Benny Poedjono

High definition geomagnetic models: A new perspective for improved wellbore positioning

Earths gravity and magnetic fields are used as natural reference frames in directional drilling. The azimuth of the bottomhole assembly is inferred by comparing the magnetic field measured-while-drilling (MWD) with a geomagnetic reference model. To provide a reference of sufficient quality for accurate well placement, the US National Geophysical Data Center (NGDC), in partnership with industry, has developed high-definition geomagnetic models (HDGM), updated regularly using the latest satellite, airborne and marine measurements of the Earths magnetic field. Standard geomagnetic reference models represent the main magnetic field originating in the Earths liquid core, but the new models additionally account for crustal magnetic anomalies, which constitute a significant source of error in directional drilling. NGDC maintains a public archive of global ship and airborne magnetic field measurements. These are compiled into a global magnetic anomaly grid and expanded into ellipsoidal harmonics. The harmonic expansion coefficients are then included in the high-definition models to accurately represent the direction and strength of the local geomagnetic field. The latest global model to degree and order 720 resolves magnetic anomalies down to 28 km half-wavelength, achieving more than an order-of-magnitude improvement over previous models. A side-by-side comparison of different onand off-shore regions shows the high level of local detail represented in the new model. Accounting for a larger waveband of the geomagnetic spectrum significantly improves the accuracy of the reference field. This directly benefits the reliability of the well azimuth determination. We further demonstrate that model accuracy is a prerequisite for applying drill string interference corrections. Finally, an accurate reference model facilitates the validation of MWD surveys by keeping the field acceptance criteria centered on the true downhole magnetic field. Together, these factors improve well placement, prevent and mitigate the danger of collision with existing wellbores and enable real-time steering to save rig-time and reduce drilling costs. Introduction Magnetic field sensors are widely used in navigation systems and in determining the orientation of devices such as satellites, solar panels and antennas. Such electronic compasses play a particular important part below the sea and earth surface where the global positioning system (GPS) is unavailable. Measurement while drilling (MWD) employs a combination of gravity and magnetic field sensors to determine the inclination and azimuth of the bottom hole assembly (BHA). Conversion from magnetic azimuth to true azimuth requires knowledge of the direction and strength of the ambient magnetic field, which is provided by a geomagnetic reference model. Such a model specifies the declination angle (measured in degrees positive east of true north), the dip angle (measured positive downward), and the total field strength (measured in nT). Geomagnetic reference models are empirical models produced from global magnetic field measurements by satellite, aircraft and ships. The US National Geophysical Data Center (NGDC) has a long term commitment to collect, archive and 2 IADC/SPE-151436-PP disseminate geomagnetic data and reference models for use by academia, industry and the general public. Here, we first provide some background on the three data types and then describe how they are blended into a global reference model. The final section shows the benefits of using High Definition Geomagnetic Models (HDGM) in directional drilling. Satellite magnetic measurements Uniform global coverage of the geomagnetic field is provided by polar-orbiting satellites. They are particularly suited for monitoring the secular variation of the main field originating in the Earth’s liquid outer core. Low-orbiting satellites further enable mapping the long-wavelength portion of the crustal magnetic field, caused by ferrous minerals such as magnetite. The smallest wavelengths resolved by a satellite roughly correspond to its orbital altitude. The recently completed German CHAMP mission (CHAllenging Microsatellite Payload, Reigber, 2002) was launched in July 2000 into an orbit at an initial altitude of 450 km, which gradually decayed to 250 km before re-entry in September 2010. Due to its low altitude and advanced instrumentation, CHAMP was the primary satellite data source for global geomagnetic reference models, such as the World Magnetic Model (Maus et al., 2010), the International Geomagnetic Reference Field (Finlay et al., 2010) and NGDC’s HDGM (http://www.ngdc.noaa.gov/geomag/hdgm.shtml). CHAMP further enabled the production of a series of crustal magnetic field models. The latest being MF7 (http://geomag.org/models/MF7.html) with a full wavelength resolution of 300 km. The corresponding grid resolution or half-wavelength resolution is 150 km. A snapshot of MF7 is shown in Fig.1. The only dedicated magnetic satellite presently in orbit is the Danish Ørsted satellite. It was launched before CHAMP in February 1999 into a higher-altitude orbit at 850 km. After the star cameras failed in 2003, it now only provides measurements of the strength of the magnetic field. Furthermore, due to the weakness of the batteries, the data coverage has become somewhat sparse. These measurements do not completely determine the global magnetic field. However, they are nevertheless sufficient to extend geomagnetic field models for a few years, bridging the present gap in satellite magnetic coverage. The next geomagnetic satellite mission, scheduled for launch in the second half of 2012, is the European Space Agency’s Swarm constellation mission. It consists of three identical satellites in low altitude orbits, which will monitor the geomagnetic field over a mission life time of at least four years. Swarm’s accurate magnetometers and its design to measure gradients between side-by-side flying satellites will offer unprecedented opportunities to improve the specification of the long wavelength crustal magnetic field. Fig. 1–Vertical component of the Earths magnetic field as seen by the CHAMP satellite. The large anomaly on the upper right is due to the iron ore deposits at Kursk, Russia. Snapshot of animation by Maus and Rother, 2011. IADC/SPE SPE-151436-PP 3 Aeromagnetic compilations Due to their high altitude, satellites can only be used to infer the long wavelengths (> 300 km) of the geomagnetic field. Over land, the most effective means of surveying smaller-scale features is by using aircraft. Such surveys have been conducted for more than 60 years. They typically cover areas with sidelengths of the order of tens to hundreds of kilometers. Significant efforts are being made to stitch these surveys together into continental-scale magnetic compilations (Fairhead et al., 1997, Minty et al., 2003). The primary challenge in producing continental scale compilations is to accurately represent the intermediate to long wavelengths. The global satellite magnetic missions have contributed significantly to overcoming these difficulties. Nevertheless, there is still considerable uncertainty in the intermediate wavelengths of about 50 km to 300 km. To address this issue, Geoscience Australia flew a mesh of long-range aeromagnetic profiles which were used to correct the long wavelengths for the 5 edition of the Australian magnetic anomaly map (Milligan et al, 2010). A similar correction of long wavelengths by long-range aeromagnetic profiles was carried out for North America (Ravat, 2009). For NGDC’s global magnetic reference field models, the continental scale compilations, together with some isolated smaller surveys, were merged into a common global grid, displayed in Fig. 2. Fig. 2–Global grid of merged continental-scale aeromagnetic compilations Marine and airborne magnetic trackline data The second source of near-surface geomagnetic survey information is from ship and aircraft trackline data. Areas far from the shore are usually surveyed by ships on marine scientific cruises. The US Naval Research Laboratory has further flown extensive aeromagnetic surveys under the Project Magnet program, which was conducted for several decades. NGDC maintains a global Geophysical Data System (GEODAS) archive of marine trackline data. This archive will be augmented with the available aeromagnetic trackline data. Both trackline data types are shown in Fig. 3. 4 IADC/SPE-151436-PP Fig. 3–GEODAS trackline magnetic data (blue) and Naval Research Lab aeromagnetic trackline data (green) Earth magnetic anomaly grid The continental-scale compilations were merged with the ship and airborne trackline data into a common Earth Magnetic Anomaly Grid (EMAG2), with a grid cell size of 2 arc minutes (Maus et al., 2009). The trackline data were first line leveled onto the combined continental-scale compilations. Then, the data were merged using least-squares collocation, also sometimes referred to as kriging. In order to better represent linear sea-floor spreading anomalies, an anisotropic covariance model was employed over the oceans. The direction of strike was inferred from isochrones of the age of the oceanic crust by Müller et al. (2008). This directional gridding technique proved particularly successful in filling data gaps in the southern oceans by interpolation and extrapolation. After merging the near-surface data, the long wavelengths were substituted with the MF6 model (Maus et al., 2009) from CHAMP satellite measurements. EMAG2 is available in digital form as grid and GeoTiff at http://geomag.org/models/emag2.html. A version for visualization in NASA World Wind can be downloaded from http://www.getech.com/downloads/EMAG2.htm. Further implementations are available for Google Maps (http://ngdc.noaa.gov/geomag/data/Google_Maps/EMAG2/index.html) and as a KMZ file for Google Earth (http://bbs.keyhole.com/ubb/ubbthreads.php?ubb=showflat&Number=1205597). A Mercator projectio

Geomagnetic Referencing in the Arctic Environment

Geomagnetic referencing is becoming an increasingly attractive alternative to north-seeking gyroscopic surveys to achieve the precise wellbore positioning essential for success in todays complex drilling programs. However, the greater magnitude of variations in the geomagnetic environment at higher latitudes makes the application of geomagnetic referencing in those areas more challenging. Precise, real-time data on those variations from relatively nearby magnetic observatories can be crucial to achieving the required accuracy, but constructing and operating an observatory in these often harsh environments poses a number of significant challenges. Operational since March 2010, the Deadhorse Magnetic Observatory (DED), located in Deadhorse, Alaska, was created through collaboration between the United States Geological Survey (USGS) and a leading oilfield services supply company. DED was designed to produce real-time geomagnetic data at the required level of accuracy, and to do so reliably under the extreme temperatures and harsh weather conditions often experienced in the area. The observatory will serve a number of key scientific communities as well as the oilfield drilling industry, and has already played a vital role in the success of several commercial ventures in the area, providing essential, accurate data while offering significant cost and time savings, compared with traditional surveying techniques.

Probing Earth’s conductivity structure beneath oceans by scalar geomagnetic data: autonomous surface vehicle solution

The electric conductivity distribution of the Earth’s crust and upper mantle provides a key to unraveling its structure. Information can be obtained from vector data time series of the natural variations of the magnetic and electric field in a directional stable reference frame. Applying this method, known as magnetotellurics, to oceanic regions is challenging since only vector instruments placed at the sea bottom can provide such data. Here, we discuss a concept of marine induction surveying which is based on sea-surface scalar magnetic field measurements from a modern position-keeping platform. The concept exploits scalar magnetic responses that relate variations of the scalar magnetic field at the survey sites with variations of the horizontal magnetic field at a reference site. A 3-D model study offshore Oahu Island (Hawaii) demonstrates that these responses are sensitive to the conductivity structure beneath the ocean. We conclude that the sensitivity, depending on the bathymetry gradient, is typically largest near the coast offshore. We show that such sea-surface marine induction surveys can be performed with the Wave Glider, an easy-to-deploy, autonomous, energy-harvesting floating platform with position-keeping capability.Graphical abstract.