BACKGROUND

[0001] This invention relates generally to hydrocarbon-producing wells, and more particularly to apparatus and methods for determining well integrity with respect to formation of micro-annulus in different material layers, and cement de-bonding associated with the well.

[0002] In hydrocarbon-producing wells such as oil and gas wells, it is important to ensure that there is no oil or gas leakage from the well into the surrounding rock formation. This is a critical safety requirement for the hydrocarbon-producing wells. The obvious consequences are blowouts or leaks that can cause material damage, personnel injuries, loss of production and environmental damages, resulting in costly and risky repairs. To ensure well safety, the well casing is surrounded by a cement wall as a part of construction of the hydrocarbon-producing well. There are different defects that can occur during the construction process of the well leading to well integrity issues. But once the hydrocarbon-producing well has been commissioned after clearance of any defects occurring during the construction process, the well integrity requires to be monitored, as new defects like micro-annuli and cement de-bonding start occurring during an operational life of the hydrocarbon-producing well. And the presence of the micro-annulus, as well as cement de- bonding creates a risk of leakage of oil and gas from the well bore into the rock formation.

[0003] Some of the reasons for formation of micro-annulus for example include, the thermal expansion of cement during the initial process of wall formation, that expands the steel casing, and subsequent cooling and contraction of cement during the setting process which creates annulus or micro-annulus at the steel-cement interface. Sometimes, during drilling, the hydrostatic pressure in casing reduces, leading to contraction of the casing. This can result in creation of annulus in the casing cement interface. The outer surface of casing may be covered by an oil film or a corrosion inhibitor which may not allow proper bonding with cement and annulus may be formed due to improper bonding. Moreover, cyclic pressure and temperature variations during hydrocarbon production also lead to the de-bonding of cement from the casing.

[0004] It is important to detect the presence of micro-annulus and know the extent of cement de-bonding to take timely preventive actions. Some of the techniques to detect the micro-annulus and cement de-bonding in oil and gas wells include use of acoustic waves in the ultrasound region that travel through the well bore and casing-cement interfaces and are reflected back. The reflected waves are studied for their attenuation, amplitude, impedance, time of flight to determine the presence of annulus and the extent of penetration of the annulus into the cement wall. The limitation of the ultrasound based technique being used is that, the oil and other well fluids, and cement signatures for attenuation are similar, and therefore it is difficult to accurately determine the presence of micro-annulus, or the extent of damage. Also, ultrasound wave of a particular frequency may not be able to pass through all the layers, thereby limiting its use when multiple layers are present between the transmitter and receiver. Further some of these techniques employ complex statistical signal processing, making it a complex procedure.

BRIEF DESCRIPTION

[0005] In one aspect, an apparatus for determining well integrity of a hydrocarbon-producing is provided. The apparatus includes an acoustic frequency generator for generating a plurality of acoustic frequencies, a modulator for selecting a set of acoustic frequencies from the plurality of acoustic frequencies, and for applying a transmission sequence, a transceiver for transmitting the set of acoustic frequencies in the transmission sequence into a well bore of the hydrocarbon- producing well, at a predetermined depth of the hydrocarbon-producing well, and for receiving reflected resonances with respect to a sub-set of acoustic frequencies from the set of acoustic frequencies, from one or more well integrity layers in a field of view of the set of acoustic frequencies; and a processor for determining one or more well integrity features based on the reflected resonances.

[0006] In another aspect, a method for determining well integrity of an oil and gas well is disclosed. The method includes generating a plurality of acoustic frequencies; modulating a set of acoustic frequencies from the plurality of acoustic frequencies, and applying a transmission sequence for the set of acoustic frequencies; transmitting the set of acoustic frequencies through a well bore at a predetermined depth of the oil and gas well; receiving reflected resonances with respect to a sub-set of acoustic frequencies from the set of acoustic frequencies, from one or more well integrity layers in a field of view of the set of the acoustic frequencies; and processing the reflected resonances to determine one or more well integrity features.

DRAWINGS

[0007] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: [0008] FIG. 1 is a diagrammatic representation of a typical well bore where the apparatus and method of the invention are deployed;

[0009] FIG. 2 is a block diagram representation of an exemplary embodiment as an apparatus for determining the well integrity according to one aspect of the invention;

[0010] FIG. 3 is a diagrammatic representation of an exemplary wave propagation representation of the set of frequencies transmitted into the well bore and the reflected resonances from different well integrity layers;

[001 1] FIG. 4 is a diagrammatic representation of an experimental set-up to represent an implementation of the apparatus in representative well integrity layers;

[0012] FIG. 5 is an illustrative representation of waveform of reflected resonances obtained in the experimental set-up of FIG.4;

[0013] FIG. 6 is an illustrative representation of waveforms for steel layers in the experimental set-up of FIG. 4;

[0014] FIG. 7 is an illustrative representation of a waveform for annulus filled with water in the experimental set-up of FIG. 4;

[0015] FIG. 8 is a representation of modelled reflected resonances in the resonance model described herein above;

[0016] FIG. 9 is a diagrammatic representation of select processing steps performed in the processor of apparatus of FIG. 2; and

[0017] FIG. 10 is a flowchart representation of a method for testing well integrity according to one aspect of the invention.

DETAILED DESCRIPTION

[0018] Detection of cement de-bonding or occurrence of micro-annuli in one or more of well integrity layers, which include casing, cement, and the interface layers of casing-cement, and casing-bedrock is important for determining well integrity in oil and gas wells, as mentioned herein above. The apparatus and method for determining well integrity of a hydrocarbon-producing well described herein uses uniquely, the resonances reflected from the well integrity layers to determine well integrity features and in some embodiments a geometry characterization image of the different well integrity layers. Well integrity features include but are not limited to a presence or absence of micro-annuli, length and thickness of micro-annulus and cement de-bonding. Micro- annulus referred herein implies generally an annulus of about 1.5mm or less. It would be appreciated by those skilled in the art that the dimensions of annulus to be detected can be varied based on user requirement.

[0019] The reflected resonances described herein are acoustic resonances, where a frequency of resonance matches one of the natural frequencies that are transmitted through the well integrity layers. The well integrity layers may include oil or gas or water or air, or different combination of these, besides a material of the well integrity layer, such as steel or cement or rock, and these are referred generally as a "medium" for transmission and reflection purpose.

[0020] Aspects of the invention use the principle that when sound wave of a particular frequency is incident on a cavity, it bounces back and forth between the cavity walls. If the frequency of the sound wave is such that an integral number of wavelengths fit in the round trip distance, then the incident and reflected waves constructively interfere, causing resonances. Therefore, the resonance criterion is:

2d=nλ Equation 1

Here d is the cavity length, n is an integer and λ is the wavelength of the sound wave. If the speed of the sound wave in the medium is c, then the frequency of the sound wave f = c/λ. Therefore, Equation 2

Af= f(n+i)-fn=c/2d Equation 3

[0021] This means that the spacing between two consecutive resonances depends only on the speed of sound in that medium and the length of the medium. If the length of the medium is known, the speed of sound in the medium can be determined by finding the frequency spacing between the resonances. From this, the medium may be identified.

[0022] Different embodiments of the invention based on the above principle are described herein below in more detail in reference to the drawings. [0023] FIG. 1 is a diagrammatic representation of a well bore part of hydrocarbon-producing well 10 showing a series of cylindrical casings 12, 14, 16. As is seen in FIG. 1, as the depth increases, the number of layers around the casings reduce, and at depths of about 1000 - 3000 meters, the number of layers around the casing is one or two. Casing 18, for example has only a cement wall 20 surrounding an outer surface of the casing, and bedrock 22 beyond the cement wall. In one example, the well bore hole may have dimensions of about 80-360 mm, and may be filled with oil, gas, water or brine and have a temperature up to 180 degrees and a pressure of 15000 - 20000 psi. The steel casing 18 may be 8-36mm thick and have a carbon content of 0.1- 0.5%, and the cement wall may have a thickness of 25-55mm. The apparatus and method of the invention are used at the increased depths of 1000-3000 meters.

[0024] FIG. 2 is a diagrammatic representation of an embodiment of the apparatus 30 for determining well integrity of a hydrocarbon-producing well. It would be appreciated by those skilled in the art, that the apparatus 30 is deployed at the given depth inside the casing 18 shown in FIG. 1, and therefore has material and structural integrity to withstand the high pressures and high temperatures at these depths.

[0025] In one exemplary embodiment, the apparatus 30 is a downhole apparatus, where 'downhole' implies that the apparatus is deployed at a predetermined depth inside the well bore. In some other embodiments, part of apparatus 30 may be provided as a downhole tool, and a part may be provided as a surface module that is in communication with the downhole tool.

[0026] The apparatus 30 includes an input module 24 that is used to provide user defined or sensor based inputs or pre-programmed inputs to select components/modules of the apparatus 30. The input module 24 includes codes and routines configured to receive the inputs and is implemented using a processor and a memory, as tangible non-transitory components.

[0027] The apparatus 30 includes an acoustic frequency generator 32 for generating multiple acoustic frequencies 34 using typically a bulk wave in a range of about 1-10 Mega Hertz (the input module 24 provides the instruction to the frequency generator 32 for the desired bulk wave generation). The acoustic frequency generator 32 in one embodiment is implemented using an oscillator and amplifier through known circuitry.

[0028] The apparatus 30 further includes a modulator/de-modulator 36 for selecting a set of acoustic frequencies 38 from multiple acoustic frequencies 34, and applying a transmission sequence to the set of acoustic frequencies. A transceiver 40 is used for transmitting the set of acoustic frequencies 38 in the selected transmission sequence through the well 10 with well bore fluids 44, and the surrounding well integrity layers- steel casing wall 46, interface of steel casing wall and cement wall 48, cement wall 50, interface of cement wall and bedrock 52, and bedrock 54.

[0029] It would be appreciated by those skilled in the art that different transmission sequences may be used, for example but not limited to, a sequence where the set of acoustic frequencies are transmitted one at a time by using for example a frequency modulated signal like a chirp signal. In another implementation, the set of acoustic frequencies are transmitted as a coded signal, which is a digital signal, in yet another example, a set of pre-selected frequencies are transmitted simultaneously.

[0030] The transceiver 40 also receives reflected resonances 58 corresponding to at least a subset of natural frequencies of the set of acoustic frequencies, that are reflected from different well integrity layers in a field of view of the set of transmission frequencies, shown generally by reference numeral 56. It would be appreciated by those skilled in the art that the field of view maybe different for different implementations, and may be dependent to some extent on a choice of a user (user may be an owner or a customer or a technical expert associated with the hydrocarbon-producing well), as well as on choice of transmission frequencies and transmission sequence.

[0031] It would be understood by those skilled in the art that once the set of acoustic frequencies penetrate the well bore, and the well integrity layers, the resonances that are effected are based on the physical properties of the layers that are encountered by these set of acoustic frequencies. For example, if a first frequency travels through a metal surface (i.e. casing wall), it will reflect a particular resonance, say a first resonance that will be a function of the first frequency. Similarly, if a second frequency travels through a micro-annulus (could be present in the casing- cement interface, or in the cement wall, or in the cement-bedrock interface), it will reflect a different resonance, say a second resonance that will be a function of second frequency. Still further, if the micro-annulus is filled with a fluid (also sometimes referred to as 'medium', for example, oil or water), a third frequency will be reflected back as a third resonance, which will be a function of the third frequency, from the micro-annulus. Still further, if a fourth frequency travels through cement, a fourth resonance is reflected back, which is a function of the fourth frequency. Furthermore, the same resonance, for example, the first resonance, will be reflected multiple times depending on the continuity of the particular layer, in the case of first frequency has the metal layer, in its field of view. It may also be noted that there will be some frequencies that do not have any reflected resonances.

[0032] It would be appreciated by those skilled in the art, that knowing what are the possible materials of the well integrity layers, the set of frequencies for transmission can be pre-determined. For example, the layers may include, steel in case of well casing, air in case of unfilled micro- annuli, fluid (oil or water) in case of filled or partially filled micro-annuli, cement, and bed-rock form other layers. Thus using this prior knowledge, frequencies that are known to have reflected resonances of particular characteristics for specific layers, can be pre-selected for transmission. This allows for very quick assessment of reflected resonances.

[0033] The transceiver 40 described herein above, in one exemplary embodiment is implemented by using broadband piezoelectric crystal. Further, in one implementation an air coupled piezo electric crystal may be used as the transceiver 40, that will work for any medium or material, and in a different implementation a conventional couplant based piezoelectric crystal maybe used that works for oil and other fluid medium. In some embodiments, a phased array of piezoelectric crystals may be used. Piezoelectric crystal as a transceiver has several advantages, including ability to simultaneously transmit multiple frequencies. However, any other transducer, that is capable of transmitting multiple frequencies and receiving reflected resonances from the different layers, may be used. In some embodiments electromagnetic acoustic transducer (EMAT) may be used to remove a necessity of any couplant required for placing the transceiver 40 close to the casing wall. In case EMAT transducer is used, it may be used in an array format to enable transmission of multiple frequencies as a set of frequencies and for receiving reflected resonances from the different well integrity layers. In some embodiments an array of sensors is used as a transceiver to allow a three - reconstruction of an image of the different well integrity layers, described herein below.

[0034] Referring back to FIG. 2, a processor 60 is coupled to the transceiver 40 via the modulator-de-modulator 36 for processing the reflected resonances. The output of the processing yields a geometry characterization image of each of the layers and well integrity features based on the reflected resonances (this is processor output, and referred generally by reference numeral 68). In one implementation, the geometry characterization image received as processor output 68 is a three-dimensional image. The geometry characterization image includes thickness of each layer derived using the reflected resonances. The well integrity features include, but are not limited to, a presence or absence of micro-annuli, the location and the extent of penetration of micro-annuli, as well as extent of cement de-bonding. These well integrity features are then used for any maintenance operation or any other control action for the oil and gas well.

[0035] The processor 60 referred herein above, may include filters to estimate resonant frequencies for example, a matched filtering correlator (for analog signals) 62 for detecting the reflected resonances. It would be appreciated by those skilled in the art that either analog or digital processing techniques will be employed based on the nature of transmitted signal. Further, the processing may include either time domain analysis or a frequency domain analysis.

[0036] In an exemplary implementation, the processor 60 includes a resonance model 64 comprising modelled resonances that is used for comparing the reflected resonances received from the transceiver 40 and for correcting for errors based on the modelled resonances. For selecting resonant peaks, based on the comparison, cepstrum analysis known in the art, may be used. In one example a correction factor is determined through the resonance model to correct for errors in the reflected resonances. Correction factor corrects errors present in the reflected resonances due to speed of sound variation with temperature and/or pressure at the depths where the transmission of the set of frequencies and the reflection of resonances occurs. Correction factor in some cases may also be provided to account for any material oxidation, or any operating parameter of the hydrocarbon-producing well, such as flow rate, fluid property such as oil/gas ratio, well pipe property like density, that impact the transmitted frequencies or reflected resonances. The well parameters referred herein above - temperature, pressure, material oxidation, operating parameters, and the like, are provided through the input module 24.

[0037] In one exemplary implementation, the modelled resonances are pre-defined resonances for different well integrity layers at different depths beyond 1000 meters. These modelled resonances are generated based on pre-selected frequencies of transmission and their respected known reflected resonances for different well integrity layers. The resonance model 64 in some implementations, also includes different modelled geometry characterization images of each layer based on modelled resonances and modelled physical characteristics of the respective well integrity layers. The resonance model, in some implementations will include a look-up table for storing (in a tangible memory in a computer implemented storage medium) the modelled resonances, modelled geometry characterization images, and modelled physical characteristics, and other such contents of the resonance model 64. The resonance model 64 may further include modelled well integrity characteristics that are derived from the modelled geometry characterization images in the resonance model. [0038] The physical characteristics referred herein above include but are not limited to, a smoothness factor of each layer, material properties of each layer, and the like. The modelled well integrity characteristics referred herein above include but are not limited to, a presence of micro- annulus, different dimensions of the micro-annulus, presence of cement de-bonding, extent of de- bonding and other related well-integrity characteristics.

[0039] The resonance model 64 described herein includes codes and routines configured for implementing the functionality of the resonance model and is implemented using a processor and a memory, as tangible non-transitory components.

[0040] The output 68 of the processor 60 may be communicated to an external or integrated display unit 70 for further control and maintenance actions. The display unit 70 may be implemented as a graphical user interface accessible for a user/operator or another communication device.

[0041] It would be understood by those skilled in the art that the different components of the apparatus 30 are in appropriate communication with each other, and the communication network, along with electrical and power network is provided for implementing the above functionalities of the different components of apparatus 30 shown in FIG. 2. Standard industry protocols may be used for implementing the connections between the different components of the apparatus 30.

[0042] FIG. 3 is a diagrammatic representation 80 to show the transmission of a set of frequencies 38 into different well integrity layers as referred herein above. As is shown in FIG. 3, arrows represented by reference numerals, 82-90 are representative resonance frequencies reflected from different material or medium of the well integrity layers such as well bore fluids such as oil or water 44, steel casing wall 46, air or water in annulus created in an interface 48 of steel casing wall and concrete (or cement) wall, concrete (or cement) wall 50, and bedrock 54. The densities of different layers and corresponding velocities of sound in that medium are shown as pi..n, and Cl..tiin FIG. 3.

[0043] The processor 60 as described herein above in reference to FIG. 2, is used to determine a match between the representative frequencies in the set of frequencies 38 and their resonances 82-90 as shown in FIG. 3. Further analysis is done using known signal processing techniques such as match filtering, de-chirping, Hilbert Transform method, or other methods that are known in the art, to detect specific resonances. Once the filtered resonances are available, the distance between subsequent peaks for a given resonance is used to determine the thickness of each layer and to obtain other well integrity features as referred herein above.

[0044] FIG. 4 is a diagrammatic representation 100 of an experimental set-up for transmitting a set of frequencies represented by waveform 102 through a transceiver 104 into a steel layer 106, and 108, that has a pre-fabricated annulus 1 10 filled with water, created using a spacer material 112 disposed at two ends of the steel layer 106, as shown in FIG. 4. This experimental set-up emulates the steel casing with a water filled annulus in a hydrocarbon-producing well, and the well integrity layers are represented by the steel layers 106, and the water filled annulus 1 10 in this experimental set-up. FIG. 4 experimental setup shows that the transceiver 104 is in contact with the steel layer 106. However, it would be understood by those skilled in the art that the transceiver 104 may not be required to be in physical contact with the steel layer in some implementations in an actual hydrocarbon-producing, and as such both embodiments where the transceiver is in contact with the well casing and embodiments where the transceiver is not in contact with well casing is covered within the scope of the invention described herein.

[0045] FIG. 5 a waveform representation 114 of reflected resonances received back from the steel layers 106 and 108 (in the experiment stainless steel blocks of thickness 5.87 mm were used), and water filled annulus 110 (in the experiment the annulus was created using a spacer of 1.14mm thickness). As is seen in FIG. 5 the peaks of the waveform are representative of the layer (or medium or material) that is responsible for the reflected resonances. Few of the peaks have been marked for illustrative purpose as 116-126, however all the peaks are processed to determine parameters referred earlier as well integrity features, such as thickness of each layer, presence and extent of annulus, and detection of medium or material present in the annulus. The distance between two subsequent peaks of the resonances from the same layer, can be used to determine a thickness of that layer. Also, the peaks occurring at different heights are indicative of reflected resonances from different layers.

[0046] FIG. 6 is an illustrative representation showing waveforms 128 and 130 that are processed for reflected resonances for the steel layers 106 and 108 respectively (also referred as SS (Stainless Steel) block 1 and SS Block 2 respectively in the FIG. 6). The reflected resonances for the steel layer 106 and 108 are resonances for 0.478 Mega Hertz (MHz). The distance between peaks of this resonances is and calculated as 5.97 mm for steel layer 106, and 6.06mm for steel layer 108. FIG. 7 is a waveform representation 132 that is processed for the water filled annulus layer 110. The reflected resonances for frequency of 0.595 MHz is indicative of presence of water, and the distance between peaks of these resonances is indicative of the diameter of the annulus in which water is present. This distance is calculated as 1.25 mm using the distance between the peaks.

[0047] FIG. 8 is a representation of modelled reflected resonances, shown as a graphical output 134 of reflected pressure against swept frequencies in the resonance model described herein above. As seen in FIG. 8, the peaks related to Af _caS mg are indicative of reflected resonances from the steel casing layer, and Afcasing, as a difference between the consecutive peaks associated with steel casing layer, indicate the thickness of the steel casing layer. Similarly, peaks related to Af mied -annulus are indicative of reflected resonances from the annulus, and Affiiied -annulus aS a difference between the consecutive peaks associated with annulus, indicate the thickness of the annulus layer.

[0048] FIG. 9 is a diagrammatic representation of processor 60 showing select processing steps to generate a geometry characterization image 136 and annulus dimension 138 described herein above that are obtained as an output of the processor described in reference to FIG. 2.

[0049] FIG. 10 is a flowchart representation 200 of a method for determining well integrity of a hydrocarbon-producing. The method is implemented at a pre-determined depth inside a well bore of the hydrocarbon-producing. The method includes a step 212 for generating multiple acoustic frequencies. As explained herein above, the acoustic frequencies are derived from a bulk wave having frequencies in the range of about 1-10 Mega Hertz.

[0050] The method includes a step 214 for modulating a set of frequencies from the multiple frequencies, for transmitting them in a transmission sequence through well integrity layers, and a step 216 for receiving reflected resonances from different well integrity layers, for at least a subset of acoustic frequencies from the set of transmitted frequencies. The method includes a step 218 for processing the reflected resonances, as described herein above in reference to the exemplary apparatus.

[0051 ] The method further includes a step 220 for determining one or more well integrity features based on reflected resonances (referred also as output of processing or processor in some embodiments). The method also includes a step 222 for communicating the output of processing step to a display unit. The processor output may be further communicated to an external communicating device for any control and maintenance actions based on the processor output. The different techniques for transmission of multiple frequencies, reception of reflected resonances, and processing of the reflected resonances have already been described in reference to the exemplary apparatus of the invention.

[0052] Thus the apparatus and method described herein provide a non destructive testing method for determining presence or absence of micro-annuli in any of the material layers or in the interfaces of different material layers present at depths of beyond 1000-3000 meters of an oil and gas well, as well as cement de-bonding, and extent of such de-bonding at the well casing and cement interface. The apparatus and method also further includes determining one or more of a thickness of each of the layer, a presence of a micro-annulus, detection of medium or fluid in the annulus, and a thickness of the micro-annulus based on the reflected resonances.

[0053] It would be understood by those skilled in the art that the apparatus and method employ electronics and computing circuitry for the functional operation of the apparatus and the method. The electronics and computing circuitry may include processor, microprocessor, controller, general purpose processor of digital signal processor, based on the functional requirement of the components. The processing of data or signals is done in one example according to computer programs encoded with instructions on non-transitory computer readable medium. There can also be memory coupled to the processors to store the computer programs, test results, analysis, characteristic outputs, as well as historical data. Further, an integrated or a separate communication circuitry may be provided that is configured to transmit the signals and data between different components in the apparatus and for communication with an external display unit or an external communication device.

[0054] The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope and the spirit of the invention. It is intended that the following claims be interpreted to embrace all such modifications and changes.

[0055] While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.