Magnetic ranging refers to the determination of the relative position of one well with respect to another using magnetic measurements. Magnetic ranging is used for several oil field applications including positioning blow-out relief wells, steam-assisted gravity drainage and heavy oil applications, and planned intersection of one well with another. The procedure is also applicable to horizontal distance measurement and TVD correlation between observation wells and nearby horizontal wells. This paper is a discussion of the theories, methodologies, and pro's and con's of both "active" and "passive" magnetic ranging.
A comparison of a number of specific technologies currently available in the marketplace will be presented. The paper will discuss the following trademarked and proprietary items: Magnetic Guidance Tool™, Single Wire Guidance™, Rotating Magnet Ranging Service™, MagTraC™, and Parallel Well Tracker™. The paper also includes a discussion of potential future directions in magnetic ranging.
Field experience and history will be presented as well as methodology for each technology. The paper will focus mainly on technologies that are currently available with discussions of the theory of operation of each technology, operation parameters, specifications, experience with drilling different types of wells with the various technologies, and finally, the advantages and disadvantages of each technology will also be discussed.
The Alberta Oil Sands Technology Research Authority (AOSTRA) began experimenting with Steam-Assisted Gravity Drainage technology ("SAGD") in the 1980s, and the first ever twin well SAGD pairs were drilled in 1987 at their Dover Underground Test Facility near Fort McMurray, Alberta. These first well pairs were drilled from an underground mine shaft, as the technology did not exist to drill the well pairs from surface using conventional drilling rig technology. (See Fig. 1.) It was not until 1993 that the technology to drill these wells from surface began to be developed. In June 1993, Amoco Canada drilled the first ever SAGD well pair from surface using the Magnetic Guidance Tool™ at Wolf lake1 in Northern Alberta. As of the date of writing, 150 well pairs have been drilled in Alberta, Saskatchewan, Wyoming, California, and Venezuela. There are currently several SAGD pairs currently being drilled and many more projects in the planning stages.
For the SAGD process to work effectively, inter-well spacing is critical. If the wells are placed too close together, steam can short circuit from the injector to the producer. If too far apart, the steam cannot heat enough bitumen to allow gravity drainage to occur. For typical SAGD projects, the well pairs are usually spaced between 4 and 10 metres apart. Thus, for SAGD well pairs drilled from the surface, the stringent separation requirements cannot readily be met by conventional wellbore surveying tools and methods1.
Magnetic Ranging is the determination of distance and orientation from the well being drilled to a target well or reference well. The determination is made based on measuring the magnetic signature from the target/reference well. The magnetic signature in the target well may be induced and measured by a number of methods. The most commonly used method employs a precisely positioned electromagnetic source. This procedure is referred to as "Active Ranging." Another method involves measurement of the current state of magnetisation of the casing in a target well. This procedure is referred to as a magnetostatic method. Both types will be discussed at greater length later in the paper.