Marine deltas
The mouths of rivers fall into two general categories. A river with a low sediment load may reach the sea in an estuary, where the effect of current, waves, and tides keeps sediment from accumulating. A heavily laden river, however, usually creates a marine delta, a seaward extension of land at or near sea level caused by the accumulation of sediments at the river's mouth. A delta is composed mostly of sediments brought down by the river, but mixed with them are fine wave-borne sediments...
Aspects Of The Transitory Field
The transitory field is responsible for the following variations of the magnetic field. Secular variations of approximately 15 gammas per year - a minor effect. Diurnal solar variation on the order of 30 to 40 gammas per day a minor effect. The cyclical Eleven Years variation - a minor effect. Magnetic storms which may reach several hundreds of gammas - a major effect. The Earth's own magnetic field extends out to approximately 8 times the radius of the planet. Beyond this prevails the Magneto...
Faults
Many structural traps, especially those near the surface, can be located by careful study of the surface geology. Stratigraphic traps, however, are usually unrelated to surface features. Their elusiveness has stimulated the development of more and more sophisticated exploration techniques and devices. Many stratigraphic traps have been discovered accidentally while drilling structural traps. A good example of why stratigraphic traps are hard to find is the shoestring sand. This type of...
Vertical Wells
The term straight hole loosely describes a borehole that a drilling contractor has drilled vertically, from top to bottom. In reality, practically all wellbores deviate from the vertical. It is virtually impossible to drill a perfectly straight hole. Drilling contracts recognize this fact and allow a variation from the strict term. A better description of modern drilling practices is controlled deviation drilling because industry now accepts a straight hole as one that meets two qualifications...
Anticlines
A short anticline plunging in both directions along its strike is classified as a dome. A dome is distinguished on structural maps by its nearly circular shape. Many domes are the result of diapirism, the penetration of overlying layers by a rising column of salt or other light, mineral. Salt domes are common along the U.S. Gulf Coast and in the Middle East, northern Germany, and the Caspian Volga area of Russia. When deformational forces exceed the breaking strength of rock, the result is a...
Secondary Migration
Hydrocarbons are moved through permeable rock by gravity. This force works in several ways by compressing pore spaces containing fluid, by causing water containing hydrocarbons to flow, and by causing water to displace less dense petroleum fluids upward. Saying that water flows through formations does not mean that it flows in underground rivers. Flow can mean movement of a few inches a year, which can add up to many miles in a geologically short time. What causes water to flow is a difference...
Directional Sensor Package Spacing
In order to avoid magnetic interference, non-magnetic drill collars must be used and empirical charts are used to estimate the length of non-magnetic material needed. Experiments have shown that mud motors can produce a magnetic field from 3 to 10 times greater than components such as steel stabilizers and short drill collars. As a rule of thumb, anytime a mud motor is run, a non-magnetic short drill collar of 3m to 5m should be placed between the motor and sensor package. It may even be...
Magnetic Declination
Many people are surprised to learn that a magnetic compass does not normally point to true north. In fact, over most of the Earth it points at some angle east or west of true geographic north. The direction in which the compass needle points is referred to as magnetic north, and the angle between magnetic north and the true north direction is called magnetic declination. You will often hear the terms variation, magnetic variation, or compass variation used in place of magnetic declination,...
Survey Accuracy
The first paper that successfully dealt with this subject was prepared by Wolf and de Wardt, Borehole Position Uncertainty - Analysis of Measuring Methods and Derivation of Systematic Error Model. The latest paper released is a summary of work completed by a small joint-industry group and a steering committee on wellbore survey accuracy - Accuracy Prediction for Directional MWD, SPE 56702. This section summaries some of the main points from these papers on sources of error with examples. The...
Average Angle
When using the average angle method, the inclination and azimuth at the lower and upper survey stations are mathematically averaged, and then the wellbore course is assumed to be tangential to the average inclination and azimuth. The calculations are very similar to the tangential method and the results are as accurate as the balanced tangential method. Since the average angle method is both fairly accurate and easy to calculate, it is the method that can be used in the field if a programmable...
Sedimentary Rock
When any type of rock is exposed at the surface, it becomes subject to weathering and erosion. Weathering processes are those that break down the structure of the rock by chemical and physical attack. Erosion is the removal of weathered rock or soil particles by flowing water, wind, moving ice, or other agents. When weathering has proceeded far enough, the erosion process may complete the job of separating the particles from the parent rock. The rock and soil particles carried away by erosion...
Combination Traps
Many petroleum traps have both stratigraphic and structural features. Some, in which both types of characteristics are essential in trapping petroleum, are difficult to classify as either primarily structural or primarily stratigraphic. For instance, originally horizontal formations that now pinch out updip can trap hydrocarbons that might not otherwise have accumulated. Secondary porosity in a shattered brecciated fault zone or anticlinal crest is a stratigraphic trapping mechanism caused by...
Drilling Fluids
Most drilling motors are designed to operate effectively with practically all types of drilling fluids. In fact, the stator or power-section of most PDM's are supplied by the same one or two manufacturers with the same general elastomer type. Successful runs have been achieved with fresh or salt water, oil based fluids, fluids with additives for viscosity control or lost circulation, and with nitrogen gas. However, some consideration should be taken when selecting a drilling fluid, as elastomer...
Kick Pads
Most drilling motors can incorporate wear pads directly above and below the adjustable bend for improved wear resistance. Eccentric kick pads can also be used on most motors ranging from 121 mm 4 3 4' to 245 mm 9 5 8 in size. This kick pad is adjustable to match the low side of the motor to increase build rate capabilities. It will also allow lower adjustable settings for similar build rates, thereby reducing radial stresses applied to the bearing assembly, and permit safer rotation of the...
Trapping
The permeability of the formation that seals off a petroleum reservoir is never absolutely zero, but just low enough to reduce the flow rate effectively to zero under reservoir conditions. Given enough pressure and fluidity as opposed to viscosity , hydrocarbons may seep into a tight formation that under less extreme conditions would totally exclude them. Effective permeability is the rock's permeability to a given fluid when another fluid is also present. Water has seven times the ability of...
Balanced Tangential
The balanced tangential method is similar to the tangential method in that the tangent to the angle determines the wellbore course. The difference between the two methods is the balanced tangential uses both the upper and lower surveys stations. The top half of the wellbore course is approximated by the upper inclination line I1A in the figure below and the lower half of the wellbore course is approximated by the lower inclination line AI2. The azimuth is approximated in the same manner. Both...
Tool Face
Tool face TF is an angular measurement of the orientation of the BHA versus the top of the hole gravity tool face or magnetic north magnetic tool face . Reference for tool face is usually the Scribe mark on the non-magnetic drill collar. Computation for tool face angles are made from magnetometers. Accuracy requirement for tool face typically - 1 to 2 degrees is not at all the same as the one for Azimuth typically 1- 0.5 degrees . None of the directional sensors in common use have any moving...
Conglomerates
Conglomerates are rocks most of whose volume consists of particles more than 2 mm in diameter. They are formed from the sediments found in alluvial fans, debris flows, glacial out- wash, and other high-energy depositional environments. Most conglomerates occur in thin, isolated layers they are not very abundant. In common usage, the term conglomerate is restricted to coarse sedimentary rock with rounded grains conglomerates made up of sharp, angular fragments are called breccia. The rounding of...
Azimuth References
Azimuth, AZ used in directional drilling, may be defined as the direction of the wellbore at a given point projected into the horizontal plane measured in a clockwise direction from Magnetic North, True North or Grid North after applying a North Reference system. Azimuth should be expressed as a value on a 0 -360 compass system. Quadrant or bearing systems i.e. N45 20'E may be easier to visualize, but make the probability of convergence mistakes higher than in an azimuth system. It is therefore...
Radius of Curvature
The radius of curvature method is currently considered to be one of the most accurate methods available. The method assumes the wellbore course is a smooth curve between the upper and lower survey stations. The curvature of the arc is determined by the survey inclinations and azimuths at the upper and lower survey stations as shown below. The length of the arc between I and I2 is the measured depth between surveys. In the previous methods, the wellbore course was assumed to be one or two...
Power Section
The drilling motor power section is an adaptation of the Moineau type positive displacement hydraulic pump in a reversed application. It essentially converts hydraulic power from the drilling fluid into mechanical power to drive the bit. The power section is comprised of two components the stator and the rotor. The stator consists of a steel tube that contains a bonded elastomer insert with a lobed, helical pattern bore through the center. The rotor is a lobed, helical steel rod. When the rotor...
Directional Drilling Limitations
Any drilling limit described in a textbook written today would be simply broken tomorrow by some operator. We have drilled horizontal wells with laterals over 6,100m long extended reach wells with over 10,000m of horizontal reach horizontal to vertical ratio of 6 or 7 to 1 multi-lateral horizontal wells with 10 legs purposefully turned horizontal wells 180 in bearing drilled 27 wells off a single land based pad location re-entered just about every wellbore configuration to drill to a new target...
Electromagnetic Mwd
Directional surveying with the EM MWD tool has become a reliable and cost effective means for surveying both directional and horizontal wells. Advances made over the past years have significantly improved tool reliability when drilling in harsh underbalanced air mist environments, and have also overcome some of the earlier obstacles associated with operational depth. Additionally since the rig pumps do not have to be cycled to receive a survey, the overall survey cycle time can be reduced and...
Jetting
The jet bit method of deflecting a well was, at one time, the most common method used in soft formations. Jetting has been successfully used to depths of 8,000 feet 2,400m however the economics of this method and the availability of other directional drilling tools limit its use. Geology is the most important influence on where jetting can be used next in importance is the amount of hydraulic energy available for jetting. Sandstones and oolitic limestones that are weakly cemented are the best...
Differentiation
With few exceptions, petroleum reservoirs are water-wet - that is, the oil is not in contact with the rock grains because they are coated with a film of water. Most oil fields have 50 to 80 maximum oil saturation. Above 80 oil saturation, the oil can be produced with very little water mixed in below 10 , the oil is not recoverable. A hydrocarbon reservoir is divided into two or more zones. If only oil and water are present, the oil occupies the upper zone. Although water still lines the pores,...
CrossHair Pendulum Compass
One of the most common types of angle units, for inclination and direction up to twenty degrees. The compass card is free to rotate inside the housing and maintain a reference to magnetic north. The inclinometer is an independent and free swinging pendulum cross-hair. The compass card is printed in reverse in order for the pendulum, which naturally falls to the low side, to depict the direction as it should be on the high side. The survey disk is read as correct. Care should be taken when...
Directional Sensor Package
The directional sensor package of any MWD tool consists of a set of triaxial inclinometers and triaxial magnetometers to measure respectively hole inclination drift and hole direction azimuth . The triaxial inclinometer measures the 3 orthogonal axes components of the earth gravity vector 'G'. The triaxial magnetometer measures the three orthogonal axes components of the earth magnetic field vector 'H'. The reference axes for measurements are usually as follows but each vendor's tools can have...
Openhole Whipstock
The whipstock was the first widely used deflection tool for changing the wellbore trajectory, and is seldom used in open-hole deflections today. A whipstock is selected according to the wedge needed to effect the desired deflection. A bit that is small enough to fit in the hole with the whipstock is then chosen at the start of the running mode, the bit is locked to the top of the whipstock. When the whipstock is positioned at the kickoff depth, whether it is the total depth of the wellbore or...
Optional Topics
Most commercial MWD Systems use mud pulse or electromagnetic telemetry to transmit survey data during tool operation. In Mud Pulse the mud pressure in the drill string is modulated to carry information in digital form. Pressure pulses are converted to electric voltages by a transducer installed in the pump discharge circuit standpipe . Then this information is decoded by the surface equipment. Tool measurements toolface, inclination, azimuth etc. are digitized downhole and then the measured...
How do I determine the Declination diagrams on maps
Most topographic maps include a small diagram with three arrows magnetic north, true north and Universal Transverse Mercator grid north. The given value of declination, corresponding to the center of the map, does not take local anomalies into account. The value is usually out of date, since it may have drifted several degrees due to secular change, especially on maps of remote regions with several decades between updates. Some maps, such as the 1 50,000 scale topographic maps by the Canadian...
What factors Influence Declination Location
Each position on the Earth has a particular declination. The change in its value as one travels is a complex function. If a navigator happens to be traveling along a rather straight line of equal declination, called an isogonic line, it can vary very little over thousands of kilometers. However for one crossing isogonic lines at high latitudes, or near magnetic anomalies, the declination can change at over a degree per kilometer 6 10 mile . Predictive geomagnetic models such as the World...
Igneous Rock
Igneous rocks are those formed directly from molten rock material, or magma. When the planet was formed, the original crust was entirely igneous today the percentage is 65 . Two principal types of igneous rock are intrusive plutonic , those that have solidified below the surface, and extrusive volcanic , those that have formed on the surface. Granite, the most common intrusive igneous rock, has crystals that are easily seen by the unaided eye. Magma that reaches the surface is called lava...
Motor Selection
Four configurations of drilling motors provide the broad range of bit speeds and torque outputs required satisfying a multitude of drilling applications. These configurations include 4. Low Speed High Torque -Gear Reduced The high speed drilling motor utilizes a 1 2 lobe power section to produce high speeds and low torque outputs. They are popular choices when drilling with a diamond bit, tri-cone bit drilling in soft formations and directional applications where single shot orientations are...
Magnetic Field Strength
The total magnetic field strength may be referred to as the H value, HFH, magnetic field strength or TTL field. The Geological Society Electromagnetic Units are used for measuring the strength of the Earth's magnetic field and are called Gammas. Some useful conversions 1 gamma 1 microtesla 1 tesla 1 gauss 1 gauss 1 gauss 1 tesla 1 x 10 Tesla 1000 gammas 1 x 109 gammas 1 x 105 gammas 1 x 10-4 Tesla 1 oersted The magnetic field intensity recorded at ground level is of a much smaller magnitude...
Methods of Deflecting a Wellbore
There are several methods of deflecting a wellbore. Deflecting means changing the inclination and or direction of a wellbore. Three common methods of accurately kicking off a well are whipstock operations, jetting, and downhole motors. The mud motor techniques are most commonly used because they are fast and accurate, however the whipstock is still used. Jetting is rarely used today, but it is still a valid and inexpensive technique. Rotary bottomhole assemblies are the least expensive method...
Dump Sub Assembly
As a result of the power section described below , the drilling motor will seal off the drill string ID from the annulus. In order to prevent wet trips and pressure problems, a dump sub assembly is utilized. The dump sub assembly is a hydraulically actuated valve located at the top of the drilling motor that allows the drill string to fill when running in hole, and drain when tripping out of hole. When the pumps are engaged, the valve automatically closes and directs all drilling fluid flow...
Scale Inclinometer Compass
Similar in principle to the pendulum cross-hair, this angle unit has an independent weighted inclinometer which appears as a scale superimposed onto the compass card on the survey photo disc. This type of angle unit is normally used for higher inclinations above twenty degrees . Depending on the manufacturer, gravity toolface is interpreted either as read or is reversed. Care should be taken to establish the correct method of determining gravity toolface, before using the single shot for...
Bioherm
Another type of stratigraphic trap is the reef. A wave-resistant accumulation of coral or shells serves as an anchor for calcareous debris that forms limestone. If deeply submerged faster than it can accrete it may become buried beneath marine shales. Such an isolated reef is called a bioherm. It may be porous enough to hold large accumulations of hydrocarbons, especially if it has been dolomitized. Limestone is especially vulnerable to dissolution by groundwater, particularly if raised above...
Drill String Magnetic Interference
The drill string can be compared to a long slender magnet with its lower end comprising one of the magnetic poles. Even if the components of a drilling assembly have been demagnetized after inspection, the steel section of the drill string will become magnetized by the presence of the Earth's field. Drill string magnetism can be a source of error in calculations made from the supplied magnetometer data. This may happen as the angle builds from vertical Figure 5-4 or as the azimuth moves away...
Azimuth Reference System Conversions
Most well proposals are generated from rectangular coordinates derived from the UTM or local grid system. The surface location to target direction will therefore be referenced to Grid North. Since wells must be surveyed with sensors that reference direction to either Magnetic or True North, it will be necessary to convert between these references. 92 301 w 30 401 N 91 201 28 40' 91 201 25 40' Magnetic declination correction converts azimuth values between the Magnetic North and True North...
Magnetic Declination Angle
The Earth can be thought of as having a magnetic dipole running through its center with north and south poles at either end. This dipole does not correspond with the Earth's rotational axis tilted approximately 12o relative to earth's axis of rotation . The angle between magnetic north and geographic north True North is defined as the magnetic declination or the angle of declination refer to illustration . The magnetic declination is dependent upon the location latitude and longitude and may...
Analog Circuit
The Analog Circuit provides an interface with the inclinometer, magnetometer, and pressure transducer sensors. The 16 channel multiplexer on the analog circuit takes input from various sensor outputs and sends the data to the logic circuit for transmission. A sensor power switch takes power from the 12 volt regulator and selectively powers up the accelerometers and magnetometers. A 5 volt excitation supply from the 12 volt regulator is used to power the pressure transducer. The status voltages...
Directional Surveying Measurements And Sensors
In order to guide a wellbore to a desired target, the position and direction of the wellbore at any particular depth must be known. Since the early days of drilling, various tools have been developed to measure the inclination and azimuth of the wellbore. To calculate the 3D path of the wellbore, it is necessary to take measurements along the wellbore at known depths of the inclination angle from vertical and azimuth direction normally relative to true north . These measurements are called...
Cementation
As compaction brings individual particles into closer contact, the process of lithification is completed by cementation. Minerals in solution - mainly calcite CaCO3 , the basic constituent of limestone -crystallize out of solution to coat the grains. Other common cementing agents include silica SiO2 , which is less soluble and therefore less abundant in groundwater and iron oxide Fe2O3 , which colors the rock yellow or red. These coatings grow together and may eventually fill the pore spaces....
Grid Systems
One of the oldest systematic methods of location is based upon the geographic coordinate system. While this information is basic, a short review is included for reference. By drawing a set of east-west rings around the globe parallel to the equator , and a set of north- south rings crossing the equator at right angles and converging at the poles, a network of reference lines is formed from which any point on the earth's surface can be located. The distance of a point north or south of the...
Planning
Once the information has been collected from the various departments, a directional plan is prepared that meets all the requirements if possible . A good well planner also tries to incorporate operational issues that contribute to the success of the well and can have a dramatic impact on the length of time required for directional equipment. This can be very important on pad layouts for multiple directional wells and can save the operator considerable expenses if properly utilized. Anyone can...
Magnetic Interference
There are two types of magnetic interference drill string and external magnetic interference which can include 1 interference from a fish left in the hole 2 nearby casing 3 a magnetic hot spot in the drill collar 4 fluctuation in the Earth's magnetic field and 5 certain formations iron pyrite, hematite and possibly hematite mud . Any deviation from the expected magnetic field value can indicate magnetic interference. External magnetic interference can occur as the drill string moves away from...
Applications of Magnetics and Gravity
In the MWD sensor package, two sets of sensors are used. One set magnetometers uses an XYZ system to measure orientation with respect to the earth's magnetic field Hx, Hy, Hz . The other set accelerometers uses an XY or XYZ system to measure orientation with respect to the earth's gravitational field Gx, Gy, Gz . From the magnetic sensors we can learn inclination, azimuth, and tool face angles. From the gravity sensors we can learn inclination and tool face angle. For hole inclinations of 0 to...
Reactive Torque
Drilling motors drive the bit with a right-hand clockwise rotation. As weight is added to the bit, reactive torque acting on the drilling motor housing is developed. This left-hand counter-clockwise torque is transferred to the drill string and may cause the joints above the motor to tighten. Reactions of this type increase with larger weight on bit values and reach a maximum when the motor stalls. This reactive torque also affects the orientation of the motor when it is used in directional...
Directional Drilling Azimuth Reference Systems
This section discusses the primary azimuth reference systems currently used in directional drilling. This will include True North and Magnetic references with particular detail given to Grid Coordinate systems i.e. UTM, Lambert, Geographic, and Local . A simple field-proven method is also presented to help avoid confusion when converting from one system to another. More than one multi-million dollar directional drilling project has missed its intended target s due to errors and or...