GB2252992A

GB2252992A - Downhole tool

Info

Publication number: GB2252992A
Application number: GB9103699A
Authority: GB
Inventors: Frank A S Innes
Original assignee: Halliburton Logging Services Inc
Current assignee: Halliburton Logging Services Inc
Priority date: 1991-02-22
Filing date: 1991-02-22
Publication date: 1992-08-26
Anticipated expiration: 2011-02-22
Also published as: GB9103699D0; GB2252992B

Abstract

A downhole tool (30) for positioning in a drill collar (31) to generate pressure pulses in a drilling fluid comprising an elongate body (3, 18) and a plurality of blades spaced around the body. The blades are each divided into an independent front section (4) and rear section (6), one of the front and rear sections being mounted on a rotatable member (8) such that the front and rear sections are angularly displaceable relative to each other between a first position in which the sections are aligned and a second position in which the rear sections obstruct fluid flow between the front sections to generate a pressure pulse. The tool includes a mean (13) for generating a torque on the rotatable member, and an escapement means (10) which is radially movable to alternately engage and disengage with teeth (12) on the rotatable member to permit stepwise rotation thereof, and thus to move the blade sections between the first and second position. <IMAGE>

Description

DOWNHOLE TOOL The invention relates to a downhole tool such as a well-logging tool, and more particularly to a tool of the measure-while-drilling (MWD) type.

When oil wells or other boreholes are being drilled it is frequently necessary to determine the orientation of the drilling tool so that it can be steered in the correct direction. Additionally, information may be required concerning the nature of the strata being drilled, the temperature or the pressure at the base of the borehole, for example. There is thus a need for measurements of drilling parameters, taken at the base of the borehole, to be transmitted to the surface.

One method of obtaining at the surface the data taken at the base of the borehole is to withdraw the drill string from the hole, and to lower the instrumentation including an electronic memory system down to the base.

The relevant information is encoded in the memory to be read when the instrumentation is raised to the surface. The disadvantages of this method are the considerable time, effort and expense involved in withdrawing and replacing the drill string. Furthermore, updated information on the drilling parameters is not available while drilling is in progress.

A much-favoured alternative is to use a measurewhile-drilling tool, wherein sensors or transducers positioned at the lower end of the drill string continuously or intermittently monitor predetermined drilling parameters and the tool transmits the appropriate information to a surface detector while drilling is in progress. Typically, such MWD tools are positioned in a cylindrical drill collar close to the drill bit, and use a system of telemetry in which the information is transmitted to the surface detector in the form of pressure pulses through the drilling mud or fluid which is circulated under pressure through the drill string during drilling operations. Digital information is transmitted by suitably timing the pressure pulses. The information is received and decoded by a pressure transducer and computer at the surface.

The drilling mud or fluid is used to cool the drill bit, to carry chippings from the base of the bore to the surface and to balance the pressure in the rock formations. Drilling fluid is pumped at high pressure down the centre of the drill pipe and through nozzles in the drill bit. It returns to the surface via the annulus between the exterior of the drill pipe and the wall of the borehole.

In a number of known MWD tools, a negative pressure pulse is created in the fluid by temporarily opening a valve in the drill collar to partially bypass the flow through the bit, the open valve allowing direct communication between the high pressure fluid inside the drill string and the fluid at lower pressure returning to the surface via the exterior of the string. However, the high pressure fluid causes serious wear on the valve, and often pulse rates of only up to about 1 pulse per second can be achieved by this method.

Alternatively, a positive pressure pulse can be created by temporarily restricting the flow through the downpath within the drill string by partially blocking the downpath.

EP-A-0325047 (Russell et al) describes a measurewhile-drilling tool employing a turbine with curved impeller blades, wherein the impeller rotates continuously under the action of the high pressure downward flow. Each impeller blade is split into two portions in a plane normal to the axis of rotation of the impeller. An electric generator is driven by the impeller assembly and the two portions of impeller blade are angularly displaceable relative to one another about the axis of rotation in response to a change in the load of the generator. When the two portions of the impeller blade are out of normal alignment, they provide increased resistance to the flow of the drilling fluid, so that as the angular displacement of the two portions varies, so will the pressure drop across the impeller assembly.The restoring force for returning the two portions of the impeller blades to normal alignment is provided by a spring or an elastomeric seal: if the restoring force is too weak a large pressure pulse can be developed, but there is a long delay before the portions are realigned so that the pressure pulse rate can only be very low. If the restraining force is too great the pulse rate can be sufficiently rapid for efficient data transmission, but the pressure pulses will be much weaker.

Furthermore, the blades cannot be retained in the nonaligned position for long as there will be a natural tendency for the blade portions to realign.

US-A-4914637 (Positec Drilling Controls Ltd) discloses a number of embodiments of MWD tool having a pressure modulator for generating positive pressure pulses.

The tool has a number of blades equally spaced about a central body, the blades being split in a plane normal to the longitudinal axis of the body to provide a set of stationary half-blades and a set of rotary half-blades. A temporary restriction in the fluid flow is caused by allowing the rotary half-blades to rotate through a limited angle, so that they are out of alignment with the stationary half-blades, the rotation being controlled by a solenoid-actuated latching means. In one embodiment, the drilling fluid is directed through angled vanes in front of the split blades in order to impart continuous torque to the rotary half-blades, such that the rotary half-blades rotate through a predetermined angle each time the latch is released, thus being rotated successively into and out of alignment with the stationary half-blades.The rotary blades are mechanically linked to a rotatable cylindrical housing via a central shaft. The latching means comprises an axially slidable actuator rod having detent means extending perpendicularly thereto, the detent means engaging successive pins protruding from the interior of the cylindrical housing as the rod slides between two axial positions, allowing the housing to rotate through a predetermined angle.

The latching means is actuated by movement of the detent means in the axial direction only, and the pins and the detent means are subject to considerable stress as the housing reaches the end of its rotation and the detent means engages the next successive pin. Accordingly, the detent means requires a substantial support on the slidable actuator rod to withstand the torque, and the pins and the detent means are susceptible to significant wear and stress. The present invention provides an escapement means which is actuated by radial movement of the detent means, such that the torque exerted on the escapement means is considerably reduced, and the escapement means does not require such a bulky and substantial support on the actuator rod.

Furthermore, the mechanical linkage between the rotary blades and the latching means in US-A-4914637 is complex and includes a number of torque transfer points where stress and ultimate failure of the device may occur.

In a preferred embodiment, the present invention aims to provide a much more direct linkage between the latching or escapement means and the rotary blades.

According to the present invention there is provided a downhole tool for generating pressure pulses in a drilling fluid, the tool comprising an elongate body for positioning in a drill collar of a drill string; a plurality of blades equally spaced around said body, each blade being divided into an independent front section and rear section, one of said front and rear sections being mounted on a rotatable member such that said front and rear sections are angularly displaceable relative to one another between a first position in which the sections are aligned and a second position in which the rear blade sections obstruct the fluid flow between the front sections to generate a pressure pulse; means whereby a torque is developed on the rotatable member; and escapement means radially movable to alternately engage and disengage with teeth on the rotatable member to permit stepwise rotation thereof to move the blade sections thereon between said first and second positions.

Preferably, the escapement means are radially movable in response to camming means. The escapement means are preferably supported in at least one slot in a stationary sleeve positioned within the rotatable member.

In one embodiment, the escapement means comprise at least one rod, the rod being longitudinally disposed in said body and being radially movable in response to the camming means.

In a further embodiment, the escapement means comprises a pivotable lever, and the lever preferably comprises a pawl which is pivotable in response to the camming means, the camming means being preferably operable by an electric actuator such as a solenoid.

We prefer to mount the rear blade sections on said rotatable member, and to develop the torque by means of angled vanes supported on the rotatable member. The rear blade sections preferably each have a generally planar forward end surface extending generally normal to the direction of fluid flow.

Preferably, means are provided for reducing torsional vibration of the rotatable member by a damping fluid contained within the rotatable member.

An embodiment of the invention will now be described in greater detail by way of example with reference to the accompanying drawings, in which: Figure 1 is a longitudinal cross-section of one embodiment of a downhole tool for generating pressure pulses in a drilling fluid; Figure 2 is a section taken on line II-II of Figure 1; Figure 3 shows detail of the blade arrangement on the tool of Figure l; and Figure 4 is a longitudinal cross-section of another embodiment of a downhole tool for generating pressure pulses in a drilling fluid.

Referring first to Figure 1, one embodiment of a downhole tool generally indicated by reference numeral 30 fits within a drill collar 31 of a drill string. The tool has a streamlined fairing or casing 2 facing into the downward flow of drilling fluid, and a standard fishing end 1 extends from the end of the fairing and permits the tool to be manipulated or to be retrieved should the tool need to be brought to the surface. The tool has a central fixed shaft 3, and rigidly attached to the shaft is a collar 32 supporting front blade sections 4. The shaft 3 has an extension 33 which contains a counterbore 34. A rotatable member 8 is supported coaxially on the shaft behind the collar 32, and closely adjacent thereto.

The rotatable member 8 is in the form of a sleeve held coaxially on the shaft 3 and extension 33 by means of bearings 7 situated towards the end of the sleeve nearest the collar 32 (hereinafter called the upstream end), and by means of bearing 15 situated towards the other end of the sleeve (hereinafter called the downstream end). A seal 5 is provided to prevent the drilling fluid from passing between the rotatable member 8 and the shaft 3 and into the bearings 7. A further seal 16 is provided to protect the bearing 15 from ingress of drilling fluid in a similar manner.

Rear blade sections 6 are provided on the upstream end of the rotatable member 8, such that they can be rotated about the central shaft 3. Each rear blade section 6 can be aligned with a front blade section 4 to form a streamlined blade lying substantially parallel with the direction of flow of the drilling fluid as indicated by arrow 35. The streamlined shape of each aligned blade is illustrated more clearly by the solid outlines shown in Figure 3. Effectively, the front and rear blade sections form a set of blades, equally spaced around a central shaft, the blades being split in a plane normal to the axis of the central shaft. When the front and rear sections of the blades are aligned, the drag coefficient of the streamlined blades is very low and the drilling fluid passes between the blades with little resistance.

However, the rotatable rear blade sections can be rotated out of line with the fixed front blade sections, thus greatly increasing the drag coefficient and hence the pressure drop, thus allowing a pressure pulse to be transmitted through the drilling fluid. The position which causes maximum drag coefficient is achieved when the rear blade sections are midway between two streamlined positions as shown by the broken outlines of the out-of-line rear blade portions 6' in Figure 3.

Two or more propeller blades 13 are supported on the downstream end of rotatable member 8 in the embodiment shown in Figure 1. Alternatively, the propeller blades may be placed upstream of the split blade sections 4 and 6.

The propeller blades are placed at an angle of incidence relative to the direction of flow of the drilling fluid so that the resulting lift component tends to rotate member 8 on its bearings about shaft 3. Thus, a continuous torque is supplied to the rotatable member 8, and the main driving force for creating the pressure pulses is derived directly from the energy in the drilling fluid, so that the additional energy requirement from downhole batteries or a turbine is very low.

The propeller blades 13 rotate with the rotatable member 8, and the position of the propeller blades when the front and rear blade sections are aligned is shown by the solid outline of blade 13 in Figure 3, while the broken outline of blade 13' indicates the position of the propeller blades when the rear blade sections 6' are out of line with the front blade sections 4.

The rotatable member 8 carries internal teeth 12 which form part of an escapement mechanism. A pawl 10 has a protruding tooth 36 at either end thereof adapted to cooperate with the internal teeth 12. The pawl is supported in a longitudinal slot 37 in the extension 33 of the shaft, the sides of the slot preventing lateral movement of the pawl, and supporting the pawl against any lateral forces applied to teeth 36 from the internal teeth 12. The pawl is pivotable at its centre about a pin 11. A rotatable camshaft 17 extends within the counterbore 34 and supports a cam 9 which cooperates with one end of the pawl 10 and a second, oppositely directed, cam 14 which cooperates with the other end of the pawl.Thus, as the camshaft 17 rotates the pawl is rocked about pin 11 by means of the cams 9 and 14 such that the pawl moves radially and alternately engages and disengages the internal teeth 12. The continuous torque on the rotatable member 8 means that, each time the pawl is disengaged from internal teeth 12, the rotatable member turns until the pawl is again engaged with the internal teeth to stop the blade sections 4 and 6 either in a streamlined position, or with the rear blade sections midway between two streamlined positions.

Figure 2 illustrates the escapement mechanism more clearly. Each time the escapement is operated by the camshaft 17, one end of the pawl is disengaged from the internal teeth 12, and the rotatable member 8 will rotate to the next position at which it is arrested by the pawl 10. The force exerted on the pawl when it engages with teeth 12 and brings the rotatable member to a stop, produces a circumferential torque which depends on the relatively small radial distance between a tooth 36 and the pin 11.

The camshaft 17 is rotated through a fixed angle by a rotary solenoid or other electric actuator 19. The electric actuator is housed in a f *o-d body 18 which is attached to the extension 33 of the shaft. The fixed body 18 and the internal spaces of the rotatable member 8 are filled with oil or other fluid which may be selected to provide the optimum combination of lubrication and viscous damping of torsional vibrations of the rotatable member.

To balance the high hydrostatic pressure in the borehole, a diaphragm 21 supported in the body 18 separates the oil 20 from the drilling liquid, but keeps the oil at the same pressure as the drilling fluid, which is admitted to a chamber 22 on the other side of the diaphragm to the oil via a small opening 23 in the housing of the body 18. A sealed piston or bellows can be used in place of the diaphragm to achieve a pressure balance.

Pressure pulses within the drilling fluid can thus be produced at a rate of up to 10 pulses per second.

A useful pulse rate is about 4 to 5 pulses per second, effectively generating a square wave of varying pressure in the fluid which can be detected at the surface by a pressure transducer which can detect, for example, a change of 40 to 50 psi in 3000 psi (270 to 350 kPa in 20.7 MPa).

Figure 4 illustrates another embodiment of a downhole tool according to the present invention. The tool generally indicated by reference numeral 40 has, as before a streamlined fairing 41 and a standard fishing end 42. A rotatable sleeve 43 extends downstream of the fairing. A stationary inner sleeve 44 extends coaxially with the rotatable sleeve 43, the rotatable sleeve being supported on the inner sleeve 44 by ball bearings 45 towards the upstream end and by roller bearings 46 towards the downstream end of the rotatable sleeve. The inner sleeve 44 is joined at its downstream end to an actuator housing 47. A ring seal 48 is provided between the rotatable sleeve 43 and the fairing 41, to prevent ingress of drilling fluid to the bearings 45 and a C-ring seal 49 is provided between the rotatable sleeve 43 and the actuator housing 47, to protect the bearings 46.

At least one propeller blade 50 is supported on the exterior of the upstream end of the rotatable sleeve 43, the blade being placed at an angle of incidence relative to the direction of flow of the drilling fluid so that the resulting lift component tends to rotate sleeve 43 on its bearings about inner sleeve 44.

A number of front blade sections 51 are provided on the exterior of the downstream end of the rotatable sleeve 43, while the same number of rear blade sections 52 are provided on a stationary collar 63 fitted to the exterior of the actuator housing 47. Each front blade section 51 can be aligned with a rear blade section 52 to form a streamlined blade lying substantially parallel with the direction of flow of the drilling fluid. Effectively, the front and rear blade sections form a set of blades equally spaced around a central shaft, the blades being split in a plane normal to the axis of the central shaft.

When the front and rear blade sections are aligned, the drag coefficient of the streamlined blades is very low as described with reference to Figures 1 to 3, and when the front blade sections are rotated out of line with the fixed rear blade sections, the drag coefficient is greatly increased, and a pressure pulse is transmitted through the drilling fluid.

The rotatable sleeve 43 carries internal teeth 53. Longitudinal slots or recesses 54 in the exterior of inner sleeve 44 house rollers 55, while holes 56 in the base of recesses 54 hold balls 57. An actuator push rod 58 slides within the inner sleeve 44 and operates a camming device 59. The push rod 58 is operated by means of a solenoid 60.

The camming device has recesses 61 in its exterior surface, such that when the recesses 61 are aligned with holes, 56 in the inner sleeve 44, the balls 57 are seated in the recesses 61 and the roller 55 is fully seated in longitudinal recess 54. The recesses 61 are shaped with sloping camming surfaces 62, such that as the camming device slides longitudinally the balls 57 may be lifted out of the recesses 61, thus being moved radially outwards through holes 56 to lift the roller 55 partially out of longitudinal recess 54. When the roller is lifted out of recess 54 it protrudes to an extent where it can cooperate with internal teeth 53 on the rotatable sleeve.

Thus the balls and rollers and camming device provide an escapement mechanism. As the camming device slides backwards and forwards, the balls and rollers move radially to alternately engage and disengage the internal teeth 53.

The continuous torque on the rotatable sleeve 43 means that each time a roller is disengaged from the teeth the rotatable sleeve turns until the next roller is engaged by the teeth, to stop the blade sections 51 and 52 either in a streamlined position or with the front blade section midway between two streamlined positions. When the rotatable sleeve is stopped by a roller 55, the stopping force is spread over the length of the roller and is absorbed by the sides of recesses 54, so that this escapement means is very hard-wearing.

In the embodiment shown in Figure 4 the recesses 61 on either side of the camming device are staggered such that only one or other roller is lifted out of its recess 54 at any one time to engage with teeth 53. Alternatively, the corresponding recesses 61 on either side of the camming device may be axially aligned such that both rollers are in the outer position at any one time. The camming device then has to be operated by sliding it to disengage the teeth 53 and allow the sleeve 43 to rotate until the camming device is operated by sliding it in the opposite direction to re-engage the teeth and prevent rotation. A further alternative is to provide only one roller in the escapement mechanism.

As in the first embodiment (shown in figures 1 to 3), the internal spaces of the rotatable sleeve 43 are filled with oil or other lubricant, the pressure being balanced with that of the drilling fluid in the borehole by a diaphragm or other suitable means as described in relation to the first embodiment.

Claims

CLAIMS:

1. A downhole tool for generating pressure pulses in a drilling fluid, the tool comprising an elongate body for positioning in a drill collar of a drill string; a plurality of blades spaced around said body, each blade being divided into an independent front section and rear section, one of said front and rear sections being mounted on a rotatable member such that said front and rear sections are angularly displaceable relative to one another between a first position in which the sections are aligned and a second position in which the rear blade sections obstruct the fluid flow between the front sections to generate a pressure pulse; means for generating a torque on the rotatable member; and escapement means radially movable to alternately engage and disengage with teeth on the rotatable member, to permit stepwise rotation thereof to move blade sections thereon between said first and second position.

2. A tool according to claim 1, wherein said front and rear blade sections are positioned on the exterior of said body.

3. A tool according to claim 1 or claim 2, wherein said escapement means are radially movable in response to a camming means.

4. A tool according to claim 3, wherein said escapement means are supported in at least one slot in a stationary sleeve positioned within said rotatable member.

5. A tool according to claim 3 or claim 4, wherein said escapement means comprise at least one rod, said rod being longitudinally disposed in said body and being radially movable in response to said camming means.

6. A tool according to claim 3 or claim 4, wherein said escapement means comprises a pivotable lever.

7. A tool according to claim 6, wherein said lever is a pawl pivotable in response to the camming means.

8. A tool according to claim 7, wherein said teeth are formed on the interior of said rotatable member, and said pawl and said camming means are supported within said rotatable member.

9. A downhole tool according to claim 7 or 8, wherein said camming means comprises cams formed on a rotatable camshaft.

10. A tool according to claim 7, 8 or 9, wherein said camming means is operable by an electric actuator.

11. A tool according to claim 10, wherein said electric actuator is a solenoid.

12. A tool according to any preceding claim wherein the front blade sections are mounted on said rotatable member.

13. A tool according to claim 12, wherein the torque is developed by means of angled vanes supported on the rotatable member.

14. A tool according to claim 13, wherein the angled vanes are supported on the rotatable member upstream of the rear blade sections.

15. A tool according to any preceding claim, wherein each said rear blade section has a generally planar forward end surface extending generally normal to the direction of fluid flow.

16. A tool according to any preceding claim, additionally comprising means to reduce torsional vibration of the rotatable member by a damping fluid contained within said rotatable member.

17. A downhole tool substantially as hereinbefore described with reference to the accompanying drawings.