Directional Drilling Glossary

The glossary of terms used in directional drilling has been developed by the API Subcommittee on Controlled Deviation Drilling under the jurisdiction of the American Petroleum Institute Production Department’s Executive Committee on Drilling and Production Practice. The most frequently used terms listed below.

Angle of inclination (angle of drift). The angle, in degrees, taken at one at several points of variation from the vertical as revealed by a deviation survey, sometimes called the inclination or angle of deviation.

Angle of twist. The azimuth change through which the drillstring must be turned to offset the twist caused by the reactive torque of the downhole motor.

Anisotrospic formation theory. Stratifiedor antisotropic formations are assumed to posses different drill abilities parallel and normal to the bedding planes with the result that the bit does not drill in the direction of the resultant force.

Azimuth. Direction of a course measured in a clockwise direction from 0◦ to 360◦; also called bearing.

Back-torque. Torque on a drill string causing a twisting of the string.

Bent sub. Sub used on top of a downhole motor to give a non straight bottom assembly. One of the connecting threads in machined at an angle to the axis of the body of the sub.

Big-eyed bit. Drill bit with one large-sized jet nozzle, used for jet deflection.

Bit stabilization. Refers to stabilization of the downhole assembly near the bit; a stabilized bit is forced to rotate around its own axis.

Borehole direction. Refers to the azimuth in which the borehole is heading.

Borehole directional survey. Refers to the measurements of the inclinations, azimuths and specified depths of the stations through a section of borehole.

Bottom-hole assembly (BHA). Assembly composed of the drill bit, stabilizers, reamers, drill collars, subs, etc., used at the bottom of the drillstring.

Bottomhole location. Position of the bottom of the hole with respect to some known surface location.

Bottomhole orientation sub (BHO). A sub in which a free-floating ball rolls to the low side and opens a port indicating an orientation position.

Build-and-hold wellbore. A wellbore configuration where the inclination is increased to some terminal angle of inclination and maintained at that angle to the specified target.

Buildup. That portion of the hole in which the angle of inclination is increased.

Buildup rate. Rate of change (◦/100 ft) of the inclination angle in the section of the hole where the inclination from the vertical is increasing.

Clearance. Space between the outer diameter of the tool in question and the side of the drilled hole; the difference in the diameter of the hole and the tool.

Clinograph. An instrument to measure and record inclination.

Closed traverse. Term used to indicate the closeness of two surveys; one survey going in the hole and the second survey coming out of the hole.

Corrective jetting runs. Action taken with a directional jet bit to change the direction or inclination of the borehole.

Course. The axis of the borehole over an interval length.

Course bearing. The azimuth of the course.

Crooked-hole. Wellbore that has been inadvertently deviated from a straight hole.

Crooked-hole area. An area where subsurface formations are so composed or arranged that it is difficult to drill a straight hole.

Cumulative fatigue damage. The total fatigue damage caused by repeated cyclic stresses.

Deflection tools. Drilling tools and equipment used to change the inclination and direction of the drilled wellbore.

Departure. Horizontal displacement of one station from another.

Fulcrum technique. Utilizes a bending moment principle to create a force on that the bit to counteract reaction forces that are tending to push the bit in a given direction.

Mechanical technique. Utilizes bottomhole equipment which is not normally a part of the conventional drillstring to aid deviation control. This equipment acts to force the bit to turn the hole in direction and inclination.

Packed-hole technique. Utilizes the hole wall to minimize bending of the bottomhole assembly.

Pendulum techniques. The basic principle involved is gravity or the “plumb-bob effect.”

Directional drilling contractor. A service company that supplies the special deflecting tools, BHA, survey instruments and a technical representative to perform the directional drilling aspects of the operation.

Direction of inclination. Direction of the course.

Dogleg. Total curvature in the wellbore consisting of a change of inclination and/or direction between two points.

Dogleg severity. A measure of the amount of change in the inclination and/or direction of a borehole; usually expressed in degrees per 100 ft of course length.

Drag. The extra force needed to move the drill string resulting from the drill string being in contact with the wall of the hole.

Drainholes. Several high-angle holes drilled laterally form a single wellbore into the producing zone.

Drift angle. The angle between the axis of the wellbore and the vertical.

Drop off. The portion of the hole in which the inclination is reduced.

Drop-off rate. Rate of change (◦/100 ft) of the inclination angle in the section of the wellbore that is decreasing toward vertical.

Goniometer. An instrument for measuring angles, as in surveying.

Gyroscopic survey. A directional survey conducted using a gyroscope for directional control, usually used where magnetic directional control cannot be obtained.

Hole curvature. Refers to changes in inclination and direction of the borehole.

Hydraulic orienting sub. Used in directional holes with inclination greater than 6◦ to find the low side of the hole. A ball falls to the low side of the sub and restrict an orifice, causing an increase in the circulating pressure. The position of the tool is know with relation to the low side of the hole.

Hydraulically operated bent sub. A deflection sub that is activated by hydraulic pressure of the drilling fluid.

Inclination angle. The angle of the wellbore from the vertical.

Inclinometer. An instrument that measures an angle of deviation from the vertical.

Jet bit deflection. A method of changing the inclination angle and direction of the wellbore by using the washing action of a jet nozzle at one side of the bit.

Keyseat. A condition wherein the borehole is abraded and extended sideways, and with a diameter smaller than the drill collars and bit; usually caused by the tool joints on the drill pipe.

Kickoff point (kickoff depth). The position in the well bore where the inclination of the hole is first purposely increased (KOP).

Lead angle. A method of setting the direction of the wellbore in anticipation of the bit walking.

Magnetic declination. Angular difference, east or west, at any geographical location, between true north or grid north and magnetic north.

Magnetic survey. A directional survey in which the direction is determined by a magnetic compass aligning with the earth’s magnetic field.

Measured depth. Actual length of the wellbore from its surface location to any specified station.

Mechanical orienting tool. A device to orient deflecting tools without the use of subsurface surveying instruments.

Methods of orientation

Direct method. Magnets embedded in the nonmagnetic drill collar are used to indicate the position of the tool facewith respect to magnetic north. A picture of a needle compass pointing to the magnets is superimposed on the picture of a compass pointing to magnetic north. By knowing the position of the magnets in the tool, the tool can be positioned with respect to north.

Indirect method. A method of orienting deflecting tools in which two survey runs are needed, one showing the direction of the hole and the other showing the position of the tool.

Surface readout. A device on the rig floor to indicate the subsurface position of the tool

Stoking. Method of orienting a tool using two pipe clamps, a telescope with a hair line, and an aligning bar to determine the orientation at each section of pipe run in the hole.

Monel (K monel). A nonmagnetic alloy used in making portions of downhole tools in the bottomhole assembly (BHA), where the magnetic survey tools are placed for obtaining magnetic direction information. Monel refers to a family of nickel-copper alloys.

Mud motor. Usually a positive displacement or turbine-type motor, positioned above the bit to provide (power) torque and rotation to the bit without rotating the drillstirng.

Mule shoe. A shaped form used on the bottom of orienting tools to position the tool. The shape resembles a mule shoe or the end of a pipe that has been cut both diagonally and concave. The shaped end forms a wedge to rotate the tool when lowered into a mating seat for the mule shoe.

Multishot survey. A directional survey in which multiple data points are recorded with one trip into the wellbore. Data are usually recorded on rolls of film.

Near-bit stabilizer. A stabilizer placed in the bottomhole assembly just above the bit.

Ouija board (registered trademark of Eastern Whipstock). An instrument composed of two protractors and a straight scale that is used to determine the positioning for a deflecting tool in a inclined wellbore.

Permissible dogleg. A dogleg through which equipment and/or tubulars can be operated without failure.

Pendulum effect. Refers to the pull of gravity on a body; tendency of a pendulum to return to vertical position.

Pendulum hookup. A bit and drill collar with a stabilizer to attain the maximum effect of the pendulum.

Rat hole. A hole that is drilled ahead of the main wellbore and which is of a smaller diameter than the bit in the main borehole.

Reamer. A tool employed to smooth the wall of a wellbore, enlarge the hole, stabilize the bit and straighten the wellbore where kinks and abrupt doglegs are encountered.

Rebel tool (registered trademark of Eastman Whipstock). A tool designed to prevent and correct lateral drift (walk) of the bit tool. It consists of two paddles on a common shaft that are designed to push the bit in the desired direction.

Roll off. A correction in the facing of the deflection tool, usually determined by experience, and which must be taken into consideration to give the proper facing to the tool.

Setting off course. A method of setting the direction of the wellbore in anticipation of the bit walking.

Side track. An operation performed to redirect the wellbore by starting a new hole; at a position above the bottom of the original hole.

Slant hole. A non vertical hole; usually refers to a wellbore purposely inclined in a specific direction; also used to define a wellbore that is nonvertical at the surface.

Slant rig. Drilling rig specifically designed to drill a wellbore that is non vertical at the surface. The mast is slanted and special pipe-handling equipment is needed.

Spiraled wellbore. A wellbore that has attained a changing configuration such as a helical form.

Spud bit. In directional drilling, a special bit used to change the direction and inclination of the wellbore.

Stabilizer. A tool placed in the drilling assembly to Change or maintain the inclination angle in a wellbore by controlling the location of the contact point between the hole and drill collars. Center the drill collars near the bit to improve drilling performance. Prevent wear and differential sticking of the drill collars.

Surveying frequency. Refers to the number of feet between survey records.

Target area. A defined area, at a prescribed vertical depth, that is planned to be intersected by the wellbore.

Tool azimuth angle. The angle between north and the projection of the tool reference axis onto a horizontal plane.

Tool high-side angle. The angle between the tool reference axis and a line perpendicular to the hole axis and lying the vertical plane.

Total curvature. Implies three-dimensional curvature.

True north. The direction from any geographical location on the earth’s surface to the north geometric pole.

True vertical depth (TVD). The actual vertical depth of an inclined wellbore.

Turbodrill. A downhole motor that utilizes a turbine for power to rotate the bit.

Turn. A change in bearing of the hole; usually spoken of as the right or left turn with the orientation that of an observer who views the well course from the surface site.

Walk (of hole). The tendency of a wellbore to deviate in the horizontal plane.

Wellbore survey calculation method. Refers to the mathematical method and assumptions used in reconstructing the path of the wellbore and in generating the space curve path of the wellbore from inclination and direction angle measurements taken along the wellbore. These measurements are obtained from gyroscopic or magnetic instruments of either the single-shot or multishot type.

Whipstock. Along wedge and channel-shaped piece of steel with a collar at its top through which the subs and drillstring may pass. The face of the whipstock sets an angle to deflect the bit.

Woodpecker drill collar (indented drill collar). Round drill collarwith a series of indentations on one side to form an eccentrically weighted collar.