Cement evaluation through dual-string pipes

Abstract

To provide a method or a system for evaluating soundness and separation of cement in a wellbore.SOLUTION: A method for cement evaluation through dual-string pipes includes the steps of: acquiring waveform data using a cement bond logging tool; processing the acquired waveform data to eliminate one or more potential DC offsets; selecting a first receiver to further process the processed acquired waveform data in a common-receiver domain; performing in-situ calibration using the processed acquired waveform data from the common-receiver domain to identify free-pipe and wall-bonded waveforms; calculating normalized amplitudes from monopole waveforms and azimuthal waveforms acquired from the free-pipe and wall-bonded waveforms to determine bond index and generate cement map; and calculating statistical averages and variances using the calculated normalized amplitude to generate a final presentation.SELECTED DRAWING: None

Description

[Background technology]

[0001] There comes a point in a well's lifecycle when the well is no longer producing or profitable, marking the end of its life. Accordingly, oil companies may consider burying the well and temporarily halting production for a few months or years, or, in certain circumstances, abandoning the well entirely. In most regions, the reclamation and abandonment of such wells is regulated by national agencies to ensure that there are no surface leaks and that no migration of formation fluids can occur, even after years of abandonment. All of this is to ensure that there are no harmful effects on the environment.

[0002] Depending on the condition and location of the well, fill and abandonment (P&A) operations can vary and can be very time-consuming as well as expensive. For example, older wells can often suffer from annular leaks or loss of casing integrity, further complicating P&A operations; in these cases, operators must cut and pull the tubular or use section mills on the casing to reach and seal off the leaking section. Furthermore, in some situations, wells may be located offshore, which can make P&A operations too costly.

[0003] During P&A operations, evaluation of the cement sheath behind the casing is important for abandoning oil and gas wells after extended production. However, traditional evaluation methods rely primarily on acoustic logging, which requires unobstructed access to the outer casing for conventional cement-bond logging. This often requires costly and time-consuming removal of the production tubing or cutting through the inner casing. Summary of the Invention

[0004] Therefore, there is a need for a method or system for assessing cement integrity and isolation in a wellbore while keeping multi-string casing intact that can simplify traditional P&A operations and significantly reduce operation time and costs while minimizing the carbon footprint associated with rig demobilization costs, tubing cuts, or cuttings.

[0005] The present disclosure is best understood from the following detailed description when read in conjunction with the accompanying drawings. It is emphasized that, according to standard industry practice, various features are not drawn to scale. In fact, the dimensions of various features may be arbitrarily increased or decreased for clarity of illustration. [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 1 is a schematic diagram of a logging scenario used to evaluate a cement sheath during P&A operations without removing production or internal tubing, according to one or more examples of the present disclosure. [Figure 2] 2 is a cross-sectional view of a simulation of the scenario of FIG. 1 in accordance with one or more examples of the present disclosure. [Figure 3] Array of 13 waveforms from Geophone 1 for a case with tubing OD of 3.5 inches and casing OD of 7 inches, showing the progressive increase in cement cover from bottom to top in the left panel and the corresponding residual waveform after subtraction of the free-pipe waveform in the right panel, according to one or more examples of the present disclosure. [Figure 4] 4 depicts a graph plotting normalized amplitude calculated using equation (2) against corresponding cement coverage for the 13 cases shown in FIG. 3 , according to one or more examples of the present disclosure. [Figure 5] 1 depicts a graph plotting calculated normalized amplitude against corresponding cement coverage for two tubing sizes (i.e., 3.5 inches and 4.5 inches) in 7-inch casing, according to one or more examples of the present disclosure. [Figure 6]1 depicts a graph plotting calculated normalized amplitude versus cement coverage for three tubing sizes (i.e., 3.5 inches, 4.5 inches, and 5.5 inches) in 9 5 / 8 inch casing, according to one or more examples of the present disclosure. [Figure 7] The example shown in FIG. 3 is used to illustrate a sequence of steps for selecting a free pipe and a good sticking waveform, according to one or more examples of the present disclosure. [Figure 8] 1 illustrates a sequence of steps for selecting free pipe and good stuck waveforms for field data acquired from an SKK / CAT well using a CETT (Cement Evaluation Through Tubing) acoustic logging tool, according to one or more examples of the present disclosure. [Figure 9] 1 shows results obtained from analyzing azimuthal waveforms using a simulated scenario involving 4.5-inch tubing and 9 5 / 8-inch casing, according to one or more examples of the present disclosure. The left side shows three snapshots, while the right panel shows interpolated cement images for 13 cases. [Figure 10] 1 is a flowchart illustrating a dual string pipe cement evaluation method according to one or more examples of the present disclosure. [Figure 11] 10 shows a set of plots from a first field example involving running 3.5 inch tubing with a SATT in a 7 inch SKK CAT well, according to one or more examples of the present disclosure. [Figure 12] 10 shows a set of plots from a second field example involving moving a SATT while deploying 5.5 inch tubing within 9 5 / 8 inch casing in the Norce U3 exploration well operated by Equinor in the North Sea, according to one or more examples of the present disclosure. [Figure 13]To illustrate the combination of computational results from multiple geophones according to one or more examples of the present disclosure, a third field example is shown utilizing the SKK CAT well data shown in FIG. 11 . DETAILED DESCRIPTION OF THE INVENTION

[0007] Illustrative examples of the below-claimed subject matter are disclosed herein. In the interest of clarity, not all features of an actual implementation are described herein. It will be appreciated that the development of such an actual implementation may involve numerous implementation-specific decisions to achieve developer-specific goals, including compliance with system- and business-related constraints that may vary from implementation to implementation. Moreover, it will be appreciated that such a development effort may be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

[0008] Furthermore, as used herein, the article "a" is intended to have its ordinary meaning in the patent art, i.e., "one or more." As used herein, the term "about," when applied to a value, generally means within the tolerance range of the equipment used to generate the value, or in some instances, plus or minus 10%, or plus or minus 5%, or plus or minus 1%, unless expressly specified otherwise. Furthermore, as used herein, the term "substantially" means most, or nearly all, or all, or an amount in the range of, for example, about 51% to about 100%. Furthermore, the examples herein are intended to be illustrative only and are presented for purposes of explanation, not limitation.

[0009] Evaluating the cement sheath behind the casing is important in reclamation and abandonment (P&A) operations to abandon oil and gas wells after extended production. However, traditional evaluation methods rely primarily on acoustic logging, which requires unobstructed access to the external casing for traditional cement bond logging. This often requires costly and time-consuming removal of production tubing or cutting of the internal casing. To address this challenge, the present invention can eliminate the need to remove production or internal casing to evaluate the integrity and isolation of the cement in the wellbore while leaving the multi-string casing intact. Such an approach can simplify the traditional P&A process and significantly reduce operational time and costs while minimizing the carbon footprint associated with rig demobilization costs, tubing cutting, or cutting.

[0010] The logging scenario is shown in Figure 1. In the figure, the configuration includes two tubulars, annulus A positioned between the tubing and the outer casing, and annulus B located between the outer casing and the formation. This logging scenario presents a means of assessing the cement sheath during P&A operations without requiring the removal of production or inner tubing, and an acoustic logging tool (with a transmitter and geophone array) is utilized for this purpose.

[0011] Traditionally, conventional cement-bond logging tools (CBLs) utilize monopole waveform data acquired by acoustic logging tools. Within the recorded waveform, the complex wavefield can be divided into at least four propagating wave components, as shown in Figure 1: 1) extensional waves within the tubing, 2) Stoneley waves within the tubing, 3) extensional waves within the outer casing, and 4) Stoneley waves within annulus A. Due to a blockage within annulus B, the recorded waveform may also contain additional propagating wave components. The dominant events among these wave components are Stoneley waves of categories 2 and 4, which propagate at slightly different velocities due to the different internal diameters of the tubing and casing. The first arrival in the waveform corresponds to the extensional wave within the tubing. However, this wave is insensitive to the cement / fluid fill conditions within annulus B and therefore cannot be utilized to assess these conditions as is typically done for conventional cement-bond logging. Extensional waves within the outer casing (i.e., Category 3) have the potential to convey information about cement / fluid conditions, but are relatively weak and often obscured and obscured by other wave components.

[0012] To solve the above problem, the present inventors have observed a strong correlation between the amplitude of these residual waveforms and the cement / fluid condition (specifically, the sticking index) obtained without removing the production tubing or inner casing. Alternatively, a different reference condition can be established by finding waveforms recorded in a well-sticking pipe where annulus B is 100% filled with cement.

[0013] In the present case, a model was developed based on the simulation scenario presented in Figure 1 to validate the observed phenomena through numerical simulation of wave propagation in a dual-string casing situation. Figure 2 shows a cross-section of the simulation, featuring an acoustic logging tool positioned at the center of the tubing, which itself is also centered within the casing. The logging tool incorporates four azimuthal receivers per station (designated R1, R2, R3, and R4). In total, there are 12 axial stations (also known as 12 geophones) distributed at regular intervals of 4 inches, with a monopole oscillator located 2 feet from the nearest station. Additionally, behind the casing, there is a fluid channel within annulus B, positioned so that its center is aligned with receiver R1. The remaining section of annulus B is filled with cement to form a complete bond with the casing.

[0014] To investigate various scenarios, 13 waveform simulations were performed with the channel aperture angle ranging from 0° (good sticking case) to 360° (free pipe case) in 30° increments. For each case, an array of waveforms was generated at 12 stations by activating monopole oscillators. At each station, four azimuth waveforms were recorded by each receiver element, and the monopole waveform at each station was obtained by summing the four azimuth waveforms. This memorandum refers to the waveform recorded by a geophone as the monopole waveform corresponding to that respective station. For example, the waveform at geophone 1 refers to the monopole waveform recorded at station 1.

[0015] To demonstrate the proposed method of the present invention, an example involving a tubing inner diameter (ID) of 3.5 inches and a casing OD of 7 inches, both with nominal thickness, is used. The approach involves subtracting the free-pipe waveform from the raw waveform. This is done within a common geophone region, i.e., within waveforms from the same geophone across different scenarios and, in the case of field data, across various depths. Figure 3 shows the waveforms at the first geophone station (geophone 1) for all 13 cases, both before and after subtraction. The index on the y-axis, denoted "i," corresponds to the case number and is also related to the cement coverage (α), also known as the agglutination index, as follows:

number

[0016] Thus, Figure 3 displays a series of 13 waveforms from geophone 1, representing the gradual increase in cement coverage from bottom to top. Waveforms labeled with index 1 correspond to a free-pipe condition, while index 13 represents a good-stuck condition. Note that the raw waveforms in the left panel appear nearly identical, regardless of the change in cement coverage behind the casing. However, when the free-pipe waveforms are subtracted, the residual waveforms shown in the right panel exhibit significant variations in amplitude. These variations exhibit a strong correlation with the percentage of cement coverage. From this correlation, a normalized amplitude is calculated to further exploit the strong correlation.

[0017] Free pipe waveform is wfm FP (t) and a good adhesion waveform is wfm WB (t), and the waveform with unknown adhesion index α is wfm α (t). The normalized amplitude is calculated as follows:

number

[0018] In this calculation, the numerator represents the root mean square (RMS) amplitude of the residual waveform, and the denominator corresponds to the root mean square (RMS) amplitude of the difference waveform between the good sticking waveform and the free pipe waveform. Both RMS amplitudes are calculated within a specified time window T and frequency band B.

[0019] Meanwhile, Figure 4 shows the normalized amplitudes derived from the waveforms, calculated using Equation (2), for the corresponding cement coverage of the 13 cases shown in Figure 3. The results show a nearly linear relationship between the normalized amplitudes and the cement coverage. Furthermore, these findings suggest that the normalized amplitudes can serve as an estimate of the cement coverage (i.e., the agglutination index).

[0020] Additional cases were added to the study to further explore the impact of different tubing and casing sizes. Table 1 provides a study to identify potential candidates for in-site P&A (fill and disposal) operations, indicated by an "x." Based on the information presented in this table, simulations were performed and the same analysis was performed for other candidates, indicated by an "O."

[0021] [Table 1]

[0022] Figure 5 shows a crossplot between normalized amplitude and corresponding cement coverage for two cases involving 7-inch casing. As shown in Figure 4, the results from the 4.5-inch tubing case are overlaid on the findings from the 3.5-inch tubing case. Similar to the 3.5-inch tubing case, the results from the 4.5-inch tubing case show a strong linear relationship with respect to cement coverage. Within the 9 5 / 8-inch casing, three possible tubing sizes are identified: 3.5 inches, 4.5 inches, and 5.5 inches. Waveform simulations are performed for these three tubing sizes within the 9 5 / 8-inch casing. Following the same processing and analysis methods described above, the results of normalized amplitude versus cement coverage are shown in Figure 6. While the calculated normalized amplitude does not strictly follow a linear relationship along the diagonal in all three scenarios, a close linear approximation is observed.

[0023] Therefore, based on the results shown in Figures 4 to 6, the following approximate model can be established as a basis for further development.

number

[0024] Features of the present invention Features of the present invention, including the algorithms used in the present invention, the in-situ calibration of the present invention, the sticking index, and the creation of cement maps, are described in further detail below.

[0025] The algorithm of the present invention In one embodiment of the present invention, the algorithm is further developed based on equation (3) and starts by subtracting the free pipe waveform from the recorded waveform with unknown cement coverage α, which is equal to:

number

[0026] An estimate of α was proposed early on using the root mean square (RMS) approach, i.e.,

number

[0027] The RMS approach proves effective when dealing with noise-free simulated data. However, when applied to field data, its performance degrades due to the presence of noise. Equation (5) is unable to adequately filter out noise during the calculation. To address this limitation, a superior robust approach is proposed that utilizes the Minimum Mean Squared Error (MMSE) criterion.

number

[0028] It is important to note that in equation (6), the model is incorporated into the minimization criterion, allowing noise that does not fit the model (i.e., equation (4)) to be effectively filtered out.

[0029] In-situ calibration of the present invention In one embodiment of the present invention, estimating the cement coverage α using either the RMS approach (Equation (3)) or the MMSE approach (Equation (6)) requires knowledge of the free-pipe waveform and the good-sticking waveform under the same dual-string conditions (i.e., the same tubing and casing size) as the recorded waveform with unknown α. Both the free-pipe waveform and the good-sticking waveform can be estimated from the recorded waveform by an in-situ calibration procedure.

[0030] The algorithm is illustrated using the example shown in Figure 3. The sequence of steps for selecting a free-pipe waveform and a good sticking waveform is presented in Figure 7. The algorithm starts with an initial guess for the free-pipe waveform and then selects the unknown value of γ.

number

number

[0031] For example, in the first panel, the first step involves selecting the raw waveform of the free pipe with index 3 (indicated by the arrow). Moving to the second panel, the residual waveform is calculated by subtracting the raw waveform of the selected free pipe (i.e.,

number

number

number

[0032] In the third panel, the raw waveforms of the estimated good sticking (i.e.

number

[0033] Specifically, the residual waveforms obtained after subtracting the free-pipe waveform show larger amplitudes for waveforms associated with better cement sticking, but show smaller amplitudes as the waveform approaches the free-pipe condition. Conversely, the residual waveforms obtained after subtracting the good sticking waveform (shown in the third panel) show a consistent opposite trend: waveforms show smaller amplitudes as the cement sticking condition improves, but show larger amplitudes as the waveform approaches the free-pipe condition. A consistent check is to ensure β > γ.

[0034] Mathematically, the residual waveform in the second panel can be expressed as:

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number

number

number

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[0035] The final step of the algorithm involves projecting the residual waveform from equation (9) onto the difference waveform defined in equation (10) over a specified time window (T) and frequency band (B). Mathematically, this projection, denoted as P(α), corresponds to the dot product between the two quantities:

number

[0036] The final panel of Figure 7 shows the projection P(α) for the given example. The magnitude and polarity of P(α) are primarily determined by the first term (α-γ) on the right side of equation (11), since the last two terms are fixed and positive. As a result, a good conglutination raw waveform that yields a maximum, most often positive projection is considered the best candidate for a good conglutination waveform. Conversely, a free-pipe raw waveform that yields a minimum, most often negative projection is identified as the best candidate for a free-pipe waveform.

[0037] Applying this criterion, the algorithm determines the raw waveform with index 1 as the free-pipe raw waveform and the raw waveform with index 13 as the good-sticking waveform, both of which are highlighted by red arrows in the last panel of Figure 7.

[0038] To demonstrate the effectiveness of the algorithm, it was applied to field data acquired from the SKK / CAT well using the CETT (Cement Evaluation Through Tubing) acoustic logging tool. The tool configuration matched the setup described in the simulation. Specifically, it was deployed with 3.5-inch tubing inside a 7-inch cased section covering approximately 120 meters to 270 meters.

[0039] Figure 8 shows a step-by-step process for identifying free-pipe and good-sticking waveforms following a similar approach as shown in Figure 7. However, instead of using a wiggle trace plot for the waveforms, the waveforms are displayed using a color variable-density logging log (VDL).

[0040] In the first panel of Figure 8, the raw waveform from geophone 1 is displayed, highlighting the initial free pipe interval. Note that for field data, an appropriate free pipe interval may be selected to allow the use of a robust averaging scheme, such as a trimmed mean or median filter over the selected interval, to obtain an initial estimate of the free pipe waveform.

[0041] Moving to the second panel, a variable density well log (VDL) of the residual waveform is displayed after subtracting the estimated free-pipe waveform from the raw waveform shown in the first panel. Based on the residual waveform in the second panel, a better stuck interval with significant residual amplitude is selected to estimate a better stuck waveform. Again, a robust averaging scheme can be used over the selected interval to obtain a better stuck waveform estimate.

[0042] The third panel shows the VDL of the residual waveform obtained by subtracting the estimated better agglutination waveform from the raw waveform shown in the first panel. As observed in the example simulation data (Figure 6), the amplitude of the residual waveform in both the second and third panels consistently reveals a consistent inverse trend in the amplitude of the residual waveform.

[0043] The final panel of Figure 8 presents the projections calculated using Equation (11). Additionally, the display highlights (highlighted in yellow) the two intervals selected for the calculation of the free-pipe waveform and the good-glued waveform. Based on these projections, it is clear that the zone ranging from 260 to 265 m is the best candidate for estimating the free-pipe waveform, and the zone from 130 to 135 m is identified as the best candidate for estimating the good-glued waveform. These estimates are consistent with our understanding of the cement-glued state of the CAT well.

[0044] As a final note, the in-situ calibration algorithm described herein has the ability to identify the best candidates for free-pipe and good-stuck waveforms from the waveforms available within the desired depth interval. If no free-pipe section exists, the algorithm must identify the closest waveform that resembles the free-pipe condition. The same principles apply to identifying good-stuck waveforms.

[0045] Creation of adhesion index and cement map In the same embodiment of the present invention, as mentioned earlier, the normalized amplitude of the monopole can be used to estimate the cement coverage, also known as the agglutination index. Monopole waveforms are obtained by combining the waveforms of azimuth geophones at different levels. As the waveforms captured by the azimuth geophones contain useful information about the distribution of cement around the hole, the possibility of using these azimuth waveforms to create a cement map was investigated.

[0046] For demonstration purposes, a simulated scenario involving 4.5-inch tubing and 9 5 / 8-inch casing was utilized. The simulation example involves a tool with 12 geophones positioned within the tubing. Each geophone is accompanied by four azimuth geophones evenly distributed around the tool with a 90-degree separation. As with the previous simulation, 13 cases were considered, varying the channel aperture angle from 0° (representing a good jamming case) to 360° (representing a free pipe case) in 30° increments. The waveforms acquired from the azimuth geophones are employed to calculate the azimuth-normalized amplitude using the same method applied to monopole waveforms.

[0047] In Figure 9, we observed results obtained from analyzing the azimuth waveform at geophone 1. The left side shows three subplots as red dots showing snapshots of estimated cement coverage plotted against azimuth angles. For comparison, the true cement coverage is represented by the yellow band, the cement coverage estimated using the monopole waveform is represented by the black line, and the cement distribution-based agglutination index is shown by the red dashed line. Notably, the cement coverage estimated using the monopole waveform closely matches the true agglutination index.

[0048] Azimuthal cement coverage exhibits a smooth variation with azimuthal angle, peaking at the center of the cement channel. This smooth variation is the result of an averaging effect caused by the acoustic logging wavelength, which is comparable to the size of a cased hole, in the range of approximately 1 to 2 feet. Despite this averaging effect, azimuthal cement coverage can still provide useful qualitative insight into the distribution of cement around the hole.

[0049] Moving to the right panel of Figure 9, a cement map constructed using 13 cases of azimuthal cement coverage is shown. Each case is identified by the angle of cement coverage, shown on the y-axis. To generate the image, four measurements of azimuthal cement coverage are interpolated and displayed in azimuthal angles. The resulting cement map allows for a qualitative interpretation of the distribution of cement around the hole, with the center of the cement channel located at approximately 180 degrees.

[0050] Process flow of the present invention In one embodiment of the present invention, the process flow of the present invention is summarized in Figure 10. To be effectively used, the input waveform must correspond to a specific depth interval where identical dual string conditions exist, which requires that the tubing and casing be the same size, with their respective thicknesses.

[0051] In the first step, the acquired waveforms are processed to remove any potential DC offset present in their raw waveforms. This process is straightforward because the waveforms have a quiet interval that precedes the first arriving wave, usually the tubing stretching wave. During this quiet interval, the waveform amplitude is ideally zero. Therefore, any constant, non-zero amplitude observed is considered a DC offset and is then subtracted from the corresponding waveform.

[0052] The primary implementation of the method is shown within the block marked with a yellow outline. The first step within this block involves selecting a geophone for processing. Throughout this memorandum, we chose to utilize the first geophone waveform to illustrate common geophone processing for two specific reasons. First, the spacing between the SATT oscillator and the first geophone is 2 feet. By selecting the first geophone, the processed results exhibit a vertical resolution of 2 feet, which aligns with the vertical resolution of conventional CBL sticking index logging logs. Second, the first geophone is located closest to the oscillator, resulting in fewer reverberations (i.e., round-trip reflections) within the wellbore. As a result, the waveform received by the first geophone is cleaner and more easily interpretable.

[0053] After selecting the geophones, the next step in the process involves performing an in-situ calibration using waveforms within the common geophone area. This calibration utilizes the algorithms described above to help identify waveforms associated with free-pipe and good-stuck conditions. Once the free-pipe and good-stuck waveforms have been identified, the next step involves calculating normalized amplitudes from the monopole waveforms to determine a sticking index, and from the azimuth waveforms to generate a cement map.

[0054] While the waveforms from the first geophone were used for demonstration purposes, the proposed approach can also be applied to waveforms acquired from other geophones. The main difference lies in the vertical resolution of the acquired results. Based on the analysis of the SATT field test data, it was observed that the calculated normalized amplitudes show similarity among the geophones located closest to the oscillator, even when the spacing from the oscillator to the geophones varies. Therefore, an opportunity exists to merge outcomes from multiple geophones by calculating the statistical mean and variance to generate a final presentation, as outlined in the process flow. Figure 13 shows such an example.

[0055] Examples of on-site implementation Example of the first site In the first field example, we present a collection of plots derived from the same example shown in Figure 8, intended to illustrate in situ identification of free-pipe waveforms and well-stuck waveforms. The first panel shows the VDL of the raw waveforms recorded at the first geophone over a depth range of 120 to 270 meters. As previously mentioned, this particular depth interval involved running 3.5-inch tubing with SATT in a 7-inch SKK CAT well.

[0056] In the second panel of Figure 11, we observe the color locator, which is obtained by processing the raw waveform from the first panel. Specifically, this display represents the VDL of the residual waveform, which is derived by subtracting the spatially filtered output of the raw waveform from the original waveform. The spatial filter used can be a moving average or median filter applied to several adjacent depth levels.

[0057] The residual amplitudes shown on the VDL are the result of wave reflection and mode conversion at the casing collars. These amplitudes therefore serve as an indicator of the location of the casing collars, which are spaced approximately 10 meters apart. To aid in identifying collar joints, a red curve representing the root-mean-square value of the residual waveform is superimposed on the VDL.

[0058] In the third panel of Figure 11, we can observe the VDL of the residual waveform, which was obtained by subtracting the estimated free-pipe waveform using the procedure described in Figure 8. Notably, the residual waveform exhibits significant amplitude at shallower depths (e.g., 120-140 m), indicating that a cemented stuck state is most favorable. Conversely, the residual waveform closer to the bottom free-pipe state (e.g., 260-265 m) exhibits very small amplitudes. This observation is consistent with previous findings and serves as further confirmation of the selection of the free-pipe and favorable stuck waveforms.

[0059] In the fourth panel of Figure 11, we can observe the monopole normalized amplitude, represented by the blue curve, calculated using Equation (4). Additionally, we overlay the DBI log (red curve) obtained from moving a Sonic Scanner [4] without tubing in a CAT well in 2006. Overall, there is a high degree of correspondence between the SATT normalized amplitude and the DBI, except for a slight depth shift between the two. However, in the 120-130 m interval, there appears to be a small discrepancy, which may be due to the eccentric positioning of the tubing within the cased hole, as suggested by the dashed red circle in the raw waveform VDL displayed in the first panel.

[0060] In the fifth panel of Figure 11, the cement map is presented using normalized amplitude derived from the azimuth waveform at the first geophone. It is important to note that only four azimuth measurements, spaced 90 degrees apart, were available, resulting in a low-resolution display. To enhance visualization, the cement image was interpolated over the entire 360-degree circumference of the borehole. Additionally, the image was corrected using relative bearing measurements corresponding to the rotation of the SATT during downhole SATT logging.

[0061] In comparison, the last panel shows a high-resolution acoustic impedance map obtained from an Isolation Scanner [7] on a previous exploration run without tubing in the CAT well several years ago. Despite the large difference in resolution between the two images, both show good qualitative correspondence by showing a well-bounded section at the top and a nearly free-pipe section at the bottom.

[0062] In summary, it is important to emphasize that the Sonic Scanner logs and Isolation Scanner images utilized for comparison in this example were acquired without the presence of tubing inside the casing. They were selected to illustrate that the method outlined in this patent memorandum can produce desirable results in a dual string casing scenario.

[0063] Example of the second site The second field example relates to an Equinor exploration well (Norce U3) in the North Sea. SATT testing was conducted within a depth interval ranging from 570 meters to 730 meters. The wellbore had a deviation of approximately 24 degrees and was surrounded by 9 5 / 8-inch casing. Multiple SATT movements were performed during testing utilizing three different tubing sizes: 3.5 inches, 4.5 inches, and 5.5 inches. This particular example focuses on results obtained from moving the SATT with 5.5-inch tubing deployed within 9 5 / 8-inch casing.

[0064] FIG. 12 presents a set of plots displayed in a similar manner to FIG. 11. The first panel displays the VDL of the raw waveform at the first geophone, and the second panel shows the VDL of the collar locator. Notably, in this example, collar locations are more frequently visible. This can be attributed to the presence of two types of collars: tubing collars and casing collars. Distinguishing between these two types of collars can be achieved by examining their respective early onset times. Early occurring collars are associated with tubing collars, while later occurring collars indicate casing collars. This differentiation allows for easy identification and differentiation of the two types of collars.

[0065] The third panel shows the VDL of the residual waveform after subtracting the estimated free-pipe waveform from the raw waveform. The free-pipe depth interval identified using the procedure described in Figure 8 is approximately 580-600 m, while the favorable jamming interval is approximately 700-710 m. The amplitude of the residual waveform qualitatively indicates that the upper part of the hole is close to a free-pipe condition, while the lower part exhibits a favorable jamming condition.

[0066] In the fourth panel of Figure 12, the monopole normalized amplitude, calculated using Equation (4), is presented as a blue curve. Additionally, the sticking index log (DBI) obtained from moving a Sonic Scanner [4] without tubing in the wellbore is overlaid as a red curve. Overall, there is good correspondence between the SATT normalized amplitude and the sticking index, except for a slight depth shift between the two.

[0067] However, between 610 and 630 m depth, a discrepancy appears to be observed between the normalized amplitude and the log of the sticking index. We suspect this discrepancy is due to formation creep (also known as formation squeeze) occurring outside the casing over several years. As a result, some partial sticking is detected by the normalized amplitude. This concept is further supported by the notable residual amplitude present in the 610–630 m range after subtracting the free-pipe waveform, as shown in the third panel. A similar argument can be applied to the interval between 640 and 690 m, where the normalized amplitude exhibits higher values ​​compared to sticking index measurements obtained several years earlier.

[0068] In the fifth panel of Figure 12, the cement map is presented using normalized amplitude derived from the azimuth waveform at the first geophone. It is important to note that only four azimuth measurements, spaced 90 degrees apart, were available, and the cement image was obtained by interpolating the four measurements around the hole. Additionally, the image was corrected using relative bearing measurements corresponding to the rotation of the SATT during downhole logging.

[0069] The final panel presents a high-resolution acoustic impedance map obtained from an Isolation Scanner [7] during a prior exploration run without tubing in the well several years ago. While there is a notable difference in resolution between the two images, both show good qualitative correspondence by showing a well-consolidated section at the bottom and a nearly free-pipe section at the top. Of note is that the effects of formation creep are also visible on the cement map, such as in the 610-630 m range.

[0070] Third field example The third field example presents an illustrative example utilizing data from the SKK CAT well, shown in Figure 11. The first three panels of the vertical display log (VDL) show waveforms obtained from the first geophone, mirroring the corresponding panels in Figure 10. The fourth panel shows the calculated normalized amplitude, obtained by combining the normalized amplitudes from the first through fifth geophones. The blue curve represents the average of the five estimates, and the yellow band encompasses the range corresponding to one standard deviation of the five estimates. Notably, the yellow band observed within certain intervals, such as 170–190 m, becomes wider, potentially indicating uneven distribution of cement around the borehole. Conversely, in zones with free pipe regions (e.g., 260–270 m) or well-aggregated areas (e.g., 130–135 m), the yellow band consistently narrows and becomes nearly invisible. The fifth panel shows the averaged cement map derived from the five geophones, which shows a slightly different distribution compared to the cement map in the fifth panel of Figure 11.

[0071] Embodiments of the present disclosure may also be directed to non-transitory computer-readable media that store computer-executable instructions and that are executable by one or more processors of a computer that access the computer-readable medium. A computer-readable medium may be any available medium that a computer can access. By way of example, such computer-readable media may comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that a computer can access and that can be used to carry or store desired program code in the form of instructions or data structures. As used herein, disk and disc include compact discs (CDs), laser discs, optical discs, digital versatile discs (DVDs), floppy disks, and Blu-ray discs; typically, these discs reproduce data magnetically, while discs reproduce data optically using lasers.

[0072] It should also be noted that software-implemented aspects of the hereinafter claimed subject matter are typically encoded on some form of program storage medium or implemented via some type of transmission medium. The program storage medium is a non-transitory medium and may be magnetic (e.g., a floppy disk or hard drive) or optical (e.g., a compact disk read-only memory, or "CD ROM"), and may be read-only or random-access. Similarly, the transmission medium may be twisted wire pairs, coaxial cable, optical fiber, or some other suitable transmission medium known in the art. The claimed subject matter is not limited by these aspects of any given implementation.

[0073] The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the present disclosure. However, it will be apparent to those skilled in the art that specific details are not required to practice the systems and methods described herein. The foregoing description of specific embodiments has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form described. Obviously, many modifications and variations are possible in light of the above teachings. The embodiments have been shown and described to best explain the principles of the present disclosure, thereby enabling those skilled in the art to best utilize the disclosure and various embodiments, with various modifications, as suited to the particular uses contemplated. It is intended that the scope of the present disclosure be defined by the following claims and their equivalents.

Claims

1. A method for evaluating cement using a dual string pipe, comprising: i) acquiring waveform data using a cement bond logging tool; ii) processing the acquired waveform data to remove one or more potential DC offsets; iii) selecting a first geophone for further processing the processed acquired waveform data within a common geophone region; iv) performing an in-situ calibration using the processed acquired waveform data from the common geophone area to identify free-pipe and wall-stick waveforms; v) calculating normalized amplitudes from monopole and azimuthal waveforms obtained from the free pipe and wall sticking waveforms to determine a sticking index and generate a cement map; vi) using the calculated normalized amplitudes to calculate statistical means and variances to generate a final representation; The method includes the steps of:

2. 10. The method of claim 1, wherein a second or more geophones can also be selected within a common geophone region for further processing of the processed acquired waveforms, followed by the remaining steps.

3. The method of claim 1 , wherein the one or more potential DC offsets are of any constant and non-zero amplitude.

4. The method of claim 1 , wherein the in-situ calibration is performed using an algorithm.

5. 1. A limited autonomy navigation system, comprising: a processor; a memory accessible to said processor; processor-executable instructions stored in said memory; Including, The processor-executable instructions may be configured to cause the system to: i) acquiring waveform data using a cement bond logging tool; ii) processing the acquired waveform data to remove one or more potential DC offsets; iii) selecting a first geophone for further processing the processed acquired waveform data within a common geophone region; iv) performing an in-situ calibration using the processed acquired waveform data from the common geophone area to identify free-pipe and wall-stick waveforms; v) calculating normalized amplitudes from monopole and azimuthal waveforms obtained from the free pipe and wall sticking waveforms to determine a sticking index and generate a cement map; vi) using the calculated normalized amplitudes to calculate statistical means and variances to generate a final representation; the system being executable by the processor to instruct:

6. 6. The system of claim 5, wherein a second or more geophones can also be selected within a common geophone region to further process the processed acquired waveforms, followed by the remaining steps.

7. The system of claim 5 , wherein the one or more potential DC offsets are of any constant and non-zero amplitude.

8. The system of claim 5 , wherein the in-situ calibration is performed using an algorithm.

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