Electrically sequenced tractor

Details Electrically sequenced tractor Electrically sequenced tractor

2002-11-05

2004-06-08

Pezzuto, Robert E. (Department: 3672)

Boring or penetrating the earth

With self-acting cyclic advance and retraction of tool or...

Reexamination Certificate

active

06745854

ABSTRACT:

BACKGROUND
1. Field of the Invention
The present invention relates to downhole drilling and, in particular, to an electrically sequenced tractor (EST) for controlling the motion of a downhole drilling tool in a borehole.
2. Description of the Related Art
The art of drilling vertical, inclined, and horizontal boreholes plays an important role in many industries, such as the petroleum, mining, and communications industries. In the petroleum industry, for example, a typical oil well comprises a vertical borehole which is drilled by a rotary drill bit attached to the end of a drill string. The drill string is typically constructed of a series of connected links of drill pipe which extend between ground surface equipment and the drill bit. A drilling fluid, such as drilling mud, is pumped from the ground surface equipment through an interior flow channel of the drill string to the drill bit. The drilling fluid is used to cool and lubricate the bit, and to remove debris and rock chips from the borehole, which are created by the drilling process. The drilling fluid returns to the surface, carrying the cuttings and debris, through the annular space between the outer surface of the drill pipe and the inner surface of the borehole.
The method described above is commonly termed “rotary drilling” or “conventional drilling.” Rotary drilling often requires drilling numerous boreholes to recover oil, gas, and mineral deposits. For example, drilling for oil usually includes drilling a vertical borehole until the petroleum reservoir is reached, often at great depth. Oil is then pumped from the reservoir to the ground surface. Once the oil is completely recovered from a first reservoir, it is typically necessary to drill a new vertical borehole from the ground surface to recover oil from a second reservoir near the first one. Often a large number of vertical boreholes must be drilled within a small area to recover oil from a plurality of nearby reservoirs. This requires a large investment of time and resources.
In order to recover oil from a plurality of nearby reservoirs without incurring the costs of drilling a large number of vertical boreholes from the surface, it is desirable to drill inclined or horizontal boreholes. In particular, it is desirable to initially drill vertically downward to a predetermined depth, and then to drill at an inclined angle therefrom to reach a desired target location. This allows oil to be recovered from a plurality of nearby underground locations while minimizing drilling. In addition to oil recovery, boreholes with a horizontal component may also be used for a variety of other purposes, such as coal exploration and the construction of pipelines and communications lines.
Two methods of drilling vertical, inclined, and horizontal boreholes are the aforementioned rotary drilling and coiled tubing drilling. In rotary drilling, a rigid drill string, consisting of a series of connected segments of drill pipe, is lowered from the ground surface using surface equipment such as a derrick and draw works. Attached to the lower end of the drill string is a bottom hole assembly, which may comprise a drill bit, drill collars, stabilizers, sensors, and a steering device. In one mode of use, the upper end of the drill string is connected to a rotary table or top drive system located at the ground surface. The top drive system rotates the drill string, the bottom hole assembly, and the drill bit, allowing the rotating drill bit to penetrate into the formation. In a vertically drilled hole, the drill bit is forced into the formation by the weight of the drill string and the bottom hole assembly. The weight on the drill bit can be varied by controlling the amount of support provided by the derrick to the drill string. This allows, for example, drilling into different types of formations and controlling the rate at which the borehole is drilled.
The inclination of the rotary drilled borehole can be gradually altered by using known equipment such as a downhole motor with an adjustable bent housing to create inclined and horizontal boreholes. Downhole motors with bent housings allow the ground surface operator to change drill bit orientation, for example, with pressure pulses from the surface pump. Typical rates of change of inclination of the drill string are relatively small, approximately 3 degrees per 100 feet of borehole depth. Hence, the drill string inclination can change from vertical to horizontal over a vertical distance of about 3000 feet. The ability of the substantially rigid drill string to turn is often too limited to reach desired locations within the earth. In addition, friction of the drilling assembly on the casing or open hole frequently limits the distance that can be achieved with this drilling method.
As mentioned above, another type of drilling is coiled tubing drilling. In coiled tubing drilling, the drill string is a non-rigid, generally compliant tube. The tubing is fed into the borehole by an injector assembly at the ground surface. The coiled tubing drill string can have specially designed drill collars located proximate the drill bit that apply weight to the drill bit to penetrate the formation. The drill string is not rotated. Instead, a downhole motor provides rotation to the bit. Because the coiled tubing is not rotated or not normally used to force the drill bit into the formation, the strength and stiffness of the coiled tubing is typically much less than that of the drill pipe used in comparable rotary drilling. Thus, the thickness of the coiled tubing is generally less than the drill pipe thickness used in rotary drilling, and the coiled tubing generally cannot withstand the same rotational, compression, and tension forces in comparison to the drill pipe used in rotary drilling.
One advantage of coiled tubing drilling over rotary drilling is the potential for greater flexibility of the drilling assembly, to permit sharper turns to more easily reach desired locations within the earth. The capability of a drilling tool to turn from vertical to horizontal depends upon the tool's flexibility, strength, and the load which the tool is carrying. At higher loads, the tool has less capability to turn, due to friction between the borehole and the drill string and drilling assembly. Furthermore, as the angle of turning increases, it becomes more difficult to deliver weight on the drill bit. At loads of only 2000 pounds or less, existing coiled tubing tools, which are pushed through the hole by the gravity of weights, can turn as much as 90° per 100 feet of travel but are typically capable of horizontal travel of only 2500 feet or less. In comparison, at loads up to 3000 pounds, existing rotary drilling tools, whose drill strings are thicker and more rigid than coiled tubing, can only turn as much as 30°-40° per 100 feet of travel and are typically limited to horizontal distances of 5000-6000 feet. Again, such rotary tools are pushed through the hole by the gravity force of weights.
In both rotary and coiled tubing drilling, downhole tractors have been used to apply axial loads to the drill bit, bottom hole assembly, and drill string, and generally to move the entire drilling apparatus into and out of the borehole. The tractor may be designed to be secured between the lower end of the drill string and the upper end of the bottom hole assembly. The tractor may have anchors or grippers adapted to grip the borehole wall just proximal the drill bit. When the anchors are gripping the borehole, hydraulic power from the drilling fluid may be used to axially force the drill bit into the formation. The anchors may advantageously be slidably engaged with the tractor body, so that the drill bit, body, and drill string (collectively, the “drilling tool”) can move axially into the formation while the anchors are gripping the borehole wall. The anchors serve to transmit axial and torsional loads from the tractor body to the borehole wall. One example of a downhole tractor is disclosed in U.S. Pat. No. 6,003,606 to Moore et al. (“Moore '606”). Moore '606 t

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