MXPA05013891A - Determination of the impedance of a material behind a casing combining two sets of ultrasonic measurements - Google Patents

Abstract

The invention provides a method for estimating an impedance of a material behind a casing wall, wherein the casing is disposed in a borehole drilled in a geological formation, and wherein a borehole fluid is filling said casing, the material being disposed in an annulus between said casing and said geological formation, said method using a logging tool positionable inside the casing and said method comprising:exciting a first acoustic wave in said casing by insonifying said casing with a first pulse, the first acoustic wave having a first mode that is either of a flexural mode or an extensional mode;receiving one or more echoes from said first acoustic wave, and producing a first signal;extracting from said first signal a first equation with two acoustic properties unknowns for respectively said material and said borehole fluid;exciting a second acoustic wave in said casing by insonifying said casing with a second pulse, the second acoustic wave having a thickness mode;receiving one or more echoes from said second acoustic wave, and producing a second signal;extracting from said second signal a second equation with said two acoustic properties unknowns;extracting the acoustic properties of said material behind the casing wall from said first and said second equations.

Description

DETERMINATION OF THE IMPEDANCE OF A MATERIAL BEHIND AN ACCOMMODATION COMBINING TWO ULTRASONIC MEASUREMENT GAMES FIELD OF THE INVENTION This invention relates generally to acoustic investigation of a borehole and to the determination of cement and mud impedances located in a well of sounding. DESCRIPTION OF THE PREVIOUS PAMO At the completion of a well, a housing string or pipe is established in a hole and a filling material called cement is forced into the annulus between the housing and the ground formation. After the cement has hardened in the annulus, it is common practice to use non-destructive acoustic test methods to assess its integrity. This evaluation is of main importance since the cement must guarantee the zone isolation between different formations in order to avoid fluid flow of the formations (water, gas, oil) through the annulus. Several cement evaluation techniques that use acoustic energy have been used in the previous branch to investigate the quality of the cement with a tool placed inside the housing.

A first cement evaluation technique, called thickness mode, shown in Figure 1 is described in greater detail in US Patent 2,538,114 to Mason and US 4,255,798 to Havira. The technique consists of investigating the quality of a cement bond between a housing 2 and an annular crown 8 in a hole 9 formed in a formation 10. The measurement is based on an ultrasonic pulse echo technique, whereby a single transducer 21 mounted on a recording tool 27 is lowered in the perforation by a shielded multiconductor cable 3, acoustic wave 23 sounds the housing 2 at almost normal incidence, and receives reflected echoes 24. The acoustic wave 23 has a frequency selected to stimulate a radial segment selected from the housing 2 towards a thickness resonance. A portion of the acoustic wave is transferred to the housing and reverberates between a first interface 11 and a second interface 14. The first interface 11 exists in the joint of a drilling fluid or sludge 209 and the housing 2. The second interface 14 is shape between the housing 2 and the annular crown 8 behind the housing 2. A further portion of the acoustic wave is lost in the annular crown 8 in each reflection in the second interface 14, resulting in a loss of energy for the acoustic wave. The acoustic wave loses more or less energy depending on the state of the material 12 behind the housing 2. The reflections in the first interface 11 and second interface 14, give rise to a reflected wave 24 that is transmitted to the transducer 21. A corresponding received signal the reflected wave 24 has an amplitude that decays with time. This signal is processed to extract a measurement of the amplitude declination regime. From the amplitude declination regime, an acoustic impedance value of the material behind the housing 2 is calculated. The value of the water impedance is almost 1.5 MRayl, while the cement impedance value is typically higher (for example, this impedance is almost 8 MRayl for a class G cement). If the calculated impedance is lower than a predefined threshold, the material is considered to be water or mud. And if the calculated impedance is higher than the predefined threshold, it is considered that the material is cement, and that the quality of the bond between cement and housing is satisfactory. This technique uses ultrasonic waves (200 to 600 kHz). The excited housing thickness mode involves vibrations of the housing segment confined to an azimuth scale, therefore, the values of the impedance of the material 12 behind the housing 2 can be plotted on a map as a function of a depth and a azimuth angle, when the characteristics of the mode and the accommodation are known. This technique provides information mainly on the state of the matter placed in the second interface 14. The impedance, as discussed above, are linked to the state of the material and, therefore, inform about the quality of the cement. Another cement evaluation technique, called flexural mode, is described in US Pat. No. 6,483,777 to Zeroug. In Figure 2, a recording tool 37 comprising an acoustic transducer for transmitting 31 and an acoustic transducer for receiving 32 mounted thereon is lowered in a borehole by a shielded multiconductor cable 3. The transducer to transmit 31 and the transducer to receive 32 are aligned at an angle?. The angle ? is measured with respect to the normal to the local inner wall of the housing N. The angle? is greater than a critical cutting wave angle of a first interface 11 between a housing 2 and a drilling fluid or slurry 20 therein. Therefore, the transducer to transmit 31 excites the flexural wave moon A in the housing 2 by sounding the housing 2 with an excitation aligned in the angle? greater than the critical cutting wave angle of the first interface 11.

The bending wave A propagates inside the housing 2 and pours energy into the sludge 20 inside the housing 2 and to the filling material 12 behind the housing 2. A portion B of the bending wave propagates within an annular crown 8 and is may reflect backward to a third interface 15. An echo 34 is recorded by the transducer to receive 32, and a signal is produced at the echo output 34. A measurement of the bending wave attenuation can be extracted from this signal and the impedance of the cement behind the housing 2 is extracted from the bending wave attenuation. The values of the impedance of the material 12 behind the housing 2 can be plotted on a map as a function of a depth and an azimuth angle, when the mud and housing characteristics are known. Since the portion B of the bending wave propagates within the annular crown 8, the corresponding signal provides information about the whole matter within the annulus 8, ie, through a complete distance separating the housing 2. and the third interface 15. Another cement evaluation technique,, called extension mode, is described in US patent 3,401,773 a Synott, et al. Figure 3 contains a schematic diagram of this cement evaluation technique involving acoustic waves having an extension mode within a housing 2. A recording tool 47, comprising a longitudinally spaced sonic transducer for transmitting 41 and a transducer for receiving 42 , it is lowered into a borehole by a shielded multiconductor cable 3. Both transducers operate on the frequency scale between approximately 20 kHz and 50 kHz. A filler material 11 insulates the housing 2 from a formation 10. The sonic transducer for transmitting 41 insonizes the housing 2 with an acoustic wave 43 that propagates along the housing 2 as an extension mode whose characteristics are determined primarily by the cylindrical housing geometry and its elastic wave properties. A refracted wave 44 is received by the transducer to receive 42 and is transformed into a received signal. The received signal is processed to extract a portion of the signal affected by the presence or absence of cement 12 behind the housing 2. The extracted portion is then analyzed to provide a measure of its energy, as an indication of the presence or absence of cement outside the housing 2. If a cement, which is solid is in contact with the housing 2, the amplitude of the acoustic wave 45 propagating as an extension mode along the housing 2 is partially decreased; consequently, the energy of the portion extracted from the received signal is relatively small. On the other hand, if a sludge, which is liquid, is in contact with the housing 2, the amplitude of the acoustic wave 45 propagating as an extension mode along the housing 2 is much less diminished; consequently, the energy of the portion extracted from the received signal is relatively high. The cement characteristics behind the housing 2 are evaluated in this way of the value of the received energy. This technique provides useful information about the presence or absence of cement next to the second interface 14 between the housing 2 and the annular crown 8. However, this cement evaluation technique uses low frequency sonic waves (20 to 50 kHz) and involves vibrations of the complete cylindrical structure of housing 2. As a result, there is no azimuth resolution. The characteristics of the material 12 behind the housing 2 can be traced in a curve as a function of depth only, when the characteristics of the mud and the housing are known. All these cement evaluation techniques require, before extracting impedance from the material behind the housing, to know the characteristics of the drilling fluid or mud and of the housing. The geometrical and physical properties of the housing must be known with sufficient precision, if we consider that the accommodation did not suffer excessive corrosion or transformation during the termination. The acoustic characteristics of mud (density and ultrasonic velocity) can be calculated excessively or below because they are subjected to pressure and temperature effects. An object of the invention is to develop a method for determining the impedance of matter behind the housing independently of the mud characteristics. COMPENDIUM OF THE INVENTION The invention provides a method for calculating an impedance of a material behind a housing wall, wherein the housing is disposed in a perforation drilled in a geological formation, and wherein a drilling fluid is filling the housing, the material being arranged in an annular crown between the housing and the geological formation, the method using a registration tool that can be placed within the housing and the method comprising: ß exciting a first acoustic wave in the housing by sounding the housing with a first impulse, the first acoustic wave having a first mode that can be one of flex mode or extension mode; • receive one or more echoes of the first acoustic wave, and produce a first signal; • extract from the first signal the first equation of two strangers, where the first unknown is an acoustic property of the material and the second unknown is an acoustic property of the drilling fluid; • exciting a second acoustic wave in the housing by sounding the housing with a second pulse, the second acoustic wave having a thickness mode; • receiving one or more echoes of the second acoustic wave, and producing a second signal; • extract from the second signal a second equation of the two unknown; • extract the acoustic property of the material from the first and second equations. Generally, the first unknown and the second unknown are acoustic properties taken from the list of: acoustic impedance, density, shear wave velocity or compression wave velocity. In a preferred embodiment, the first unknown is the impedance of the material and the second unknown is the impedance of the drilling fluid and the method further comprising extracting the impedance of the drilling fluid from the first and second equations. In another preferred embodiment, the first equation is a linear dependence between the impedance of the material and the impedance of the drilling fluid; and the second equation is also a linear dependence between the impedance of the material and the impedance of the drilling fluid. This simplification reduces complexity and processing time. The method described here is preferably done with a material such as cement if the goal is to evaluate the integrity of the cement termination. And to ensure an image of the entire perforation, the method comprises guiding and rotating the registration tool within the housing in order to evaluate the description of the material behind the housing within a scale of depths and azimuth angles. However, the method is still applicable if the material is different from cement. BRIEF DESCRIPTION OF THE DRAWINGS Additional embodiments of the present invention can be understood with the accompanying drawings: Figure 1 shows a schematic diagram of a cement evaluation technique using the thickness mode of the previous branch. Figure 2 shows a schematic diagram of a cement evaluation technique using the bending mode of the Previous Branch.

Figure 3 shows a schematic diagram of a third cement evaluation technique using the extension mode of the Previous Branch. Figure 4 shows a schematic diagram of the tool according to the invention in a first mode. Figure 5 shows a schematic diagram of the tool according to the invention in a second embodiment. DETAILED DESCRIPTION Figure 4 is an illustration of the tool according to the present invention in a first embodiment. A description of an area behind a housing 2 is evaluated by calculating a quality of a filling material within an annular crown between the housing 2 and a geological formation 10. A registration tool 57 is lowered by cable 3 shielded multiconductor inside housing 2 of a well. The registration tool is raised by surface equipment not shown and the depth of the tool is measured by a depth gauge not shown, which measures cable displacement. In this way, the recording tool can be moved along a vertical axis within the housing, and can be rotated about the vertical axis, thereby providing an evaluation of the description of the area behind the housing within a depth scale and azimuth angle. Typically, the quality of the filling material depends on the state of the material inside the annulus. And the different acoustic properties can inform about the state of the material and therefore the quality of the filling material: acoustic impedance, density, shear wave velocity or compression wave velocity. In the modality described here, to evaluate the quality of cement and its integrity, the acoustic impedance of the material inside the annulus, which informs about the state of matter (solid, liquid or gas), is measured. If the measured impedance is below 0.2 MRayls, the state is gas: it is considered that the filling material behind the housing has holes, the cement is not present. If the measured impedance is between 0.2 MRayls and 2 MRayls, the state is liquid: matter is considered to be water or mud. And if the measured impedance is greater than 2 MRayls, the state is solid: the material is considered to be cement, and the quality of the bond between the cement and the housing is satisfactory. Finally, the values of the impedance of the matter within the annulus are plotted on a map as a function of the depth and azimuth angle. In the continuation, the impedance of the matter inside the annulus will be called the cement impedance (Zcem), even if the material inside the annulus does not have the cement composition; and the impedance of the drilling fluid is the mud impedance (Zlodo). The material inside the annular crown can be any type of filling material that ensures insulation between the housing and the land formation and between the different types of layers of the land formation. In the embodiment described herein, the filling material is cement, in other examples the filling material can be a granulated material or solid compound chemically activated by encapsulated activators present in material or physically by additional registration tool present in the housing. In a further embodiment, the filling material may be a permeable material, the isolation between the different types of layers of the terrestrial formation is no longer assured, but its integrity can still be evaluated. The tool 57 comprises a first transducer for transmitting 51, which insounds the housing 2 with a first acoustic wave. The first acoustic wave is emitted with an angle? relative to a housing normal 2 greater than the critical cutting wave angle of the first interface 11. Therefore, the first acoustic wave propagates within the housing 2 predominantly as a bending mode. A portion of the energy of the first acoustic wave is transmitted to the annular ring 8. An additional portion of the energy is reflected inside the housing 2. A first transducer for reception 52 and an additional transducer for reception 522 respectively receive a first echo and respectively produce a first signal and an additional signal corresponding to the first acoustic wave. The first transducer to receive 52 and the additional transducer to receive 522 may be placed on a vertical axis in the registration tool 57. The first signal and the additional signal are recorded and analyzed by processing means, not shown. A measurement of an additional amplitude is extracted from the additional signal, and a measurement of a first amplitude is extracted from the first signal. A value of a bending wave attenuation of the first acoustic wave along the housing 2 is calculated from the measurement of the additional amplitude and a measurement of the first amplitude. It has been observed that when the cement velocity is less than a threshold value of approximately 2600 m / s for typical cement, there is an approximate linear relationship between the bending wave attenuation and the sum of cement impedance and mud impedance. . Since the acoustic impedance is equal to the product of density per velocity, the condition on the cement velocity can be interpreted, for typical cement (1 to 2 g / cm3) as a condition in the cement impedance less than approximately 2.6 to 5.2 MRayls . The approximate linear relation is given by: Att = ki - (Zcßm + Zlodo) (1) The term Zcem is the true impedance of cement, the term Zlodo is the true impedance of mud, Att is the attenuation of bending and the coefficient kx It is the proportion factor. The first equation (1) links the true cement impedance and the true mud impedance, which refer to the two unknown variables. The tool 57 further comprises a second transducer for transmitting 511, which insonises the housing 2 with a second acoustic wave 53. The second transducer for transmitting 511 can also be used as a second transducer to receive 511 and is substantially directed to a housing normal 2. The second acoustic wave 53 has a frequency selected to stimulate a radial segment selected from housing 2 towards a resonance of thickness. The second acoustic wave has a thickness mode. The second transducer to receive 511 receives one or more echoes 55 corresponding to the second acoustic wave 53 and produces a second signal corresponding to the second wave 53 acoustics. The second signal is recorded and analyzed by processing means, not shown. The processing means extracts the resonance group delay width a, and this group delay width can be approximated by a second linear relationship: a = k2 - Zcem + k3 - Zlodo (2) The term ZCBm is the true impedance of cement, the term Zlodo is the true impedance of mud and k2, k3 are factors of proportion. These factors are of different sign and magnitude, with k3 being negative. The second equation (2) links the true cement impedance and the true mud impedance, which refer to the two unknown variables. The factors of proportionality k2r k3 are of different sign, and therefore, the system of equations (1) and (2) is non-singular and always provides a unique solution. The processing means combine the first and second equations (1) and (2) and values of the true impedance (3) of cement and the true impedance (4) of mud are extracted: a- yAtt Zcem = X (3) ? -Att- a Ziodo = j () Finally, the values of the impedance of the matter inside the annulus, in this case the cement impedance are plotted on a map as a function of the depth and the azimuth angle. The quality of cement in the annulus is, therefore, evaluated. In a further embodiment, the processing means may consider that the mud impedance is further restricted to only slowly changing with the depth to reflect the fact that the mud properties are only affected by pressure and temperature. In another additional embodiment, the processing means may consider that the mud impedance may also change rapidly for example at the interface between two segregated slurries with different densities. For example, a Kalman filter can be used to define Zlsdo at depth z depending on Zlodo at depth z - 1; the processing means will combine the first and second equations (1) and (2) and values of the true cement impedance and the true mud impedance will be extracted in the same way but with a condition on the variation of Z-depth depth z -1 to z. In another additional embodiment, when the linear approximations are no longer valid, the processing means uses two equations: respectively a first equation (5) of the first signal and additional equations for a bending mode and a second equation (6) of the second signal for a thickness mode: Att = F '(Zcsmr Z? odo) (5) a = G (Zcemr Z? oo) (6) For cement velocity lower than the threshold value, it has been observed that the system of two Equations still have a unique pair of solution. And the system can be solved by a minimization process between the measured values of the Att bending attenuation and the group delay width a, and the expected values. And the processing means combine first and second equations (5) and (6) and the values of the true cement impedance (7) and the true mud impedance (8) are extracted: Zcem = M (Att, a) (7) Zio or = N (Att, a) (8) Figure 5 is an illustration of the tool according to the present invention in a second embodiment. A description of an area behind a housing 2 is evaluated by calculating a quality of a filling material within an annular crown between the housing 2 and a geological formation 10. A recording tool 67 is lowered by shielded multiconductor cable 3 within the 2 accommodation of a well. The tool 67 comprises a first transducer for transmitting 61, which insonises the housing 2 with a first acoustic wave 63. The first acoustic wave propagates within the housing 2 primarily as an extension mode, whose characteristics are determined primarily by the cylindrical geometry of the housing and its elastic wave properties. A portion of the energy of the first acoustic wave 63 is transmitted to the annular corona 8. An additional portion of the energy is propagating as an acoustic wave 65 along the housing 2. The amounts of energy transmitted to the annular corona 8 and propagated along the housing 2 depend on the state of the material behind the housing 2. A refracted wave 64 is received by the transducer to receive 62 and is transformed into a first signal corresponding to the first acoustic wave 63. The first signal is recorded and analyze by means of processing, not shown. The processing means extracts a first equation corresponding to the first signal for Attext measured extension attenuation with extension mode. Attext = F '(Zcsm, Zlodo) (9) The first equation can be approximated by a linear equation dependent on Zcsm, the true cement impedance, and Zlodor the true impedance of mud. The tool 67 further comprises a second transducer for transmitting 611, which insonises the housing 2 with a second acoustic wave 603. The second transducer to transmit 611 is also used as a second transducer to receive 611 and is substantially directed to a housing normal 2. The second acoustic wave 603 has a frequency selected to stimulate a radial segment selected from housing 2 towards a thickness resonance . The second transducer to receive 611 receives one or more echoes 604 corresponding to the second acoustic wave 603 and produces a second signal corresponding to the second acoustic wave 603. The second signal is recorded and analyzed by processing means, not shown. The processing means extracts a second equation corresponding to the second signal for the group delay width measured with a thickness mode: a = G (zcem rZ? Odo) (10) The second equation can be approximated to a linear equation Zcemr depends on the true impedance of cement, and Zlodor the true impedance of mud: the second equation becomes in this way in equation (2) as already used above. Measurements of extension mode and thickness mode measurements, because they involve different unlinked waves, produce a system of two non-collinear equations and, therefore, having a single pair of solutions. If the system is non-linear, the system can be solved by means of a minimization process between the Zflexlán and Zespesor measured values, and the expected values. And processing means combine first and second equations (9) and (10) and the values of the true cement impedance (11) and the true mud impedance (12) are extracted: Zcem = M '(? Ttext, a ) (11) Zlodo = N '(Attext, a) (12) Finally the values of the impedance of the matter inside the annulus, that is, the cement impedance, are plotted on a map as a function of the depth and the azimuth angle. The quality of cement in the annulus is, therefore, evaluated.

CLAIMS 1.- A method for calculating an impedance of a material behind a housing wall, wherein the housing is disposed in a borehole drilled in a geological formation, and where a drilling fluid is filling the housing, the material being arranged in an annular crown between the housing and the geological formation, the method using a registration tool positionable within the housing and the method comprising: (i) exciting a first acoustic wave in the housing by sounding the housing with a first pulse, the first acoustic wave having a first one that can be one of flex mode or extension mode; (ii) receiving one or more echoes of the first acoustic wave, and producing a first signal; (iíi) extract from the first signal a first equation with two unknown ones, where the first unknown is an acoustic property of the material and the second unknown is an acoustic property of the drilling fluid; (iv) exciting a second acoustic wave in the housing, by sounding the housing with a second pulse, the second acoustic wave having a thickness mode; (v) receiving one or more echoes of the second acoustic wave, and producing a second signal; (vi) extract from the second signal a second equation with the two unknown ones; (vii) extract the acoustic property of the material of the first and second equations. 2. The method according to claim 1, wherein the first unknown and the second unknown are acoustic properties taken from the list of acoustic impedance, density, shear wave velocity or compression wave velocity. 3. The method according to claim 1, wherein the first unknown is the impedance of the material and wherein the second unknown is the impedance of the drilling fluid, and the method also comprising extracting the impedance of the drilling fluid from the drilling fluid. the first and the second equations. 4. - The method according to claim 3, wherein the first equation is a linear dependence between the impedance of the material and the impedance of the drilling fluid. 5. The method according to claim 3 or 4, wherein the second equation is a linear dependence between the impedance of the material and the impedance of the drilling fluid. 6. - The method according to any of claims 1 to 5, wherein the material is cement. 7. - The method according to any of claims 6, further comprising guiding and rotating the registration tool within the housing in order to evaluate the description of the material behind the housing within a scale of azimuth depths and angles .