Conventional Mud Logging Practices

Routine Services

With a diversity of services at hand, an operator has a number of decisions to make when selecting the types and level of services needed for a specific drilling program. Conventional mud-logging services generally include the following capabilities:

• Monitoring, calculating, and plotting drill depth and drill rate;
  
• Determining lag time (stroke count) and making continuous adjustments while drilling;
  
• Describing, interpreting, and plotting lithologies from cuttings and cores;
  
• Detecting, plotting, and evaluating concentrations of total combustible gas in mud and cuttings;
  
• Detecting, plotting, and evaluating concentrations of individual hydrocarbon compounds in mud and cuttings gas;
  
• Measuring basic physical and chemical properties of drilling mud;
  
• Making basic pressure evaluation measurements, calculations, and log presentations;
  
• Making borehole deviation calculations to produce logs. and reports with data adjusted to true vertical depth;
  
• Providing notification and interpretive functions to drilling supervisors concerning drilling events (e.g., oil show, gas kick, and mud cut); Interpreting formation log data and plots and correlating them to reference wireline logs and core data.
  

Supplemental services generally available and compatible with conventional mud-logging units and programs include these capabilities:

• Monitoring work sites for dangerous and nonhydrocarbon gases;
  
• Monitoring mud pits for dissolved acid gases, mud surges, and in/out properties;
  
• Providing basic geochemical and petrophysical analyses of cuttings and sidewall cores.
  

Standard Equipment

Based on observations published in 1983 by the Society of Professional Well Log Analysts (SPWLA) and information provided by established mud-logging service companies, a number of instruments and support systems are considered basic to a conventional mud-logging unit. These are listed below. Individual pieces of equipment may be omitted from a mud-logging unit if the functions they perform are not needed during a specific drilling program.

In addition to the items listed, it is desirable that preparatory arrangements assure that the equipment has been designed and installed to provide continuous, reliable performance under any adverse operating or climatic conditions expected at the wellsite. The equipment should also satisfy any environmental or safety regulations of the area.

It also is advisable to ensure that logging personnel are sufficiently qualified and experienced both to operate and maintain the mud-logging equipment specified for the site and to assemble and interpret mud-logging data for designated geological and engineering purposes.

 

Mud-logging Equipment Commonly Used During Conventional Mud Logging

Total combustible gas analyzer using a catalytic combustion detector (COD) or flame ionization detector (FID)

Gas sample dilution system to obtain linear detector response at all gas sample concentrations

Automatic sampling gas chromatograph (GO) capable of routinely isolating and detecting the following:

lightest combustible gases (hydrogen and methane)

other hydrocarbon gases (ethane, propane, butane, and isobutane)

lightest hydrocarbon liquids (pentane+)

Gas calibration system that allows for the following:

· Total hydrocarbon detector to be calibrated to respond in units of percentage equivalent methane in air(% EMA)

Gas chromatograph calibrated to read directly in % EMA or parts per million of the types of individual hydrocarbons it detects

Backup catalytic combustion or thermal conductivity detector (TCD) compatible with operating detector systems

Batch total hydrocarbon analyzer equipped with blender and vacuum line for the extraction and analysis of cuttings gas

Microscope and ultraviolet (UV) light inspection chamber for description of cuttings lithology and evaluation of oil shows

Bulk density apparatus for measuring relative density of shale (clay) cuttings

Depth and drill rate recorder for determining sample depth and preparation of a rate-of-penetration (ROP) log for lithology and porosity interpretation

Pump stroke counters (2) to record drilling (circulating) time, calculate lag time (tracer test) and indicate arrival-at-surface stroke count

Surface mud pit volume sensor system for use in conjunction with the total hydrocarbon detector to provide immediate recognition of well kicks for well control and rig safety

Mud test equipment for evaluating general physical and chemical mud properties, which will include the following:

Mud balance to determine mud density

Marsh funnel to estimate mud viscosity

Sand test kit to isolate and identify fine solids in circulating mud

Salinity titration equipment to confirm the presence of undesirable or dangerous constituents in returning mud

Test chemicals and glassware to confirm identification of cement, salt, carbonates, sulfates, etc., in cuttings

Processing equipment for sieving, washing, and drying samples

Chemical apparatus to detect trace quantities of dissolved hydrogen sulfide in drilling mud; this includes an automatic hydrogen sulfide gas detector capable of signaling an alarm upon the appearance of such gas in the gas trap. (Generally, this is a minimal requirement; in an area of known hydrogen sulfide occurrence, more stringent safety provisions will probably be required.)

Text processing, log preparation, and related support items, such as typewriter, dyeline or electrostatic copier, drafting supplies and appropriate forms, sample bags, envelopes, boxes, shipping materials, and other materials needed in the routine of mud logging. In some modern conventional mud-logging units this function may include computer storage and plotting capabilities.

Formation Mud Log

An SPWLA recommended formation mud log format is shown in. In this log, as with nearly all formation mud logs, data are presented in vertical, well-profile tracks for easy comparison with wireline and related down hole logs. Mud log headings are similar to those of other downhole logs, but they generally present information that summarizes the well at total depth
(TD).

The typical sequence of tracks from left to right is:

Track One Penetration rate

Track Two Cuttings lithology with depth and show columns

Track Three Total combustible gas

Track Four Hydrocarbon gases and geochemical data

Track Five Descriptive information

Track One

Drilling-related comments and data are also reported descriptively in this column, particularly if they may explain changes in ROP. For example, this column may include (1) bit type, diameter, and jet sizes; (2) bit-operating statistics, such as average weight on bit, rotary speed, torque, and pump pressure; and (3) final bit footage, hours, and grade. Track One commonly records trips and shut-in times, and it may carry directional survey data. This track also denotes the drilling chronology of the well by the placement of each day's date at "midnight" bottomhole depth.

Track Two

In general, Track Two carries three sets of data — depth, lithology, and shows .Depth generally is on the left of this track and is shown numerically and graphically. Standard down hole log scales, such as 2 in = 100 ft or 1 m = 500 m, are used. Cuttings lithology is represented graphically, both in terms of individual rock types and their gross relative percentages. Visible porosity may also be depicted in this track. Hydrocarbon shows and cored intervals are generally indicated by symbols on the right of the track. The former will be numbered and qualitatively rated. Both may reference specific reports.

Track Three

On a five-track log hydrocarbon analytical data typically make up both Track Three and Track Four. On a four-track log , it is common to combine these two tracks and present all geochemical data as the single third track.

Track Three generally carries the type of data that was the basis for early mud-logging services-measurement of total combustible gas arriving at surface in the drilling mud. Such measurements as connection gas and trip gas routinely appear here, as can total cuttings gas. Gas data generally are plotted in percent equivalent methane in air (% EMA). When basic formation-pressure data, such as shale density and mud temperature, are included in the formation log, these typically appear as gradient plots in Track Three for comparison with the total combustible gas curve.

 

Track Four

The second track of gas analytical data, Track Four, usually contains specific compositional information.The second gas track generally is scaled to show concentrations of individual hydrocarbon compounds, usually in parts per million (ppm), that were recovered from the drilling mud. Concentrations are determined through some form of gas chromatography.

Track Four may also present relative gas wetness by comparing the content of methane with that of heavier gases. In some cases ratios between individual or groups of hydrocarbon gases are plotted in Track Four. Additionally, compositional data plotted in this track may come from cuttings gas analysis. Analytical data for the elemental gas hydrogen
plus nonhydrocarbon gases like carbon dioxide
may also be included in this track; however, it is more common to find the latter on a supplementary log.

Track Five

In virtually all conventional mud logs the right-hand track — Track Five in our example — serves as a concise diary or summary of the drilled well .

This track contains abbreviated sample descriptions of the lithology depicted in Track Two and will indicate cuttings porosity if that is not shown in Track Two. Track Five can include data on drilling mud, such as weight and viscosity, as well as comments and various details on borehole conditions and deviations. Any factors that can influence overall log quality can be entered here for reference. When space is a problem, or convention so dictates, some Track Five data can show up instead in Track One for use with ROP evaluation.

Morning Report

An operator should expect to receive a morning or periodic report from the mud logger once per day, per twelve-hour logger shift, or per eight-hour tour shift, whichever is specified in the mud-logging agreement. All drilling, geologic, and hydrocarbon "events" of the period should be noted on the periodic report, along with more routine items regarding instrument performance and calibration, log print distributions, and sample shipments.

Show Report

The morning report format may be used when reporting an oil or gas show, or the operator or service company may provide a special form . In the gas show report, the background value determined before the show and, if available, after the show should be included; the magnitude of the show can be validly estimated only relative to such established values.

Reports of any special occurrence, such as a show, should always be in writing, even if the wellsite geologist is present during the event. The report should be signed by both mud-logging and operator parties in acknowledgment of transmittal and receipt. This is a basic procedure to ensure good communications and prevent confused or erroneous reporting. It is the type of routine procedure that should be confirmed during the prespud meeting.

Pigtail Logs and Attachments

Certain data, records, and tabulations require more space than is available in the right-hand track of a mud log. These may be drafted or typed onto plain log film or vellum and attached to the end of the mud log after it is completed. These are called pigtail logs. Typical pigtail logs and attachments include zone of interest logs, bit records, conventional core recovery, sidewall core recovery, drillstem test (DST) results, and fishing efforts.

Invoices and Manifests

During drilling, the mud-logging crew must prepare documents for the shipping of logs and samples from the wellsite. The mud logger and operator should take extra care in preparing these invoices and manifests. Copies should be retained at the wellsite until written confirmation of delivery is received.

For "tight hole" samples, description and depth-coding systems may be used for both labels and manifests. These systems must be established ahead of time by the operator and the mud logger, preferably in time to be confirmed at the prespud meeting.

Summary Reports

On completion of well drilling, the mud-logging company normally is required to provide a final report. This is a written geologic, hydrocarbon, and possibly formation-pressure report, summarizing evaluations and recommendations made from the various logs. The final report should be delivered with the final log prints and always filed with the logs.

Additionally, after "rig-down" of the logging unit, the mud-logging crew should prepare an end-of-well report, which should be signed by the wellsite geologist or an operator representative. A copy should be filed by both the operator and the mud logger. This report will be used to prepare final billing for the service, but, more importantly to operator personnel, it will contain details of the final distribution of all logs, samples, and documents kept in the logging unit during well drilling. The listing should include all mud-logging unit copies of reports, worksheets, analog recorder charts, computer disks and tapes, and any offset wireline or mud logs provided for correlation purposes. The end-of-well report is not complete — and the mud logger is not relieved of responsibility — until all listed items are accounted and signed for.

Mud Log Preparation and Presentation

A side-by-side comparison- of a formation mud log and a single wireline log shows that there are considerably more data contained on a mud log. This is partly because many physical, chemical, and operational factors are presented within the mud log format. The main cause, however, is the fact that, unlike wireline logs — which are produced in a few hours, perhaps at interim drilling stages of the well — mud logs are produced over the drilling life of a well. A mud log is built day by day, recording continuously changing drilling, borehole, mud, and ambient conditions. In addition, the mud log contains many supplemental details that are necessary to interpret the basic ROP, geologic, and analytical data accurately.

In years past, all formation mud log data were plotted on the log by hand at the wellsite. Typically they were inked in final form at the end of each shift or prior to the periodic report. Hand drafting often led to poor use of a mud logger's time, particularly if a number of special analyses were being performed during the shift. More recently, simple, relatively inexpensive computers and plotters have been incorporated in mud-logging units as on-site log-drafting machines ( Figure 1 and Figure 2 , Typical product). Such equipment is becoming common where full formation-logging services are involved; consequently, computer-prepared logs may be provided by mud-logging companies in place of more conventional hand-drafted varieties.

Most conventional logging operations that use computers require manual entry of data. From this assemblage of data, selected values may be extracted through routine computer programs, or as needed, for plotting, replotting, or cross-plotting. This flexibility, plus the accuracy and speed inherent in computer manipulation of data, allows broader presentation and use of conventional mud log data while drilling is in progress.

Exercise 1.

What are the functions of the following pieces of mud-logging equipment?

a. total combustible gas analyzer

b. gas chromatograph

c. blender and vacuum line

d. pump stroke counter

e. mud pit volume sensor

f. mud balance

g. ultraviolet light inspection chamber

Solution 1:

a. Detects total concentration of all combustible gases in mud.

b. Separates concentrations of individual hydrocarbon compounds in gas sample.

c. Liberates and extracts gas entrapped in cuttings.

d. Lags mud circulation; determines volume of mud in circulating system.

e. Detects a surge in mud return from well kick

f. Measures drilling mud weight and shale density

g. Checks cuttings and cores for fluorescence (shows)

Exercise 2.

What is the function of a pigtail log?

Solution 2:

The pigtail log adds information that is not routinely plotted on the formation log.

It provides additional details on events and drilling developments that:

1. aid in the interpretation of the mud log, or
   
2. provide the operator with information on specific wellsite conditions.
   

Exercise 3.

List at least three parameters that generally would be plotted or recorded on each track of the following five-track formation log.

(Note: Numerous variations are to be expected; most basic information should be included in final distribution depicted.)

a. ROP track

b. lithology track

c. total gas track

d. chromatography gas track

e. descriptive track

Solution 3:

a. Rate of penetration, bit record, bit operating conditions, directional surveys, trips, midnight date (and a number of other items if not carried elsewhere, such as porosity and shut-in periods).

b. Percentage lithology, shows, porosity, cored intervals, casing points.

c. Total mud gas, basic shale density plot, mud temperature curve, total cuttings gas if taken, nonhydrocarbon gases (H2S, CO2), notations on connections, trips, and lag gas.

d. Concentration of individual hydrocarbons in mud gas and cuttings gas, percentage plot of gases, nonhydrocarbon gases if not in "c."

e. Abbreviated sample description, mud information (e.g., weight, viscosity, chlorides), fluorescence, general information not carried elsewhere.