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The method used to obtain the measurements needed to calculate and plot the 3D well path is called directional survey.
Three parameters are measured at multiple locations along the well path:
Measured depth (MD)
MD is the actual depth of the hole drilled to any point along the wellbore or to total depth, as measured from the surface location. Inclination is the angle, measured in degrees, by which the wellbore or survey-instrument axis varies from a true vertical line.
An inclination of 0° would be true vertical, and an inclination of 90° would be horizontal. Hole direction is the angle, measured in degrees, of the horizontal component of the borehole or survey-instrument axis from a known north reference. This reference is true north, magnetic north, or grid north, and is measured clockwise by convention.
Hole direction is measured in degrees and is expressed in either azimuth (0 to 360°) or quadrant (NE, SE, SW, NW) form.
Application of parameters
Each recording of MD, inclination, and hole direction is taken at a survey station, and many survey stations are obtained along the well path. The measurements are used together to calculate the 3D coordinates, which can then be presented as a table of numbers called a survey report. Surveying can be performed while drilling occurs or after it has been completed.
Purpose of directional survey
The purpose of directional survey is to:
- Determine the exact bottomhole location to monitor reservoir performance
- Monitor the actual well path to ensure the target will be reached
- Orient deflection tools for navigating well paths
- Ensure that the well does not intersect nearby wells
- Calculate the TVD of the various formations to allow geological mapping
- Evaluate the Dog Leg Severity (DLS), which is the total angular inclination and azimuth in the wellbore, calculated over a standard length (100 ft or 30 m)
- Fulfill requirements of regulatory agencies, such as the Minerals Management Service (MMS) in the U.S.
Survey quality control
The nature of downhole directional surveying is that it can never be independently verified. It is very difficult to go down the well to check if the bottom is located where the calculations claim. Practically, the best way of verifying survey results is to have surveys obtained from two different sources, preferably from two different sensor types, such as a magnetic measurement while drilling (MWD) survey checked by a rate gyro or inertial navigation system.
Survey tool performace
Survey-tool performance often is dependent on how it is run. Regardless of the system or sensor type, the quality of the survey is controlled by the surveyor. The surveyor must follow the procedures and verification checks specified by the survey company and possibly even apply additional procedures and checks, as specified by the operating company, to ensure the best possible survey. Unless the proper procedures and checks are adhered to, the quality of the survey is questionable. These checks should include pre- and post-job calibration checks and paperwork and procedures verification by someone other than the original surveyor.