Figure I-4 gives some pictures of drilling data acquisition system fixed inside mud logging unit and some of its sensors. The data from specific equipment/machinery is transmitted after being measured by means of sophisticated transducers/sensors. By means of a data acquisition unit (left picture) all of the required data is recorded for analysis. Hook-load sensor is shown (right picture) which is attached to the dead-line of the rig. The available data has been collected through the MLU which was the centre at which the drilling parameters were being acquired by means of the sensors placed at various points. Mud logging units are mainly instrumented for the following tasks , Table I-2. Table I-2: Tasks of mud logging units . Measure drilling parameters (ROP, WOB, RPM, Flow Rate). Record properties of drilling cuttings. Transmit and record acquired drilling data. Provide drilling monitoring services, on site and/or at remote locations. ROP: This parameter is the most important parameter, since all of the calculations in this study are based on estimations of Rate of Penetration (ROP). It is measured through the relative change of the position of the block in time. Accurate calibrations are very important in order to have a representative ROP parameter. WOB: It is the abbreviation for “Weight on Bit”. It represents amount of weight applied onto the bit, that is then transferred to the formation which
in turn is the energy created together with string speed that advances drillstring. It is measured through the drilling line, usually by means of having attached a strain-gauge which measures the magnitude of the tension in the line itself, and gives the weight reading based on the calibration. This sensor measures a unique value, which is the overall weight (Hook-load) of the string including the weight of the block and Top Drive System (TDS). For all of these circumstances correct calibration is required in order to have proper reading for this drilling parameter. RPM: This parameter stands for “revolution per minute”. It represents the rotational speed of the drillstring. With the invention of TDS; the reading is directly linked to the electronics of the unit itself. It is considered that the measurements for this parameter are accurate as long as the acquisition system set-up has been thoroughly made up. The optimization process flow applied in this study was composed of using drilling parameters data collected at the mud logging units. Figure VI-4 gives the flow chart of the process flow applied. The data was prepared accordingly in order to provide suitable means for the multiple regression application based on the defined general rate of penetration equation. Pore pressure, wellbore inclination, equivalent circulating density, mud
rheological properties, and additional rotation of the bit due to the motor for each data point was calculated and included in the database. Once the required dataset was stored, multiple regression was run, and regression coefficients were determined. The determined coefficients are used in
calculating the predicted rate of penetration and determine the optimum parameters. The reported weight on bit ranges are the magnitudes recorded through mud logging system, for this reason the instrument readings are included in the database, some ranges could be extremely large due to scale input. This sort of data ranges could be attributed to noise. To obtain the total gas measurements a gas trap is installed at the mud pit that is able to capture a sample of gas from the mud. Figure 1.5 illustrates the concept of a gas trap. An impeller agitates the mud releasing gas into the air. A mixture of this gas-air sample is then sent to the mud logging unit for analysis. The total gas reading is a measure of the relative concentration of all hydrocarbons combined. The unit of measurement for total gas is simply called a “unit.” Each contractor has their own definition of a unit, but, regardless of the conversion factor, the unit is effectively a concentration (i.e., ppm). The total gas measurements are recorded on a time basis and then converted to a depth basis through knowledge of the ROP. It is then corrected to account for the lag time, which is the time it takes the drilling mud to travel from the bottomhole to the surface.
Figure 1.5 Illustration of the gas trap system. The stream of circulating drilling mud enters the gas trap where an impeller agitates the mud, releasing trapped gasses into the air. A portion of this gas-air sample is pumped to the gas logging unit where the total gas measurements are recorded. (Geosearch Logging Inc.). Conventionally, the total gas readings have two main purposes. An obvious application of the mud log is that total gas “shows” help to delineate
productive, hydrocarbon rich zones along the wellbore. Completion engineers will use this information to design effective completion strategies in order to maximize production. The second purpose is perhaps the more critical application. Total gas measurements can provide a means for the early detection of large gas kicks that can threaten to blow out a well. The drilling engineer will usually adjust the density of the drilling mud to overcome the formation pore pressure. A gas kick is a dangerous situation that can occur if the well intercepts an unexpected overpressured zone. A gas kick implies that a large gas bubble has entered the annular section of the wellbore and may or may not be rapidly rising towards the surface. “Loss of well control during drilling operations constitutes one of the major hazards faced by rig personnel, and can lead to loss of life and major destruction of a drilling installation,” (Wang et al. 1994). Figure 1.6 illustrates the importance of the mud log data. The picture shows a flare from a zone that was previously unknown to contain hydrocarbons and exhibited no change in drilling characteristics. However, the mud logging unit detected a large presence of gas in the drilling fluid and the drilling engineer and company man were immediately notified. The well was controlled and the gas was flared with no impact to the safety of the crew. Figure 1.6 Photograph of a gas flare. The mud log indicated that the well had taken a dangerous kick and the well was subsequently controlled. (Geosearch Logging Inc.).
Interpretation of the mud log requires a deep understanding of the fundamental processes that that contribute to the recorded total gas measurements. Many considerations must be accounted for if the mud log is to provide meaningful information. The present study is heavily reliant upon high quality mud log interpretations so the following example of a mud log analysis is well deserved. The example shown in Figure 1.7 represents an ideal mud log report. This particular mud log contains measurements or estimates of ROP, mud density (mud weight), static and circulating mud pressure, reservoir pore pressure, and total gas. Additionally, it is observed that information regarding the points at which drillstring connections were made is reported. In fact, knowledge of the measured depth locations of where the connections are made is one of the most critical pieces of data necessary to perform a mud log analysis, especially for the purposes of this study. As a drillstring connection is made the circulation of drilling fluid is halted. During the time it takes to make the connection any produced or liberated fluids will accumulate at the bottom of the well. Once the drilling mud returns to circulation, this gas will cause an artificially high total gas peak on the mud log. This process is called a connection gas event. Another common event is called trip gas, which occurs whenever a trip is performed. These events are manifestations of the drilling operation and must not be mistaken as responses to reservoir conditions.
The first criterion is the use of total gas concentration measurements from mud logs. Using a gas chromatograph, the mud logging unit is able to determine the concentration of gas present in the drilling fluid at any given time. The hypothesis is that as a natural fracture is encountered by the drill bit, the volume of gas contained within the fracture will rapidly flow into the wellbore and enter the stream of circulating drilling fluid. This will be seen as a sudden “spike” in total gas concentration in the mud log, which will eventually return to normal operating levels after all of the trapped gas has exited the fracture (see Figure 3.1). For the purposes of the present research, these spikes have been expressed as “gas peaks”, and offer the potential for reliably locating conductive natural features. ---------------------------------------------------------------------------------------MUD LOGGING Another kind of logging techniques used is ―mud logging A wellsite geologist (usually called a Mudlogger or ―Mudlog geologist) work when drilling is going on. The geologist Analyzes the rock sample coming circulating mud/fluids off “flow line” from the drill sting /pipe. Similar to a well log, a Mud Log is prepared by the Mud logging company. A mud log displays the gas present in the formation by using gas chromatograph techniques. A mud log also describes the formation geology. USE OF INFORMATION The well log and the mud log are usually transferred in “real time” to the operating, which uses these logs to make operational decisions about the
well and to make interpretations about the quantity of hydrocarbons present. MUD LOGGING Overview Solids Liquids Gasses
Overview: Mud logging is the process of collecting, analyzing and recording the meaningful solids, fluids, and gasses brought to the surface by the drilling fluid (mud). The mud logger keys all of his data to the geolograph on the rig floor. Solids: The mud logger collects sample of the cutting from the downhole strata on a regular basis, usually every ten feet. Collections are usually made at the shaker table for the mud system. The logger then washes and dries the cutting, keeping them properly labeled as to depth that they represent. In order to know what depth the samples actually come from it takes the samples to reach the surface from the time they were cut. The greater the depth, the greater the time for the samples to reach the surface after they were cut. In order to help with the lag calculations, the logger tracks hole size, pump strokes and occasionally sends distinctive sample down the pipe to measure the actual lag.
The logger examines the dried sample under a binocular microscope and records the predominant rock types on a depth strip chart that has drill times and chromatograph reading also recorded on it. Rock types are often correlated with the drill time for a particular rock type e.g. fast drill time for porous sandstone obviously properly recording what rock type is being drilled at what depth is not an exact science. Some factors that affect the accuracy are longer experience in an area, improper lag times, interbedded thin layers of multiple rock types, finely-ground rock fragments sloughing of uphole rock material, and diligence of the logger. Liquids: The logger analyzes for liquids primarily in two ways: chloride content and fluorescence. Chloride content of the mud is constantly monitored. An increase I chloride from a certain depth can indicate a strong salt water flow from a permeable formation. Samples are also viewed under ultraviolet light to check for fluorescence because most oils fluorescence. In the event that fluorescence is detected. The sample is subjected to cleaning fluid to determine whether it is hydrocarbon or mineral fluorescence. Gasses A key component of a mudlogging unit is a gas chromatograph. This unit continuously samples the gases coming out the mud and analyzes them for methane and the heavier hydrocarbons. The presence of ‗show„ of hydrocarbons on the chromatograph alerts the logger to be more alert for
other evidences of hydrocarbons. Sometimes a separate ―lag‖ must be determined for gasses as they may rise faster to the surface than the samples. A bag of carbide dropped down the drill pipe is usually used for this purpose. MUD LOGGING What is Mud Logging? Mud logging provides subsurface geological information while drilling a well. Mud logging examines and analyzes geological information contained in formation cutting and drilling mud, to determine if oil and gas are encountered during well drilling. Mud logging also provides critical safety function such as determine pore pressure, kick control and ambient gas monitoring. Mud logging is used whole drilling most exploratory and much development wells, both on and off shore. Mud logging service range from basic unmanned gas detection to sophisticated full serviced, manned operations. Services have evolved to become the inform the well and include electronic monitoring of drilling parameters. These parameters include torque, penetration rate, mud levels, pump speed and third party service provider data. Basic unmanned services use cost effective mud logging unit that provides simple gas de potentially productive pay zones and enhances drilling safety. The system is configurable for sulphide Hs2 monitoring. The units
are equipped with alarms to alerts the user if gas increase problems occur and works in conjunction with data logging and drilling penetration rates. Mud logging services provides: Advance drilling monitoring systems capable of gas detection, electronic monitoring and database recording for a full range of drilling parameters on manned or unmanned operations. Experience crews and additional tools to monitor and analyzes both the geology of the subsurface information and the drilling parameters. Various combinations of services and crew sizes depending on customer
requirement and well complexity. A portable laboratory (mud logging unit) that houses the hardware and software (mud logging system) Analysis conducted at wellsite during the drilling process and samples are returned to the operator after analysis. BENEFIT OF MUD LOGGING Mud logging lowers costs and improves the success rates. Manned mud logging uses sophisticated system and experience crews, which range from people. Mud Loggers analyzed geological data as well as drilling parameters to identify and or productive hydrocarbon reserves, subsurface formation porosity and formation fractures. The used to make recommendation that optimize drilling paths (geosteering) for maximum reset production. Mud Loggers also look at the filling parameters in conjunction with formation and provide recommendations to improve drilling rates reduce costs and enhance safety. Advanced Mud Logging Equipment and Services:
Formation evaluation: Analysis of the physical and chemical properties of formation or determine the classification of the subsurface rock formation. Geosteering: Assisting operators in determining the optimum path of drilling a in order to maximize hydrocarbon reserves and well productivity. Drilling Optimization: Analysis of drilling parameters to optimize drilling rates and redu Fracture Identification: Identifying existing fracture in the rock
surrounding a well that r and gas. Thin Zones: Identifying thin zones in the well, which contain producible hydro zones often are overlooked and require sophisticated Mud Logg help identify them. Oil/Water/Gas CONTACT Us: Determining CONTACT US points of oil, water and gas in order quantifying the location of hydrocarbon reservoirs. Gas Chromatograph: Tool that analyzes gas entrained in the drilling mud for its precis composition, which is used to perform complex analyses such a oil/water/gas CONTACT Us, identify thin zones, identify fracture geosteer horizontal wells. High speed units can analyze the gas every 30 seconds. Calcimeter: Tool that determines the amount of limestone and dolomite press formation in order to assist formation evaluation. EXPERIENCED CREW ILI„s experience crews are specialist in manned Mud Logging Services.
ILI has developed its experience workforce primarily through internal training and promotion hires junior personnel as sample catchers and trains them in accordance with our document program. Sample Catchers are promoted to Mud Logger, Data Engineers and pressure Eng meet specific training and experience criteria required for advancement. Ill„s requirements include internal and external training classes with centrally examinations. Also, they have minimum number of wells logged with excellent safety records and job performance. Mud logging crew functions Pressure Engineer: Predicts and interprets pore pressure, which is used for drilling safely and casir seat selection. Usually has at least one year experience as a data engineer. The individual also must have completed the abnormal formation pressure training class and has shown
competence in pressure evaluation. Data engineer: Analyzes drilling and logging data to make
recommendations on drilling Parma documents probable hydrocarbon rich pay zones. Usually has at least two tear as mud logger. This position requires a completed advanced mud logging, evaluation, drilling
engineering and well controls training classes. The person proven competence in the analysis of drilling operations. Mud Logger: Prepares a cutting samples used for drilling and geological interpret with monitoring drilling parameters. Usually the mud logger has
6 to 12 months a sample catcher, worked as a trainee and has passed the basic two part mud course. Sample catcher: Typically an entry level training position for mud loggers. Retrieve sample for system for analysis and assists mud loggers, data engineers and pressure Eng. OUR SERVICES MUD LOGGING -------------------------------------------------------------------------------------------Mud logging and core analysis are direct methods of formation evaluation. Wireline well logging is the indirect analysis of downhole features by electronic methods. To log a well by wireline (actually conductor line), an instrument called a sonde is put on bottom, and a recorder plots a graph at the surface as the sonde is raised to the surface. The numerous logs offered by wireline companies today gather data in many different ways under many different conditions. Many, but not all, logging devices can be run in a single sonde in one wireline trip. A specific combination is usually chosen for the types of formation data needed. Correlation between the curves gives a clear picture of lithology, porosity, permeability, and saturation up and down the wellbore. Caliper logs, spontaneous potential logs, resistivity logs, radioactivity logs, or acoustic logs may be included in a typical logging run. Mud logging is a useful evaluation technique that has developed since the advent of rotary drilling in the 1920‟s. Since mud circulates constantly
during drilling, mud logging can provide information on a continuous formation sample. A mud logger checks mud for oil and gas and collects bit cuttings for analysis. Bit cutting analysis is very useful, since it can tell much about rock types and formation characteristics, which must be known for mapping formation beds. Information gathered by mud logging is recorded on a mud log. Basic mud logging involves: Depth and ROP determination Lag time determination Cuttings sampling and lithological description Gas sampling and analysis INTERNATIONAL LOGGING OVERSEAS Mud Logging Unit The basic INTERNATIONAL LOGGING OVERSEAS mud logging unit is 27‟ x 8‟ x 8‟ 6” and weighs 10 tons. It contains the following standard equipment: • Gas equipment (2 FID THD, 2 FID Chromatographs, CO2 Detector, H2S Detectors) • 1 HP Integrator • 4 computers • 1 intelligent chart recorder or 2 continuous recording charts • 1 Laser jet for printing reports and plot logs • 1 Epson inkjet printer to plot out logs
2 Microscopes • 1 Microgas blender • Pit Sensors • Rig floor sensors (pressure transducers, RPM proximity and torque sensors) • Depth sensors (either drawworks, geolograph encoders or block height proximity sensors) • Mud property sensors (mud weight, mud temperature and mud resistivity sensors) • Mud press, laboratory glassware and chemicals to allow chemical tests on cuttings and mud • Equipment for catching samples • External monitors for data display On offshore floating rigs additional equipment is installed: • Rig-motion compensated depth system The unit could be equipped handle coring jobs, shale density, shale factor, mineral staining and other specialized tests. The International Logging Overseas mud logging unit is be able to provide: • Personnel trained in the interpretation of all derived data, capable of providing interpretation and observations to the wellsite geologist and to the company man
Personnel trained to monitor drilling parameters, monitor mud properties, pit volumes, and monitor gas and its components using FID THA, Chromatograph-Integrator, CO2 and H2S • Data processing, storage, display and transmission to off wellsite locations • Record on charts drilling parameters • Print out, store and display mud logs, pressure logs and other logs needed at the wellsite • Personnel trained to do pore pressure monitoring and estimation • Personnel trained to perform specialized tests and analyses as required by the client • Personnel trained to handle core samples Lag Time Determination 11.3.1 Introduction A definite time interval is always required for pumping the samples from a particular depth to the surface where they can be collected. This time interval is called lag time and can be measured in terms of pump strokes and in terms of time. This lag applies to all downhole information, including formation cuttings and the fluids (gas, oil and water) that they contain. The lag always exists and changes continuously as the hole becomes deeper. As such, it is necessary to know the lag and apply it continuously
to returning samples. Due to the factors that cause it to change, the lag must be frequently checked and corrected. With the above in mind, lag determination is one of the most fundamental concepts that must be mastered by the mud logger. Lag time will allow for correct correlation with real-time parameters such as depth and drill rate. Lag time is dependent on two factors: the volume of drilling fluid in the annulus and the flow rate of the drilling fluid. The annulus is the space around a pipe in the wellbore, the outer wall being the wall of either the hole or casing. It is sometimes called annular space. The faster the mud is pumped into the borehole, the quicker it returns to the surface. Similarly, an increase in depth means an increase in annular volume, and hence an increase in lag time. As the annular diameter gets larger (due borehole washouts), the annular volume increases and so will lag time. For a given annular volume, the lag time (in minutes) can be calculated by dividing the annular volume (bbls) by the flow rate (bbl/min). When using a time value, it should be remembered that if the flow rate changes, so will the lag. To compensate for such changes, lag time is also converted into pump strokes, so a change in pump speed (spm) will not affect lag time. Formation Gas Determination Mud logging is performed by using the returning mudstream as a medium of communication with the bottom of the borehole. There is a general relationship between the kind and amount of hydrocarbons in the drilling
fluid arriving at the surface, and the hydrocarbons in the formation as it was drilled, with that portion of mud passing across the bottom. The gases, if present, will be released from the cuttings into the mudstream and entrained, probably in solution, in the drilling fluid. At the surface, it is necessary to remove and detect these hydrocarbons. To do this, the following equipment is used: A gas trap which continuously samples the drilling fluid and
simultaneously removes gases from the fluid Equipment to transport and regulate the air-gas mixture from the trap to the detector in the logging unit Gas detector and chromatograph which process the air-gas mixture into concentration and compositional gas readings Equipment / Hardware The Gas Trap To meet the unique requirements of mud logging, the gas trap must perform the following important functions: Extract gases contained in the drilling fluid, independent of such variables as density, viscosity, and gel strength of the mud Sample consistently, regardless of the flow rate through the circulating system The International Logging Overseas gas trap consists of a cylinder that sits in the possum belly, as near to the flowline exit as possible, or in the flowline, either if the possum belly is not available or if the flowline is long.
It allows the mud to continuously pass through it by means of a hole near its base. An agitator motor sits on top of the gas trap and has a propeller shaft extending into the mud. The propeller continually agitates the drilling fluid as it passes through the trap. A continuous flow of air enters through a vent in the top of the trap and is whipped through the mud where the maximum mud surface is exposed. This air-gas mixture is subsequently drawn into the logging unit. The Vacuum and Pneumatic System After the gases are removed by agitation from the drilling fluid, they are transported via a length of hose to the gas detectors in the logging unit by a vacuum pump. The pump pulls a continuous measured stream of sample gas through the vent in the trap. Formation gases, if present, are continuously extracted from the drilling fluid in the gas trap, and are mixed with air and carried into the logging unit via a condensate bottle, where water vapor condenses. The flow of air, or air-gas mixture, passes through additional flow-regulation equipment, plumbing, and instruments and arrives at the detector where a continuous gas reading is obtained. Moisture should be kept out of the gas detection equipment. Total Hydrocarbon Analyzer This system uses a continuous sample fed into a regulated, constant temperature, hydrogen flame. The flame is situated in a high potential (850 volt) atmosphere between two electrodes. As combustion occurs, the gas
predictably constant ratio of these charged particles moves immediately to the positive electrode (anode), inducing a current at that probe. The amount of current induced is proportional to the total ion charge produced in the flame and increases as the percentage of hydrocarbons in the sample increases. The ion charge becomes a measure of the total number of carbon-hydrogen bonds present in the air-gas mixture. The FID detector meter displays the percentage of methane-equivalent (C1) hydrocarbons present in the gas sample. It is calibrated to read 1.00 when a 1% methane calibration gas burns in the FID. When burning a ditch sample containing heavier hydrocarbons (those with a greater number of carbon-hydrogen bonds in the molecular structure than in methane), the meter displays a reading reflecting the proportionately greater number of carbon-hydrogen bonds. For example, when burning a 1% concentration of pentane (C5), the meter reads 5.00; when burning a 2% pentane or a 10% equivalent methane mixture, the meter reads 10.00 (2% pentane = 2 x 5 =10 carbon-hydrogen bonds; 10% methane = 10 x 1 = 10 carbon-hydrogen bonds). Each of these readings indicates that the relative concentration of combustible
hydrocarbons is 10 times greater than that in the calibration gas. Therefore, it is possible to read more than 100% or 5000 methaneequivalents units. FID Chromatograph
Gas chromatography is the physical separation of gases into its components. It has the same functional description as many other analytical instruments: chemical compounds in, signal voltage out. Wellsite gas chromatography has made great advances since the early 1920's. The use of a chromatograph to separate out the components of formation gas has enhanced the ability to perform accurate interpretation of reservoir quality. Previous advancements in gas chromatography have dealt with the type of detector used (CCD, TCD, FID, etc.), the type of column packing materials (diatomaceous earth, polymers, etc.), and the medium through which the gases were separated (Gas-Liquid, GLC or Gas-Solid, GSC). A chromatograph separates and analyzes hydrocarbons in the ditch gas sample to determine how much of each hydrocarbon is contained in the sample. The ILO gas chromatograph is a flame ionization detector (FID). The FID gas chromatograph, utilizes two stainless steel columns, packed with large surface area polymer beads, to separate the gases. Once separation has occurred, the individual hydrocarbons go to a circular chamber inside an aluminum block for detection. This chamber (the FID chamber) completely encloses a hydrogen flame that is not affected by logging unit pressure or by normal amounts of carbon dioxide and nitrogen. Hydrocarbons entering the high potential hydrogen flame are ionized. The detector response will be essentially proportional to the carbon content of
the molecule and will depend on the quantity of gas entering the flame per unit of time. The response to ion flow is sent to a high-gain amplifier, then to a chart recorder and recording integrator. The FID has a greater dynamic range and is more linear to higher gas concentrations than the older catalytic chromatograph. It is also less likely to be affected by temperature change. HP Integrator Integrators are small modular microprocessors, specifically designed to process the output signal from chromatographic instruments. This signal is plotted as a function of time, forming a plot of symmetric peaks, known as a chromatogram. This plot is useful for both qualitative and quantitative analysis. The positions of the peaks serve to identify the components of the sample; the areas under the peaks can be related to concentration. For chromatograms, zero on the time axis corresponds to the moment the sample was injected into the chromatograph's columns and elution was started. The average rate for the molecules to reach the detector is the retention time. Chromatogram The recording (or “signature”) of the gas-air mixture is termed a chromatogram. The sensitivity of the detector to each gas is established on a regular basis by passing a calibrated sample through the columns. This
calibration mixture contains known concentrations of methane through pentane. CO2 Detector To be published at a future date. H2S Detector To be published at a future date. Blender Gas Analysis Some clients may require the logging personnel to perform blender gas analysis. The gas extracted from this type of analysis is called cuttings gas. The cuttings gas is extremely important as it may form the basis for further evaluation as an indicator of reservoir porosity and permeability. INTERNATIONAL LOGGING utilizes a blender hooked up to one of the THA in the unit to perform the blender gas analysis. The detector is used to check the amount of combustible hydrocarbons in the drilling mud and drill cuttings. It differs from the online THA in that it is a batch system. Samples of the mud and unwashed samples are collected and checked periodically (always during any ditch gas shows). The sample
(approximately 200 cc) is placed in a blender jar and agitated for a standard length of time. The resultant air-gas mixture is drawn into the gas detector. The gas readings are read directly and recorded in the worksheet as gas units/percent. Gas Show Determination and Analysis Definition
A gas show is a significant occurrence of hydrocarbon gases detected from the mud stream and identifiable as being the result of the drilling of a specific increment of formation. It is any deviation in gas amount or composition from the established background. To decide whether a gas show is “good” or “poor” requires a total evaluation of all mud logging parameters plus a consideration of many other variables. Sources of Gas in the Mud Gas can originate from a formation via a number of mechanisms. It is necessary for the mud logger to isolate and monitor these causes to draw the appropriate conclusion. Gas originating from other sources or indirectly from the formation will also be seen in the mud stream. These must be recognized and removed from consideration. There are four sources of gas in the mud. These are: Liberated Gas Produced Gas Recycled Gas Contamination Gas Liberated Gas One source of gas in the mud stream is liberated gas. This is the gas released from the formation as the bit crushes it. Not all the gas contained in the cuttings is liberated to the mud stream. Some will be trapped in the
cuttings because of poor permeability and others will stay in solution dissolved in the mud stream and will not be released at the surface. Produced Gas Produced is gas entering the borehole from the walls or bottom of the well when there is no drilling activity. There are two distinct types: Produced gas that occurs when a condition of underbalance exists: If there is sufficient permeability, there is a natural tendency for the formation fluids to flow into the borehole because of the negative differential pressure. This “feed-in” of gas, if not controlled, may result in a kick. Examples are trip and connection gases. In a condition of balance or even some overbalance there will be a continual diffusion of fluids between the formation and the hole: This is encouraged by the removal of filter cake by pipe movement and by the flow of mud past the exposed wall. Recycled Gas Recycled gas is gas that is not liberated at the surface and is their entrained in the mud stream that is pumped back into the borehole. The recycled gas will present a more diffused gas having a larger proportion of heavy gases, since the more volatile components are often liberated to the atmosphere. Recycled gas usually comes back to the surface after one complete circulation (down time + lag time). Gaseous Contaminants
When petroleum products are added to the mud or mud additives degrade, we may get anomalous gas shows called contamination gas. Diesel, whether in the form of an oil-based mud or a mud additive to reduce drill string torque or as a spotting fluid to free stuck pipe, is the major contaminant. Gas Types in the Mud Log Two types of gas are found on the mud log. These are continuous and discontinuous gases. Continuous gases are entered as a smooth curve, examples being background gas and gas shows. Discontinuous gases are entered either as a smooth curve or in histogram fashion, or with an alphanumeric designation, examples being
chromatograph gas and cuttings gas, and connection gas and trip gas. Other Mud Logging Equipment Microscope A binocular microscope with several magnifications is used for lithological evaluation and descriptions. Ultraviolet-Light Box The UV box is used for determining the percentage, physical character, color, and intensity of hydrocarbon fluorescence in drilling fluid, cuttings and core chips. It consists of a box with a viewer on the top, containing ultra-violet tubes and white light bulbs. The 3600Å, long ultraviolet source used by ILO is effective in producing fluorescence in the visible region, since it is itself on the threshold of visibility.
Pit Level Indicators International Logging Overseas uses two types of pit-level indicators. These are: delaval pit sensors and ultrasonic sensors. The delaval sensor has an array of reed switches enclosed in a metal rod. A ball or float rises or falls with the mud level. This movement is detected by a change in resistance in the electrical current from the sensor to the logging unit. Pit level data is continuously recorded in a chart using the maximum allowable chart span to ensure maximum sensitivity so that any changes in pit levels are immediately recognized. Pump Stroke Sensors Pump stroke counters are used to monitor all active mud pumps, primarily for correct lagging of samples to surface. The sensor is a proximity switch installed in a mud pump, which is tripped by the action of the pump's rod. The DLS has a digital counter for each pump, and a stroke per minute (spm) meter. Sensors for Monitoring Mud Properties ILO uses three types of sensors to monitor mud properties of the mud stream going into the hole and leaving the hole. Those monitoring the properties of mud going to enter the hole are emplaced in the suction pit. While those monitoring the properties leaving the hole are emplaced in the shale shakers. The mud temperature sensor is used to monitor the temperature of the mud going into the hole and coming out of it.
Rig Floor Sensors Rig floor sensors that are used are pressure transducers, proximity sensors and torque sensors (either electrical or mechanical). Pressure transducers are used to monitor the hookload, pump pressure and the choke or casing pressure. They come in two ratings of 800 psi (for hookload) and 4000 psi (for pump and casing/choke pressures). Transducers with higher ratings of 10000 and 15000 psi are also used. Mud logging: the recording of information derived from examination and analysis of formation cuttings made by the bit and mud circulated out of the hole. A portion of the mud is diverted through a gas-detecting device. Cuttings brought up by the mud are examined under ultraviolet light to detect the presence of oil or gas.