The Expanding Role of Mud Logging Peter Ablard For decades, samples and measurements obtained at the surface have provided mud Chris Bell Chevron North Sea Limited loggers with insights into conditions at the bit face. Information captured through Aberdeen, Scotland mud logging gave operators early indications of reservoir potential and even warned David Cook of impending formation pressure problems. New sampling and analysis techniques, Ivan Fornasier Jean-Pierre Poyet along with advances in surface sensor design and monitoring, are bringing the Sachin Sharma science of mud logging into the 21st century. Roissy-en-France, France Kevin Fielding Laura Lawton Hess Services UK Limited The mud logging unit has long been a common against deployment of wireline logging tools. In London, England wellsite fixture. First introduced commercially in such wells, analysis of mud gas and cuttings often 1939, these mobile laboratories carried little provided the first, and perhaps only, indication George Haines more than a coffee pot, a microscope for examin- that a formation might be productive. Today, Houston, Texas, USA ing formation cuttings and a hotwire sensor for although LWD technology is able to give the first detecting the amount of hydrocarbon gas encoun- glimpses of near-bit conditions in real time, Mark A. Herkommer tered while drilling. The mud logger’s job was to adverse wellbore conditions sometimes preclude Conroe, Texas record the depth and describe the lithology of the use of downhole logging tools. In such cases, formations that the drill bit encountered, then the mud log continues to inform operators of the Kevin McCarthy determine whether those formations contained producibility of their wells. At a minimum, the BP Exploration any oil or gas. mud log gives the operator an early indication of Houston, Texas Outside the logging unit, the mud logger’s zones that merit special attention, additional log- Maja Radakovic domain ranged from the shale shaker to the drill ging services or production tests. Sinopec-Addax floor. The shale shaker yielded formation cuttings The mud log serves a variety of functions. As a Geneva, Switzerland and gas—both liberated by the drill bit—which correlation tool, the mud log’s ROP and total gas were transported to the surface in the drilling curves exhibit a remarkable correspondence to Lawrence Umar fluid. Periodic visits to the shaker permitted the gamma ray and resistivity curves, respectively.1 Petronas Carigali Sdn Bhd mud logger to collect cuttings for microscopic Throughout the drilling process, mud logs pro- Kuala Lumpur, Malaysia examination, while a suction line between the vide real-time correlations with logs from neigh- Oilfield Review Spring 2012: 24, no. 1. shaker and the logging unit carried gas from the boring wells and help the operator track the bit’s Copyright © 2012 Schlumberger. gas trap to the logging unit’s hotwire gas detec- position in relation to target formations. Because For help in preparation of this article, thanks to Justin tion system. Visits to the drill floor allowed the mud log is based on physical samples, it pro- Ashar, Kamel Benseddik, Regis Gallard, Willie Stoker and Craig Williamson, Houston; John Christie, Paris; Gen important exchanges of information between the vides direct, positive identification of lithology Herga and Denise Jackson, Gatwick, England; Mark Jayne, mud logger and driller. Using basic surface mea- and hydrocarbon content. This information can Francois Le buhan, Remi Lepoutre, Jacques Lessi, Audrey Malmin, Keith Ross and Philippe Verdenal, Roissy-en- surements, the mud logger was able to produce a be helpful when formation characteristics make France, France; Nikhil Patel, Singapore; and Irwan Roze, concise account of drilling activity. wireline or LWD log interpretation complicated Conroe, Texas. FLAIR, MDT, PreVue, RFT, StethoScope and Thema are For decades, gas measurements, lithology and or ambiguous. It can also fill gaps where other marks of Schlumberger. rate of penetration (ROP) provided the earliest such measurements have not been obtained. 1. Like electric logs, most mud logs conform to format indications of reservoir potential. Before the Thus, when integrated with wireline or LWD mea- standards set forth by the Society of Professional Well Log Analysts (SPWLA). For mud log standards advent of measurements while drilling (MWD) surements, cores and well test data, the mud log promulgated by SPWLA: Mercer RF and McAdams JB: and logging while drilling (LWD), mud loggers provides independent evidence for a more com- “Standards for Hydrocarbon Well Logging,” Transactions of the SPWLA 23rd Annual Logging Symposium, were able to obtain valuable formation data from prehensive understanding of reservoir conditions Corpus Christi, Texas, USA, July 6–9, 1982, paper LL. wells in which drilling conditions, formation and geology. characteristics or well trajectory conspired 24 Oilfield Review Drill floor Mud mixing pit Mud pumps Suction pit Sand trap Settling pit Mud logging cabin Reserve pit The scope of the basic mud logging service The mud logger’s role takes on added impor- From a drilling standpoint, the mud logger’s has broadened over time, as additional sensors tance when there is a drilling break, or signifi- most important task is gas monitoring. Mud gas brought more data into the logging unit— cant increase in ROP. Then the mud logger alerts trends that develop while drilling are integral to expanding in diversity from gas chromatographs the company representative and requests that evaluation of mud balance and identification of to weight-on-bit and mud pit level indicators. The drilling be stopped until mud and cuttings from potentially overpressured formations. By care- mud logging service now typically tracks ROP, the bit face can be circulated to the surface. If fully tracking gas and drilling parameters, the lithology, visual hydrocarbon indicators, total mud analysis indicates the presence of hydrocar- mud logger can recognize impending deviations combustible gas in mud and individual hydrocar- bons—called a show—the mud logger informs from normal trends and give advance warning so bon compounds in the gas, along with numerous the geologist, who may elect to core or test the the driller and company representative can miti- drilling parameters. As a hub for monitoring drill- interval. Geochemists and biostratigraphers also gate the problem by adjusting the density of the ing operations and rig sensors, the mud logging rely on mud loggers to collect representative drilling fluid or shutting in the well. Thus, the unit has become a source of crucial information samples needed to make correlations and develop success of a well and the safety of the drilling for the company representative, the driller and geologic models. operation may hinge on how quickly a mud logger the geologist. can synthesize and interpret myriad pieces of data. Spring 2012 25 Mud logging capabilities have evolved over Advances in computing and networking tech- This article describes how a basic mud log is the years. By the mid-1950s, gas samples were nology, surface sensor design and sample analysis assembled, reviews sampling and analysis tech- being analyzed by wellsite gas chromatography. are bringing the mud logging unit into the 21st niques used in formation evaluation and dis- In the 1960s, mud logging companies began offer- century. Today, even more sensors lead into the cusses basic methods for monitoring pressure. An ing geopressure detection services.2 Automated logging unit, each acquiring data at a frequency of overview of recent sensor technology maps the event recorders, made possible through use of several times per second. To handle this increase evolutionary path from basic formation evalua- robust microelectronic components, were incor- in data volume, a context-aware processing sys- tion to advanced analysis of mud gases for geolo- porated into mud logging processes during the tem—based on computer-generated trend lines gists and well integrity services for drillers. 1970s. During the following decade, mud logging and a library of established models—makes the units entered the computer age. Computers took data easier for the mud logger and other end users Mechanics of Mud Logging the burden of printing the log away from the mud to comprehend. Digital images of samples viewed In the oil field, drilling fluid is integral to every drill- logger, who used to laboriously compile the data under the microscope can be rapidly transmitted ing project. Be it water-base, oil-base or gas-base, and then draw the log by hand. In addition, com- from the wellsite to the client office. And new drilling fluid is vital to the process of making hole: puters allowed mud loggers to organize and approaches to gas sampling and analysis have s)TSTREAMSUNDERPRESSURETHROUGHJETNOZZLES track data from multiple sources without sen- been developed to extract geochemical properties on the drill bit to clean the bit and carry heat sory overload. at the wellsite. away from the bit face. s)TTRANSPORTSDRILLCUTTINGSFROMTHEBITFACETO the surface, thus playing an essential role in the hole cleaning process. s)TOFFSETSBOTTOMHOLEPRESSURETOHELPMAINTAIN Traveling block wellbore stability and prevent the influx of for- mation fluids that could cause blowouts. Over time, numerous variations on the basic Standpipe mixture of clay- and freshwater-base drilling flu- ids have been developed (and sometimes dis- Kelly Flowline carded). Well-known variations are based on Rotary table saltwater, mineral oil, diesel oil, polymers, nitro- Mud logging unit Gas trap gen, mist and foam. Each type has specific prop- Drill floor Suction line Shale shaker erties that deliver superior performance in Bell nipple certain drilling environments. And each requires Blowout preventer special accommodations when it comes to mud logging: Some require custom sampling tech- niques; others require special sample rinsing pro- Mud Suction Shaker Reserve cedures. This section focuses on the simplest of pump pit pit pit Casing environments, in which freshwater-base drilling muds are used. The practice of mud logging relies heavily on the mud circulation system, which carries forma- tion cuttings and fluids to the surface.