Patent Application
20130043076
Some wells may need to be drilled using a complex trajectory to reach multiple target areas or to perform other operations. Therefore, operators must be able to precisely “steer” the drilling direction. To do this, operators can remotely operate a directional drilling device near the drill bit to control the drilling direction. Various types of directional drilling devices are known in the art. One such device uses a variable stabilizer, such as disclosed in U.S. Pat. No. 4,821,817, to control the drilling trajectory. The variable stabilizer has stabilizer blades that center the drill string within the borehole. Drilling mud pumped downhole is used to control the variable stabilizer by retracting the blades. When selected blades are retracted, the device permits the drilling angle of the drill bit to be changed.
Another directional drilling device is commonly referred to as a bent housing mud motor. This device uses a mud motor disposed on a housing that has an axis displaced from the axis of the drill string. In use, circulated drilling fluid hydraulically operates the mud motor, which has a shaft connected to a rotary drill bit. By rotating the drill bit with the motor and simultaneously rotating the motor and bit with the drill string, the device produces an advancing borehole trajectory that is parallel to the axis of the drill string. However, by rotating the drill bit with the motor but not rotating the drill string, the device can produce a borehole trajectory deviated from the axis of the non-rotating drill string. By alternating these two methodologies, operators can control the path of the borehole.
Another directional drilling device is a rotary steerable system that can change the orientation of the drill bit to alter the drilling trajectory but does not require rotation of the drill string to be stopped. One type of rotary steerable system is disclosed in U.S. Pat. No. 6,116,354, which is incorporated herein by reference. Although effective, rotary steerable systems during certain operations can suffer from vibrations and oscillations that can be extremely damaging and hard to control. These uncontrolled vibrations can especially occur when the rotary steerable system is run below a high torque mud motor with a reasonably high speed (i.e., a total bit RPM of about 110). Generally the higher the RPM, the higher the likelihood of CCW whirl.
In particular, a bottom hole assembly having a rotary steerable system essentially acts as a series of rotating cylindrical spring mass systems with variable support points (typically stabilizers or extended blades). The natural frequencies of these spring mass systems can create a variety of damaging vibrations during operation. Ideally, the bottom hole assembly experiences concentric rotation so that drill bit has sliding contact with the borehole wall. Although the assembly may initially be in sliding contact, the assembly eventually tries to ride up the wall in a horizontal borehole, but gravity and bending strain tend to throw the assembly back downslope.
The riding and dropping of the assembly in the borehole can intensify and becomes more violent with increasing impact loads propelling the assembly back and forth across the borehole. Eventually, the multiple impacts can develop into counterclockwise (CCW) bit whirl in which the drill bit is in continuous rolling contact with the borehole wall. At this stage, the frequency of the whirl action jumps dramatically, and the bottom hole assembly oscillates in a counterclockwise direction opposite to the rotation of the drill string. In general, the resulting motion can be defined by a Hypocycloid sub form of general Hypotrochoids. (This is true for a point on the outer surface of the BHA because the center describes a circle of diameter equal to the borehole clearance). The whirl action from the drill bit can travel up the drill string and can affect multiple points on the assembly.
As expected, counterclockwise bit whirl can unevenly wear the drill bit's cutters and can create fatigue in the various components of the bottom hole assembly and drill string. For this reason, operators need a way to reduce or minimize the development of counterclockwise bit whirl in a bottom hole assembly having a rotary steerable system or any other rotary drilling assembly.
A bottom hole assembly for directional drilling avoids damaging vibrations that conventional assemblies may experience during operation. The assembly has a drill bit, a first collar that rotates with the drill bit, a rotary steerable tool that can control the trajectory of the drill bit, and a second collar that rotates with the drill string used to deploy the assembly.
The rotary steerable tool can use point-the-bit or push-the-bit technology. For example, the rotary steerable tool can have a center shaft that drives the drill bit and can have a non-rotating sleeve disposed about the center shaft and configured to remain rotationally stationary relative to the shaft. Hydraulically actuated pistons on a mandrel disposed in the sleeve can deflect the center shaft relative to the sleeve to direct the drill bit, and a stabilizer disposed on the first collar can act as a fulcrum point for the tool. During operation, both the drill string and the bit are rotated, and a mud motor on the assembly can impart rotation to the drill bit.
In one arrangement, the first collar coupled between the drill bit and the rotary steerable tool defines a bend that deflects the drill bit from an axis of the first collar. The bend can be predefined in the collar or can be adjustable. During operation, this bend causes a portion of the bottom hole assembly to engage the borehole wall. In this way, the bend can inhibit counterclockwise (CCW) bit whirl from developing at the drill bit by promoting clockwise whirl in a portion of the bottom hole assembly, generating friction against the borehole wall, and dampening vibrations generated at the assembly. By inhibiting or even preventing CCW bit whirl at the bottom hole assembly, other damaging vibrations such as CCW whirl in the drill string can also be prevented from forming up the borehole. In other arrangements, only the second collar between the tool and the drill string can define a bend, or both the first and second collars can define bends.
A directional drilling system 10 in
During operation, a rotary drilling rig 20 at the surface rotates the drill string 22 connected to the bottom hole assembly 50, and a mud system 30 circulates drilling fluid or “mud” through the drill string 22 to the bottom hole assembly 50. The mud operates the mud pump 56, providing torque to the drill bit 58. As the drill string 22 rotates, the drill bit 58 and lower collar 66 also rotate. Eventually, the mud exits through the drill bit 58 and returns to the surface via the annulus.
During drilling, the rotary steerable tool 60 can be operated to direct the drill bit 58 in a desired direction using point-the-bit technology discussed later so that the bottom hole assembly 50 can change the drilling path. As noted previously, however, the bottom hole assembly 50 with the rotary steerable tool 60 can suffer from undesirable vibrations in some circumstances, and the resulting motion from the vibrations can be extremely damaging and hard to control, especially when the rotary steerable tool 60 is run below a high torque mud motor 56 with a reasonably high speed (i.e., a total drill bit RPM of about 110). It is believed that damaging vibrations that begin as counterclockwise (CCW) bit whirl starting at the bottom hole assembly 50 and that can travel up the assembly 50 and drill string 22. The frequencies involved in CCW bit whirl can be at least an order of magnitude higher than the drill string's RPM and can be a function of the borehole's diameter, the drill bit's diameter, and dimensions of other components of the bottom hole assembly 50 that act as the driving surfaces for whirl.
Regardless of the frequencies involved, the whirl once CCW bit whirl develops can migrate up the drill string 22 where it changes frequencies as the casing/drill string traction diameters change. This migrating whirl can eventually lead to CCW whirl in the drill string 22. The frequency of this whirl is believed to be established by the relative diameter of tool joints and the casing's internal diameter and is believed to be driven by the bottom hole assembly's CCW bit whirl, which can occur at a different frequency.
To alleviate the problems associated with CCW whirl, the rotary steerable tool 60 has a bend 67 in its rotating lower collar 66 near the drill bit 58. As the collar 66 and bit 58 rotate, the bend 67 in the collar 66 can prevent CCW bit whirl from developing and evolving into other uncontrolled motions, such as whirl in the drill string 22 uphole. The bend 67 can prevent this evolution by clamping portions of the bottom hole assembly 50 in the borehole 40, creating friction between the assembly 50 and the borehole wall, creating clockwise (CW) whirl in the assembly 50, or producing a combination of these actions.
During operation, for example, the rotating bend 67 produces frictional damping as the bent collar 66 is forced straight in the borehole 40. This friction inhibits the drill bit 58 from moving into rolling contact with the borehole wall, which could lead to CCW bit whirl. In addition, the bend 67 preloads the assembly 50 against the borehole wall and dampens harmful vibrations that may develop during operation and attempt to travel uphole. When this bend 67 is forced straight in the borehole 40, for example, the bend 67 clamps portions of the bottom hole assembly 50 and adjacent drill string 22 against the borehole 40. This clamping prevents resonant frequencies from developing and makes it harder for bit whirl to develop and travel uphole, because the traction of the drill bit 58 around the borehole wall cannot be maintained for an entire 360 degrees.
Finally, by engaging the borehole wall, the bend 67 also tends to create clockwise (CW) whirl that inhibits the extremely damaging hypocycloidal CCW bit whirl from developing. As expected, CCW whirl of the bit 58 cannot coexist with CW whirl in the assembly 50 generated by the collar 66. In this way, any CW whirl created by the collar 66 occurring at the collar's rotational frequency forces the drill bit 58 out of continuous rolling contact with the borehole wall and breaks up any CCW bit whirl that may develop.
As shown in more detail in
A suitable system for the rotary steerable tool 60 is the Revolution® Rotary Steerable System available from Weatherford. As shown, the rotary steerable tool 60 has an upper end 62 coupled to the upper collar 54. A center shaft (72;
As shown in the cross-section of
As shown in
As shown in
In one arrangement, the bend 67 may be disposed a length (L) of a several feet or less from the drill bit 58, although the actual distance may vary given a particular implementation, size of the assembly 50, etc. In general, the bend 67 may define an angle (θ) of from 0 to 3-degrees, although the angle may depend on variables of the particular implementation. In addition, the bend 67 may deflect the drill bit 58 by a deflection (D) of about 3/16 inch off axis or more. For example, the deflection (D) of the drill bit 58 may be about ¼-inch from axis of the tool 60, although again the deflection (D) depends on the particular implementation. [Para 33] Given the deflection (D) by the bend 67, the drill bit 58 when rotated sweeps a circular path that drills a borehole slightly larger than the diameter of the drill bit 58. As shown in
The bend 67 may even tend to dampen string vibration even in over gage holes. For example, the bottom hole assembly 50 having a ¼-inch off axis bend 67 may be effective even in a ⅜-inch over gage borehole. The bend 67 may also dramatically reduce the tendency of the assembly 50 to engage in stick slip oscillation, which are pumped rotational oscillations caused by forcing functions at the drill bit 58. Although the actual amount of deflection required to be effective depends on the stiffness of the bottom hole assembly 50, the deflection load is preferably sufficient to assure that at least a portion of the bottom hole assembly 50 engages and stays in contact with the borehole wall.
As discussed above, the lower collar 66 near the near-bit stabilizer 52B can define the bend 67. In an alternative shown in
In another alternative shown in
In this specification, terms such as “upper”, “lower” and “bottom” may be used for convenience to denote parts which have such an orientation in the drill string when the drill string extends vertically in a borehole. However, it will be understood that these parts may have a different orientation when the bottom hole assembly is in a section of borehole that deviates from the vertical and may even be horizontal.
Although discussed as being used with the rotary steerable tool 60 that uses point-the-bit technology (namely a center shaft deflected by a mandrel with pistons in a non-rotating sleeve), the teachings of the present disclosure are also applicable to rotary steerable tools that use push-the-bit technology. A push-the-bit rotary steerable tool can use external pads extendable from a non-rotating sleeve to engage the borehole wall to direct the drill bit. Thus, this form of tool can have a center shaft driving the drill bit and can have a sleeve disposed about the center shaft that is configured to remain rotationally stationary relative to the shaft. At least one pad disposed on the sleeve is extendable therefrom to engage the borehole wall to change the trajectory of the drill bit.
The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the Applicants. In exchange for disclosing the inventive concepts contained herein, the Applicants desire all patent rights afforded by the appended claims. Therefore, it is intended that the appended claims include all modifications and alterations to the full extent that they come within the scope of the following claims or the equivalents thereof.
