GPS (Oilfield Applications)

Key Takeaways

• GPS has replaced conventional surveying methods for most oilfield positioning applications — before GPS became commercially available in the late 1980s and early 1990s, well locations, pipeline routes, and seismic line positions were determined by conventional ground surveying methods (total station, triangulation, tape and compass surveys) that were slow, labor-intensive, and subject to cumulative error over long distances; GPS dramatically accelerated and simplified the positioning workflow, enabling real-time position determination anywhere in the world without line-of-sight to fixed reference points; the transition from conventional surveying to GPS in the oilfield has been essentially complete for over two decades, and GPS is now assumed to be the positioning technology in virtually all oilfield geospatial applications.
• Seismic vessel and streamer positioning with GPS determines the accuracy of the resulting seismic image — in a 3D marine seismic survey, the position of each hydrophone in each streamer (which may be up to 12 km long and require positioning hundreds of sensor groups) must be known accurately to construct the correct geometry for seismic processing; GPS positioning of the vessel tow point, combined with streamer heading sensors (acoustic positioning and compass sensors distributed along the streamer) and real-time modeling of streamer geometry under currents and sea-state, provides the acquisition geometry that processing algorithms use to assign source-receiver positions to each recorded seismic trace; errors in streamer positioning of more than a few meters can cause smearing in the final migrated seismic image, reducing the resolution and accuracy of structural maps and reservoir characterization.
• Offshore platform geodetic surveys use differential GPS to monitor structural movement — large floating and fixed offshore structures (semisubmersibles, tension-leg platforms, spars, FPSOs, jackups) are subject to environmental loading from waves, wind, and current that causes continuous motion; continuously operating DGPS receivers on offshore platforms provide real-time position and orientation data that is used for dynamic positioning (keeping the platform or vessel on station with thruster control), mooring system monitoring (detecting anchor drag or mooring line failures), riser tension management, and structural health monitoring; the sub-decimeter accuracy of DGPS and RTK systems is required to detect the relatively small but operationally significant position excursions that indicate mooring or structural issues requiring attention.
• Directional drilling survey programs combine GPS well location with downhole survey instruments — the GPS coordinates of the wellhead (determined with DGPS accuracy) provide the starting position for the wellbore trajectory calculation; as the well is drilled, magnetic multishot surveys (inclination, azimuth, and depth measurements at regular intervals) or gyroscopic surveys (particularly in areas with magnetic interference from casing strings or mineral deposits) are used to track the wellbore's path from the surface GPS position; the accuracy of the resulting wellbore position at depth depends on both the surface GPS accuracy and the accumulated uncertainty of the downhole survey measurements; in directional drilling to specific targets (particularly in multi-well pad drilling where wellbore separation must be maintained for safety), the combination of precise surface GPS and downhole survey accuracy determines the confidence interval on the wellbore's landing position relative to the target.
• GPS underpins pipeline inspection and integrity management — intelligent pig (in-line inspection) surveys for pipeline corrosion, dent, and crack detection produce data that must be spatially referenced to pipeline coordinate systems derived from GPS surveys of pig trap locations, valve locations, and above-ground markers; pipeline GPS surveys using survey-grade instruments map the pipeline route to sub-meter accuracy, providing the coordinate system against which all inspection anomalies are located and against which repair activities are planned; for new pipeline construction, GPS guides the right-of-way staking, machine guidance for excavation and pipe laying, and the as-built surveys that document the final pipeline location for regulatory filing and future excavation safety programs.