Volumetric piezoelectric seismic wave source and related methods

Seismic wave sources and related methods are provided. A seismic wave source includes a housing, plural pillars and an excitation system. The housing is split in two halves along a plane including a longitudinal axis of the housing. The plural pillars are made of piezoelectric elements and are positioned inside the housing to have one end in contact with a semi-cylindrical middle portion of one half of the housing and another end in contact with a semi-cylindrical middle portion of the other half of the housing. The excitation system connected by wires to the plural pillars and is configured to provide electrical signals to the piezoelectric elements. Upon receiving the electrical signals from the excitation system, the pillars generate forces on the housing thereby generating seismic waves.

BACKGROUND

1. Technical Field

Embodiments of the subject matter disclosed herein generally relate to devices and methods for generating seismic waves in an underground formation and, more particularly, to mechanisms and techniques for generating seismic waves with volumetric piezoelectric sources.

2. Discussion of the Background

Seismic wave sources may be used to generate seismic waves in underground formations for investigating the underground structure based on seismic images formed from reflections of the seismic waves at interfaces between formation layers that have different acoustic properties. The reflections are detected by seismic receivers. A seismic survey investigating the underground structure may be performed on land or water.

Focusing now on land seismic sources, in early such sources, a force was applied to the formation through a bell-shaped mass attached to the top of a pillar, the pillar being mounted on a metal plate resting directly on the formation or bolted to a concrete slab (i.e., a coupled reactive mass). This type of coupling was abandoned because of inadequate repeatability of the seismic waves obtained for a surface source, even when fixed on a concrete slab, partly due to variations in temperature and humidity in the weathering zone (WZ). Another reason for abandoning this arrangement was that while the energy delivered peaks around 80-100 Hz (depending on the mass), it decays abruptly at low frequencies.

A conventional seismic wave source10(described in U.S. Pat. No. 7,420,879 to Meynier et al.) is illustrated inFIG. 1. The source10includes plural vibrators (electromechanical, electromagnetic, hydraulic, piezoelectric, magnetostrictive, etc.) forming a pillar1in contact with plates2and3. A force is applied to the formation by the pillar1via the plates2and3, thereby generating a seismic wave.

The pillar1, which is covered with a deformable membrane4, is connected by a cable5to a signal generator6. The source10is placed in a cavity or well W, for example, of 15 to 20 cm in diameter, at a desired depth under the WZ, usually between 20 and 200 m. A coupling material7, such as cement or concrete, is injected into the well to be in direct contact with pillar1over the total length thereof and with the plates2and3. In order to allow the coupling material7to be homogeneously distributed in the space between plates2and3, the plates may have perforations8. The diameter of plates2and3must correspond substantially to the diameter of the cavity or well W so as to achieve maximum coupling surface area.

The signal generator6generates an excitation signal in a frequency sweep or a single frequency, causing elements forming the pillar1to expand or contract temporarily along the pillar's longitudinal axis.

The metal plates2and3are mounted on the pillar ends to improve the coupling of the pillar1with the coupling material7. The coupling material7intermediates the coupling between the source and the formation. For example, the plates2and3have a thickness of 10 cm and a diameter of 10 cm. The pillar1has a length exceeding 80 cm. The membrane4may be made of polyurethane and surround the pillar1to decouple it from the coupling material (cement)7. Thus, only the end portions of the pillar1and the plates2and3are coupled with the coupling material (cement)7. Upon receiving an excitation (electrical signal) from the signal generator6, the source10generates forces along the pillar's longitudinal axis. This conventional source provides good repeatability and high reliability, once a good coupling is accomplished.

A typical pillar has a cylindrical shape with a radius of 5 cm and a length of 95 cm. This pillar may consist of 120 ceramics made, for example, of lead-zirconate-titanate (PZT) known under the commercial name NAVY type I. Each ceramic may have a ring shape with 20 mm internal diameter, 40 mm external diameter and 4 mm thickness. The maximum length expansion obtainable for this pillar in the absence of constraints is 120 μm, corresponding to a volume change of about 1000 mm3. The electrical signals fed to the pillars have 5-300 Hz, 2500 V peak maximum and 2 A peak maximum.

However, the conventional source has the disadvantage of producing a large amount of energy corresponding to transverse waves (i.e., S-waves, in which the displacement of the medium is perpendicular to the direction of wave propagation) relative to the energy corresponding to the more desirable longitudinal waves (i.e., P-waves, in which the displacement of the medium is parallel to the direction of wave propagation, and the P-waves propagation speed is nearly twice the S-waves propagation speed). Another disadvantage is related to the source's repeatability and reliability. It depends on the impedance adaptation between the source, the coupling material and the formation. Due to the strong impedance of the source and the low generated displacements, the best efficiency is obtained when the source is coupled in a hard environment with cement. Therefore, a conventional source is not necessarily suitable for very soft formations. Yet another disadvantage is that the radiation pattern of the conventional source (long pillar of ceramics) is adapted for vertical wells but not for horizontal wells.

Thus, there is a need to develop a seismic source capable of generating seismic waves carrying a larger fraction of the energy as P-waves than conventional sources, better adapted to soft formations and/or suitable for deployment in horizontal wells.

BRIEF SUMMARY OF THE INVENTION

A seismic wave source to be operated in a well inside a formation is configured to reduce energy carried by the S-waves relative to the P-waves and, thus, to increase the amount of energy of seismic waves used for evaluating the formation's structure based on reflections of the seismic waves. This source is also better adapted for horizontal wells.

According to an exemplary embodiment, a seismic wave source for generating seismic waves includes a housing, plural pillars inside the housing, and an excitation system. The housing is split in two halves along a plane including a longitudinal axis of the housing. The plural pillars are made of piezoelectric elements and are positioned to have one end in contact with a semi-cylindrical middle portion of one of the two halves of the housing and another end in contact with a semi-cylindrical middle portion of the other one of the two halves of the housing. The excitation system is connected by wires to the plural pillars and is configured to provide electrical signals to the piezoelectric elements. Upon receiving the electrical signals from the excitation system, the pillars generate forces on the housing thereby generating the seismic waves.

According to another embodiment, a method for generating seismic waves in a formation includes placing a seismic wave source inside a hole drilled in the formation, filling the space around the housing with a coupling material, and providing electrical signals to piezoelectric elements of pillars in the source. The source includes (A) a housing split in two halves along a plane including a longitudinal axis of the housing, (B) plural pillars made of piezoelectric elements positioned inside the housing with one end in contact with a semi-cylindrical middle portion of one of the two halves of the housing and another end in contact with a semi-cylindrical middle portion of the other one of the two halves of the housing, and (C) an excitation system connected by wires to the pillars and configured to provide electrical signals to the piezoelectric elements. The longitudinal axis of the housing is substantially parallel to a drilling direction of the hole.

According to another embodiment, a seismic wave source configured to be lowered in a well comprises plural piezoelectric elements arranged at multiple locations along an insertion direction to generate forces in a plane substantially perpendicular to the insertion direction.

DETAILED DESCRIPTION OF THE INVENTION

The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a land seismic source used to perform a seismic survey to evaluate the structure of a solid formation.

Seismic wave sources, according to various embodiments, are configured to be inserted in a well (along the well's drilling direction) and include a plurality of piezoelectric elements arranged to generate forces at multiple locations along the drilling direction, the forces being substantially perpendicular to the drilling direction. Some of these seismic wave sources have the advantage of reducing the energy carried by the S-waves relative to the energy carried by the P-waves and are better suited for use in horizontal wells because the directivity of the P-waves toward the target (i.e., the formation) is increased. Moreover processing and interpreting P-waves is preferred to processing and interpreting S-waves.

The sources configured to be inserted in a well are often designated as land sources. However, a person of skill in the art would understand that such a designation is not a limitation, being possible to use such a source by placing it in a well drilled in a formation under the seabed.

FIG. 2is a schematic diagram of a seismic wave source100according to an exemplary embodiment. The source100has a housing110split in two halves112and114by a plane including a longitudinal axis116. This seismic source100is placed in a well120drilled in a formation130such that the longitudinal axis116is substantially parallel to the drilling direction122of the well120. The source100is placed below the weathering zone and an unconsolidated layer132to avoid the source being affected by meteorological conditions (i.e., humidity, temperature, surface water, etc.).

FIG. 3shows a cross-section of a land seismic source200(which is similar to the source100) in a plane including the source's longitudinal axis216according to another exemplary embodiment. Each of the two halves212and214of the housing210has a semi-cylindrical middle portion213or215, respectively. The halves212and214may taper off at the ends along the longitudinal direction in conical shapes217,218,219and220. The two halves212and214of the housing210may be made of metal or other appropriate materials.

Inside the housing210, plural pillars240a,240b, . . . ,240nmade of piezoelectric elements are arranged substantially perpendicular to the plane including the longitudinal axis216. One end (e.g.,241a,241b, . . . ,241n) of each pillar is in contact with the semi-cylindrical middle portion213, and the other end (e.g.,242a,242b, . . . ,242n) of the pillar is in contact with the semi-cylindrical middle portion215. The ends (e.g.,241a,241b, . . . ,241n,242a,242b, . . . ,242n) of the pillars (e.g.,240a,240b, . . . ,240n) may be fixedly attached to the respective semi-cylindrical portion (e.g., glued) or may be only maintained in a fixed position (e.g., using pre-stressing rods, not shown) while the source is active. The ends (e.g.,241a,241b, . . . ,241n,242a,242b, . . . ,242n) of the pillars240a,240b, . . . ,240nare positioned such that to enable applying forces F to the formation130. In one embodiment, the pillars may be arranged such that elongation axes of at least two pillars to have an angular offset in a plane perpendicular to the longitudinal axis216.

In order to be usable inside typical drilling holes, the housing's diameter (perpendicular to the drilling direction) is preferably less than 20 cm. The two semi-cylindrical middle portions preferably have a length much shorter than the seismic wavelength, e.g., between 0.5 and 5 m.

FIGS. 4A and 4Billustrate in more detail alternative structures of the pillars. InFIG. 4A, the pillar340is made of disk or ring piezoelectric elements345. Electrodes348are connected between the piezoelectric elements345to enable applying the electrical excitation signals generated by a signal generator350. The pillar340is in fact a layered sequence of piezoelectric elements and electrodes (only a pair shown). The electrical excitation signals are transmitted via wires360to the electrodes348.

InFIG. 4B, the pillar440is made of rectangular piezoelectric elements445. Electrodes448are placed between the piezoelectric elements445to enable applying the electrical excitation signals generated by a signal generator450. The pillar440is also a layered sequence of piezoelectric elements and electrodes (only a pair of which are shown). The electrical excitation signals are transmitted via wires460to the electrodes448. In other embodiments, the piezoelectric elements may have shapes other than those illustrated inFIGS. 4A and 4B.

FIG. 5illustrates a seismic source400having a body410according to another exemplary embodiment. The semi-cylindrical middle portions412and414of the housing410have a length of L=2 m and a radius R=5 cm. Inside the housing are ten pillars440, each including 12 ceramics. The pillars440expand upon applying the electrical excitation signal, causing a volume change of about 2500 mm3. The pillars440may be equally spaced along the longitudinal axis of the housing, to have a distance equal to L/9 between neighboring pillars. Compared to the conventional seismic source, the configuration of the source400allows a larger variation of the volume enclosed by the housing associated with a lower physical displacement and, thus, lower shear energy loss. Moreover, a seismic source with variable volume like the source400may be compatible with a flexible coupling and, thus, is easier to use inside a soft formation.

The mechanical impedance Z of the source is determined by the product of the mass corresponding to the displaced volume and sound velocity. The capacity to adjust the source's impedance favors source/cement/formation coupling, leading to better energy transmission from the seismic source to the formation. The source's impedance may be varied by adjusting the longitudinal distance between pillars. The larger the longitudinal distance between pillars, the more flexible the source and easier to adjust to couple to soft formations. In hard formations like limestone, the distance between the pillars may be reduced.

The length of each pillar440may be about 6 cm. Each pillar440may have one end in contact with the semi-cylindrical portion of one of the halves412and414, and the other end in contact with the semi-cylindrical portion of the other one of the halves412and414of the housing410. The pillars440may be mounted inside the semi-cylindrical portions via pre-stressing rods. A space of d=1 mm may be left in between the halves412and414to allow compression of the housing410. All the volume inside the housing410and between the pillars440may be filled with polyurethane (PU) resin455, but no PU is applied outside the housing410.

FIG. 6is a cross-section A-A′ perpendicular to the longitudinal axis of the housing410, illustrating the forces F generated upon feeding an electrical excitation signal to the pillars440and resulting in seismic waves propagating in the surveyed formation.

The electrical signals fed to the pillars may have 5-300 Hz, 2500 V peak maximum and 2 A peak maximum. The excitation system providing electrical signals to the piezoelectric elements may include a signal generator and cables carrying electrical signals to the pillars. The signal generator may be located at the top of the well.

In one embodiment, the excitation system generates signals having a predetermined frequency. In another embodiment, the excitation system generates a signal sweeping a predetermined range of frequencies. In some embodiments, the excitation system generates trains of signals at predetermined time intervals.

A flow diagram of a method500for generating seismic waves in a formation according to an exemplary embodiment is illustrated inFIG. 7. The method first includes placing a seismic wave source inside a hole drilled in the formation at S510. The source has (A) a housing split in two halves along a plane including a longitudinal axis of the housing, (B) plural pillars made of piezoelectric elements that are positioned inside the housing to have one end in contact with a semi-cylindrical portion of one half of the housing and another end in contact with a semi-cylindrical portion of the other half of the housing, and (C) an excitation system configured to provide electrical signals to the piezoelectric elements. The longitudinal axis of the housing may be substantially parallel to a drilling direction of the hole.

The method500further includes filling a space around the housing with a coupling material, at S520, and providing electrical signals to the piezoelectric elements, at S530. Upon receiving electrical signals from the excitation system, the pillars generate forces toward the outside of the housing, resulting in seismic waves.

The coupling material used at S520may be cement but, in a different embodiment, may be a material other than cement.

The source's piezoelectric elements placed at S510may have substantially the same shape, which may be a disk, a ring or a rectangle. The source may include several pillars (e.g., 10 pillars). The pillars may be substantially parallel to one another. The pillars may be arranged at substantially equal distances along the longitudinal axis.

The source's excitation system may include a signal generator and cables carrying electrical signals to the pillars. In one embodiment, the excitation system may generate a signal having a predetermined frequency. In another embodiment, the excitation system may generate a signal sweeping a predetermined range of frequencies. Yet in another embodiment, the excitation system may generate trains of signals at predetermined time intervals.

The source's housing placed at S510may have at least one end tapering off along the longitudinal axis into a conical shape.

In one embodiment, at S510, the seismic wave source may be placed inside the hole at a predetermined depth at which the formation is not affected by meteorological conditions.

Method500may further include (A) placing a plurality of seismic wave detectors at various positions relative to the source, (B) acquiring reflections of seismic waves in the plurality of seismic wave detectors, and (C) extracting information about the structure of the formation based on the acquired reflections.

In contrast to the conventional source in which the pillar's expansion causes P-waves for the material (formation) above and below the plates, S-waves being produced only as a consequence of vertical constraint, in embodiments illustrated inFIGS. 2-6, the pillars expand perpendicular to the drilling direction of the well, for a smaller distance than the conventional pillar, and the force applied to distal portions of the housing decreases due to the tapering off along the longitudinal axis into a conical shape. Therefore, the energy carried by the S-waves is reduced relative to the energy carried by the P-waves and, increasing the amount of energy of seismic waves (P-waves) used for evaluating the formation's structure.