Glossary of Geosteering terms

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.

Glossary of geosteering terms
TermAlso namedUnitDefinition
AzimuthAzi, Azm, DriftdegreesCompass heading of trajectory, angle between wellpath and North axis (true north or magnetic North) measured in plan view, clockwise from North. 0º for heading North, 90º for heading East, 180º for heading South, 270º for heading West. Also called “Drift direction”.
BenddegreeAngle between downhole mud motor and drill bit, used to achieve build or turn while sliding. Usually a mud motor is “dialled” at 1.8º to 2.8º on the build section, and 1.5º to 1.8º on a horizontal section.
Buildº or metersIncrease of inclination; in horizontal drilling also decrease in TVD as a direct result of building angle. As opposed to drop.
Build rateº/30mInclination 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 6-8º/30m.
Build sectionBuildSection of a well where the wellpath is changed to a different inclination, usually from vertical (0º) to horizontal (90º).
ClosureClosure distancemetersLength of wellpath projected on horizontal plane (plan view) .
Closure directionClosure azimuth
DoglegDogleg severity, DLSº/30mCombination 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.
Dropº or metersDecrease 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/+EmetersHorizontal distance between survey point and North-South axis in plan view.
GeosteeringDirectionally controlled drilling of a well, usually with the intent of keeping the hole in the pay zone, achieving maximum exposure to the reservoir.
HeelLandingpointFirst point in a horizontal well trajectory where the inclination reaches 90º. Usually an intermediate casing is set at this landing point to isolate the pay zone.
High sideHSToolface oriented up (or 0º) in horizontal drilling when attempting to increase inclination angle (build wellpath).
Hold sectionHoldSection of a well where the wellpath is maintained along a pre-defined inclination and azimuth. If the inclination ids different from 0º (vertical) or 90º (horizontal), it is also called a tangent section.
Horizontal sectionLateral sectionSection of a wellpath where the inclination is maintained at around 90º, 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 93º range. If steeper inclinations are planned, a more general term used is “Lateral section”.
InclinationInc, IncldegreesAngle 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 90º in a perfectly horizontal well, and can exceed 90º in a horizontal well that points up. A vertical well has inclinations close or equal to 0º.
Kick off pointKOPmetersMeasured depth from where a well is steered (usually beginning of the build section).
LandingLanding point, HeelLanding 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 85º) 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 90º, horizontal), and many times a casing string is run at this point.
Low sideLSToolface oriented down (or 180º) in horizontal drilling when attempting to decrease inclination angle (drop wellpath).
Measured DepthMDmetersDepth of well measured as the bit goes.
Mud motorDownhole motorMud driven downhole motor which rotates the bit below the bend.
MWDMeasured while drillingSet of physical parameters measured in a wellbore while drilling the well. Typically measurement of inclinationazimuth, gamma, sometimes resistivity (if more parameters are recorded, the operation is called LWD, Logging while drilling).
Northing+N/-SmetersHorizontal distance between survey point and East-West axis in plan view.
Rotary steerableDevice used to geosteer while rotating from surface, usually driven by mobile pads.
RotatingRotation 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).
SidetrackLegA 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.
SlidingSlideDrilling 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 depthMDmetersSurvey tool depth, measured depth where survey is recorded, usually 10-15m behind the bit.
SubseaSS, Subsea levelmetersVertical distance between see level and survey point.
SurveyDirectional surveyA geometric measurement of inclination and azimuthat a certain depth (station depth) along the well trajectory. A series of other parameters (such as TVDVSdogleg, etc are then determined using calculations such as minimum curvature method.
Tangent sectionTangent, hold sectionSection 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 drillProcedure where well is oriented in order to start a sidetrack.
ToeTotal depth of a horizontal well. A toe-up profile is achieved when the inclination is more than 90º throughout the horizontal section (the well rises, “builds“), a toe-down profile is achieved when the inclination angle is below 90º in the horizontal section (the well points down, “drops“, and the toe is located at a lower TVD than the heel).
ToolfacedegreeOrientation of motor bend: in horizontal drilling measured relative to vertical line (up is 0º, down is 180º, right is 90º or 90R, left is 270º or 90L); in vertical drilling relative to magnetic north (N is 0M, E is 90M, S is 180M, W is 270M).
Total DepthTD, Final TD, FTDmetersMeasured depth to which a well is drilled.
True Vertical DepthTVDmetersVertical distance between KB and survey point.
Turn rateWalk rateº/30mAzimuth angle difference between two consecutive survey points, extrapolated to 30m meters (or 100 feet); measured in horizontal plane.
Turn sectionTurnSection of a well where the wellpath is steered in the horizontal plane to a different azimuth.
Vertical SectionVSmetersHorizontal distance from wellhead to survey point, measured along a pre-defined azimuth in the horizontal plane.
Vertical section planeVetrical section azimuthdegreesPre-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.
WellpathTrajectoryThe trajectory of a directionally drilled well in three dimensions.


Calin Dragoie

Posted On:
August 26, 2010

Technical Articles, Uncategorized