The present invention relates, in general, to a method of locating the borehole of a deep well at a substantial distance below the earth's surface, and, more particularly, to a method for locating such a well bore by means of a magnetic field produced by current flowing in the casing of such a well bore, and without the need to use the earth's magnetic field for orientation purposes.
It is well known that in drilling deep wells for oil and gas exploration, precise control of the path followed by the well is extremely difficult, so that it is virtually impossible to know the exact location of the well at a given depth. For example, a drilling tolerance of plus or minus one quarter of a degree will allow the bottom of a 10,000-foot well to be positioned anywhere within a circle 100 feet in diameter, and numerous factors can increase the deviation. This is not of particular concern in normal drilling operations, but if an accident should occur, such as a rupture in the casing of the well which causes the well to blow out, it may become necessary to plug the well at or below the point of the rupture to bring it under control. In order to do this, a relief well must be drilled to intersect the original well at the desired level, and since such blowouts produce extremely hazardous conditions at the surface in the vicinity of the original well, the relief well must be started a considerable distance away and drilled at an incline in order to protect the personnel and the equipment used.
Because the same problems of control of the direction of drilling are encountered in the relief well as were encountered in the original well, the location of the relief well bore cannot be determined with precision; accordingly, it is extremely difficult to determine the distance and direction from the relief well to the blown out well. A further difficulty is that in some cases the force of the explosion resulting from the rupturing of a well casing can do so much damage at the surface that it may be difficult to say exactly where the well head is, much less where the well is located at a depth of 10,000 feet or more. Because of the danger at the well head, the relief well is started one-half mile or more away from the area of the original wellhead, angles toward the original well until it approaches relatively closely, and then follows the original well to the location where the wells are to intersect. Thus, the relief well usually is very complex, compounding the problem of knowing exactly where it is located with respect to the original well.
Numerous attempts have been made to provide accurate measurements of well location, and many surveying techniques are available which can give general information as to the location of a relief well with respect to a target well. However, such survey techniques are not capable of providing accurate data concerning the relationship of the relief well to the original well until the relief well has approached very near the original well; i.e., within a few tens of feet. At that point, equipment such as the "Magrange" (magnetic gradient ranging) system developed by Tensor, Inc., of Austin, Tex., and Houston Oil and Minerals Research and Development, Inc., can be used with considerable accuracy; but it has been found that outside a radius of a few tens of feet, the Magrange system is usually inadequate.
In an attempt to extend the distance at which accurate information can be obtained, a variety of electrical well logging techniques have been used which treat the target well as an anomaly in the geologic structure of the earth surrounding the relief well. The art of electrical well logging is highly developed, and although systems such as the "ULSEL" (ultra-long-spaced electrode logging) system have been successfully used in locating geologic anomalies of certain types, they have been less than satisfactory in guiding relief well bores to locate and intersect existing boreholes. The ULSEL system is described extensively in the patent literature; particularly in U.S. Pat. Nos. 3,256,480; 3,479,581; 3,697,864; and 3,778,701, all issued to Richard J. Runge. As described in these patents, the system is directed to the measurement of the apparent resistivity of the earth across a pair of electrodes and, since no directionality is given by this method, it is ineffective for directing a relief well toward an existing well.
In addition to the resistive-type logging of geological strata described in the foregoing patents, there have been attempts to obtain similar data through the use of electromagnetic prospecting. Such a system is illustrated, for example, in U.S. Pat. No. 2,723,374, to Williams, which is directed to lateral exploration from a borehole to determine the magnitude and direction of resistive anomalies in the earth. Such resistive anomalies result in variations in the pattern of an electrical current injected into the earth from electrodes mounted in the same borehole as a pair of induction sensing coils mounted at right angles to each other. These sensing coils detect massive resistive anomalies such as ore bodies, salt domes, reefs, or dipping strata which have conductivities different from the conductivities of adjacent beds, and which therefore disturb the electrical current flow. The sensing coils are used in conjunction with other conventional well logging systems to determine the probable location of mineral deposits. However, such systems do not suggest the possibility of locating relatively small targets such as well bores.
Other systems have been developed for directing a second well with respect to a first well to permit intersection of the two. One such system is described in U.S. Pat. No. 3,282,355, to Henderson, wherein a relief well is controlled to intersect a blown-out well by the use of sonic detectors responsive to the sound produced by fluids flowing out of the blown well formation. However, such a system will not operate when there is no sound emanating from the target well, and, in addition, does not provide the required degree of directional and distance accuracy. Another proposal in the prior art is the use of a signal transmitter in one well and a signal receiver in the other well. For example, in patents such as U.S. Pat. Nos. 3,285,350, to Henderson, 3,722,605, to Isham, and 4,016,942, to Wallis, et al., soundwaves are used as the signals, while in U.S. Pat. No. 3,731,752, to Schad, and U.S. Pat. No. 3,406,766 to Henderson, the signal is a magnetic field produced by an electromagnet. In all of these systems, however, the target well must be accessible so that the signal source can be placed in one well and the receiver in the other, and they are not effective where the target well is not open, or where there is a chance of an explosion or fire that would endanger personnel.
Many of the difficulties outlined above were overcome by the methods and apparatus disclosed in U.S. Pat. No. 4,323,848, and U.S. Pat. No. 4,372,398, of Arthur F. Kuckes, the disclosures of which are incorporated herein by reference. In accordance with the '398 patent, an electric current flow is produced in the casing of a target well, or borehole, by injecting a current into the earth surrounding the target well through the use of an electrode located in the relief borehole. This electrode is carried by the logging cable and contacts the uncased sides of the relief borehole so that a current flow is established in the vicinity of the relief hole. This current flow extends between the downhole electrode and a second electrode which, in a preferred form of the invention, is located at the earth's surface in the vicinity of the head of the relief well. The injected ground current finds a path of least resistance through the casing or other current-conducting material in the target borehole, and the resulting concentration of current produces a magnetic field surrounding the target well which can be detected by a field sensor such as that described in the '848 patent. The sensor disclosed in the '848 patent is extremely sensitive to very small magnetic fields, and permits an accurate detection of currents flowing in borehole casings located a considerable distance away from the relief hole. The current generated in the target borehole casing is an alternating current of relatively low frequency which produces a characteristic magnetic field that can be detected, and which produces a corresponding output signal from the AC magnetic field sensor.
The signals obtained from the AC magnetic field sensor, in accordance with the aforesaid patents, provide a measure of the direction and distance to the target well casing. In the course of drilling the relief well, such measurements are taken periodically at selected depths, and the resulting measurements, when taken with simultaneous measurements of the earth's static magnetic field to provide a stable reference point for orientation of the sensor, will, in accordance with U.S. Pat. No. 4,372,398, permit a continuous calculation of the presumed location of the target well with respect to the location of the AC magnetic field sensor in the relief well. This information can be used to guide further drilling of the relief well. Thus, as the relief well approaches a desired depth, its approach to the location of the target well can be guided so that the target well is intersected at the desired depth below the earth's surface in a rapid and effective manner without ambiguity, and with resulting savings in the cost of drilling a relief well.
Although the method and the apparatus set forth in the aforesaid patents of Arthur F. Kuckes have been found to be extremely effective, it has been found that the method of establishing the location and orientation of the magnetic field sensor within the relief well may, in some circumstances, be better carried out through methods and apparatus other than those required to detect and measure the earth's magnetic field, since in some cases that field is distorted or the measurements become unreliable.
In order to determine the direction of the target well from the data received from an AC magnetic field sensor in a relief well, a complete specification of the sensor orientation in space is needed at each station where measurements are made. This orientation is obtained, in accordance with U.S. Pat. No. 4,372,398, by a measurement of two vector components of the earth's magnetic field which lie in the plane perpendicular to the axis of the sensor. Under many conditions, this method of using the earth's magnetic field for sensor orientation is very effective. However, if, for example, the borehole axis nearly coincides with the earth's magnetic field lines, this method is imprecise. Further, if the earth's magnetic field is perturbed by ferromagnetic effects around the relief well or the target well, the determination of sensor orientation may be faulty. Thus, alternate methods of determining sensor orientation in order to precisely locate a target well are required.