Method and apparatus for measuring distance and direction by movable magnetic field source

Abstract

Methods for determining the distance from a borehole to a nearby, substantially parallel target well for use in guiding the drilling of the borehole, including positioning a magnetic field sensor in the borehole at a known depth and providing a magnetic field source in the target well. The wells may be vertical or horizontal, and the source preferably is a solenoid movable in the target to a location at approximately the same depth as that of the sensor. The distance between the borehole and the solenoid is determined, in one embodiment, by moving the solenoid to depths above and below the depth of the sensor, and detecting maximum field vectors. The distance the solenoid moves between the maxima is the distance between the wells. In another embodiment, the ratio of solenoid magnetic field vectors is used to determine the ratio of the difference in depth z between the solenoid and the sensor to the lateral distance R between the wells. This ratio z / r is then used to determine the separation of the wells.

Description

BACKGROUND OF THE INVENTIONThe present invention is directed, in general, to a method for guiding the drilling of wells at a substantial depth in the earth, and more particularly to methods for determining the distance and direction to a target-well from a borehole being drilled.The difficulties encountered in guiding the drilling of a borehole to intersect, to avoid, or to parallel an existing well at distances of thousands of feet below the surface of the earth are well known. Such guidance may be required when it is desired to avoid existing wells in a field, or when existing oil or gas wells have blown out and it becomes necessary to drill intersecting relief boreholes to prevent serious damage to underground gas or oil fields. Various electromagnetic methods for the precise drilling of such relief boreholes have been developed and have met with significant success during the past few years. Such methods and the instruments used are described, for example, in U.S. Pat. No. 4,323...

AI Technical Summary

Benefits of technology

The solenoid produces different horizontal target magnetic fields in the earth at the different locations of the MWD magnetometer array, which change from the initial zero to maximum values as it is positioned at locations both above and below the location of the magnetometer array. The horizontal components of the target fields are measured and the field maxima are correlated with the axial positions of the solenoid. The distance which the solenoid moves between the lowermost and uppermost locations which correspond to horizontal magnetic field maxima below and above the zero field location, is the same as the horizontal distance (or separation) between the target well and the magnetometer at the depth of the magnetometer. Since the vertical position of the solenoid can be measured precisely, the distance between its locations at maximum magnetic field values can be determined exactly and the precise horizontal distance between the relief borehole and the target can be measured directly, with no mathematic manipulation of the values being required.

The target well may be a cased well, but even though a magnetic field produced by a solenoid located within a casing will be attenuated by a factor of up to 25, the sensitivity of the magnetometer is adequate for accurate distance measurement as far away as 25 meters or more. The solenoid appears as a point source for the target magnetic field at greater distances, but as the borehole being drilled moves close to the target well, the point source characterization of the solenoid changes. This is due to the finite length of the solenoid and the "pole smearing" effects caused by the ferromagnetism of the target well casing, which can result in the poles of the solenoid being smeared out over a length of up to four feet. Although this will cause the magnetic field source to be less well defined, and may introduce some inaccuracy, the method of measuring field maximum values described above produces distance measurements which allow reliable guidance of the drilling of the borehole. Well known mathematical methods are available for compensating for these effects.

This latter technique can be carried out since the electrical resistance between the two wells in an oil producing sand is very low so that the injection of a large current into a wellhead is relatively easy. Furthermore, the bleed-off rate of the current from such wells is slow and calculable so that at the depths of interest, enough current is still on the well casing to have a significant effect on the magnetic field sensors in the MWD package to permit precise drilling control of the well.

Problems solved by technology

Even though the guidance of boreholes with respect to existing wells is, in general, well developed, special problems can occur where existing techniques are not sufficient to provide the precise control required.

For example, when it is desired to locate and to either avoid or to intersect a particular target well in a field which includes numerous other wells, problems can occur.

However, in multiple-well fields, the use of such a current injection system results in a target current being induced in all of the wells in the region, not just the target well.

This produces multiple magnetic fields which are superimposed at the borehole magnetometer, making it extremely difficult to obtain accurate distance measurements to the target well of interest, thus interfering with drill guidance.

Problems are also encountered in drilling non-parallel wells, such as a horizontal well through a field of vertical wells, or vice versa, where it is desired to avoid the existing wells, or in the alternative to intersect a specific well.

Another area of difficulty occurs in the drilling of multiple horizontal wells, particularly where a well being drilled must be essentially parallel to an existing well.

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