not carrier gas. Common carrier gases used with FID sensors are helium and nitrogen, which also produce negligible ions in a ?ame. Molecules of sample consumed by the ?ame will ionize, causing the ?ame to become more electrically conductive than it was with only hydrogen and carrier gas. This conductivity causes the detector circuit to respond with a measurable electrical signal.

Mass Spectrometer

Measures mass-to-charge ratio of ions. The sample is bombarded with a beam of electrons, and the resultant ions are separated according to their mass-to-charge ratio. The results are displayed as a spectra.?

There are a few drawbacks to a Mass Spec gas system. It can't differentiate among isomers of a molecule with the same m/z ratio (ex: Normal and Iso Butane). Many hydrocarbons produce similar m/z ratios as Nitrogen and often get 'swallowed up' or lost in the noise of the overabundance of N2 in the air.

Modern Mud Logging Presentations

The reports and graphics utilized today go far beyond the old 'strip log' the wellsite geologist once prepared. Computerized log drawing software is now standard, offering vertical, horizontal, and hybrid logs, plotting any parameter in any track. A good mud logging company should have a pre-spud meeting with the client to determine what logs are required and requested, what formats, scales, and supplemental reports (show reports, bit reports, survey reports, etc), and when and to whom are those various logs transmitted electronically.

Other Services Now Available

There are many other measurements available utilizing both the collected sample as well as the captured (via isotubes) gas. X-ray diffraction, X-ray fluorescence, Pyrolysis, Scanning Electron Microscopy, Carbon Isotope gas analysis, and higher-order mineralogy can all be effectively run in partner labs for quick turnaround.

The key for those is proper depth control, proper sampling techniques, and tracking of those samples properly until the client gets results.

The Reality of US Land Operations

Ideally, a geologist would love thousands of feet of conventional core…too expensive and time-consuming. Next would be thousands of sidewall cores…too expensive. LWD/Wireline full suite of logs…too expensive and horizontal drilling too risky. So…mudlogging is used to obtain traditional petrophysical measurements

The driller can be helped by identifying bit wear (Mass Spec: Hydrogen, Ethene detection helps the drillers. Wearing bit produces hydrogen and Ethene from the superheating around the bit, resulting in the cracking of hydrocarbons.

Inexpensive (but powerful) Data

?A properly used mud logging service can obtain data from the drilling process because the results of that process produce the gas and cuttings anyway. The collection and analysis of that gas and those cuttings can then be used by the team to drill more effective wells and result in better frac jobs (KNOW YOUR ROCK)

While with a major OFS company, the fracing division came to me (in the drilling/mud logging division) and asked for help in analyzing why some frac jobs went south. Turns out, we analyzed well over 1,000 wells in various US land basins and discovered the ‘sweet spot’ the geologist anticipated was not where expected. Up to 85% of the expected zones were not conducive to a frac stage. Those zones contained high clay content and had crossed natural or induced faults from other laterals in the area. Often the proppant and fluid went behind the pipe, out through faults, anywhere except in the formation. By utilizing engineered frac jobs, utilizing better analysis of what the rock we penetrated, more effective frac jobs were designed to maximize good zones, resulting in longer-lasting wells and less produced water.

Objections to Mud Logging

Over the years, I have heard many of the common objections as to why a client won't use Mud Logging. Here are a few along with the case to use Mud Logging

?We have drilled hundreds (thousands) of wells in the area…we know exactly where we are. Rocks don't behave! Mother Nature is NOT our friend. Clay is the enemy of a good frac job. KNOW YOUR ROCKS

?Not in the budget. A well today costs from $3-$5 million for a 15-day well. Mud loggers on location for 8 days. $5 million / 15 days = approx. $300k per day. Mud logger costs approx. ? of 1% of daily cost.

?Depth correlation is bad. Lag is constantly checked against gamma, drill breaks, gas breaks, and lithology changes. Other things can be used, rice, paint, etc to adjust lag.

?I trust traditional petrophysical measurements. All logging tools are approximations using indirect measurements of rock and fluid, whereas mud logging measures the rock, the gas, etc.

?Not ‘real time’. Are decisions made in real time? How quickly can a team make a decision based on data obtained? All mudlogging can be ‘near real-time’ (lag + analysis time).?

?Data density is not sufficient (I get 6-inch readings with LWD). Data density in lateral can be small TVT (true vertical thickness), perhaps less than one-inch TVT between samples.

The Future

The future is the past. In the 1970’s, and today in deepwater operations, there was a second component…data engineering. The data engineer was the flip side to the logging geologist. The data engineer collected and analyzed drilling parameters to assist with safety optimization and operational awareness (24/7)…even when not drilling. Most problems occur while the drilling operation is halted.

Why has this gone away? Sadly, cost constraints in US Land operations are the main reason for the elimination of that position. Too many people considered it a redundant position, the collection of data required a full set of surface measurement hardware to produce the data.

Hasn’t that been replaced by software and AI? When EDR systems became the 'accepted' gathering method of drilling data, the redundant systems in mud logging operations went away. At this same time, very clever programmers started developing software that could provide remote monitoring, some calculations, and even early stages of Machine Learning and Artificial Intelligence. However, it is being discovered that with the hectic pace of today's 'factory drilling' and reduced staff, many operators have found that the company representative as well as the drilling engineer has neither the time nor stamina to personally watch drilling operations on a 24/7 basis, and if they are looking after several wells, there is no way for any of these to be monitored properly.

How can this be done today? There is an alternative.? By utilizing the various real-time visualization software packages, you can have the real-time center of the mud logging company monitor your wells. They can also assist in data quality (is all the BHA entered correctly, does the pipe tally make sense, help the driller determine tripping speed at critical depths, making sure mud is being displaced correctly, is torque or drag increasing beyond expectations, is there a better WOB/RPM combination for optimal MSE.

The Future Can Be Now

All of what has been shown is possible at a cost-effective price point. By working with your mud logging vendor, and giving them a peek behind the curtain, these things are obtainable. The value of actually looking at, touching, and analyzing the rock and gas, a ‘throw-away’ commodity (the cuttings and produced gas) allows the client to have a better data set for fully understanding their reservoir. The proper selection of the right mud logging company can not only provide the geologist valuable information for posterity, redundancy, and the only opportunity to actually look at the physical rock and gas, but that same mud logging company can act as the bridge, helping the drillers reach their goals in a safe, effecient and better coordinated program.