Boring or penetrating the earth – With signaling – indicating – testing or measuring – Tool position direction or inclination measuring or...
Reexamination Certificate
2002-04-02
2004-05-18
Bagnell, David (Department: 3672)
Boring or penetrating the earth
With signaling, indicating, testing or measuring
Tool position direction or inclination measuring or...
C175S040000, C175S061000, C166S066500
Reexamination Certificate
active
06736222
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates, in general, to measurement while drilling (MWD) methods and apparatus, and more particularly to methods and apparatus for relative drilling direction measurement using a drill stem carrying a rotating magnet drill head.
BACKGROUND OF THE INVENTION
A typical drill stem, of the type which may be used in drilling boreholes such as wells for oil or gas exploration or production, or boreholes for the installation of cables and pipelines, in addition to many other purposes, carries at its lower end a drill head which includes a motor-driven rotary drill bit. Such drill bits are mounted on an angled drill bit shaft, or bent sub, which is driven by a motor under the control of a drilling operator at the earth's surface. The longitudinal axis of the bent sub is typically set at a small angle; for example, three-fourths of a degree, with respect to the axis of the drive motor and drill stem to allow directional drilling. The drive motor is typically mounted to the lower end of, and is coaxial with, a nonmagnetic section of the drill stem in which well survey electronics are located for measurement of well direction and location during drilling. Such drills are typically operated in one of two modes; a sliding mode or a rotary mode. In the sliding mode, the drill motor is activated to cause the drill bit to rotate while the rotational angle of the drill stem is held steady, and thus does not rotate. Since the axis of the bent sub, or drill bit shaft, is at a slight angle with respect to the axis of the drill stem and the drill motor, rotation of the bit causes the borehole to be drilled at the angle of, and in the direction of, the angle of the bent sub with respect to the drill stem axis, and this causes the borehole to change direction. The direction of the bent sub is controlled by the angular position of the drill stem and manifests itself by changing the down hole drilling direction by a small amount.
In the rotary mode of drilling, the drill stem is rotated as the down hole motor is powered to give the drill bit a compound rotation i.e. a component due to drill stem rotation and a component from the motor. This produces a continuous precession of the bent sub around the axis of the drill stem and causes the borehole to be drilled with a slight helicity. The average drilling is in the direction of the drill stem axis and with the diameter of the borehole being slightly larger than the diameter of the drill stem.
To achieve accurate directional control of the drilling, the drill operator needs to know with precision the borehole curvature being achieved. However, the measurement while drilling (MWD) equipment typically is located in a drill stem section above the drive motor, about 15 meters behind the drill bit. This means that if standard MWD equipment is relied on for the measurement of the borehole inclination and azimuth, the drill system will have advanced 15 meters before any measurement of a change in borehole direction can be obtained. In many applications, such as in the drilling of intersecting wells or in the drilling of closely spaced parallel wells such as those used in steam assisted gravity drainage (SAGD) wells, where parallel wells are spaced, for example, by approximately 5 meters over a kilometer of length, the problems of accurate directional measurement and drilling control are recurring and very serious.
In current practice, there are several systems used for overcoming this delay in the measurement of borehole inclination, but none for borehole azimuth. Typically, accelerometers and transmitters are located at or near the drill bit, and these transmit data past the motor to the MWD equipment, using either acoustic or electromagnetic transmission signals. However, these systems have serious problems, since such communication links are unreliable and the drilling must be stopped to measure the drill bit inclination. Such stoppages not only delay the drilling, but can result in the drill stem sticking in the borehole.
The current practice of drilling steam assisted gravity drainage (SAGD) well pairs is based upon a system disclosed in U.S. Pat. No. 5,485,089 and IADC/SPE paper 27466 which allows precise location determination of the MWD sensor package relative to a reference point approximately opposite the MWD package in a reference well. However, this method gives no information about the current relative drilling direction, i.e., whether the current drilling path is parallel to the reference well. It is only after drilling has proceeded to far beyond the point where the current measurement is being made that this evaluation can be carried out.
U.S. Pat. No. 5,589,775, which discloses a method of utilizing a drill bit with a rotating magnet to measure the azimuthal direction to an adjacent parallel wellbore, complements the present invention for obtaining a relative direction determination to a remote point. However, to produce parallel well pairs, knowledge of the current relative drilling direction relative to the direction of the reference well is very important, and there is a serious need to do this better than has previously been possible.
Similar concerns arise when it is necessary to drill precisely to a predetermined, distant point. Present practice is based upon determining the coordinates of the present drill bit location and those of the target and adjusting the drilling as it proceeds. Until recently these coordinates were determined by integrating a large ensemble of survey measurements from a surface location to the drill bit in conjunction with land surveys, with an ensemble of survey measurements to the target location. Recent developments have focused on making in situ determination of the apparent target location relative to the current drill bit location (A. G. Nekut, A. F. Kuckes and R. P. Pitzer, Rotating Magnet Ranging—a new drilling guidance technology, 8
th
One Day Conference on Horizontal Well Technology, Canadian Sections SPE/Petroleum Society, Nov. 7, 2001).
U.S. Pat. No. 5,258,755 discloses a method for determining relative drilling direction utilizing a drill bit with a rotating magnet in conjunction with an axial electromagnet as part of the drilling assembly. While the physical principles of the method are sound, the encumbrance associated with incorporating an electromagnet into a drilling assembly has inhibited its development.
SUMMARY OF THE INVENTION
The present invention overcomes the problems of previous approaches to directional drilling control by providing an in situ determination of relative direction from a current drilling direction to a target.
One embodiment of the present invention is directed toward a method for measuring borehole curvature near a drill bit by measuring relative borehole direction at the drill bit during drilling of a borehole with respect to the direction of the axis of an MWD sensor package usually located approximately 15 meters behind the drill bit. An MWD package typically includes magnetometers for measuring the three vector components and includes inclinometers for measuring the three vector components of the earth's gravity. The measurement of direction is accomplished by mounting a permanent magnet on the drill bit for rotation, with the magnetic axis of the magnet lying in a plane perpendicular to the axis of rotation of the drill bit, and by providing alternating magnetic field sensors in a nonmagnetic section of the drill stem above the drive motor. These A.C. sensors, which may be incorporated into the conventional MWD equipment, detect and measure the x, y and z vector components of the alternating magnetic field produced by the rotating permanent magnet when the bit is driven by the bit motor. If the shaft connecting the drill bit to the motor is straight, i.e., coaxial with the axis of the motor and drill stem, the permanent magnet will be in a plane perpendicular to the axis of the drill stem, and would produce a uniform magnetic field in an x y plane perpendicular to the axis of the drill stem at the location of th
Kuckes Arthur F.
Nekut Anthony G.
Pitzer Rahn G.
Bagnell David
Jones Tullar & Cooper PC
Stephenson Daniel P
Vector Magnetics LLC
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