GB2489227A

GB2489227A - Resonance enhanced drill test rig

Info

Publication number: GB2489227A
Application number: GB1104677.8A
Authority: GB
Inventors: Marian Wiercigroch
Original assignee: ITI Scotland Ltd
Current assignee: ITI Scotland Ltd
Priority date: 2011-03-21
Filing date: 2011-03-21
Publication date: 2012-09-26
Also published as: GB2504017A; GB201104677D0; GB201316664D0; GB2504017B; WO2012126898A3; WO2012126898A2

Abstract

Apparatus for testing a resonance enhanced rotary drilling module. The rig comprises a resonance enhanced rotary drilling module with oscillator, a fixed frame, a movable frame for moving the rotary drilling in an axial direction relative to a sample and means for generating relative rotary motion between drilling module and sample. A torsion restraint unit reduces torsional loading on the vibrator. The torsional restraint may be a membrane torsion restraint unit. Load cells can be used to measure the axial and oscillatory loading. Closed loop real time control of the oscillator may be provided using a controller connected to load cells

Description

TEST APPARATUS

The present invention relates to high frequency percussion enhanced rotary drilling, and in particular to resonance enhanced drilling. Embodiments of the invention are directed to an apparatus for testing a resonance enhanced rotary drilling module, and methods of testing using the apparatus. Further embodiments of this invention are directed to resonance enhanced drilling equipment which may be controllable according to these methods and apparatus.

Percussion enhanced rotary drilling is known per Se. A percussion enhanced rotary drill comprises a rotary drill-bit and an oscillator for applying oscillatory loading to the rotary drill-bit. The oscillator provides impact forces on the material being drilled so as to break up the material which aids the rotary drill-bit in cutting though the material.

Resonance enhanced rotary drilling is a special type of percussion enhanced rotary drilling in which the oscillator is vibrated at high frequency so as to achieve resonance with the material being drilled. This results in an amplification of the pressure exerted at the rotary drill-bit thus increasing drilling efficiency when compared to standard percussion enhanced rotary drilling.

US 3,990,522 discloses a percussion enhanced rotary drill which uses a hydraulic hammer mounted in a rotary drill for drilling bolt holes. It is disclosed that an impacting cycle of variable stroke and frequency can be applied and adjusted to the natural frequency of the material being drilled to produce an amplification of the pressure exerted at the tip of the drill-bit. A servovalve maintains percussion control, and in turn, is controlled by an operator through an electronic control module connected to the servovalve by an electric conductor.

The operator can selectively vary the percussion frequency from 0 to 2500 cycles per minute (i.e. 0 to 42 Hz) and selectively vary the stroke of the drill-bit from 0 to 1/8 inch (i.e. 0 to 3.175mm) by controlling the flow of pressurized fluid to and from an actuator. It is described that by selecting a percussion stroke having a frequency that is equal to the natural or resonant frequency of the rock strata being drilled, the energy stored in the rock strata by the percussion forces will result in amplification of the pressure exerted at the tip of the drill-bit such that the solid material will collapse and dislodge and permit drill rates in the range 3 to 4 feet per minute.

There are several problems which have been identified with the aforementioned arrangement and which are discussed below.

High frequencies are not attainable using the apparatus of US 3,990,522 which uses a relatively low frequency hydraulic oscillator. Accordingly, although US 3,990,522 discusses the possibility of resonance, it would appear that the low frequencies attainable by its oscillator are insufficient to achieve resonance enhanced drilling through many hard materials.

Regardless of the frequency issue discussed above, resonance cannot easily be achieved and maintained in any case using the arrangement of US 3,990,522, particularly if the drill passes through different materials having different resonance characteristics. This is because control of the percussive frequency and stroke in the arrangement of US 3,990,522 is achieved manually by an operator. As such, it is difficult to control the apparatus to continuously adjust the frequency and stroke of percussion forces to maintain resonance as the drill passes through materials of differing type. This may not be such a major problem for drilling shallow bolt holes as described in US 3,990,522. An operator can merely select a suitable frequency and stroke for the material in which a bolt hole is to be drilled and then operate the drill. However, the problem is exacerbated for deep-drilling through many different layers of rock. An operator located above a deep-drilled hole cannot see what type of rock is being drilled through and cannot readily achieve and maintain resonance as the drill passes from one rock type to another, particularly in regions where the rock type changes frequently.

Some of the aforementioned problems have been solved by the present inventor as described in WO 2007/141550. WO 2007/141550 describes a resonance enhanced rotary drill comprising an automated feedback and control mechanism which can continuously adjust the frequency and stroke of percussion forces to maintain resonance as a drill passes through rocks of differing type. The drill is provided with an adjustment means which is responsive to conditions of the material through which the drill is passing and a control means in a downhole location which includes sensors for taking downhole measurements of material characteristics whereby the apparatus is operable downhole under closed loop real-time control.

US2006/0 157280 suggests down-hole closed loop real-time control of an oscillator. It is described that sensors and a control unit can initially sweep a range of frequencies while monitoring a key drilling efficiency parameter such as rate of progression (ROP). j\j oscillation device can then be controlled to provide oscillations at an optimum frequency until the next frequency sweep is conducted. The pattern of the frequency sweep can be based on a one or more elements of the drilling operation such as a change in formation, a change in measured ROP, a predetermined time period or instruction from the surface. The detailed embodiment utilises an oscillation device which applies torsional oscillation to the rotary drill-bit and torsional resonance is referred to. However, it is further described that exemplary directions of oscillation applied to the drill-bit include oscillations across all degrees-of-freedom and are not utilised in order to initiate cracks in the material to be drilled.

Rather, it is described that rotation of the drill-bit causes initial fractioning of the material to be drilled and then a momentary oscillation is applied in order to ensure that the rotary drill-bit remains in contact with the fracturing material. There does not appear to be any disclosure or suggestion of providing an oscillator which can import sufficiently high axial oscillatory loading to the drill-bit in order to initiate cracks in the material through which the rotary drill-bit is passing as is required in accordance with resonance enhanced drilling as described in WO 2007/14 1550.

None of the prior art provides any detail about how to monitor axial oscillations. Sensors arc disclosed generally in the U52006/0157280 and in WO 2007/141550 but the positions of these sensors relative to components such as a vibration isolation unit and a vibration transmission unit is not discussed.

Despite the solutions described in the prior art, there has been a desire to make further improvements to the methods and apparatus it describes. It is an aim of embodiments of the present invention to devise a test apparatus (also termed a test rig) and test methods to facilitate the development of improvements to drilling efficiency, drilling speed and borehole stability and quality, while limiting wear and tear on the drill module so as to increase the lifetime of the module, Accordingly, the present invention provides an apparatus for testing a resonance enhanced rotary drilling module, which apparatus comprises: (i) a resonance enhanced rotary drilling module comprising an oscillator; (ii) a fixed frame for fixing the apparatus to a base surface; (iii) a movable frame for moving the rotary drilling module in an axial direction relative to a sample; (iv) a means for generating relative rotary motion between the drilling module and a sample; and (v) a torsion restraint unit for reducing the torsional loading on the oscillator.

The orientation of the apparatus of the present invention is not especially limited, provided that it enables testing of a resonance enhanced rotary drilling module. Horizontal orientations are envisaged, however typically the apparatus is arranged in a vertical configuration, i.e. with the fixed frame fixed to its location -e.g. the ground and/or other solid surface in a field trial, or to the floor and/or other solid surface of a test building or laboratory.

The fixed frame is not especially limited provided that it serves to fix the apparatus relative to its location, i.e. relative to the earth in a field trial, or alternatively relative to the test building or laboratory. In this context fixed means that movement of the apparatus relative to its location is limited such that testing of the drilling module is not unduly impeded.

The moveable frame is not especially limited, provided that testing of the drilling module is not impeded. In typical embodiments of the invention, the movable frame is configured to move the drill module so that when a sample is present the drill bit of the drilling module will contact the sample and drilling into the sample will be possible. Typically the movable frame moves in an axial direction with respect to the drill module. Also in a typical embodiment, the moveable frame is located within the fixed frame, and may (for example) slide up and down within the fixed frame.

The means for generating the rotary motion between the drilling module and a sample is not especially limited, provided that the relative rotational motion is sufficient for drilling into the sample to be possible. Thus, the drilling module may be caused to rotate, whilst the sample is held stationary, or alternatively the sample may be rotated whilst the drilling module is not rotated (the drilling module will not be stationary, since it will undergo axial oscillation in order to perform resonance enhancement). In typical embodiments a lathe system is employed so that the sample is caused to rotate. In a typical vertical configuration of the apparatus, a vertical lathe will be employed.

The torsion restraint unit is an important part of the apparatus. As has been mentioned above, in resonance enhanced drilling operations, it is necessary to ensure that the drill bit is both rotating and oscillating. However, the inventors have discovered that typical means for imparting oscillatory loading (such as magnetostrictive oscillators) are sensitive to torsional loads, and may quickly cease to function if these are not controlled. This is not a problem in known test apparatus for non-resonance enhanced methods which are not employing highly controlled combinations of rotational and oscillatory loading. Thus, the torsion restraint unit is required in order to reduce the torsional loading on the oscillator.

The nature of the torsion restraint unit is not especially limited, provided that it is capable of reducing the torsional load on the oscillator, as compared with the torsional load in the absence of the torsion restraint unit. In typical embodiments, the torsion restraint unit comprises a membrane torsion restraint unit. In the present context, membrane means a configuration wherein the torsion restraint unit comprises a broad, flat structure. Generally, but not exclusively, the torsion restraint unit is in the form of a disc, with an axial dimension that is less than its radial dimension. A typical example of such a unit is shown in Figure 3. In typical embodiments, the torsion restraint unit has greater torsional stifihess than axial stifihess. To maximise this capability, further structure may be provided, such as a disc with holes defining a number of spokes, radii, or spars for spreading the forces applied to the unit across its structure. A typical example of this is shown in Figure 3.

The invention will now be described in more detail, by way of example only, with reference to the following Figures, in which: Figure 1 shows a photograph of a typical test apparatus of the present invention. It is in a vertical configuration with the moveable frame sliding vertically inside the fixed frame. A vertical lathe is employed for rotating a rock sample, which is held in place by a sample fixing means located on the rotating plate of the lathe. Various further components of the drilling module are also shown.

Figure 2 shows a schematic section view of the apparatus, depicting the location of the torsion restraint unit. The magnetostrictive oscillator is a PEX-30, whilst the structural spring operates as a vibration transmission unit.

Figure 3 shows an exemplary torque restraint unit, detailing the main dimensions of a specific exemplary embodiment.

In a typical vertical configuration, with a rotating sample, the torque restraint unit is designed to prevent the rotation of the drill-bit (due to the rotation of the rock sample). It is designed to link to the oscillator (e.g. to a PEX-30). The basic requirement of the component is to reduce, prevent, isolate and/or block the torsional load to the oscillator and have a minimal effect on the axial oscillations of the drill-bit. Therefore the structure should have a large torsional stiffness and a small axial stiffness, The stiffness, k, of a body is a measure of the resistance offered by an elastic body to deformation. For an elastic body with a single degree of freedom (for example, stretching or compression of a rod), the stiffness is defined as: where: F is the force applied on the body o is the displacement produced by the force along the same degree of freedom (for instance, the change in length of a stretched spring) The geometric parameters of restraint were designed by following the strength requirements of the apparatus components. For an exemplary apparatus such as that of Figure 1, the procedure can be described as follows: The maximum static and dynamic load is proximately P3+P3O kN, the radius of drill-bit is Rr=2% in=O.073025 m, and the friction coefficient of drill-bit and rock is p=l, so the maximum torque will be: Mmax =4iiR [P+PdJ=146O.5 [Nm] If an M8 type bolt is used, the shear stress on each bolt in inner circle shown in Figure 3 is: r=_-_= MMax AMS 6 rOAM8 Since rrO.04 m, AM8=32.8 l06 m2: M =185.531 MPa.

6 rOAMS In this example, the material of bolt can be: -Property class 4.6, materials: low or medium carbon, head marking 4.6.

-Property class 4.8, materials: low or medium carbon, head marking 4.8.

-Property class 5.8, materials: low or medium carbon, head marking 5.8.

Similarly, following the strength requirements, the following parameters may be. employed in some embodiments: -The thickness of the membrane may be h=O.002 m.

-Width of the trip may be b=O.02 m.

-The diameter of the holes for M8 bolt may a bit greater than 0.008 m (Diameter MS is 0.008 m).

-Radius of the inner hole d177 may be less than 0.03 m -Radii of chamfer angle can be 0.Ol-0.015 m (to avoid stress concentration) In typical embodiments of the invention, the rotary drilling module comprises: (i) an upper load-cell for measuring static and dynamic axial loading; (ii) a vibration isolation unit; (iii) optionally an oscillator back mass; (iv) an oscillator comprising a dynamic exciter for applying axial oscillatory loading to the rotary drill-bit; (v) a vibration transmission unit; (vi) a lower load-cell for measuring static and dynamic axial loading; (vii) a drill-bit connector; and (viii) a drill-bit, wherein the upper load-cell is positioned above the vibration isolation unit and the lower load-cell is positioned between the vibration transmission unit and the drill-bit, and wherein the upper and lower load-cells are connected to a controller in order to provide down-hole closed loop real time control of the oscillator.

In such embodiments, the torsion restraint unit is typically situated between the oscillator and the vibration transmission unit.

It is envisaged that the drilling module being tested may eventually be employed as a resonance enhanced drilling module in a drill-string. The drill-string configuration is not especially limited, and any configuration may be envisaged, including known configurations.

The module may be tumed on or off as and when resonance enhancement is required.

In this apparatus arrangement, the dynamic exciter typically comprises a magnetostrietive exciter. The magnetostrictive exciter is not especially limited, and in particular there is no design restriction on the transducer or method of generating axial excitation. Preferably the exciter comprises a P]EX-30 oscillator from Magnetic Components AB.

The dynamic exciter employed in the present arrangement is a magnetostrictive actuator working on the principle that magnetostrictive materials, when magnetised by an external magnetic field, change their inter-atomic separation to minimise total magneto-elastic energy.

This results in a relatively large strain. Hence, applying an oscillating magnetic field provides in an oscillatory motion of the magnetostrictive material.

Magnetostrictive materials may be pre-stressed uniaxially so that the atomic moments are pre-aligned perpendicular to the axis. A subsequently applied strong magnetic field parallel to the axis realigns the moments parallel to the field, and this coherent rotation of the magnetic moments leads to strain and elongation of the material parallel to the field. Such magnetostrictive actuators can be obtained from MagComp and Magnetic Components AB.

As mentioned above, one particularly preferred actuator is the PEX-30 by Magnetic ComponentsAB.

It is also envisaged that magnetic shape memory materials such as shape memory alloys may be utilized as they can offer much higher force and strains than the most commonly available magnetostrictive materials. Magnetic shape memory materials are not strictly speaking magnetostrictive. However, as they are magnetic field controlled they are to be considered as magnetostrictive actuators for the purposes of the present invention.

In this arrangement, the vibration transmission unit is not especially limited, but preferably comprises a structural spring. It may be, for example, a torroidal unit with a concertina-shaped wall, preferably a hollow metal can with a concertina-shaped wall. The vibration isolation unit is also not especially limited, and may comprise a structural spring. It may be, for example, a torroidal unit with a concertina-shaped wall, preferably a hollow metal can with a concertina-shaped wall.

In this arrangement, the positioning of the upper load-cell is typically such that the static axial loading from the drill string can be measured. The position of the lower load-cell is typically such that dynamic loading passing from the oscillator through the vibration transmission unit to the drill-bit can be measured. The order of the components of the apparatus of this embodiment is particularly preferred to be from (i)-(viii) above from the top down.

In further embodiments of the invention, the rotary drilling module comprises: (i) an upper load-cell for measuring static loading; (ii) a vibration isolation unit; (iii) an oscillator for applying axial oscillatory loading to the rotary drill-bit; (iv) a lower load-cell for measuring dynamic axial loading; (v) a drill-bit connector; and (vi) a drill-bit, wherein the upper load-cell positioned above the vibration isolation unit and the lower load-cell is positioned between the oscillator and the drill-bit wherein the upper and lower load-cells are connected to a controller in order to provide down-hole closed loop real time control of the oscillator.

In such embodiments, the torsion restraint unit is typically situated below the oscillator.

The module may be turned on or off as and when resonance enhancement is required.

In this apparatus arrangement, the oscillator typically comprises an electrically driven mechanical actuator. The mechanical actuator is not especially limited, and preferably comprises a VR2S 10 actuator from Vibratechniques Ltd. An electrically driven mechanical actuator can use the concept of two eccentric rotating masses to provide the needed axial vibrations. Such a vibrator module is composed of two eccentric counter-rotating masses as the source of high-frequency vibrations. The displacement provided by this arrangement can be substantial (approximately 2 mm).

Suitable mechanical vibrators based on the principle of counter-rotating eccentric masses are available from Vibratechniques Ltd. One possible vibrator for certain embodiments of the present invention is the VR25 10 model. This vibrator rotates the eccentric masses at 6000 rpm which corresponds to an equivalent vibration frequency of 100 Hz. The overall weight of the unit is 41 kg and the unit is capable of delivering forces up to 24.5 kN. The power consumption of the unit is 2.2 kW.

This drilling module arrangement differs from the first drilling module arrangement in that no vibration transmission unit is required to mechanically amplify the vibrations. This is because the mechanical actuator provides sufficient amplitude of vibration itself Furthermore, as this technique relies on the effect of counter-rotating masses, the heavy back mass used in the magnetostrictive embodiment is not required. The vibration isolation unit is not especially limited, but preferably comprises a structural spring. It may be, for example, a torroidal unit with a concertina-shaped wall, preferably a hollow metal can with a concertina-shaped wall.

In this arrangement, the positioning of the upper load-cell is typically such that the static axial loading from the drill string can be measured. The position of the lower load-cell is typically such that dynamic loading passing from the oscillator to the drill-bit can be monitored. The order of the components of the apparatus of this embodiment is particularly preferred to be from (i)-(vi) above from the top down.

The apparatus of all of the arrangements of the invention gives rise to a number of advantages in the drilling modules being tested. These include: increased drilling speed; better borehole stability and quality; less stress on apparatus leading to longer lifetimes; and greater efficiency reducing energy costs.

The preferred applications for all embodiments of the drilling modules being tested are in large scale drilling apparatus, control equipment and methods of drilling for the oil and gas industry. However, other drilling applications may also benefit, including: surface drilling equipment, control equipment and methods of drilling for road contractors; drilling equipment, control equipment and method of drilling for the mining industry; hand held drilling equipment for home use and the like; specialist drilling, e.g. dentist drills.

During resonance enhanced drilling module test operation, the rotary drill-bit is rotated relative to the sample, and an axially oriented dynamic loading is applied to the drill-bit by the oscillator to generate a crack propagation zone to aid the rotary drill-bit in cutting though material.

The oscillator and/or dynamic exciter is controlled in accordance with preferred methods of the present invention. Thus, the invention ftrther provides a method for testing a resonance enhanced rotary drill module comprising an apparatus as defined above, the method comprising: controlling frequency (f) of the oscillator in the resonance enhanced rotary drill whereby the frequency (f) is maintained in the range: (D2 U/(8OOOnAm))112 S f 5 S1(D2 U/(8OOOAm))"2 where D is diameter of the rotary drill-bit, U5 is compressive strength of material being drilled, A is amplitude of vibration, m is vibrating mass, and Sf is a scaling factor greater than 1;and controlling dynamic force (Fd) of the oscillator in the resonance enhanced rotary drill whereby the dynamic force (Fd) is maintained in the range: [(/4)D2effUs] < Ed < SFd[(/4)D2effUs] where Doff is an effective diameter of the rotary drill-bit, U is a compressive strength of material being drilled, and SFd is a scaling factor greater than 1, wherein the frequency (f) and the dynamic force (Ed) of the oscillator are controlled by monitoring signals representing the compressive strength (U5) of the material being drilled and adjusting the frequency (f) and the dynamic force (Ed) of the oscillator using a closed loop real-time feedback mechanism according to changes in the compressive strength (U5) of the material being drilled.

The ranges for the frequency and dynamic force are based on the following analysis.

The compressive strength of the formation gives a lower bound on the necessary impact forces. The minimum required amplitude of the dynamic force has been calculated as: F=-DU Doff is an effective diameter of the rotary drill-bit which is the diameter D of the drill-bit scaled according to the fraction of the drill-bit which contacts the material being drilled.

Thus, the effective diameter Doff may be defined as: Doff = where Scontact is a sealing factor corresponding to the fraction of the drill-bit which contacts the material being drilled. For example, estimating that only 5% of the drill-bit surface is in contact with the material being drilled, an effective diameter Deff can be defined as: Dejj = VD.

The aforementioned calculations provide a lower bound for the dynamic force of the oscillator. Utilizing a dynamic force greater than this lower bound generates a crack propagation zone in front of the drill-bit during operation. However, if the dynamic force is too large then the crack propagation zone will extend far from the drill-bit compromising borehole stability and reducing borehole quality. In addition, if the dynamic force imparted on the rotary drill by the oscillator is too large then accelerated and catastrophic tool wear and/or failure may result. Accordingly, an upper bound to the dynamic force may be defined as: SFci[(7t/4)D2effUs] where SFd is a scaling factor greater than 1. In practice 5Fd is selected according to the material being drilled so as to ensure that the crack propagation zone does not extend too far from the drill-bit compromising borehole stability and reducing borehole quality.

Furthermore, SFd is selected according to the robustness of the components of the rotary drill to withstand the impact forces of the oscillator. For certain applications SFd will be selected to be less than 5, preferably less than 2, more preferably less than 1.5, and most preferably less than 1.2. Low values of SFd (e.g. close to 1) will provide a very tight and controlled crack propagation zone and also increase lifetime of the drilling components at the expensive of rate of propagation. As such, low values for SEd are desirable when a very stable, high quality borehole is required. On the other hand, if rate of propagation is the more important consideration then a higher value for 5Fd may be selected.

During impacts of the oscillator of period; the velocity of the drill-bit of mass m changes by an amount zlv, due to the contact force F=F(t): mAy = JF(e)dt, where the contact force F(t) is assumed to be harmonic. The amplitude of force F(t) is advantageously higher than the force Fd needed to break the material being drilled. Hence a lower bound to the change of impulse may be found as follows: mAV= fFdsin(t=iLIcO.O5D2v.

Assuming that the drill-bit performs a harmonic motion between impacts, the maximum velocity of the drill-bit is v,=A w, where A is the amplitude of the vibration, and co=2irf is its angular frequency. Assuming that the impact occurs when the drill-bit has maximum velocity v,, and that the drill-bit stops during the impact, then Av=vrn=2A1 Accordingly, the vibrating mass is expressed as O.05D2Ur m=-S 4ifA This expression contains r, the period of the impact. The duration of the impact is determined by many factors, including the material properties of the formation and the tool, the frequency of impacts, and other parameters. For simplicity, r is estimated to be 1% of the time period of the vibration, that is, z=O.Ol/f This leads to a lower estimation of the frequency that can provide enough impulse for the impacts: f/ D2U 8OOO,z4m The necessary minimum frequency is proportional to the inverse square root of the vibration amplitude and the mass of the bit.

The aforementioned calculations provide a lower bound for the frequency of the oscillator.

As with the dynamic force parameter, utilizing a frequency greater than this lower bound generates a crack propagation zone in front of the drill-bit during operation. However, if the frequency is too large then the crack propagation zone will extend far from the drill-bit compromising borehole stability and reducing borehole quality. In addition, if the frequency is too large then accelerated and catastrophic tool wear and/or failure may result.

Accordingly, an upper bound to the frequency may be defined as: Sf(D2 U/(8OOOnAm))"2 where S is a scaling factor greater than 1. Similar considerations to those discussed above in relation to SFd apply to the selection of S. Thus, for certain applications Sf will be selected to be less than 5, preferably less than 2, more preferably less than 1.5, and most preferably less than 1.2.

In addition to the aforementioned considerations for operational frequency of the oscillator, it is advantageous that the frequency is maintained in a range which approaches, but does not exceed, peak resonance conditions for the material being drilled. That is, the frequency is advantageously high enough to be approaching peak resonance for the drill-bit in contact with the material being drilled while being low enough to ensure that the frequency does not exceed that of the peak resonance conditions which would lead to a dramatic drop off in amplitude. Accordingly, S is advantageously selected whereby: fr15rf fr where fr is a frequency corresponding to peak resonance conditions for the material being drilled and Sr is a scaling factor greater than 1.

Similar considerations to those discussed above in relation to 5Fd and S apply to the selection of S. For certain applications Sr will be selected to be less than 2, preferably less than 1.5, more preferably less than 1.2. High values of 5r allow lower frequencies to be utilized which can result in a smaller crack propagation zone and a lower rate of propagation. Lower values of 5r (i.e. close to 1) will constrain the frequency to a range close to the peak resonance conditions which can result in a larger crack propagation zone and a higher rate of propagation. However, if the crack propagation zone becomes too large then this may compromise borehole stability and reduce borehole quality.

One problem with drilling through materials having varied resonance characteristics is that a change in the resonance characteristics could result in the operational frequency suddenly exceeding the peak resonance conditions which would lead to a dramatic drop off in amplitude. To solve this problem it may be appropriate to select S whereby: f (fcXI) where X is a safety factor ensuring that the frequency (f) does not exceed that of peak resonance conditions at a transition between two different materials being drilled. In such an arrangement, the frequency may be controlled so as to be maintained within a range defined by: fr/Srf(fcX) where the safety factor X ensures that the frequency is far enough from peak resonance conditions to avoid the operational frequency suddenly exceeding that of the peak resonance conditions on a transition from one material type to another which would lead to a dramatic drop off in amplitude.

Similarly a safety factor may be introduced for the dynamic force. For example, if a large dynamic force is being applied for a material having a large compressive strength and then a transition occurs to a material having a much lower compressive strength, this may lead to the dynamic force suddenly being much too large resulting in the crack propagation zone extend far from the drill-bit compromising borehole stability and reducing borehole quality at material transitions. To solve this problem it may be appropriate to operate within the following dynamic force range: Fd 5Fd [(/4)D2effUs -Y] where Y is a safety factor ensuring that the dynamic force (Fd) does not exceed a limit causing catastrophic extension of cracks at a transition between two different materials being drilled. The safety factor Y ensures that the dynamic force is not too high that if a sudden transition occurs to a material which has a low compressive strength then this will not lead to catastrophic extension of the crack propagation zone compromising borehole stability.

The safety factors X and/or Y may be set according to predicted variations in material type and the speed with which the frequency and dynamic force can be changed when a change in material type is detected. That is, one or both of X and Y are preferably adjustable according to predicted variations in the compressive strength (Us) of the material being drilled and speed with which the frequency (IT) and dynamic force (Fd) can be changed when a change in the compressive strength (Us) of the material being drilled is detected. Typical ranges for X include: X > f1/lOO; X > fr/50 or X > fr/lO. Typical ranges for Y include: Y > SFd [(7t/4)D2ffTJ]/l 00; Y> 5Fd [(7c/4)D2ffU]/50; or Y> SFd {(m/4)D2errUsJ/1 0.

Embodiments which utilize these safety factors may be seen as a compromise between working at optimal operational conditions for each material of a composite strata structure and providing a smooth transition at interfaces between each layer of material to maintain borehole stability at interfaces.

The previously described embodiments of the present invention are applicable to any size of drill or material to be drilled. Certain more specific embodiments are directed at developing drilling modules for drilling through rock formations, particularly those of variable composition, which may be encountered in deep-hole drilling applications in the oil, gas and mining industries. The question remains as to what numerical values are suitable for drilling through such rock formations.

The compressive strength of rock formations has a large variation, from around U=70 MPa for sandstone up to U5=230 MPa for granite. In large scale drilling applications such as in the oil industry, drill-bit diameters range from 90 to 800 mm (3 /2 to 32"). If only approximately 5% of the drill-bit surface is in contact with the rock formation then the lowest value for required dynamic force is calculated to be approximately 2OkN (using a 90mm drill-bit through sandstone). Similarly, the largest value for required dynamic force is calculated to be approximately 6000kN (using an 800mm drill-bit through granite). As such, for drilling through rock formations the dynamic force is preferably controlled to be maintained within the range 20 to 6000kN depending on the diameter of the drill-bit. As a large amount of power will be consumed to drive an oscillator with a dynamic force of 6000kN it may be advantageous to utilize the invention with a mid-to-small diameter drill-bit for many applications. For example, drill-bit diameters of 90 to 400mm result in an operational range of 20 to 1 500kN. Further narrowing the drill-bit diameter range gives preferred ranges for the dynamic force of 20 to 1 000kN, more preferably 20 to 500kN, more preferably still 20 to 300kN.

A lower estimate for the necessary displacement amplitude of vibration is to have a markedly larger vibration than displacements from random small scale tip bounces due to inhomogeneities in the rock formation. As such the amplitude of vibration is advantageously at least 1 mm. Accordingly, the amplitude of vibration of the oscillator may be maintained within the range 1 to 10 mm, more preferably 1 to 5 mm.

For large scale drilling equipment the vibrating mass may be of the order of 10 to 1000kg.

The feasible frequency range for such large scale drilling equipment does not strctch higher than a few hundred Hertz. As such, by selecting suitable values for the drill-bit diameter, vibrating mass and amplitude of vibration within the previously described limits, the frequency (f) of the oscillator can be controlled to be maintained in the range 100 to 500 Hz while providing sufficient dynamic force to create a crack propagation zone for a range of different rock types and being sufficiently high frequency to achieve a resonance effect.

A controller may be configured to perform the previously described method and incorporated into a resonance enhanced rotary drilling module such as those described in the various embodiments of the invention above, The resonance enhanced rotary drilling module may be provided with sensors (the load cells) which monitor the compressive strength of the material being drilled, either directly or indirectly, and provide signals to the controller which are F representative of the compressive strength of the material being drilled. The controller is configured to receive the signals from the sensors and adjust the frequency (f) and the dynamic force (Fd) of the oscillator using a closed loop real-time feedback mechanism according to changes in the compressive strength (Us) of the material being drilled.

The inventors have determined that, the best arrangement for providing feedback control is to locate all the sensing, processing and control elements of the feedback mechanism within a down hole assembly. This arrangement is the most compact, provides faster feedback and a speedier response to changes in resonance conditions, and also allows drill heads to be manufactured with the necessary feedback control integrated therein such that the drill heads can be retro fitted to existing drill strings without requiring the whole of the drilling system to be replaced.

While this invention has been particularly shown and described with reference to preferred embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appending claims.

Claims

CLAIMS: 1. An apparatus for testing a resonance enhanced rotary drilling module, which apparatus comprises: (i) a resonance enhanced rotary drilling module comprising an oscillator; (ii) a fixed frame for fixing the apparatus to a base surface; (iii) a movable frame for moving the rotary drilling module in an axial direction relative to a sample; (iv) a means for generating relative rotary motion between the drilling module and a sample; and (v) a torsion restraint unit for reducing the torsional loading on the oscillator.
2. An apparatus according to claim I, wherein the torsion restraint unit comprises a membrane torsion restraint unit.
3, An apparatus according to claim 2, wherein the torsion restraint unit is in the form of a disc, with an axial dimension that is less than its radial dimension.
4. An apparatus according to any preceding claim, wherein the torsion restraint unit has greater torsional stiffness than axial stiffness.
5. An apparatus according to any preceding claim, wherein the resonance enhanced rotary drilling module comprises: (i) an upper load-cell for measuring static and dynamic axial loading; (ii) a vibration isolation unit; (iii) optionally an oscillator back mass; (iv) an oscillator comprising a dynamic exciter for applying axial oscillatory loading to the rotary drill-bit; (v) a vibration transmission unit; (vi) a lower load-cell for measuring static and dynamic axial loading; (vii) a drill-bit connector; and (viii) a drillTbit, wherein the upper load-cell is positioned above the vibration isolation unit and the lower load-cell is positioned between the vibration transmission unit and the drill-bit, and wherein the upper and lower load-cells are connected to a controller in order to provide down-hole closed loop real time control of the oscillator.
6. An apparatus according to claim 5, wherein the torsion restraint unit is situated between the oscillator and the vibration transmission unit.
7, An apparatus according to claim 5 or claim 6, wherein the vibration transmission unit comprises a structural spring.
8. An apparatus according to any of claims 1-4, wherein the resonance enhanced rotary drilling module comprises: (i) an upper load-cell for measuring static loading; (ii) a vibration isolation unit; (iii) an oscillator for applying axial oscillatory loading to the rotary drill-bit; (iv) a lower load-cell for measuring dynamic axial loading; (v) a drill-bit connector; and (vi) a drill-bit, wherein the upper load-cell positioned above the vibration isolation unit and the lower load-cell is positioned between the oscillator and the drill-bit wherein the upper and lower load-cells are connected to a controller in order to provide down-hole closed ioop real time control of the oscillator.
9. An apparatus according to claim 8, wherein the torsion restraint unit is situated below the oscillator.
10. An apparatus according to any of claims 5-9, wherein the vibration isolation unit comprises a structural spring.
11. An apparatus according to any of claims 5-10, wherein the frequency (0 and the dynamic force (Fd) of the oscillator are capable of being controlled by the controller.
12. An apparatus according to claim 11, wherein the frequency (f) and the dynamic force (Fd) of the oscillator are capable of control according to load cell measurements representing changes in the compressive strength (Us) of material being drilled.
13. A method of testing a resonance enhanced rotary drilling module, comprising operating an apparatus as defined in any preceding claim.
14. A method according to claim 13, the method comprising: controlling frequency (f) of the oscillator in the resonance enhanced rotary drill module whereby the frequency (f) is maintained in the range: (D2 U51(8000mAm))"2 f SD U5/(8OOO))"2 where D is diameter of the rotary drill-bit, U is compressive strength of material being drilled, A is amplitude of vibration, m is vibrating mass, and Sf is a scaling factor greater than l;and controlling dynamic force (Fd) of the oscillator in the resonance enhanced rotary drill module, whereby the dynamic force (Fd) is maintained in the range: [(m/4)D2ffU] Fd Si SFJ[Ot/4)D2ffU] where Dcff is an effective diameter of the rotary drill-bit, TJ is a compressive strength of material being drilled, and 5Fd is a scaling factor greater than 1, wherein the frequency (f) and the dynamic force (Fd) of the oscillator are controlled by monitoring signals representing the compressive strength (Us) of the material being drilled and adjusting the frequency (f) and the dynamic force (Fd) of the oscillator using a closed loop real-time feedback mechanism according to changes in the compressive strength (Us) of the material being drilled.
15. A method according to claim 14, wherein S is less than 5, preferably less than 2, more preferably less than 1.5, and most preferably less than 1.2.
16. A method according to claim 14 or 15, wherein Srd is less than 5, preferably less than 2, more preferably less than 1.5, and most preferably less than 1.2.
17. A method according to any of claims 14-16, wherein Sçis selected whereby: <fr where fr is a frequency corresponding to peak resonance conditions for the material being drilled.
18. A method according to claim 17, wherein 81 is selected whereby: f (fcX) where X is a safety factor ensuring that the frequency (0 does not exceed that of peak resonance conditions at a transition between two different materials being drilled.
19. A method according to claim 18, wherein X > fr/i 00, more preferably X > fr/SO, more preferably still X> fr/i 0.
20. A method according to any of claims 14-19, wherein: 8Fd [(/4)D2ffU -Y] where Y is a safety factor ensuring that the dynamic force (Fd) does not exceed a limit causing catastrophic extension of cracks at a transition between two different materials being drilled.
21. A method according to claim 20, wherein Y> SFd [(lt/4)D2effUs]/100, more preferably Y> SFd [(m/4)D2effUsI/50, more preferably still Y> SFd ftm/4)D2effUs]/1 0.
22. A method according to any one of claims 18-2 1, wherein one or both of X and Y are adjustable according to predicted variations in the compressive strength (Us) of the material being drilled and speed with which the frequency (f) and dynamic force (Pd) can be changed when a change in the compressive strength (Us) of the material being drilled is detected.
23, A method according to any of claims 13-22, wherein the method further comprises controlling the amplitude of vibration of the oscillator to be maintained within the range 0.5 to 10mm, more preferably 1 to 5mm.
24. An apparatus or method according to any of claims 11-23, wherein the frequency ( of the oscillator is controlled to be maintained in the range 100Hz and above, preferably from lOOto500Hz.
25. An apparatus or method according to any of claims 11-24, wherein the dynamic force (Fd) is controlled to be maintained within the range up to 1 000kN, more preferably 40 to 500kN, more preferably still 50 to 300kN.