Boring or penetrating the earth – Automatic control
Patent
1997-10-02
1999-11-09
Neuder, William
Boring or penetrating the earth
Automatic control
175 45, 175 73, E21B 708
Patent
active
059795700
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to oil and gas drilling and more specifically relates to an apparatus and method for selecting or controlling, from the surface of the earth, the direction in which a wellbore proceeds utilizing standard drilling techniques.
BACKGROUND ART
The formation through which a wellbore is drilled exerts a variable force on the drill string at all times. This variable force is essentially due to the clockwise rotary motion of the bit, the weight applied to the drill bit and the strata of the formation. Formation is a general term used to define the material--namely rock, sand, shale, clay, etc.--that the wellbore will pass through in order to open a pathway or conduit to a producing formation. This variable force will result in a variable change in the direction of the wellbore.
The formation is generally layered by the action of nature over millions of years and is not necessarily level. The formation will have dips, defined as a change in direction of the layers of the formation, which can extend either upward or downward. As the drill bit moves into a dip or from one type of formation to another, the force on the drill bit will change and cause the drill bit to wander up, down, right or left. This wandering is the natural result of the reaction of the formation to the clockwise torque and forward drilling force exerted by the drill bit on the formation. Mathematically the result can be viewed as a simple vector cross product between the torque force and the drilling force or weight on bit. The cross product results in a component force towards the right of the drilling force. The industrial term given to this effect is "bit-walk" and many methods to control or re-direct "bit-walk" have been tried in the industry.
Bit-walk is predictable, but the magnitude and, frequently, the direction of bit-walk are generally unpredictable. Looking at the vector cross product model, it can be seen that as the drilling force or weight on bit is varied, the cross product varies. Or, as the RPM of the drill string is varied, the cross product varies. Or, as the formation changes, the cross product changes. In drilling a wellbore, all of these forces constantly vary; thus, the magnitude of bit-walk constantly changes. The industry has learned to control the effects of bit-walk in a vertical hole by varying the torque and weight on bit while drilling a vertical hole. However, in an inclined (non-vertical) hole bit-walk causes a number of problems.
By industry definition, once an inclined hole is established, the side of the wellbore nearest to true vertical is called the "low-side" of the hole. The opposite side of the hole is referred to as "high-side" and is used as a reference point throughout the wellbore drilling operation. The drilling force follows the longitudinal extension of the wellbore; thus, the drilling force is parallel to and spaced from the low-side of the hole. Since bit-walk is the result of applied torque and drilling force, then it can be anticipated that normal bit-walk will be to the right of the low-side of the hole. This definition applies in all wellbores.
In a vertical hole or slightly inclined hole, bit-walk may be controlled by developing as much rigidity as possible in the lower portion of the drill string near to the drill bit. This can be and generally is accomplished by using drill string components of high rigidity and weight (drill collars or heavy-weight drill pipe) and stabilizers. A stabilizer, well known in the industry, is a tubular member with a combination of radial blades, often having a helical configuration, circumferentially arranged around the tubular and extending beyond the outer diameter of the tubular. The extension of the stabilizer blades is limited to the diameter of the drill bit. Thus, the stabilizer will work in a stable hole; however, if the wellbore washes out (increases in diameter due to formation or other downhole mechanical or hydraulic effects) or where the lateral force exerted by the blades is less than the torque eff
REFERENCES:
patent: 4638873 (1987-01-01), Welborn
patent: 4770258 (1988-09-01), Falgout, Sr.
patent: 4938297 (1990-07-01), Schmidt
patent: 5220963 (1993-06-01), Patton
Chance Jack Philip
McLoughlin Stephen John
Alworth C. W.
Neuder William
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