Boring or penetrating the earth – Processes – Boring curved or redirected bores
Patent
1999-01-20
2000-08-29
Tsay, Frank S.
Boring or penetrating the earth
Processes
Boring curved or redirected bores
175 38, 175215, E21B 708
Patent
active
06109370&
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Directional controlled drilling arises from the early practices of using either a whipstock (wedge) set within a borehole to force a hole to deviate from a known trajectory, or the use of a jetting bit. Both are described in some detail in Applied Drilling Engineering, Society of Petroleum Engineers Textbook Series, Vol. 2, Chapter 8, Adam T. Bourgoyne Jr., Keith K. Millheim, Martin E. Chenevert & F. S. Young, Jr., 1991. The jetting system typically involves the use of a two-cone roller bit with a single stabilizer and a large jetting bit. When a directional adjustment is required, the drilling is interrupted and the large jet is held in the direction in which the deviation is required so that the jet erodes preferentially in that direction. Rotary drilling can resume after the desired directional change has been effected.
More recently most directional drilling has been undertaken by the use of down-hole mud motors. Turbine and positive displacement motors have been used with the latter being in more common use. Down-hole motors operate by converting energy extracted from the drilling fluid forced down the drill string and through the motor. This energy is converted into rotary motion which is used to rotate a drill bit that cuts the rock ahead of the tool. Directional change is effected by the use of a bottom hole assembly which includes a bent housing either behind or in front of the motor so that the bit does not drill straight ahead, but rather drills ahead and off to the side. This bottom hole assembly may be supported within the borehole by a series of stabilizers which assist the angle building capability of the assembly.
The bottom hole assembly so described tends to build an angle rather than drill straight ahead. Such a tendency can be halted in some drilling systems by rotating the entire drill string and bottom hole assembly so that on average the system drills straight ahead. A more common practice is to undertake repeated directional changes to the borehole trajectory by turning the rod string and hence the tool face angle. Alternatively, as is the case in coiled tubing drilling where the drill string cannot be rotated, the tool face is adjusted by incremental moves associated with fluid pressure pulses which relocate the tool at varying tool face angles. By changing the direction at which the bottom hole assembly tends to build an angle, many changes to the trajectory can be achieved. The borehole is seldom aligned in its intended direction but follows a snaking path about the planned direction. One of the consequences of this system of drilling is that the drill string is, by reason of the many changes in direction of the borehole, subject to much higher friction and stress levels. This is described in more detail in the publication Optimisation of Long Hole Drilling Equipment, Australian Mineral Industries Research Association, Melbourne, Ian Gray, March 1994. A consequence of the friction and stress is that the length of borehole is limited.
The basis for changing the direction in which drilling assemblies currently drill includes survey information measured near the bit, combined with a knowledge of the total distance drilled, and knowledge of the formation. The survey information normally provides information on the direction tangential to the survey tool located in the drill rods within the borehole. This information can be integrated with respect to the linear dimension of the borehole to arrive at the coordinates for the borehole. The formation position is either detected by prior drilling and geophysics or by geosteering equipment. The latter may comprise geophysical and drilling sensors to detect the nature of the material which is being drilled, or which are located at some distance from the drill string. The nature of the material being drilled is most likely to be detected using a torque and thrust sensor within the drill string, short focused gamma-gamma probes or resistivity probes. Alternatively, formation types may be detected at a greater dista
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Gray Ian
Tsay Frank S.
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