Glossary
of geosteering terms
Term Also named Unit Definition
Azimuth Azi, Azm, Drift degrees Compass heading of trajectory, angle between wellpath and North axis (true north or magnetic North) measured in plan view, clockwise from North. 0 for heading North, 90 for heading East, 180 for heading South, 270 for heading West. Also called "Drift direction".
Bend   degree Angle between downhole mud motor and drill bit, used to achieve build or turn while sliding. Usually a mud motor is "dialled" at 1.8 to 2.8 on the build section, and 1.5 to 1.8 on a horizontal section.
Build   or meters Increase of inclination; in horizontal drilling also decrease in TVD as a direct result of building angle. As opposed to drop.
Build rate   /30m Inclination angle difference between two consecutive survey points, extrapolated to 30m meters (or 100 feet); measured in vertical plane. Typical build rates in a build section of a horizontal well are 6-8/30m.
Build section Build   Section of a well where the wellpath is changed to a different inclination, usually from vertical (0) to horizontal (90).
Closure Closure distance meters Length of wellpath projected on horizontal plane (plan view) .
Closure direction Closure azimuth  
Dogleg Dogleg severity, DLS /30m Combination of build and turn rate; normalized measure of curvature (bend and turn) at a certain point. High doglegs pose difficulty in tripping and running casing or liner.
Drop   or meters Decrease of inclination; in horizontal drilling also increase in TVD as a direct result of dropping angle. In an "S" shape well, the section where a well is steered back to vertical after a build and hold section. As opposed to build.
Easting -W/+E meters Horizontal distance between survey point and North-South axis in plan view.
Geosteering     Directionally controlled drilling of a well, usually with the intent of keeping the hole in the pay zone, achieving maximum exposure to the reservoir.
Heel Landing point   First point in a horizontal well trajectory where the inclination reaches 90. Usually an intermediate casing is set at this landing point to isolate the pay zone.
High side HS   Toolface oriented up (or 0) in horizontal drilling when attempting to increase inclination angle (build wellpath).
Hold section Hold   Section of a well where the wellpath is maintained along a pre-defined inclination and azimuth. If the inclination ids different from 0 (vertical) or 90 (horizontal), it is also called a tangent section.
Horizontal section Lateral section   Section of a wellpath where the inclination is maintained at around 90, usually with the intent to keep the hole in the pay zone in order to maximize exposure to a reservoir. Typically inclinations are maintained in the 87 to 93 range. If steeper inclinations are planned, a more general term used is "Lateral section".
Inclination Inc, Incl degrees Angle between a tangent to the wellpath and a vertical line, measures the deviation from vertical at a certain point, measured with pendulum, accelerometer or gyroscope. Inclination increases in the build section, is 90 in a perfectly horizontal well, and can exceed 90 in a horizontal well that points up. A vertical well has inclinations close or equal to 0.
Kick off point KOP meters Measured depth from where a well is steered (usually beginning of the build section).
Landing Landing point, Heel   Landing is the process of increasing the TVD and inclination until a wellpath reaches horizontal or near-horizontal position in a desired reservoir formation. A soft landing is a landing where a specific inclination (usually 80 to 85) is maintained until a formation top is confirmed, and the wellpath is turned to horizontal only after that reservoir top was confirmed. The landing point often coincides with the heel (the point where inclination reached 90, horizontal), and many times a casing string is run at this point.
Low side LS   Toolface oriented down (or 180) in horizontal drilling when attempting to decrease inclination angle (drop wellpath).
Measured Depth MD meters Depth of well measured as the bit goes.
Mud motor Downhole motor   Mud driven downhole motor which rotates the bit below the bend.
MWD Measured while drilling   Set of physical parameters measured in a wellbore while drilling the well. Typically measurement of inclination, azimuth, gamma, sometimes resistivity (if more parameters are recorded, the operation is called LWD, Logging while drilling).
Northing +N/-S meters Horizontal distance between survey point and East-West axis in plan view.
Rotary steerable     Device used to geosteer while rotating from surface, usually driven by mobile pads.
Rotating     Rotation of drilling bit is driven from surface (rotary table or top-drive), usually higher rate of penetration, but no steering (except for rotary steerable devices).
Sidetrack Leg   A leg of a well that is started from a point along the wellpath that was previously drilled. The bottomhole assembly is positioned with the bend facing the desired direction, a through is formed by working the pipe, followed by time drilling to achieve proper separation.
Sliding Slide   Drilling without rotating the entire drillstring (by way of the rotary table or top-drive), rotation of bit occurs only below the bend, achieved by the mud driven downhole motor.
Station depth MD meters Survey tool depth, measured depth where survey is recorded, usually 10-15m behind the bit.
Subsea SS, Subsea level meters Vertical distance between see level and survey point.
Survey Directional survey   A geometric measurement of inclination and azimuth at a certain depth (station depth) along the well trajectory. A series of other parameters (such as TVD, VS, dogleg, etc are then determined using calculations such as minimum curvature method.
Tangent section Tangent, hold section   Section of a well where the wellpath is maintained at a certain inclination, with the intent of advancing in both TVD and vertical section. Short tangent sections are build for housing submersible pumps for example.
Time drill     Procedure where well is oriented in order to start a sidetrack.
Toe     Total depth of a horizontal well. A toe-up profile is achieved when the inclination is more than 90 throughout the horizontal section (the well rises, "builds"), a toe-down profile is achieved when the inclination angle is below 90 in the horizontal section (the well points down, "drops", and the toe is located at a lower TVD than the heel).
Toolface   degree Orientation of motor bend: in horizontal drilling measured relative to vertical line (up is 0, down is 180, right is 90 or 90R, left is 270 or 90L); in vertical drilling relative to magnetic north (N is 0M, E is 90M, S is 180M, W is 270M).
Total Depth TD, Final TD, FTD meters Measured depth to which a well is drilled.
True Vertical Depth TVD meters Vertical distance between KB and survey point.
Turn rate Walk rate /30m Azimuth angle difference between two consecutive survey points, extrapolated to 30m meters (or 100 feet); measured in horizontal plane.
Turn section Turn   Section of a well where the wellpath is steered in the horizontal plane to a different azimuth.
Vertical Section VS meters Horizontal distance from wellhead to survey point, measured along a pre-defined azimuth in the horizontal plane.
Vertical section plane Vetrical section azimuth degrees Pre-defined azimuth angle along which the VS is calculated, usually angle between North and a line uniting wellhead and Total Depth, measured in plan view.
Wellpath Trajectory   The trajectory of a directionally drilled well in three dimensions.
Directional drilling

Directional drilling is becoming the predominant method of opening up reservoirs in the mature Western Canada Sedimentart Basin. Wellsite geologists are supervising and controlling the wellpath in order to achieve maximum exposure to the reservoir or to hydrocarbons within a reservoir. Strong understanding of directional drilling terms is paramount in this capacity.

August 2010, Chinook Consulting Services

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