Mud Logging

Introduction

Development of Mud Logging

MUD LOG: A continuous analysis of the drilling mud and cuttings to determine the presence or absence of oil, gas, or water in the formations penetrated by the drill bit, and to ascertain the depths of any oil- or gas-bearing formations. (Gary et al., 1972)

Conventional mud logging is a wellsite effort that attempts to determine as rapidly as practicable, and from materials at hand, numerous subsurface conditions that are directly related to petroleum exploration and development drilling.

Concept of Mud Logging

When a rock is penetrated by the drill bit, a quantity of rock with the fluids it contains — water, oil, and gas — is ground up, dispersed in the circulating drilling mud, and transported up the well annulus . The relative volumes of rock and fluids arriving at surface during any one sampling period will be controlled by:

the volume of the cylinder of drilled rock, which depends upon drill bit size (cylinder cross-sectional area) and penetration rate (cylinder height);

the porosity of the rock which determines how much fluid can be entrapped;

the permeability of the rock which determines how much of each fluid can escape from the cuttings into the drilling mud while in uphole transit;

the relative saturations of water, oil, and gas, which represent the proportions of the total porosity filled with each of these fluids;

the differences in temperature and pressure between the bottom of the borehole and surface conditions, which control changes in such phenomena as the solubility of gas in oil or water, the distribution of hydrocarbons between liquid and gas phases, and the bulk volume of gas present at ambient surface conditions.

The rationale for mud-logging services is that if representative samples of drilled materials are analyzed as soon as they come from the borehole, it should quickly be feasible to reconstruct the nature and composition of rocks and fluids at depth.

Two conditions prevent mud logging from achieving its ideal goal of providing immediate measurement of in situ subsurface conditions. First, none of the factors listed can be precisely measured to permit quantitative reconstruction of down-hole conditions. Second, drilling practices and analytic procedures introduce other variables into any such reconstruction. These include:

· the addition of fluids and cavings from uphole, uncased portions of the wellbore;

the introduction of mud additives as well as recycled debris and gas from the mud tanks;

the variation in operational efficiencies of surface pumping and extracting equipment.

Nevertheless, important qualitative deductions and interpretations can be made from mud log data.

Conventional Mud-Logging Services

The two basic categories of service routinely available are combustible gas detection and formation logging.

Combustible Gas Detection

Mud logging originated as an exploration service in 1939. At that time it consisted of the extraction and gross detection of combustible gases carried to surface by circulating drilling mud. To provide this combustible gas detection service, a portion of the returning drilling mud was passed through a gas trap, where it was agitated and aerated. This mixture of air and extracted gas was then drawn by vacuum from the top of the trap to a nearby detector .The original "hot wire" gas detectors were nondiscriminating and provided a single output of total combustible gases to an analog meter calibrated in arbitrary "gas units." The operator, or mud logger, manually transcribed and plotted the readings of the detector. However, the analytical instruments initially available were relatively unwieldy and troublesome and required constant attention, adjustment, and recalibration. Consequently, operation and maintenance of gas detectors required the full-time attention of early mud loggers. Interpretation of the results or their correlation with other drilling data was the responsibility of the wellsite geologist or engineer.

Basic Formation Logging

The 1940s and early 1950s saw a broadening of the simple gas-logging effort as a consequence of improvements in gas analysis instruments. As transportable gas detectors became more rugged, reliable, and automated, mud-logging personnel had more free time to work with other on-site sources of information.

As a first step, mud loggers took over the drillers' traditional task of collecting and examining cuttings samples and preparing a lithology. Combining the lithology log with the combustible gas plot created the primitive formation log.

The following quotation, from Hugh Barton of Phillips Petroleum, provides some insight into early mud logging:
A soil gas analysis apparatus for surface oil prospecting was developed around 1938by R.L. Eoan, R.W. Crawford, and B.H. Ashe of Phillips Petroleum Company. In about 1939, G.G. Oberfell, Research Vice President, requested that this system be adapted to measure hydrocarbons in drilling mud to improve the chances of finding oil and gas in geological formations where electric logs of the time sometimes missed them. T.C. Wherry was assigned the task and he set up a laboratory in Oklahoma City to analyze muds from a well being drilled in the area. The mud was put in a pressure vessel and heated until head-space pressure reached about two atmospheres. At that point the gaseous mixture was expanded into an evacuated vessel. After cooling, the mixture was passed into a "hot wire" gas analyzer consisting of a platinum wire heated in a bridge circuit. The system was calibrated in terms of equivalent normal butane concentration in air.

First Results showed an increase in gas content near the pay zone-not dramatic, but enough to encourage further development of the concept. Phillips decided to build an improved apparatus into a trailer and do analyses on site. Our first analyses in 1939 at the Billings Unit north of Perry Oklahoma, were performed by Crawford, Ashe, Bill Flatford, Wayne Peck, and me. Results were promising but, as with many drilling operations, "gas sniffing" did not go smoothly. We drilled the main pay zone in bitter cold weather and the logging unit was frozen up and inoperable at that time. We refined the apparatus and took it to Hidalgo County in south Texas, where no freezing weather was expected. A good log was obtained on the next well.

My recollection of first direct, on-site use of mud logging occurred on Christmas Eve, 1940, in the Chocolate Bayou field near Alvin, Texas. The gas content in the mud began to build up and I informed the driller, who refused to believe it because he could not see or smell it in the mud. He would not stop drilling to circulate the gas cut mud. At this point I did insist that a test be made of the blowout preventer. Naturally it was found to be stuck open. The driller did agree, reluctantly, to clean it out, probably because there had been a disastrous blowout nearby earlier. A few minutes after repairs were made the well began to kick hard and would have blown out except for the now-working blowout preventor. About 20 feet of gas zone had been penetrated so that the near-blowout stuck the pipe in the hole and required an expensive fishing operation.

Two research mud logging units were fielded with good results prior to World War II. After the war, Bob Pryor built another one in a large trailer and operated it for a few years until industry picked up mud logging.

Initially, the quality of lithologic descriptions showed little improvement over those of drillers. Lithologic nomenclature often appears to have been restricted to terms like sand, hard shale, soft shale, lime, and "Annie Hydrite." Nevertheless, as experience and training of mud loggers improved, mud log geology became a first-hand record of the depths and characteristics of downhole rock types.

The drilling rig also provided another source of data — rate of penetration (ROP) — which has become a routine inclusion in formation logging. Initially, mud loggers took depth measurements from the driller's report. Later, however, mud loggers began to attach their own sensors to the kelly and calculate depth drilled and ROP on a fine foot-by-foot scale. This was a substantial improvement over driller's ten-foot-or-more averages. The addition of accurate penetration rate data permitted improved interpretation of rock strength and porosity. The ROP plot also encouraged visual correlations to be made between the mud log and those wireline logs that reflected rock properties related to drillability (e.g., porosity).

The final developmental step in formation logging was the introduction of tracer materials, such as calcium carbide, to the mud system and the addition of a pump stroke counter. With these, the rate of mud flow in the borehole could be measured accurately. This, in turn, provided a reliable means of estimating the lag time between penetration of a rock unit at depth and the arrival of its gases and cuttings at surface.

Although a minor technological achievement, lagging was a major practical advance for the working geologist. Reporting of gas and cuttings data at lagged depths allowed direct comparisons and correlations to be made with logs from other sources.

Modern Formation Logging

Since the development of formation logging, improvements have been made in gas detection capabilities through the introduction of more accurate and discriminating gas analyzers. Gas chromatography, for example, is now used routinely in formation logging to measure the con-cent rations of individual hydrocarbon compounds in mud and cuttings gases. Other analyzers, such as for hydrogen sulfide and carbon dioxide, can be added upon request to formation logging programs. Some modern formation logging units have incorporated pyroanalyzers to evaluate cuttings as petroleum source and reservoir rocks.

Advanced Mud-Logging Services

We have broken advanced services into three categories-pressure evaluation, systems monitoring and data acquisition, and interactive evaluation and advisory.

Pressure Evaluation Services

In the mid 1960s a major expansion in mud-logging services began as mud-logging companies became more involved in engineering and operational aspects of well drilling. The first expansion was to monitor for overpressured zones. This marked the transition to advanced mud-logging services.

The need for pressure evaluation services evolved because of improvements made in drilling techniques in the 1960s. Development of more stable drilling fluids
and mud additives
allowed for much greater control of mud density and mud properties. As a result, mud weight could be increased or decreased rapidly as needed, and also be maintained at relatively constant levels, without full-time attention and treatment. At the same time, improvements in blowout prevention technology, adjustable choke design, and well-kill capabilities
took.

These factors led first to a reduced fear of well blowouts, then to increased confidence in the ability of a rig and its crew to prevent or control a well kick, and finally to a full appreciation of the economic benefits to be gained from balanced drilling.

Balanced drilling — which is drilling with a minimum or with no overbalance of drilling mud density relative to the formation pressure gradient — reduced drilling mud treatment costs, improved penetration rates, and resulted in better overall conditions in borehole walls and adjacent formations.

However, these technical advances in drilling did not entirely remove the risks related to loss of well control. The penetration of abnormally high formation pressures could rapidly dilute the balanced mud system, lower the mud density, and cause the mud to surge to the surface. The stage would be set for a blowout.

As a result, the concept of "geopressure evaluation," also called "surnormal pressure recognition" or "pressure prediction," was incorporated into some mud-logging programs.

It was realized early that data directly applicable to identification of over-pressured zones could be obtained from available mud-logging techniques. Changes in penetration rate, cuttings character, and mud gas volume are examples of such indicators. Concurrently, mud-logging service companies introduced several additional wellsite geological and petrophysical techniques that improved detection of a "geopressured formation" or of a declining mud overbalance at the bottom of the borehole. These techniques provide such measurements as shale density, clay mineralogy (shale factor), mud temperature gradient, and mud pit volume. Techniques have continued to evolve to the point that pressure evaluation programs are now used routinely to select optimum casing points in overpressure transition zones in order to both control formation pressure on the wellbore and minimize excessive mud weight damage to openhole formations.

Systems-Monitoring and Data-Acquisition Services

With the expansion of mudlogging services to include pressure evaluation, it became logical, if not necessary, to install sensors and central recorders that were able to measure continuously various drill rig, mud pit, and circulation system parameters. This led to the evolution of systems-monitoring and data-acquisition services that constantly monitored diverse drilling operations. This, in turn, resulted in drillers and operators improving overall rig performance and safety.

Advanced logging service continued to expand in the 1970s through the
introduction of mini- and microcomputers. While these were able to automate data acquisition and analysis systems, they could not be interfaced readily with older hydraulic or pneumatic sensors on the rig. As a result, independent electronic sensors were added to advanced mud-logging facilities.

Interactive Evaluation and Advisory Services

Logging units set up to perform the types of advanced mud-logging service available in the 1970s were well equipped to act as "command" centers for nearly all engineering and drilling operations. This indeed became the case in many of the more advanced logging facilities in the late 1970s and early 1980s.

This funneling of data through one facility, the mud-logging unit, encouraged further expansion of advanced mud logging into interactive evaluation and advisory services. Mud-logging companies added new types of programs, software, and reports. Well kill, mud hydraulics, optimum drilling, and inventory control programs
are examples. Specialized communications equipment also was incorporated in many advanced mud-logging units for two-way transmittal of data between the drillsite and company offices.

The 1980s introduced another expansion to interactive evaluation and advisory services — downhole measurement while drilling — commonly referred to as measurement while drilling (MWD). MWD technology involves installing common wireline logging sensors in the drillstring, a short distance above the bit. From these sensors, data are either transmitted directly to surface, generally through the mud system, or are recovered from recorders when the drillstring is pulled. Sensors are currently available for (1) directional monitoring (e.g., inclination, azimuth, tool face); (2) drill system monitoring (e.g., weight on bit, rotary torque, bottomhole pressure, mud temperature); and (3) formation evaluation (e.g., formation resistivity, mud resistivity, formation gamma ray).

While all of these advanced services are still generally classified as "mud logging," in many cases it can be seen that similarities lie only in the mode of operation-continuous monitoring in an on-site laboratory during active well drilling. In addition, availability has expanded to the point that many of the routine services and basic instruments can be acquired individually from the many smaller
companies that specialize in particular aspects of logging and supply stand-alone equipment.

Exercise 1.

How do the following parameters affect cuttings and mud gas samples arriving at surface during one sample period?

a. ROP

b. porosity of rock drilled

c. permeability of rock drilled

d. depth of bottomhole

Solution 1:

a. The greater the ROP, the greater the volume of cuttings and gas to arrive at surface during the same time interval (assuming all other factors to be constant).

b. The greater the porosity, the greater the volume of fluids that will be in the combined mud and cuttings arriving at surface.

c. The greater the permeability, the greater the percentage of pore fluids that will have escaped to the mud.

d. The greater the depth, the greater the hydrostatic pressure change during uphole travel and, therefore, gas expansion. Also, the drop in temperature relative to the local geothermal gradient will be greater, with proportional modifications to phase and solubility.

Exercise 2.

Discuss at least two factors that influence cuttings and gases during their transit from bit to mud-logging unit and will affect analytical results.

Solution 2:

• Cavings — modify apparent cuttings composition
  
• Fluid incursions — modify gas composition and concentration
  
• Recycled materials — contaminate samples
  
• Variations in sampling efficiency — yield inconsistent, and perhaps nonrepresentative samples.
  
• Variations of analytical sensitivity (inside the logging unit) — yield inconsistent or biased data.
  

Exercise 3.

What was the original function of mud logging?

Solution 3:

Originally, mud logging monitored combustible gases returning uphole in the circulating mud.

Exercise 4.

What general categories of information are commonly found on a modern formation log?

Solution 4:

ROP, lithology, shows, total combustible gas and individual hydrocarbon compounds of mud gas (and possibly cuttings gas), descriptive information, and basic geopressure plots.