Deviation survey
Encyclopedia
A Deviation surveyhttp://www.glossary.oilfield.slb.com/Display.cfm?Term=survey#2, more generally known in the oil industry as simply a 'survey', is the measurement of a borehole
's departure from the vertical
, measured in degree
s (°).
Borehole
A borehole is the generalized term for any narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water or other liquid or gases , as part of a geotechnical investigation, environmental site...
's departure from the vertical
Vertical direction
In astronomy, geography, geometry and related sciences and contexts, a direction passing by a given point is said to be vertical if it is locally aligned with the gradient of the gravity field, i.e., with the direction of the gravitational force at that point...
, measured in degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
s (°).
- When a well plan dictates the drilling of a straight boreholeBoreholeA borehole is the generalized term for any narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water or other liquid or gases , as part of a geotechnical investigation, environmental site...
, surveys are periodically taken to ensure that it will hit its target and also to ensure that it does not trespass underneath different property lines. These surveys can be taken fairly simply with a mechanical drift recorder more commonly known as a Totco or Totco barrel (named after the company that perfected the device). This device is run inside the drill stringDrill stringA drill string on a drilling rig is a column, or string, of drill pipe that transmits drilling fluid and torque to the drill bit. The term is loosely applied as the assembled collection of the drill pipe, drill collars, tools and drill bit...
attached to a wire on a wirelineWirelineIn the oil and gas industry, the term wireline usually refers to a cabling technology used by operators of oil and gas wells to lower equipment or measurement devices into the well for the purposes of well intervention and reservoir evaluation....
unit, down to the bottom of the drill pipe where the device (which is simply an inverted pendulum with a timer that punches a pin-hole onto a round paper tab delineated with concentric circles indicating increments of degrees) measures the angle of the hole and then is pulled back out to visually inspect to determine the angle. There are versions of this device that actually take a picture on film and are often used in situations where the azimuth (direction) needs to be determined also.
- When a well plan dictates a directional or horizontal borehole, more sophisticated tools are normally used. One such tool is the MWD (measurement while drilling) tool that uses electronic accelerometers and gyroscopes, to continually take surveys while drilling (as the name suggests) and also measures the azimuth (direction). This tool is attached to the drill stringDrill stringA drill string on a drilling rig is a column, or string, of drill pipe that transmits drilling fluid and torque to the drill bit. The term is loosely applied as the assembled collection of the drill pipe, drill collars, tools and drill bit...
itself and requires extra support personnel to use and interpret the data, and its costs are much more substantial. (See well loggingWell loggingWell logging, also known as borehole logging is the practice of making a detailed record of the geologic formations penetrated by a borehole. The log may be based either on visual inspection of samples brought to the surface or on physical measurements made by instruments lowered into the hole...
)