Scintillation Detectors
Detection of gamma rays using Scintillators represents a significant improvement over both the efficiency and spectral deficiencies of G-M tubes. The most common scintillation detector, and the one used in MLW LWD sensors, is the sodium iodide NaI crystal. Again, the detection occurs as the gamma ray interacts with the electrons in the NaI crystal, either through Compton scattering or the photoelectric event. However, the effective volume of the detector is that of the crystal to be compared...
Gtotal Btotal
As a general guideline, MDIP should not vary by more than - 0.3 degrees from the local magnetic dip angle or from survey to survey during a bit run. The local magnetic dip angle is determined by using magnetic modeling software or directly measuring it through infield referencing. Surveys which do not conform to this guideline should alert the field engineer that some magnetic interference or pipe movement is probable, or that there was a hardware failure. MDIP may also change abruptly from bit...
Gravity Toolface
Gravity toolface is the angular distance the bent sub scribe line is turned, about the tool axis, relative to the high side of the hole Gravity toolface Dir Probe Gravity Toolface Toolface Offset If the inclination of the wellbore is above 5 to 8 , then gravity toolface can be used The toolface will be referenced to the highside of the survey instrument, no matter what the hole direction of the survey instrument is at the time The toolface will be presented in a number of degrees either right...
Gamma Ray Applications
Determining shale volume - By using the relative response of the gamma curve compared with a 100 shale reading, the shale volume Vsh in the formation can be estimated from the following equation To calculate the shale volume Vsh of any zone, determine the denominator of the equation by subtracting the clean sand reading GRclean from the shale baseline reading GRsh . Then, determine the numerator of the equation by subtracting the reading in the zone of interest GRzone from the shale baseline...
Survey Accuracy As A Function Of Inclination
Berror IS A LARGER PERCENTAGE OF Bref AT HIGHER INCLINATIONS For systematic uncertainty, the uncertainty is along the Z-axis. This will result in decreased survey accuracy when drilling east or west as opposed to drilling north or south. This is due to the fact that Berror will tend to pull Bref in the direction of the Z-axis, away from Bn. This relationship is shown in the figure below.
Cuttings Bed Suppression
Cuttings beds formation in deviated holes can be suppressed by using muds with exceptionally good suspension characteristics. However, high flow rates are still necessary since the beds can never be totally eliminated. For water based mud, the best conventional additive for this purpose is XC polymer. All the major service companies now offer low shear enhancers sometimes referred to as rheology modifiers for oil based muds. A simple force balance approach can be used to determine the minimum...
Pump Rate
The single most crucial factor for successful hole cleaning is mud flow rate, especially for deviated holes. As a rough guide, the annular velocity needed for cleaning wells deviated 50 - 60 is approximately twice that required for the vertical case. Take all reasonable steps to reduce frictional pressure. Doing so will extend the range of available flowrate. In critical cases, careful consideration should be given to BHA design, nozzle selection, and additional losses due to mud motors MWD...
What Are Gamma Rays
Gamma rays are a form of high energy electromagnetic EM radiation. Other examples of EM radiation are visible light, radio and microwaves, infrared and ultraviolet light, and x-rays. Each of these forms is distinguished in the EM spectrum by their frequency f and wavelength A, . In free space that is, a vacuum all EM waves travel at the speed of light, c, which is 3 108 meters second, or 186,000 miles second The frequency and wavelength are related to the speed by the equation While the...
Principles of Neutron Logging
Neutrons are emitted from the source at a high energy, usually between 1 and 10 MeV. However, they are not detected until their energy has been reduced to a fraction of an eV through collisions with atomic nuclei in the formation. The rate at which energy is lost by the neutrons is a function of the elements in formation. Hydrogen is the most efficient element in slowing down, or moderating, neutrons. Porosity is inferred from the detection of these low energy, or thermal, neutrons because...
Broken Diamonds
Broken diamonds result from excessive tangential force. When the diamond's strength is exceeded, a fracture along a cleavage plane s occurs on most bits, and all or most of the exposed stone separates. This is recognizable on the used bit by the generally flat, highly reflective surface. Causes are stones too large, broken formation, junk, too few stones, excessive WOB, improper stabilization, and erosion. Downhole pressure sensors are drilling performance tools that provide continuous and...
Vertical Projection
Declination, dip and Bt are constant No drillstring interference No casing or other non-drillstring interference IFR In-Field Referencing Geomagnetic Field Measurement Method Impact to drilling survey programs Impact to drilling survey programs IFR stations, or geomagnetic observatories, measure the earth's magnetic field at constant time intervals The IFR Method is the ability to enhance survey quality at the rig site The IFR Method is of continuing value to the drilling and sub-surface...
Chipped Cutter
Description A cutter which has minor breakage. Less than half of the cutter has been broken. The chip is in both the diamond layer and tungsten carbide substrate. Cause Impact associated with bit whirl or excessive torque due to change information as well as similar variable. Description Where the diamond layer or a predominant portion of the layer has cleanly come off the tungsten carbide interface. A delamination always occurs at the interface where the tungsten carbide substrate is exposed....
Invasion Process Measurement of Rxo
Log ex. 2 displays a water zone located deeper in the same well as above. The MWD curves in Track II had a formation exposure time of only one hour, and the agreement of all four phase shift resistivity curves signified that no appreciable invasion had occurred. Track III shows the same interval logged 71 2 days later, and significant invasion Di 23 inches had occurred, as indicated by the interrelationship of the curves. This profile is commonly observed in Gulf Coast high salinity water sands...
Survey Accuracy
Survey accuracy is a function of both instrument related uncertainties and systematic uncertainties. Instrument related uncertainties include such things as sensor performance, calibration tolerances, digitizer accuracy, and resolution. This is defined as the baseline uncertainty and it is present in all survey sensors. Systematic uncertainties are a function of magnetic interference from the drill string and can be reduced by housing the instrument in a longer nonmagnetic drill collar. The...
Design Criteria
By properly accounting for the incremental increase in friction on the drillstring, the torque and drag programs can be used to design drillstrings so that the following requirements are met The drillstring has sufficient tension capacity. Conventional drillstring design calls for at least 100,000 lbs of margin between the anticipated weight of the string indicated hookload minus travelling block and kelly and the tensile capacity of the top section of drillpipe specified in API Recommended...
LS Bond Failure
Description Where the braze joint between the PDC cutter and the bonded extension shears or breaks off. Sometimes the bonded extension is still in the cutter pocket while the rest of the cutter is gone. Cause Due to LS Bond failure usually relating back to PDC manufacturing. Description Where the entire blade has broken off of a bit. The fracture must have located between the body and the height of the blade. Cause Due to severe impact. Variables include Bit whirl, severe change in formation,...
Fundamentals of Geosteering
Students will identify geosteering applications and describe methods of enhancing the geosteering effort. They will also define the applications, theory of operations, and response of the formation evaluation sensors Gamma Ray, Resistivity, Neutron, Density and interpret log data obtained during geosteering. Modeling the well using offset well data Mud Logging Data Directional Data Enhancing Geosteering with LWD Sensor Data Gamma Ray Sensor Response Data Interpretation Resistivity
Vibration Modal CouplingInvolves All Three Motions
Description. A coupling motion among axial, torsional, and lateral vibrations. It creates axial and torque oscillations and high lateral shocks of the BHA. The motion is similar to a chaos so the Vibration's average data will not be very high. Typical Environment. Vertical or near vertical wells, pendulum or unstabilized BHA and hard rock. Consequences. MWD component failures, bit cutter impact damage, collar and stabilizer wear, wash-outs and twist-offs due to connection fatigue cracks....
Statistical Correction for Large Standoffs
The single mathematical equation used for the spine and rib correction fits all points of the rib independent of mud weight for standoffs up to 1 inch. Beyond this distance, the correction fails because the curvature and extent of the rib is different for different mud weights. While in most LWD situations the standoff with which the Density must cope is less than 1 inch, there are occasions where errors due to standoff may be significant. As discussed in the preceding section, in very light...
Directional Sensor Measurements and Calculations
The measurements that we make with the DIRECTIONAL SENSOR are made relative to these axes. The X-axis is perpendicular to the tools long axis and is in the direction of the scribe line etched on the DIRECTIONAL SENSOR nonmag sub. The Y-axis is also perpendicular to the long axis. The Z-axis is along the long axis of the DIRECTIONAL SENSOR in the direction the hole is being drilled. The scribe line on the DIRECTIONAL SENSOR sub allows measurement of the relationship TOOL PHYSICAL AXIS between...
Formation Influx
Formation fluids are usually less dense than drilling muds. Therefore, any formation fluid in the mud decreases the mud density. This shows up in the EMW as a rapid and sometimes dramatic decrease see Figure 8 . The exception to this is in riser-less drilling using sea water as the drilling fluid. A water kick from an unconsolidated formation can carry sand into the annulus and increase the density of the annular fluid. This causes an increase in the EMW see Figure 9 . Figure 8 Effect of a Gas...
Mud Weight
Mud weight influences hole cleaning by affecting the buoyancy of the drilled cuttings. This applies for both vertical and deviated holes. For small changes in density, the flowrate required to maintain adequate hole cleaning is directly proportional to the cuttings mud density differential. However, the selection of mud weight in most cases is predetermined. The selection is based on pore pressures, rock mechanics, in situ tectonic stresses, density required to stabilize the hole avoid cavings...
Dull Evaluation
A dull bit is graded to provide an accurate description of the bit condition. Dull Evaluation is a study of a dull roller cone bit to determine the contributing factor or combination of factors that were responsible for the overall condition of the dull. Following is information on the dulling characteristics common to roller cone bits. Possible causes and solutions for problem wear modes commonly encountered in drilling are discussed. These should not be considered exclusive. This describes a...
APGPM MW VIS Pipe ID TFA Pipe Length SPP
15,340' 45FH 3ID 16.6 ppf Annular velocity must be 32 ft min
Short Collar Azimuth
Traditional compass type surveying instruments with their ability to sense only the direction of the local magnetic field vector must be used in conjunction with enough nonmagnetic drill collars so that the local magnetic field vector is uncorrupted by drill string magnetization. With solid state magnetometers and their ability to measure 3 orthogonal axes of the local magnetic vector it is possible to compensate for axial magnetization and use much shorter lengths of nonmagnetic material....
Rate Gyro Disadvantages
Usually more expensive than conventional gyros Must use electric wireline to hoist instrument Must have AC power available at survey site Sensitive to shock and vibration Normal drilling operations must be discontinued while surveying
Hole Angles Above
Turbulent transitional flow is most effective in cleaning and evacuating minimizing cuttings bed formation. Lower mud rheologies at the corresponding annular shear rates will be required to obtain the Reynolds numbers above 2,100. In this case the cuttings cuttings beds will be removed as ripples or dunes. However, turbulent flow cannot be achieved in most of the 445 mm 17-1 2 in holes and some of the 311 mm 12-1 4 in holes. This is due to a variety of reasons including limitation of surface...
Long Collar Azimuth
To calculate azimuth using the conventional method the following data is required 1. Bx magnetic field vector in the X direction 2. By magnetic vector in the Y direction 3. Bz magnetic vector in the Z direction Azimuth is referenced in the horizontal plane to true or grid north. The magnetic field that we measure, however, is at some angle from the horizontal, that is the magnetic dip angle. Therefore to reference our measurement to true north in the horizontal plane we must project the...
Drillstring Design for Horizontal Wells
In drilling a horizontal well, as in a conventional well, the drillstring must perform the following functions Transfer and support axial loads weight on bit Transfer and support torsional loads rotation Hydraulics will be addressed later. This section covers the considerations and calculations required to properly design a drillstring for horizontal drilling. One of the main differences between horizontal drilling and more conventional well types is that the BHA lays on its side. Thus, it...
Calculation Methods
Displacement Difference from Actual Total Survey Depth 5,985 feet Vertical hole to 4,064 feet, then build to 26 at 5,985 feet Survey Intervals approximately 62 feet
Calibration Methodology
The calibration procedure consists of rotating the sensor through the field of investigation for each axis and comparing the output with known values. Examine the ideal response of a single axis rotation, at 0 degrees the sensor axis is aligned with the field and the output voltage is at a maximum. As you rotate the sensor counter clockwise the voltage decreases until at 90 degrees the output goes to 0 volts. As you continue to rotate the sensor counter clockwise the output voltage goes...
Tripping
Always circulate the hole clean prior to tripping. Use minimum circulation times. Rotate the pipe at maximum of 60 rpm when circulating prior to tripping. Use low vis low wt pills for wells gt 30 . Calculate volumes to ensure well control. Make a rotary wiper trip after a long section is drilled with downhole motor. Make sure cementing pumps are available to pump in the case of an emergency. First developed by General Electric in 1955 Originally used in electric motors Began use in the...
Effect of Rotation on Settled Cuttings
When rotation begins, the settled cuttings and cavings are picked up by the increased turbulence and mixing see Figure 18 . The initial jump in EMW at 20 20 is caused by the rotation effect on the well hydraulics. The more gradual increase between 20 20 and 21 00 is caused by the resuspended solids reaching the vertical section of the well. The Pressure sensor does not see this increased cuttings load until the it reaches the vertical section and the increased weight of the mud affects the...
Gamma Ray Emission Spectra of Potassium Thorium Uranium
The radiation from potassium is distinct, with a single energy value of 1.46 MeV. Both thorium and uranium emit radiation with a range of energies, but with certain peak frequencies. These peaks are especially distinct at energy levels of 2.62 MeV for thorium and 1.7 MeV for uranium. Emission spectra for potassium, thorium, and uranium are shown in the figure below. Figure 1 - Gamma Ray Emission Spectra Figure 1 - Gamma Ray Emission Spectra A gamma sensor counts the gamma rays emitted by the...
Borehole Mud Salinity Correction
Chlorine is very efficient at capturing thermal neutrons. Chlorine's thermal neutron capture cross-section 33.3 Barns is 100 times that of hydrogen 0.33 Barns . Thus, increasing salinity causes a reduction in thermal neutron population. This decrease in thermal neutrons will cause lower count rates, appearing to the tool as an increase in porosity. There is also a competing effect in that the increase in chlorine causes a reduction in the hydrogen content of the mud surrounding the tool. This...
Center Coring
Center coring occurs when the diamonds in and around the apex are destroyed or lost. The formation begins to wear away the matrix material and the bit tries to cut a small core. ROP slows or stops and an increase in standpipe pressure may be evident. The causes of center coring include Incorrect diamonds too large Broken or fractured formation Poorly bonded bedding planes Using small diamonds in the apex area will usually solve this problem. A core ejector modification may also help. Improved...
Phase Shift vs CPA Resistivity Accuracy
At higher formation Resistivity, the phase shift Resistivity is more accurate than the CPA Resistivity. There are two reasons for this. First, as the formation Resistivity increases, the change in attenuation with a further increase in Resistivity becomes very small and difficult to accurately resolve. In other words, a cross-plot of attenuation vs. Resistivity see Figure 3 would show the relationship to flatten-out above about 20-50 ohm-m, whereas there remains a small but measurable change in...
Dielectric Effects
The dielectric permittivity of a material is quantified by what is called the dielectric constant. Typically, scientists ratio a material's dielectric constant, s, to the dielectric constant of free space vacuum , so, to simply change the range of typical values. This relative value is referred to as the relative dielectric constant, sr, and is defined as follows All the resistivity transforms we use assume a relative dielectric constant sr of 10. For a given formation resistivity, if the...
Equivalent Mud Weight EMW
The measurement taken by the pressure sensor is an absolute pressure measurement. In theory Hydrostatic Pressure and Equivalent Mud Weight are the same if the borehole is free of cuttings. More often than not the borehole is not free of cuttings and the Annular Pressure sensor is used to provide an accurate downhole pressure reading used to calculate equivalent mud weight EMW . True Vertical Depth TVD must be correct to effectively use EMW to make realtime drilling decisions. EMW is calculated...
Flow Regimes
Hole cleaning in horizontal holes will be enhanced by turbulent flow which promotes efficient cuttings removal. Flow in the annulus is generally laminar for hole diameters 311 mm 12-1 4 in and above. For 216 mm 8-1 2 in sections and below, flow can become turbulent depending upon mud rheology and flow rate. The mud Yield Point has a strong influence over the critical flow rate. The data in figure 27 are for a 1.6 sg 13.3 ppg mud. Lower mud weights will require higher flow rates to reach...
Magnetic Float Compass
A mechanical compass uses a compass card that orients itself to magnetic north, similar to a hiking compass needle, that always points to magnetic north The compass card uses an attached magnet to get its orientation. As the magnet is attracted to magnetic north, hole direction can be read Inclination is measured by means of a pendulum or float device In the float device, the float is suspended in a fluid which allows the instrument tube to move around it independently as inclination A...
Matrix Wear
Fluid erosion is responsible for most matrix wear. All diamond bits will exhibit fluid erosion to some extend after they have been run. The amount of war is proportional to the length of time the bit was run and the hydraulic conditions under which it was subjected. Fluid erosion is caused the drilling fluid passing over the bit face at high energy levels with solids chipping away at the bit matrix. The energy expended increases as the velocity of the fluid increases. High energy levels help...
Rop Directional
Increases in penetration rate result in higher cuttings concentrations in the annulus. Past experience has shown that 0.5 is the maximum allowable annular concentration to efficiently drill vertical and near vertical wells. For deviated wells, deeper cuttings beds form as the penetration rate increases. Removing these deeper beds require higher flowrates. It is important to control and limit instantaneous ROP's in deviated wells since deep beds are difficult to remove. Figure 20 15.8 18.9 22.1...
Bit Nozzle Selection
If sufficient flow rate is available for hole cleaning, then bit nozzles can be selected for optimum hydraulics in the normal way. When sizing bit nozzles, it is also important to note that Certain mud motors have optimum bit differential pressure ranges Nozzles should be selected to minimize potential hole erosion problems for friable formations The table, Bit Pressure Losses amp Velocities below, shows the calculated bit pressure losses and nozzle velocities for a 1.2 sg 10 ppg mud over the...
Magnetic Interference
Magnetic interference problems when surveying a well are usually due to casing or a fish that has been left in the hole. Unfortunately, the majority of the magnetic interference problems occur when the accuracy of our azimuth is very critical. A well is usually kicked off just below a casing shoe or through a window in the casing. The casing is a large concentration of magnetic material, the ends of which act like magnetic poles from which the curving flux lines cause magnetic interference. On...
Analysis in OilBased Mud
Log ex. 3 and Log ex. 4 are examples taken from one of the first Resistivtiy logs run in the North Sea. It is also the first Resistivity log run in an oil-based drilling fluid. This well was drilled with an 8.5-inch diameter drill bit and a borehole inclination of 37 degrees. The intervals shown are a sandstone formation with 25 average porosity. Connate water resistivity is 0.021 ohm-m at formation temperature. Log example 3 North Sea Oil Zone Logged After Coring Operation Log example 3 North...
Acid Bottle Inclinometer
This technique was originally used in the mining industry from about 1870 A glass cylinder filled with hydrofluoric acid was lowered down the drill string on a wireline until it rested on top of the bit or on a baffle plate at some point above the bit The acid bottle was left in this position for about 30 minutes to allow the acid to react and leave a mark on the side of the cylinder indicating the horizontal surface The glass was inspected back at the surface, and the angle of inclination was...
The Mechanics of Dull Grading
The Most efficient and accurate dull grading technique is one that is thorough and consistent. The best way to accomplish this is to examine dulls in exactly the same manner every time. In other words, develop a procedure that is repeatable and uses the same system or steps every time you look at a dull. Too many times we get caught up in focusing on one aspect of the dull and miss other factors that contributed to the total dull condition. The first step is to examine the cutting structure....
Azimuth Error Magnetic 1
1 -3.00 lt -2.00 fe -100 0.00