GB2503036A - Variable frequency resonant non-destructive borehole seismic source - Google Patents

Abstract

The borehole seismic source is based on a mechanical resonator consisting of a mass 3,23 and a pair of springs 4,24, whose stiffness can be varied by varying their length. The springs are either gas springs 4 with constant pressure or super-linear mechanical springs 24. The resonance is excited mechanically at its natural frequency e.g. by means of an electromagnetic actuator 11,12,31,32. This natural frequency is varied in a controlled manner by varying the stiffness of the springs. The reaction mass excites the casing of the device via the springs. The casing is clamped 13,33 to the well sides 1,21, and produces a strong source of P- and S-waves. The resonator provides an energy reservoir and, when energised, acts in the manner of an impedance transformer, enabling much higher forces to be exerted on the well sides than the forces generated by the electromagnetic actuator. The spring stiffness can be varied continuously to produce a swept sinusoidal output. The source can generate very low frequencies, e.g. 2 Hz.

Description

Variable frequency resonant non-destructive borehole seismic source This invention provides a high amplitude low frequency source of seismic compression and shear waves (P-and S-waves) for deployment down an oH or gas borehole for use in seismic exploration and gas/oil reservoir monitoring. The radiated signature is in the form of a sinusoid with variable frequency, or a swept sinusoid, and is predictable, controllable and reproducible. For a vertica' well, the device provides a strong source of P-waves with a downward radiation pattern and S-waves in a horizontal pattern. The invention employs an electromagnetic device to drive a reaction mass which excites the rock via a pair of springs, the excitation being at the natural frequency of the mass/spring combination. The spring has a variable stiffness.

this stiffness being varied by changing the length of the spring. To achieve this, the spring is either a gas spring of constant mean pressure (but variable length), or a super-linear mechanical spring (i.e. a spring whose stiffness increases as its length is reduced).

The envisaged applications are in reverse vertical seismic profiling (VSP), for example to image in great detafi faults, pinch-outs and salt walls, and in well-to-well tomography (with applications, for example, in reservoir monitoring).

The device will cause no significant damage to the borehole and will provide sufficient seismic energy to be recorded at useful distances. The advantage of using a sinusoidal signature rather than an impulsive signature is that the total energy of the excitation can be spread out over time compared to an impulsive source. Thus, peak stresses on the well sides are much smaller for the same overall output energy. In principle, the impulse response can be recovered through signal processing if the swept sinusoid is employed, provided the bandwidth is sufficiently wide.

During the course of seismic expthration there can arise a need to monitor seismic response down to low frequencies. For example, the accuracy with which seismic data can be inverted to estimate acoustic impedance as a function of depth or travel time is much improved by including vety low frequencies in the inversion. The frequency range covered by the proposed device is governed by the details of the design, but frequencies down to 2 Hz or lower should be feasible.

Commonly used borehole sources include piezo-electric vibrators, airguns, sparkers and noise generated by the roller cone drill bit. All have significant disadvantages.

mainly in ternis of the signature leveds produced and the frequency content.

Alternative patented devices capaNe of producing swept sinusoidal signatures incorporate some form of electromagnetic excitation and a reaction mass (see US patents 7562740, 4222455. 4715470) or hydraulic excitation and a reaction mass (see US patent 4805725).

The present invention also employs a reaction mass forced into motion by an electromagnetic actuator. The novel aspect of the invention is that the bulk of the energy is transferred to the surrounding rock from the reaction mass via a pair of springs. The combination of reaction mass and springs produces a mechanical resonator, which provides a number of advantages compared to non-resonant alternatives.

Thus the present invention is based on a mechanical resonator consisting of the combination of a reaction mass constrained between a pair of springs, all contained within an outer casing. The springs may be gas springs or mechanical springs. The resonator is excited mechanically at its natural frequency using an electromagnetic actuator. This natural frequency is varied in a controlled manner by varying the stiffness of the springs, specifically by varying the length of a gas spring whilst holding it at constant mean pressure, or by varying the length of a super-linear mechanical spring (i.e. a spring whose stiffness increases as its length is reduced). The outer casing is clamped to the well sides. The motion of the reaction mass drives the casing through the springs, which then excites both P-and S-waves in the sunounding rock. The resonator provides an energy reservoir and, when energised, acts in the manner of an impedance transfoimer, enabling much higher forces to be exerted on the well sides than the forces generated by the electromagnetic actuator alone. The stiffness of the two springs can be varied continuously, thereby producing a swept sinusoidal output with a prescribed and reproducible apportionment of energy across the specified frequency bandwidth.

The main envisaged application areas for the novel borehole seismic source are in reverse vertical seismic profiling and well-to-well tomography. Advantages of the technology include the following.

(1) The source is non-destructive.

(2) The source signature is much higher at low frequencies than other source types mentioned in common usage for this application (namely piezo-electnc vibrators, airguns, sparkers and noise generated by the roller cone dnll bit).

(3) The source signature is controllable, reproducible and known precisely from the measured dynamics of the reaction mass.

(4) The signature covers a wide band width of frequencies, which can be tailored.

(5) The source can be positioned at any number of locations within the well.

The novel source can be used to advantage in either of the above application domains, increasing signal to noise ratio, improving image detail and removing ambiguities.

The invention will now be described by way of four example embodiments of the device with reference to the accompanying drawings: Figure 1 serves to illustrate embodiments I and 2 of the invention -these employ constant mean pressure, vanable length gas spnngs; Figure 2 illustrates embodiments 3 and 4 of the invention, which employ variable length super-Unear mechanical springs in place of the gas springs.

In Figure 1, embodiment 1 or 2 of the device is shown deployed in a vertical well 1.

The main components of the device are: the outer casing 2; the reaction mass 3; the gas springs 4; the gas plenums 5; the gas spring pistons 6 incorporating wiper seals 7, a controllable orifice 8 and a piston drive motor 9; the hydrostatic bearings/seals 10 which retain the gas in the gas springs 4; the electromagnetic actuator (shown as a linear motor in the figure, consisting of one or more annular magnets ii and a set of coils 12); some means of clamping the device to the well walls 13; and a pair of isolation end-caps 14, each containing a gas isolator 15 and incorporating a polyurethane bearing/seal 16. The figure does not explicitly show high pressure gas lines, electrical power supply and electronic communication/control links which will be required to connect the device to the surface.

The outer casing 2 is cylindrical and made out of steel or some other strong material.

The reaction mass 3 is also cylindrical but with a central "waist" to house the electromagnetic actuator (here shown in the form of a linear motor consisting of magnets 11 and coils i2). It will be manufactured from some dense material such as steel and may be up to several meters in overall length to achieve the required mass.

The gas springs 4 are pressurised with a pressure differential sufficient to support the reaction mass 3 in its mean position. This can be accomplished by pumping gas (for example nitrogen) into or out of the two gas plenum volumes 5 using a gas pump via a tube. The gas pump may be located at the surface (not shown). Specifically, the at-rest position of the reaction mass 3 should be near the midpoint of its possible range of motion, such that the annular magnet 11 is centrally positioned within the set of coils 12. Therefore (P1-PjA=Mg (1) where and P, are respectively the mean gas pressures in the lower and upper gas springs 4, A is the area of cross-section of the reaction mass 3 at the beanng/seal 10, M is its mass and g acceleration due to gravity. The resonant frequency f in Hertz of the mechanical spring/mass resonance is (2) 2zVM where k is the combined effective stiflhess of the two gas springs 4. This is given by rPA __ k=TL + (3) where I and L are respectively the lengths of the lower and upper gas springs 4 and x is the ratio of specific heats for the gas (usually taken to be 1.4 for air, say, at normal temperature and pressure). A suitable gas might he nitrogen, for examp'e. lhe invention changes the combined effective stiffness of the two gas springs 4 by adjusting the lengths! and!. in accordance with equation (3).

Perhaps the greatest technological challenge is in the design of the hydrostatic combined bearing and seal 10. This consists of a narrow annular channel containing circulating oil under pressure in the manner well known to those versed in the craft of bearing design. This circumferential gas-tight sliding seal prevents escape of gas from each of the two gas springs 4. The performance of the hydrostatic bearing/seals 10 will constrain the maximum achievable velocity for the reaction mass 3. This limit combined with the frequency of operation governs the maximum excursion of the reaction mass 3 from its mean central location, which must be substantially less than either! or to preserve linearity in the dynamical behaviour of the gas springs 4.

The electromagnetic actuator assumed in this embodiment and illustrated in Figure 1 is a linear motor. This consists of one or more annular magnets 11 embodied within the waist of the reaction mass 3 (see Figure 1) and a set of separately energised coils 12 attached to the casing 2. The reaction mass 3 incorporates a position sensor (not shown) which allows a control system to measure the position of the reaction mass 3 at any time. The coils 12 are individually energised with relative phases (which depend on this measured position) in a manner such as to impart a vertical upwards or downwards force to the reaction mass 3, where the force is upwards when the measured reaction mass velocity is upwards, and downwards when the reaction mass velocity is downwards.

Before operation it should be ensured that the mass of gas in each of the two gas springs 4 is such that when stationaiy the reaction mass3remains near the midpoint of its possible range of motion. This can be accomplished, as previously described, by pumping gas into or out of the two gas plenum volumes 5 using a gas pump via a tube.

The gas spring piston orifices 8 are then closed. During operation, the linear motor is used to excite the reaction mass 3 into motion at the initial resonant frequency of the system (see equations (i), (2) and (3)), with the gas spring piston orifices 8 remaining closed. This can be done by applying an initial force for a period of time sufficient to put the mass 3 into motion. Thereafter a force proportional to the velocity of the reaction mass 3 may be applied so that it becomes negatively damped and oscillates with increasing amplitude until the radiated energy causes sufficient loss of energy to limit the oscillations, or by applying a force proportional to the sign of the velocity of the reaction mass 3 so that it becomes similarly negatively damped, or by a combination of these methods. Applying a vdocity-dependent force may be accomplished using a simple feedback system from a velocity or position sensor attached to any part of the reaction mass 3.

Iii embodiment 1 of the device, once the reaction mass 3 has achieved the desired amplitude of motion, actuation of the device is continued for a period sufficient to achieve the required accumulated energy output, the corresponding signature being a single-frequency sinusoid of the prescribed duration. The gas spring piston orifices 8 remain closed during this phase of operation. Excitation is then removed and the reaction mass allowed to return to rest.

The gas spring piston orifices 8 can now be opened and the gas spring pistons moved to change the lengths L1 and L, of one or both gas springs 4. The pressure in the gas springs 4 and the associated gas plenums 5 is allowed to equalise, following which the gas spring piston orifices 8 are re-closed, through this process, the combined effective stiffness of the two gas springs 4 is changed. The excitation cycle can now be repeated, producing a sinusoidal signature at the new resonant frequency of the reaction mass 3 on the gas springs 4.

Very low frequencies, of order 2 Hz, can be achieved by having the pressure in the gas spring 4 volumes relatively low andlor by having the lengths of the gas springs 4 long.

lii embodiment 2 of the device, once the reaction mass 3 has achieved the desired amplitude of motion, actuation of the device is again continued for period sufficient to achieve the required accumulated energy output. However, in this case the gas spring piston orifices 8 are opened to a prescribed degree and the position of the two gas spring pistons 6 slowly and continuously adjusted. The area of each orifice has to be sufficiently small that the flow of gas through the orifice during ally half period of the reaction mass 3 motion is much less than that required to equalise the pressure variation due to the motion of the reaction mass 3, but large enough such that the mean pressure difference between each gas spring 4 and the associated plenum 5 remains small over the duration of the motion of the gas spring pistons 6. These conditions together imply that the motion of the gas spring pistons 6 takes place over the course of many periods of motion of the reaction mass 3. Excitation is then removed and the reaction mass allowed to return to rest. The signature produced by this process is that of a swept sinusoid.

It is imperative that the varying forces on the gas springs 4 resulting from oscillations of the reaction mass 3 do not change the positions of the gas spnng pistons 6 as this will change the resonant frequency of the oscillations and may also damp them. The positioning motors 9 must therefore be such that the pistons 6 present veiy high mechanical impedance to the springs 4. lii embodiment 1 this may be accomplished by mechanically locking the positioning motors 9 during an excitation cycle so that the pistons 6 cannot move. In embodiment 2 the motors 9 must move during an excitation cycle and cannot be locked. They must therefore have intrinsically high mechanical impedance. This may be accomplished for example by using rotary motors combined with high-ratio rotary-to-linear motion converters from which backlash has been eliminated, in the manner well-known to those versed in the art of actuator systems.

For a range of swept frequencies covering, for example. two octaves, equations (2) and (3) together indicate that the range of gas spring 4 length vanation required is a factor of sixteen. This will not usually be a problem because the displacements of the mass 23 decrease with the square of frequency for constant output force so that the ratio of mass displacement to spring length will remain nominally constant throughout the frequency range.

Also shown illustrated in Figure 1 are clamping devices 8 for clamping the invention to the wails of the boreholes. Given the potentiai consideraNe length of the device, these may be required at more than just the two axial locations shown in the figure.

The clamping devices 8 can be driven either electrically, hydraulically or pneumatically, whichever is the most convenient.

The figure shows optional isolation end caps, incorporating polyurethane bearings/seals 9, each containing an internal volume 10 filled with pressurised gas for isolating the vibration of the casing 2 from the mud in the well, the objective being to reduce the amplitude of any tube waves excited by the device.

In Figure 2, embodiments 3 and 4 of the device are shown deployed in a vertical well 21. The main components of the device are: the outer casing 22; the reaction mass 23; the super-linear springs 24; spring supports 25; pressurised gas volumes 26 and 27 respectively above and below the reaction mass 23; spring support drive motors 29; the electromagnetic actuator (shown as a linear motor in the figure, consisting of one or more annular magnets 31 and a set of coils 32); some means of clamping the device to the well walls 33; and a pair of isolation end-caps 34, each containing a gas isolator and incorporating a polyurethane bearing/seal 36. The figure does not explicitly show high pressure gas fines, electrical power supply and electronic communication/control links which will be required to connect the device to the surface.

The overall spnng stiffness is a combination of the gas spring stiffness of the two gas volumes 26 and 27 and the two super-linear springs 24. The gas spring stiffnesses are constant, so to maximise the range of variation achievable in the frequency of the output signature, the stiffness of the super-linear springs 24 must be substantially greater than that of the gas springs.

The operation of the device is substantially the same as the operation of the gas spring version (embodiments I and 2). In embodiment 3 the spring supports 25 remain in a fixed location during an excitation cycle, and the signature will be a single-frequency sinusoid. In embodiment 4 the positions of supports 25 are slowly and continuously varied to generate a swept-frequency sinusoid. The super-linear nature of the springs 24 means that their effective stiffnesses increase as their lengths are reduced by compression. This is accomplished by driving the supports 25 towards the reaction mass 23 using the spring support drive motors 29. It is imperative that the varying forces on the springs 24 resulting from oscillations of the mass 23 do not change the positions of the spring supports 25 as this will change the resonant frequency of the oscillations and may also damp them. The positioning motors 29 must therefore present very high mechanical impedance to the springs 24. In embodiment 3 this may be accomplished by mechanically locking the positioning motors 29 during an excitation cycle so that they cmrnot move. In embodiment 4 the motors 29 must move during an excitation cycle and cannot be locked. They must therefore have intrinsically high mechanical impedance. This may be accomplished, as in embodiment 2, by using rotary motors combined with high-ratio rotary-to-linear motion converters from which backlash has been eliminated, in the manner well-known to those versed in the art of actuator systems.

The non-linear spring charactenstics mean that the "sinusoidal" output will not be a pure sinusoid but will contain some higher harmonics. This effect will be reduced if the overall length of each spring 24 is much greater than the amplitude of the motion of the reaction mass 23.

The combination of the pressure differential in the gas volumes 26, 27 respectively above and below the reaction mass 23 and the lengths of the two super-linear springs must be such that the at-rest position of the reaction mass 23 is at its required centralised location, such that the annular magnet 31 is centrally positioned within the set of coils 32.

Claims (11)

1. CLAIMSA borehole seismic source suitable for producing seismic signals over a band of frequencies, the source comprising: a reaction mass and a pair of springs of variable stiffness arranged to act on opposite faces of said reaction mass, this combination providing a mechanical resonance of variable and controllable natural frequency; an outer casing isolated from the reaction mass except via the springs, so that force is transmitted from the reaction mass to the casing via the spnngs; a means for clamping the casing to the walls of the borehole; a means for varying the stiffness of the springs by varying the length of the springs, this being achieved either by using gas springs of variable length but constant mean pressure, or by using super-linear mechanical springs; a means for causing and sustaining the reaction mass to vibrate at a frequency dependent on the stiffness of the springs.
2. 2. A borehole seismic source according to claim i incorporating gas springs with variable length but constant mean pressure.
3. 3. A borehole seismic source according to claim 1 incorporating super-linear mechanical springs with adjustable length.
4. 4. A borehole seismic source according to claims 1 and 2 or 3 in which the band of frequencies includes 2 Hz.
5. 5. A borehole seismic source according to claims I and 2 or 3 in which the band of frequencies includes 10Hz.
6. 6. A borehole seismic source according to any preceding claim in which the means for varying the stiffness is designed to do so continuously thus enabling the source to emit a swept-frequency acoustic signal.
7. 7. A borehole seismic sound source according to any of the preceding claims in which a displacement or velocity sensor is attached to the reaction mass and including means for using an output signal of the displacement or velocity sensor directly or indirectly as an input signal for an actuator acting on the reaction mass.
8. 8. A borehole seismic sound source according to claim 7 in which the said actuator is caused to apply a force to the reaction mass that is proportional to and in the same direction as the velocity of the reaction mass.
9. 9. A borehole seismic sound source according to claim 7 in which the said actuator is caused to apply a force to the reaction mass that is of constant magnitude and in the same direction as the velocity of the reaction mass.
10. 10. A borehole seismic sound source according to any of claims 7, 8 or 9 in which the said actuator is a linear electric motor.
11. 11. A sound source substantially as described herein above and illustrated in the accompanying drawings.