The 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. 4,323,848 and U.S. Pat. No. 4,372,398, both issued to the applicant herein, and in U.S. Pat. No. 4,072,200 to Morris et al. See, also, Canadian Patent No. 1,269,710 of Barnett et al, issued May 29, 1990.
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. Thus, when multiple wells lead from a single location, such as a drilling platform, it may become necessary to drill a borehole to avoid intersecting neighboring wells or, alternatively, to intersect a particular one of, for example, sixteen wells, all starting at approximately the same location and spreading downwardly and outwardly from each other. The borehole being drilled may start at the same general location as the other wellheads, or may start at a location several hundred feet from the wellhead of a target well. If intersection with a specific well is desired at, for example, three thousand feet below the surface, guidance information can be provided by a low-frequency alternating current injected into the earth, as from an electrode in the borehole being drilled, with the resulting earth current being concentrated in a casing or other electrically conductive material at the target well. The current so produced in the target well results in a magnetic field which can be detected from a highly sensitive magnetometer located in the borehole. 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. The need to provide two or more horizontal wells in close proximity, but with a precisely controlled separation, occurs in a number of contexts such as in steam assisted recovery projects in the petroleum industry, where steam is to be injected in one horizontal well and mobilized viscous oil is to be recovered from the other. This process is described, for example, in Canadian Patent No. 1,304,287 of Edmunds et al, which issued Jun. 30, 1992. Another example is in the field of toxic waste reclamation, where there is a need to drill parallel horizontal wells under waste disposal sites so that air can be pumped into one and toxic fluids forced into and recovered from the other. Again, in hot rock geothermal energy systems, there is a need to drill parallel wells so that cold water can be injected into one and not recovered from the other.
The need to drill horizontal, parallel wells is of most immediate concern in the mobilization of heavy oil sands, where a borehole is to be drilled close to and parallel to an existing horizontal well with a separation of about ten meters for a horizontal extension of a thousand meters or more at depths of, for example, 500 to 1,500 meters. A number of such wells may be drilled relatively closely together, following the horizon of the oil producing sand, and such wells must be drilled economically, without the introduction of additional equipment and personnel.