Electrically powered
electromagnetic field source beacons installed in a reference well in combination with a down-hole
measurement while drilling (MWD) electronic
survey instrument near the
drill bit in the borehole being drilled permit distance and direction measurements for drilling guidance. Each
magnetic field source
beacon consists of a coil of wire wound on a steel
coupling between two lengths of steel tubing in the reference well, and powered by an electronic
package. Control circuitry in the electronic
package continuously “listens” for, and recognizes, a “start”
signal that is initiated by the driller. After a “start”
signal has been received, the
beacon is energized for a short time interval during which an
electromagnetic field is generated, which is measured by the MWD apparatus. The generated
magnetic field may be an
AC field, or switching circuitry can periodically reverse the direction of a generated DC
electromagnetic field, and the measured vector components of the electromagnetic field are used to determine the relative location coordinates of the drilling bit and the
beacon using well-known mathematical methods. The
magnetic field source and powering
electronic packages may be integral parts of the reference well casing or may be part of a temporary work string installed therein. Generally, numerous beacons will be installed along the length of the reference well, particularly in the important
oil field application of drilling
steam assisted gravity drainage (SAGD) well pairs.