FR2643153A1 - Implosive source for borehole seismology - Google Patents

Abstract

The invention relates to an implosive source for borehole seismology. It comprises means 1 for pressurising the walls of the well P over a determined length, means 2 for controlling pressure release of the walls of the well after they have been pressurised, and means 3 for supplying the pressurisation means with hydraulic fluid. Means 4 for sequencing the pressurisation means and the control means for pressure release of the walls of the well P are provided. Application to geophysical seismic measuring and tests.

Description

 The invention relates to an implosive source for seismic this well.

 The sources for seismic wells currently known and used for seismic tests or geophysical prospecting campaigns, sources such as detonators or percussive masses, make it possible to carry out tests in all types of geological formations. However, a large part of the vibrational energy released by the latter tends to remain in the well in the form of tube waves which convert to P waves at the bottom of the hole, in cellars and at the level of all the irregularities present along drilling. These aforementioned sources therefore have the disadvantage of not allowing optimal transmission of the fraction of vibrational energy transmitted to the geological formations.

 The object of the present invention is to remedy the aforementioned drawback by using an implosive source for seismic wells allowing optimal transmission of vibrational energy to geological formations.

 Another object of the present invention is the implementation of an implosive source cane for well seismic for which a high ratio of useful seismic energy to the energy of tube waves is obtained.

 Another object of the present invention is the implementation of a seismic source capable of being used both in open drilling and in tubed drilling.

 Another object of the present invention is finally an optimization of the ratio of energy in P waves to energy in S waves by the proper configuration of the source.

 The implosive source for well seismic, according to the invention, is remarkable in that it comprises at least one hydraulic cylinder with admission-expansion comprising a cylinder body and a movable rod, means for mechanical coupling of the free end. from the rod and from the jack body to the walls of the well, means for supplying hydraulic fluid and for pressurizing said jack body, means for triggering the jack body and means for sequencing the means of .menta.ion in hydraulic fluid and for triggering the actuator body.

 It finds application to the seismic of wells during geophysical campaigns, preferably for the emission of ibrations in surface wells.

The invention will be better understood on reading the description and the observation of the drawings below in those
FIG. 1a represents a schematic view of a source by implosion for seismic wells,
FIG. 1b represents the relative arrangement of the source in the well and of a network of geophone type sensors intended to ensure the recording of the test vibrations generated by the source,
FIG. 1c represents the curve of relative displacement of the end of the rod of the jack, and therefore of the walls of the well as a function of the pressure exerted on them,
FIGS. 2a, 2b, 2c represent an advantageous non-limiting embodiment detail of the implosion source for seismic wells according to the invention,
FIGS. 3a and 3b represent a recording of the vibrations emitted by the source according to two successive tests carried out under identical conditions,
FIGS. 4a, 4b, c represent, during comparative tests, the recordings of the various sensors of the network of .a FIG. 1b, respectively in the case of the source with implosion according to the intention, of a detonator and of a striking mass,
FIGS. 5a, 5b, 5c also represent, during comparative tests, the records of the various sensors of the network of FIG. 1b. respectively in the case of the implosion source according to the invention, a detonator and a percussive mass,
FIGS. 6a, 6b, 6c represent the frequency spectra of the vibrations generated, respectively by the emission of an implosion source according to the invention, a detonator and a striking mass, these vibrations being received and detected by the same sensor of the network of figure lb.

 The implosive source for well seismic, object of the.nxention, will first be described in conjunction with Figures la, lb, lc.

 In accordance with the aforementioned figures, the implosive source for well seismic according to the invention comprises at least means I for pressurizing the walls of the well P over a determined length. It further comprises means 2 for controlling the expansion of the walls of the well P after they have been pressurized.

 In addition, means 3 for supplying hydraulic fluid are provided for controlling the aforementioned pressurizing means. Sequencing rroyens 4 are provided to ensure the sequencing of the means I for pressurizing and the expansion control means of the walls of the well P.

 According to a non-limiting embodiment of the inplosive source for seismic wells, object of the invention, the means I for pressurizing the walls of the well can be constituted by a hydraulic cylinder 15 with hydraulic fluid inlet-expansion comprising a actuator body 100 and at least one movable rod 101. It will of course be understood that any type of actuator can be used and in particular a actuator comprising two movable rods, for example. Coupling means 102, 103 are provided, in order to ensuring the mechanical coupling of the free end of the cylinder body 100 when the latter is a cylinder comprising a single rod and of the rod or rods 101 to the walls of the well P.

The operating principle of the implosive source for well seismic, object of the invention, is as follows
The hydraulic cylinder or, more generally, the pressurizing means 1 placed in the well P expand the walls of the latter up to a certain point while remaining within the limits of stress and elastic deformation of the walls of the well P and the components of the cylinder. An abrupt reduction in the pressure in the pressurizing means I or in the actuator causes the walls to return to their initial position by releasing the potential energy stored in the geological formations. including in the casing of the well when a casing is provided in the latter. The kinetic energy thus released then propagates in space in the three usual forms - P waves: - S waves associated with shearing on the edges of the area where the cylinder
or the pressurizing means 1 exert the pressure; for
increase the ratio "P energy / S energy", it is preferable
increase as much as possible the surface subjected to the pressure of
pressurizing means 1 - tube waves generated by the movements of the wall; there is a
energy transfer from energy formations to the fluid and propagation
of guided waves.

 thus it can be said that at the moment when the sudden decompression caused by the expansion control means 2 of the walls of the well occurs, the seismic energy is already transferred to the surrounding geological formations. The problem of passing a certain energy from drilling into geological formations in a very short time or by harmonic oscillations is thus resolved.

 In the case of a chemical explosion, it is estimated that a ground charge emits only rus55 o of its potential energy in the form of seismic waves.

 In the case of a vibrator coupled to the walls of the borehole, the yield is approximately 10 times higher, ie approximately 5 th of the energy converted into waves P. Confer Hardee, Elbring, Paulsson, 1987.

 In the case of the accumulation of energy in geological formations by static elastic deformation, the quantity of energy which it is possible to accumulate remains limited by the surface of contact with the means I of pressurization or the jack , the stiffness modulus p of the surrounding geological layers and, of course, by the limits of the elastic domain.

For a deformation of a cylindrical cavity due to an overpressure P over a length #z large compared to the diameter 2r, the variation in volume uV is given by the relation #V = - r # zP / (i) whence l 'we deduce an increase r of the radius
#r = Pr / 2u (2! the elastic energy then accumulated is given by W = 1/2 Fmax #r (3) either
W = (- r2 zP2) / 2 (- ')
In order to set a plausible order of magnitude, in the unfavorable case of geological layers formed by Caen Caen from Solenhofen for which = 0.231 megabars, a jack, having mechanical coupling means 102, 1.3 with a height of 1 m generating a maximum overpressure of 100 bars in a borehole constituting the 160 mm diameter well, allows an energy of
W = 174 joules.

In a less rigid formation corresponding to a S wave speed of 126G m / s, i.e. a stiffness modulus close to 0.0325 megabars, as for example in the case of the Ten Sleep Formation at
Amherst, Ohio
W = 1237 joules.

 We can then see that the orders of magnitude are therefore very close to the potential energies of sources in use or being tested in Europe and the United States of America.

 A more particularly advantageous embodiment of a measurement system making it possible to carry out tests of the implosive source, as shown in FIG. 1a, will be described in conjunction with FIG. 1b.

 In FIG. 1b, the implosive source I is seen from above, installed in a wellbore for example, a network of receivers constituted by geophones S1 to 512 being installed on the surface: in order to carry out the measurements of the vibrations collected by the latter. The geophones and the aforementioned network are interconnected with a fire control computer 4, the implosive source 1 being supplied by control means 3 comprising. as shown in particular in FIG. 1a, a hydraulic fluid reservoir 30 and a high pressure pump 31 ensuring its connection between the reservoir and the body of the actuator by means of a suitable intake 51.

 The cylinder was constituted by a hydraulic cylinder 10 equipped with a rupture diaphragm allowing the stress applied to be released in a very short time. The jack 15 was placed in a trench dug because of the sandy clay and exerted its pressure on the walls of the trench representing the borehole or well P by mechanical coupling means 1'2, 153 to which were added linings constituted by these wooden plates of 35 cm side each.

 The recording network consisted of twelve groups of geophones connected in parallel series and spaced 1 m apart.

 FIG. 1c shows the perfectly linear relationship between the transfer in the cylinder and the longitudinal deformation, the tests having been carried out inside the range of elastic deformation.

The maximum force exerted by the jack has been estimated at 5,500 Newtons, or a pressure on the geological formations of 555,500 Pascals or 5.5 bars. The accumulated elastic energy is written
W = 1/2 F max x # 1
W = 1/2 (55,000 x 1.5 x 102) = 375 joules. (5).

 In Figure Ic. the ordinate axis, relating to the elongation, is graduated in mm and the abscissa axis, related to the pressure exerted, is graduated in bars.

 A more advantageous embodiment of the implosive source for well seismic, object of the invention, will be described in connection with FIG. 2.

 The aforementioned embodiment is more particularly intended for use during geophysical survey campaigns.

 According to Figure 2 above, the means 1 for pressurizing the walls of the well P can be constituted by an envelope 1 1 substantially cylindrical in plastic material, such as polyvinyl chloride, flexible vinyl or the like. A protective frame 12 is provided, this being constituted by a metal cage formed by flexible metal blades 120. As shown in FIG. 2a, the metal blades 120 are preferably fixed to two end cups 121 , 122, the metal blades 120 and the cups 121 and 122 giving, in the absence of pressurization to the implosive source, the appearance of a cylindrical container, as shown in FIG. 2b in particular.

 As shown in FIG. 2c, the external surface of the envelope li and the flexible metal blades 120 is intended, when pressurized, to come into contact with the wall of the well P over most of the longitudinal dimension of the envelope II and flexible metal blades 120. These then make it possible to apply to the wall of the well P in a substantially radial direction of drilling a mechanical stress in the area of elastic deformation of the walls of the borehole and the metal blades 120.

 As shown in particular in FIGS. 1a and 2a, the actuator body 100 and the casing It comprise a valve 5 for admitting the hydraulic fluid and an expansion valve 20. Of course, it will be understood in the case of the embodiment of Figure 2a that the inlet valve 5 is preferably placed at the cup 122, a fJxatlon system 500 to a cable for example being provided. The cup: 1 and the body of the jack 100 are also provided with an expansion valve 2 @.

 In the case where the implosive source according to the invention is constituted by a jack, the mechanical coupling means 152, 153 of the free end of the rod 101 and of the jack body 150 to the walls of the well P can be constituted by a metal base 1020, 1030 mechanically secured respectively to a free end of the rod and of the jack body and by a contact lining 1521 and 1031 secured to each base.

 The hydraulic fluid supply means consist of the hydraulic fluid reservoir 30 consisting of an oil e- by the high pressure mode 31 already mentioned.

 in an advantageous, non-limiting embodiment of the implosive su rye for well seismic, object of the invention, the means 2 for controlling the expansion of the walls of the well are constituted by an expansion surge with pressure threshold mounted in the valve 20 relaxation.

 According to an advantageous characteristic of the implosive source for sgsmicize of wells, object of the invention, the expansion valve 20 makes it possible to ensure the passage of the intake phase and pressurization of the gold jack of the envelope il and metal blades 12 in the expansion phase in a time interval of less than 50 ms.

 As shown in FIG. 2a in particular, the casing i 1 or the jack body 150 further comprises a pressure sensor 6 intended to detect the passage from the intake and pressurization phase to the trigger to determine the origin of seismic test time.

 Finally, it will be noted that the sequencing means 4 can be formed, as described above, by the fire control computer has interconnected with the high pressure pump 31 and with the pressure sensor 6 mentioned above. Of course, the fire control computer 45, as shown in FIG. 1b in particular, is also interconnected to the measurement network formed by the groups of geophones S1 to 512.

 Tests carried out under the conditions previously described, in particular when the source was constituted by a jack, as mentioned previously, have successively made it possible to note the Darfaite repeatability of the shots fired by means of the implosive source for seismic well conforming to the object. of the invention.

 In FIGS. 3a and 3b, there have been shown successive shots fired by means of the implosive source, object of the invention, these shots being conducted using the fire control computer and the vibrations being recorded by means of the network. shown in Figure lb. We will see in Figures 3a and 3b the perfect repeatability of two successive shots thus made.

 Another comparative test was carried out, the implosive source, onset of the invention, being successively replaced by a detonator buried at the same depth as the depth of the jack, then by the percussion of a mass of 5 kg brandished normally and directed against the base or contact pad previously mentioned. In the latter case, the energy transmitted to the corresponding geological layers was estimated at 15 joules.

 As such, FIGS. 4a, 4b, 4c represent the standardized records of the three seismograms under the conditions of experimentation previously mentioned. The amplitude ratios calculated in a time window of 0 to 300 ms are respectively 1 for the percussive mass, 1.6 for the buried detonator and 5 for the implosive source, object of the invention. The seismic energy emitted by the implosive source, object of the invention, is therefore considerable compared to that emitted by the detonator, which however has a total potential energy 5 times higher. The importance of the coupling of the implosive source, object of the invention, is thus highlighted. It will of course be noted that in the case of FIGS. 3a, 3b and 4a, 4b, 4c, the ordinate axis is graduated in geophone order rmero. as shown in Figure lb and the x-axis is graduated in time, ie in ms.

 In the same way, and for analogous graduations of the recordings in FIGS. 5a. 5b, 5c, these equalized recordings clearly show the best S / N signal-to-noise ratio in the case of the Implosive source, object of the invention.

 In FIGS. 6a, 6b, 6c, the spectra of frequency 2 are respectively shown an implosive source, object of the invention, the detonator detonated and the percussive mass, the recording being of course carried out by the same group of geophones, the geophone 12 in this case. It is noted that the spectrum of the implosive source, object of the invention, is n spectrum with a lower low frequency component, this one having an attenuation of 20 dB at 40 Hz against 65 Hz for the detonator and 7 Hz for the mass impactful.

 A particularly advantageous implosive source for seismic wells has thus been described, all of the advantages being able to be summarized in a manner below.

 The implosive source according to the invention allows optimum use of the power transportable by a logging cable, which allows the transport of a power of approximately 700 W, apart from a fraction of a second at the instant of the pressure drop. , there is therefore no disadvantage in increasing the pressure in the cylinder body or in; 'envelope 11 above.

 The maximum pressure exerted on the walls of the borehole or of the well P can be modified and, consequently, damage can be avoided. It is thus possible to comply with the standards of constraints imposed on the walls of the well P or on the casing itself.

 Due to a radial force, that is to say substantially normal to the walls of the well P, the implosive source, object of the invention, does not play the role of a piston in drilling and, therefore , there is a high ratio of seismic energy useful to the energy of tube waves.

 The lengthening of the dimensions of the source in the longitudinal direction of the well allows an optimization of the energy P / energy S ratio. Indeed, the shearing effect occurs on the walls.

 The implosive source, object of the invention, can also be used in open drilling or in tubed drilling, which is not the case of the source of the weight drop type.

 The mechanical system of the implosive source, object of the invention, is much simpler than the downhole vibrator and, compared to the latter, the virtual mass. that is to say equivalent in mass of the ground affected by the force of the source, is maximum, which lowers towards the seismic frequencies the resonant frequency of the system.

Claims (10)

 pressurizing.  determined length,. means (2) for controlling the expansion of the walls of said well after their  1. Implosive source for well seismic, characterized in that it comprises at least means (1) for pressurizing the walls of said well (P) on a  means for controlling the expansion of the walls of said well.  in pressure means (4) for securing means (I) for pressurizing and  . means (3) for supplying hydraulic fluid to the setting means  2. Implosive source for seismic wells according to claim Dn I. caracoerized in that said means (1) for pressurizing the paros of said uJts are constituted by. a hydraulic cylinder (lrj) with admisslon-trigger including a body of  cylinder (100) and at least one movable rod (101), these mechanical coupling means (102, 103) of the free end of the  cylinder body (IJ) of the rod (s) (101) at the walls of the well (P).  elastic of the walls of the borehole and of the metal blades (120).  drilling mechanical stress in the deformation domain  to apply to said wall in the substantially radial direction of the  most of the longitudinal dimension of the envelope (11), so  pressure, intended to come into contact with the wall of the well (P) on the  the envelope and flexible metal blades being, when  formed by flexible metal blades (120), the external surface-ace of  3. Implosive source for well seismic according to the claims: isn 1. characterized in that said means (I) for pressurizing the walls of said well (P) consist of: - a substantially cylindrical casing (ll) made of material plastic, - a protective frame (12) constituted by a metal cage  4. Implosive source for seismic wells according to one of claims 2 or 3, characterized in that the jack body (100) and the casing (ll) comprises a valve (5) for admitting hydraulic fluid and a expansion valve (20).  5. Seismic source according to one of claims I to 3, characterized in that the mechanical coupling means (102, 153) of the free end of the rod (101) and of the jack body (I 00) to the walls of the well (P) are constituted by - a metal base (1020, 1030) mechanically integral respec  tively with the free end of the rod and of the jack body, - a contact lining (1021, 1031) secured to each base.  6. Implosive source for seismic wells according to one of claims 1 to 5, characterized in that said means for supplying hydraulic fluid consist of - a hydraulic fluid reservoir (30), - a high pressure pump ( 31) connecting said reservoir to the valve  of admission of said cylinder.  7. Implosive source for well seismic according to one of claims 4 to 6, characterized in that said means (2) for controlling the expansion of the walls of the well are constituted by a pressure relief expansion valve mounted in the valve. (2ru) of relaxation.  8. Implosive source for well seismic according to claim 7, characterized in that said expansion valve (20) allows to pass from the intake phase and pressurization to the expansion phase in an interval of time less than 50 milliseconds.  9. Implosive source for well seismic according to one of claims 1 to 8, characterized in that the casing (11) or the cylinder body (100) further comprises a pressure sensor (6) intended to detect the transition from the intake and pressurization phase to the expansion phase to determine the origin of the seismic test time.  10. Implosive source for well seismic according to one of claims 1 to 9, characterized in that the sequencing means (4) consist of a fire control computer (40) interconnected by the high pressure pump (31 ) and the pressure sensor (6).