Geomagnetic Referencing—The Real-Time Compass for Directional Drillers

Andrew Buchanan To pinpoint the location and direction of a wellbore, directional drillers rely on Eni US Operating Company Inc. Anchorage, Alaska, USA measurements from accelerometers, magnetometers and gyroscopes. In the past, high-accuracy guidance methods required a halt in drilling to obtain directional Carol A. Finn Jeffrey J. Love measurements. Advances in geomagnetic referencing now allow companies to use E. William Worthington US Geological Survey real-time data acquired during drilling to accurately position horizontal wells, Denver, Colorado, USA decrease well spacing and drill multiple wells from limited surface locations.

Fraser Lawson Tullow Ghana Ltd. Accra, Ghana For a variety of reasons, operating companies Stefan Maus need to know where their wells are as they are Magnetic Variation Services LLC being drilled. Many of today’s deviated and hori- Boulder, Colorado zontal wells no longer simply penetrate a reser- voir zone but must navigate through it laterally to Shola Okewunmi contact as much of the reservoir as possible. Chevron Corporation Precise positioning of well trajectories is required Drift indicator disk Houston, Texas, USA Plumb bob to optimize hydrocarbon recovery, determine 6° where each well is relative to the reservoir and Benny Poedjono 4° avoid collisions with other wells. Sugar Land, Texas 2° To accomplish these objectives, drillers require Disk Oilfield Review Autumn 2013: 25, no. 3. directional accuracy to within a fraction of a degree. Copyright © 2013 Schlumberger. To achieve this level of accuracy, they use measure- For help in preparation of this article, thanks to Essam Adly, Muscat, Oman; Goke Akinniranye, The Woodlands, ment-while-drilling (MWD) tools that include accel- Texas; James Ashbaugh and Robert Kuntz, Pennsylvania erometers and magnetometers that detect the General Energy Company, LLC, Warren, Pennsylvania, USA; Nathan Beck, Anchorage; Luca Borri, Jason Brink Earth’s gravitational and magnetic fields; they also and Joseph Longo, Eni US Operating Co. Inc., Anchorage; use sophisticated procedures to compensate for Punch marks show 3.5° inclination Daniel Cardozo, St. John’s, Newfoundland, Canada; Pete measurement perturbation. As drillers have found Clark, Chevron Energy Technology Company, Houston; Clock Steve Crozier, Tullow Ghana Ltd., Accra, Ghana; Mike success with these tools and become more depen- Hollis, Chesapeake Energy, Oklahoma City, Oklahoma, USA; Christopher Jamerson, Apache Corporation, Tulsa; dent on them for well guidance, the need for accu- Xiong Li, CGG GravMag Solutions, Houston; Ross Lowdon, rately quantified positional uncertainty that takes Aberdeen; Diana Montenegro Cuellar, Bogotá, Colombia; Ismail Bolaji Olalere, Shell Nigeria, Port Harcourt, Nigeria; into account all measurement error has also Irina Shevchenko, Michael Terpening and John Zabaldano, increased. For some applications, the uncertainty is Houston; Tim White, US Geological Survey, Denver; and the as important as the position itself. Government of Newfoundland and Labrador, Department of > Natural Resources, St. John’s, Newfoundland, Canada. This article reviews aspects of wellbore sur- Mechanical drift indicator. This downhole PowerDrive is a mark of Schlumberger. veying, focusing on modern techniques for mag- device measures drift, or deviation from vertical, using a pendulum, or the “plumb bob,” principle. 1. Borehole orientation may be described in terms of netic surveying with MWD tools. To understand inclination and azimuth. Inclination refers to the vertical The sharp-tipped pendulum is lowered onto a angle measured from the down direction—the down, the operation of and uncertainty associated with disk into which it punches two holes that mark horizontal and up directions have inclinations of 0°, 90° magnetic tools, we examine important aspects of an initial measurement then a verification and 180°, respectively. Azimuth refers to the horizontal measurement. In this example, the inclination is angle measured clockwise from true north—the north, the Earth’s magnetic field and its measurement. 3.5°. The technique gives no indication of azimuth east, south and west directions have azimuths of 0°, 90°, Examples from the USA, Canada, offshore Brazil 180° and 270°, respectively. For more on borehole but may be reliable for surface hole intervals and orientation: Jamieson AL: Introduction to Wellbore and offshore Ghana illustrate the application of shallow vertical wells in which dogleg severity Positioning. Inverness, Scotland: University of the new techniques that improve measurement accu- and inclination are not significant. [Adapted from Highlands and Islands, 2012, http://www.uhi.ac.uk/en/ Gatlin C: Petroleum Engineering Drilling and Well research-enterprise/wellbore-positioning-download racy and thus effect considerable reduction in Completions. Englewood Cliffs, New Jersey, USA: (accessed June 18, 2013). magnetic tool survey error. 2. Griswold EH: “Acid Bottle Method of Subsurface Well Prentice-Hall, Inc. (1960): 143.] Survey and Its Application,” Transactions of the AIME 82, no. 1 (December 1929): 41–49.

32 Oilfield Review Historical Perspective makes it easier and less expensive to complete directional driller stopped drilling to measure Traditionally, wellbores were drilled vertically wells and produce hydrocarbons. However, the wellbore inclination and azimuth.1 and were widely spaced. Well spacing decreased introduction of horizontal drilling and closer well- The oldest survey method entailed lowering a as fields matured, regulations tightened and res- bore spacing has intensified the need for accurate glass bottle of acid downhole and holding it station- ervoirs were targeted in remote areas. Over time, wellbore positioning and for processes to prevent ary long enough for the acid to etch a horizontal ring drilling multiple horizontal wells from a single collisions between the bit and nearby wellbores. in the bottle. The ring’s position was interpreted for pad became common practice. Today, more than Before the introduction of modern steerable inclination once the device was retrieved.2 a dozen wells may fan out into the reservoir from downhole motors and advanced tools to measure Another simple survey tool is the single shot a single offshore platform or onshore drilling pad. hole inclination and azimuth, directional or hori- mechanical drift indicator (previous page). Pad drilling—grouping wellheads together at zontal drilling was much slower than vertical Magnetic single shot (MSS) and multishot (MMS) one surface location—necessitates fewer rig drilling because of the need to stop regularly and surveys have also been used to record inclination moves, requires less surface area disturbance and take time-consuming downhole surveys. The and magnetic azimuth. For those surveys, the tool

Autumn 2013 33 West as the survey tool continuously traverses the well P path. Surveying engineers also use them in gyro- X compassing mode, in which the stationary tool Geographic responds to the horizontal component of the D north Earth’s rotation rate. The use of rate gyros has Y I reduced errors—such as geographic reference East H errors and unaccountable measurement drift— Magnetic that are associated with conventional gyros. F north Unfortunately, because they are taken while the Magnetic field vector tool is stationary, gyro surveys carry operational risk and rig time cost associated with wellbore conditioning when drilling is stopped.6 In some intervals, significant magnetic inter- ference from offset wellbores makes accurate magnetic surveying impossible. To address this Z limitation, scientists developed gyro-while- Down drilling methods. Tool design engineers are extending the operational limits of some com- mercial gyro-while-drilling survey systems to the > Magnetic field orientation. At any point P, the magnetic field vector (red) is commonly described in terms of its direction, its total magnitude, F, in that full range of wellbore inclinations. direction and H and Z, the local horizontal and vertical components of F. The For some situations, surveying engineers angles D and I describe the orientation of the magnetic field vector. The combine gyroscopic and magnetic surveying. One declination, D, is the angle in the horizontal plane between H and of the combined techniques—inhole referenc- geographic north. The inclination, I, is the angle between the magnetic field vector and the horizontal plane containing H. Of these measurements, D and ing—makes use of highly accurate gyroscope I are required to convert the compass orientation of a wellbore to its measurements in shallow sections to align subse- geographic orientation. The absolute magnitudes of F, Z or H are used for quent data obtained using magnetic surveys in quality control and calibration. deeper sections.7 In highly deviated and extended-reach wells, this approach delivers lev- els of accuracy comparable to those acquired took photographs, or shots, of compass cards Triaxial accelerometers measure the local with wireline gyroscopic surveys without incur- downhole while the pipe was stationary in the gravity field along three orthogonal axes. These ring the added time and costs. In these inhole slips. Photographs were taken every 27 m [90 ft] measurements provide the inclination of the tool referencing systems, gyroscopic measurements during active changes of angle or direction and axis along the wellbore as well as the toolface are used in shallow near-vertical wellbore sec- every 60 to 90 m [200 to 300 ft] while drilling relative to the high side of the tool.4 Similarly, tri- tions in the vicinity of casing until MWD mag- straight ahead. The introduction of downhole mud axial magnetometers measure the strength of the netic surveys can be obtained free of interference motors in the 1970s and the development of rug- Earth’s magnetic field along three orthogonal and in longer-reach sections in which inclina- gedized sensors and mud pulse telemetry of MWD axes. From these measurements and the acceler- tions increase. An additional benefit of using data enabled the use of continuously updated digi- ometer measurements, the tool determines azi- both gyro and MWD surveys is the detection of tal measurements for near real-time trajectory muthal orientation of the tool axis relative to gross error sources in either tool. adjustments. Most wells are now drilled using sur- magnetic north. Conversion of magnetic mea- vey measurements from modern MWD tools. surements to geographic orientation is at the Positional Uncertainty heart of MWD wellbore surveying. The key mea- Drillers use positional uncertainty estimates to Well Survey Basics surements are magnetic dip (also called mag- determine the probability of striking a geologic Today, directional drillers rely primarily on real- netic inclination), total magnetic field and target and of intersecting other wellbores.8 They time MWD measurements of gravitational and magnetic declination (above).5 base the estimates on tool error model predic- magnetic fields using ruggedized triaxial acceler- A variety of tools exploit gyroscopic princi- tions, which themselves depend on quality con- ometers and magnetometers. Other categories of ples. These systems are unaffected by ferromag- trol (QC) of survey data. Survey tool quality survey tools include magnetic multishot tools, netic materials, giving them an advantage over checks help identify sources of error, often with inclination-only tools and a family of tools based magnetic tools in some drilling scenarios. Some redundant surveys as independent cross-checks. on the use of gyroscopes, or gyros.3 Unlike MWD tools take measurements at discrete intervals of For most survey tools, the outputs are azi- tools, many of these specialty tools are run as measured depth (MD) along the well path when muth, inclination and measured depth. Errors in wireline services, thus requiring cessation of the the survey tool is stationary; others operate in a each measurement may occur because of both drilling process. Increasingly, however, gyro- continuous measurement mode. North-seeking the tool and the environment. Accuracies avail- scopic tools are also being incorporated into gyrocompasses (NSGs) make use of gyroscopes able from stationary measurements made with downhole steering and surveying instruments for and the rotation of the Earth to automatically standard MWD tools are on the order of use while drilling. find geographic north. Rate gyros provide an out- ±0.1° for inclination, ±0.5° for azimuth and put proportional to the turning rate of the instru- ±1.0° for toolface. ment and may be used to determine orientation

34 Oilfield Review A surveying engineer’s ability to determine 1,000 ft 1,000 ft borehole trajectory depends on the accumulation of errors from wellhead to total depth. Rather 200 ft than specifying a point in space, surveying engi- neers consider wellbore position to be within an ellipsoid of uncertainty (EOU). Typically, the uncertainty in the lateral direction is larger than in the vertical or along-hole directions. When dis- played continuously along the wellbore, the EOU presents a volume shaped like a flattened cone surrounding the estimated borehole trajectory (right). The combined effects of accumulated error may reach values of 1% of measured well depth, which could be unacceptably large for long wellbores.9 The Industry Steering Committee for Wellbore Survey Accuracy, ISCWSA—now the SPE Wellbore Positioning Technical Section, WPTS— has promoted development of a rigorous mathe- matical procedure for combining various error sources into one 3D uncertainty ellipse.10 External effects on accuracy include axial mis- alignment, BHA deflection, unmodeled geomag- X netic field variations and drillstring-induced Y Z interference. The latter two factors dominate the performance of magnetic tools and their error models; such models depend on the resolution of the geomagnetic reference model in use.11 > Planned well trajectories showing slices of the ellipsoids of uncertainty The Geomagnetic Field (EOUs) obtained from standard MWD (blue) and from higher accuracy MWD (red) surveys. The azimuthal and inclination uncertainties are in the XY plane To make use of magnetic measurements for find- perpendicular to the borehole. The depth uncertainty is along the Z-axis of ing direction, it is necessary to take into account the borehole. When shown at a dense series of points along the well the complexity of the geomagnetic field. The geo- trajectory, they form a “cone of uncertainty.” The high-accuracy method magnetic field surrounds the Earth and extends delivers a wellbore with smaller positional uncertainty. (Adapted from Poedjono et al, reference 32.) into nearby space.12 The total magnetic field mea- sured near the Earth’s surface is the superposition of magnetic fields arising from a number of time-

3. This family includes conventional gyros, rate gyros, horizontal and up directions have dips (inclinations) of 9. For typical well depths and step-out, or horizontal reach, north-seeking gyros, mechanical inertial gyros and ring 90°, 0° and –90°, respectively. Declination is defined the dimensions of the uncertainty envelope may be on laser inertial gyros. For more on gyros: Jamieson AL: similarly to hole azimuth. For more on magnetic field the order of 100 ft [30 m] or more unless action is taken “Understanding Borehole Surveying Accuracy,” orientation: Campbell WH: Introduction to Geomagnetic to correct error sources and run high-accuracy surveys. Expanded Abstracts, 75th SEG Annual International Fields, 2nd ed. Cambridge, England: Cambridge University This may exceed the size of the target and increase the Meeting and Exposition, Houston (November 6–11, 2005): Press, 2003. risk of unsuccessful wellbore steering. For more on the 2339–2340. 6. Gyro surveys conducted on wireline in openhol