Oil and gas well leakage monitoring and position detection method

Abstract

The invention discloses an oil and gas well leakage monitoring and position detecting method which comprises the steps that firstly, two measuring short sections are installed in a drilling tool combination and used for measuring the pressure or the pressure difference of the corresponding well depth position, and one measuring short section is close to the lower end of a drilling tool; 2, recording related basic data for later use; thirdly, whether well leakage occurs or not is judged according to the ground pressure or the liquid level condition of the drilling fluid tank; or comparing the actual well leakage index phi with a manually set well leakage index critical value to judge whether well leakage occurs or not, and if so, speculating well leakage parameters according to the pressure factor change state; and 4, when well leakage is judged to occur, the well leakage position and the leakage rate are determined according to the annulus pressure loss between the two measuring pup joints at the multi-measuring-point positions. By the adoption of the scheme, real-time monitoring and feedback of the well leakage condition can be conveniently achieved, the underground conditions such as the leakage point position and the leakage amount can be rapidly determined, good accuracy and reliability are achieved, and underground safety can be guaranteed.

Description

Technical Field

[0001] This invention belongs to the field of drilling engineering technology, specifically relating to a method for monitoring and detecting well leakage in oil and gas wells. Background Technology

[0002] Loss in wells is a common and complex downhole situation encountered in drilling operations, with most drilling processes experiencing some degree of leakage. Loss in wells causes drilling fluid loss, increases drilling costs, and in severe cases, leads to a drop in well pressure, affecting normal drilling, causing wellbore instability, and inducing formation fluid inrush into the wellbore, resulting in a blowout. Therefore, monitoring and quickly locating loss in wells are crucial for safe operation during drilling.

[0003] Existing methods for detecting well leakage mainly include drilling fluid pool level monitoring, measuring the velocity of moving pressure waves in the drilling fluid using acoustic sensors, and measuring annular drilling fluid flow rate. In recent years, with the development of machine learning and artificial intelligence algorithms, well leakage early warning technologies based on surface logging parameters and employing neural networks, case-based reasoning, support vector machines, random forests, and combinations of various algorithms have emerged. However, due to the extreme complexity of real-world conditions, their application in the field still has certain limitations. Currently, on-site judgment of well leakage mainly relies on changes in the drilling fluid pool level (some companies also consider changes in relative flow rate at the outlet), but this method has poor accuracy, significant drawbacks, and is difficult to adapt to rapid responses in complex downhole conditions.

[0004] On the other hand, determining the location and velocity of well leakage after it occurs is crucial for rapid well leakage management. While the leakage velocity can be roughly calculated based on the return of drilling fluid from the surface, pinpointing the location of the leakage is more challenging. Existing methods for detecting leakage locations mainly include the spiral flowmeter method, well temperature profile method, well logging data analysis method, and standpipe pressure test method. However, these methods are very difficult to apply to wells with complex structures and have poor adaptability, especially when multiple leakage points exist downhole, where the detection results have even greater errors.

[0005] Theoretically, annular pressure or annular pressure loss will inevitably change after well leakage, and this can be determined using downhole pressure data. However, since wellbore is typically several kilometers long, there are many factors that interfere with annular pressure. The change in annular pressure loss caused by well leakage is relatively small compared to the pressure drop due to the gravity of the drilling fluid, and it is easily overshadowed by annular pressure fluctuations. In other words, both the leakage rate and the location of the leakage will affect the downhole pressure. It is impossible to determine the leakage rate and location simultaneously using only one downhole pressure reading, especially when there are more than one leakage point in the well. In such cases, the reliability of the above method will be greatly reduced. Summary of the Invention

[0006] In view of this, the present invention provides a method for monitoring and detecting well leakage in oil and gas wells, in order to solve the problems of untimely monitoring of well leakage status, inaccurate detection and judgment of well leakage location and leakage rate, and low reliability in the prior art.

[0007] The technical solution is as follows:

[0008] A method for monitoring and detecting well leakage in oil and gas wells, the key of which includes the following steps:

[0009] S1, Two measuring subs are installed in the drill string assembly. The measuring subs are used to measure the pressure or differential pressure at the corresponding well depth, and one of the measuring subs is located near the lower end of the drill string.

[0010] S2, record basic drilling data for later use. The basic data includes at least the wellbore structure, drill string assembly, drilled wellbore trajectory, and drilling fluid density and rheological performance parameters.

[0011] S3, determine whether well leakage has occurred based on ground pressure and / or the level of drilling fluid in the tank;

[0012] Alternatively, real-time drilling parameters can be used to calculate the actual well leakage index at the anomaly point of the pressure factor δ. It compares the actual well leakage index with the manually set critical value to determine whether well leakage has occurred. If the actual well leakage index is greater than the critical value, well leakage is determined to have occurred. Well leakage parameters are inferred based on the pressure factor change status, where the pressure factor δ is the ratio of the real-time pressure loss coefficient at time t to the baseline pressure loss coefficient.

[0013] S4. When well leakage is determined to have occurred in step S3, the location and rate of well leakage are determined based on the annular pressure loss between the two measuring sections at the multi-measuring point location.

[0014] The above scheme mainly achieves the function of fixed-point pressure measurement through dual measuring subs. By combining the data, it fully utilizes the qualitative and quantitative relationship between leakage and annular pressure loss to determine the location or rate of well leakage (leakage amount). Compared with existing technologies, it has better reliability and accuracy, which helps to shorten the leakage feedback waiting time, speed up the plugging process, and ensure downhole safety.

[0015] Preferably, the distance between the two measuring sections is 30m-60m, and the drill string between the two measuring sections is of the same size. A larger distance can amplify the influence of annular pressure loss and ensure the accuracy of the equipment measurement records, but it will increase the blind zone for determining the leak point. Therefore, considering both factors, the above-mentioned distance is preferred. Based on the existing technology of measuring sections, it not only has better measurement results, but also facilitates the rapid determination of the leak point. At the same time, using a drill string of the same size between the two sections helps to further ensure the accuracy of the calculation results and reduce the calculation difficulty.

[0016] As a preferred option: the actual well leakage index mentioned in step S3 The calculation process includes the following steps:

[0017] S3.1, Calculate the positive coefficient of drilling fluid repair gravity;

[0018] S3.2, Calculate the theoretical value of real-time annular pressure loss between the two measured short sections;

[0019] S3.3, Calculate the actual real-time annular pressure loss between the two measurement sections;

[0020] S3.4 Calculate the real-time pressure loss coefficient, which is the ratio of the actual value of the annular pressure loss to the theoretical value of the annular pressure loss;

[0021] S3.5, Calculate the pressure factor δ in real time;

[0022] S3.6, Calculate the baseline pressure loss coefficient and the average pressure factor, wherein the baseline pressure loss coefficient is the average value of the pressure loss coefficients corresponding to multiple detection points within the sliding detection window;

[0023] S3.7, use the Bayesian online change point detection method to detect whether the pressure factor calculated in S3.5 changes abruptly. If an anomaly is detected, calculate the actual well leakage index at this time.

[0024]

[0025] Where α is the weighting coefficient, which is arbitrarily assigned a value of 0.1-0.2; and These represent the pressure factors at time t, between A and B, above A, and above B, respectively, where A is the measurement position of the lower measuring section and B is the measurement position of the upper measuring section.

[0026] By adopting the above scheme, after correcting the drilling fluid gravity, the subsequent annular pressure loss is calculated. The annular pressure loss is then converted into a more accurate pressure loss coefficient, which is then used to calculate the pressure factor and the actual well leakage index. This helps to improve the accuracy and reliability of the calculation results.

[0027] As a preferred option: the actual well leakage index calculated according to S3.7. If well leakage is detected, step S4 is performed as follows: assuming there is one and only one leakage point downhole, and based on... and The changes can be used to preliminarily infer well leakage parameters, such as and Both decreased, while If it remains unchanged, the leak point is above point B; for example... and If both decrease, the leakage point is below point B. Simultaneously establish the relationship formula between leakage velocity and leakage location, and use this formula to draw a well leakage parameter analysis chart:

[0028]

[0029] in, Q represents the actual annular pressure loss between points A and B, f(Q) represents the functional relationship between the annular pressure loss and displacement between A and B, and m represents other influencing factors besides Q; L Indicates leakage discharge, L L L represents the distance between the leak point and point A. If the leak point is below point A, then L... L =0,k A ′ B This is the baseline pressure loss coefficient between A and B.

[0030] Preferably, steps S3 and S4 are both calculated by the monitoring system. If well leakage is detected, an alarm signal is issued through the execution device connected to the monitoring system. This approach facilitates a further reduction in well leakage feedback time, enabling rapid and effective on-site measures to ensure downhole safety.

[0031] As a preferred option: when it is determined in step S3 that a well leak has occurred downhole based on the ground pressure or the fluid level in the drilling fluid tank, step S4 shall determine the location of the leak and the amount of leakage discharge according to the following steps;

[0032] S4.1, make a preliminary on-site assessment of the leak location, and raise or lower the drill string to a position 50-100 mm above the leak location;

[0033] S4.2, Fixed-point circulation test, with normal drilling displacement circulation, calculate the circulation pressure difference ΔP from the pressure or pressure difference data obtained from the measurement sub;

[0034] S4.3, Pump shutdown observation: Record the start and end times of the pump shutdown observation, and take the minimum pressure difference recorded within a few seconds immediately after pump shutdown as the measured drilling fluid gravity pressure drop ΔP. g ;

[0035] S4.4, Conduct a lowering test, repeating steps S4.2 and S4.3 every 10-30 meters, and record the data.

[0036] S4.5, Based on the data recorded in S4.4, perform data processing, plot the actual annular pressure loss variation between the two measurement sections, and based on the actual annular pressure loss ΔP f Formulas relating leak location to other factors are used to determine the location of leaks.

[0037] When leak point C is below point A, the corresponding annular pressure loss ΔP f1 Minimum,

[0038] ΔP f1 =mf(QQ) L )L AB ;

[0039] When the leak point C is between points A and B, the closer the leak point is to point A, the greater the corresponding annular pressure loss ΔP. f2 The smaller,

[0040] ΔP f2 =mf(Q)L L +mf(QQ L (L) AB -L L );

[0041] When leak point C is above point B, the corresponding annular pressure loss ΔP f3 Maximum, equal to normal annular pressure loss.

[0042] ΔP f3 =mf(Q)L AB ;

[0043] The formula relating the annular pressure loss to the location of the leak is:

[0044]

[0045] Where A is the measurement position of the lower measuring section, B is the measurement position of the upper measuring section, and Q represents the normal drilling displacement. L Let f(Q) represent the leakage discharge rate, f(Q) represent the functional relationship between annular pressure loss and Q, m represent other influencing factors besides Q, and L represent the leakage discharge rate. AB L represents the distance between the measurement points of two measurement sections. L Indicates the distance between the leak point and the lower measuring section;

[0046] S4.6, Determination of leakage discharge volume;

[0047] If no annular pressure loss data is recorded under normal circumstances without leakage, according to drilling hydraulics theory, f(Q) can be approximately expressed as Q y , where y is related to the rheological mode and flow regime of the drilling fluid;

[0048] If normal, leak-free annular pressure loss data is recorded, the functional relationship of f(Q) can be determined through the annular pressure loss data. Once f(Q) is determined, the formula for calculating the total leakage discharge is:

[0049]

[0050] As a preferred method: compare the amount of leakage obtained in step S4.6 with the amount of leakage detected on the surface to determine the number of downhole leaks and the corresponding amount of leakage.

[0051] If the calculated total leakage discharge is significantly less than the leakage discharge detected by the surface drilling fluid tank, it is inferred that there is a leak below the last measuring point.

[0052] If the calculated total leakage discharge is close to the bottom of the well but still too small, it is inferred that there is a leak between the lower measuring section and the bottom of the well, and its leakage discharge is equal to the surface leakage discharge minus the calculated total leakage discharge.

[0053] If the calculated total leakage discharge is close to the leakage discharge detected by the surface drilling fluid tank, then use the following formula: Calculate the leakage rate at each leak point from bottom to top;

[0054] In the formula, Q i Q represents the annular displacement below the i-th leak point; for the bottommost leak point, it equals the zero-leakage displacement. Li This represents the leakage rate at the i-th leakage point.

[0055] As a preferred method: if normal annular pressure loss data is available, when determining the number of leaks, the pressure loss at the lowest measuring point can be compared with the normal pressure loss to determine whether there are any leaks below that measuring point. Multiple judgment methods can corroborate each other to ensure the reliability of the results.

[0056] Preferably, the measurement section is a PWD measurement section. This approach facilitates acquisition and implementation, meets the data measurement, recording, and transmission requirements, and eliminates the need for additional measurement tools.

[0057] Compared with the prior art, the beneficial effects of the present invention are:

[0058] The well leakage monitoring and location detection method for oil and gas wells provided by this invention facilitates real-time monitoring and feedback of well leakage conditions, and quickly determines the well leakage location and leakage amount, etc., with good accuracy and reliability, which is conducive to ensuring downhole safety. Attached Figure Description

[0059] Figure 1 This is a schematic diagram of a well leak detection system using a dual-measuring sub.

[0060] Figure 2 This is a schematic diagram of the process logic for monitoring well leakage using the actual well leakage index in this invention;

[0061] Figure 3 This is a schematic diagram of well leakage monitoring during drilling in Example 1 (comprehensive relationship diagram of pressure factor, well leakage index, time and drill bit position);

[0062] Figure 4 This is a diagram showing the well leakage parameters in Example 1;

[0063] Figure 5A schematic diagram illustrating the change in annular pressure loss during well leakage detection using short-distance pressure differential.

[0064] Figure 6 A schematic diagram illustrating the use of short-distance pressure differential to determine the location of a leak (showing a leak point);

[0065] Figure 7 This is a schematic diagram illustrating the use of short-distance pressure differential to determine the location of leaks (showing multiple leak points);

[0066] Figure 8 A schematic diagram illustrating the use of short-distance pressure differential to determine the location of a leak (indicating the leak point is at the bottom of the well);

[0067] Figure 9 A schematic diagram of the annular pressure loss between two measuring sections at different measuring points in Example 2. Detailed Implementation

[0068] The present invention will now be described in further detail with reference to the accompanying drawings.

[0069] refer to Figures 1 to 9 The method for monitoring and detecting leaks in oil and gas wells, as shown, mainly includes the following steps: S1, installing two measuring subs (also commonly referred to as pressure measuring subs or differential pressure measuring subs) in the drill string assembly. These measuring subs are used to measure the pressure or differential pressure at the corresponding well depth. The lower measuring sub is located near the lower end of the drill string, and its measuring position is as follows: Figure 1 Point A is shown, and the measurement part of the upper measuring sub is point B. In the specific implementation of this application, the PWD sub is preferred as the measuring sub, and the distance between the two measuring subs is 30m-60m. The two measuring subs are drilled with the same size tool.

[0070] S2, record basic data such as well structure, drill string assembly, drilled well trajectory, drilling fluid density, and rheological performance parameters for future use;

[0071] S3 determines whether well leakage has occurred based on ground pressure or drilling fluid level. In actual implementation, it can also be determined based on drilling parameters or other signals from drilling equipment.

[0072] Alternatively, real-time drilling parameters can be used to calculate the actual well leakage index at the anomaly point of the pressure factor δ. It compares the actual well leakage index with the manually set critical value to determine whether well leakage has occurred. If the actual well leakage index is greater than the critical value, well leakage is determined to have occurred. Well leakage parameters are inferred based on the pressure factor change status, where the pressure factor δ is the ratio of the real-time pressure loss coefficient at time t to the baseline pressure loss coefficient.

[0073] S4. When well leakage is determined to have occurred in step S3, the location and rate of well leakage are determined based on the annular pressure loss between the two measuring sections at the multi-measuring point location.

[0074] As described above, based on the two different judgment methods in step S3, two related but distinct well leakage detection methods are derived. One is a detection method based on real-time dual pressure data, mainly used to detect whether well leakage has occurred during drilling or post-drilling playback, and to roughly estimate well leakage parameters. The other is a detection method based on short-distance pressure difference at multiple downhole measuring points, mainly used post-drilling to detect whether well leakage has occurred in suspected leakage sections by lowering the record and analyzing the short-distance pressure difference, and to accurately estimate well leakage parameters. The specific details of the two methods are as follows:

[0075] The detection method based on dual-pressure data first refers to... Figure 1 Assume a well with a depth of H has the structure shown in the figure, and the distance between the two measurement points A and B is L. AB Point C is represented as the hypothetical leakage point.

[0076] According to fluid mechanics principles, the drilling fluid pressure and pressure difference measured at points A and B by the two PWDs, respectively, have the following relationship with the drilling fluid loss:

[0077] P B =P gB +ΔP fB (1)

[0078] P A =P B +P gAB +ΔP fAB (2)

[0079] When leak point C is above point B

[0080] ΔP AB =P A -P B =P gAB +ΔP fAB (3)

[0081] When leak point C is below point A

[0082] ΔP AB =P A -P B =P gAB +ΔP fAB (4)

[0083] When leak point C is between points A and B

[0084] ΔP AB =P A -P B =P gAB +ΔP fAC +ΔP fCB (5)

[0085] In the formula, P gB P represents the annular drilling fluid static pressure above point B. gAB ΔP represents the annular drilling fluid static pressure between points A and B. AB ΔP represents the annular pressure difference between points A and B. fB ΔP represents the annular drilling fluid flow pressure loss above point B (hereinafter referred to as annular pressure loss). fAB ΔP represents the pressure loss of the drilling fluid flow in the annulus between points A and B. AC ΔP represents the annular pressure difference between points A and C. CB This represents the annular pressure difference between points CB.

[0086] Table 1 shows the pressure variation characteristics of the well with and without leakage during a certain period of the drilling process, assuming all other engineering parameters remain constant. The table also presents the surface parameter changes when leakage occurs, which can help in identifying leakage. Furthermore, by calculating the pressure reduction, the leakage rate (loss amount) and location can be determined.

[0087] Table 1. Characteristics of changes in related parameters during well leakage.

[0088]

[0089] When other engineering parameters also change, such as changes in inlet flow rate, the pressure and pressure difference at points A and B will change regardless of whether leakage occurs. In this case, the pressure and pressure difference at points A and B without leakage can be calculated using a real-time corrected annular pressure calculation theoretical model. This calculation is then compared with the measured pressure and pressure difference data at points A and B to determine whether the pressure fluctuation is caused by normal parameter changes or by well leakage.

[0090] Theoretically, it's feasible to determine well leakage by observing changes in bottom hole pressure. However, since annular pressure loss is much smaller than drilling fluid hydrostatic pressure, the reduction in annular pressure loss caused by well leakage has a very small impact on bottom hole pressure. Furthermore, due to the influence of temperature and pressure within the well, the actual drilling fluid density and performance parameters vary along the entire wellbore. Errors in wellbore trajectory, irregular well diameter, and cuttings accumulation in any section can all cause changes in bottom hole pressure, making single-point pressure readings unsuitable as the primary basis for well leakage detection.

[0091] In this application, two measuring subs are used, and the distance between the two measuring subs is relatively short. The parameters such as the annulus size, drilling fluid density, performance, and wellbore trajectory between the two subs will not vary much in space. The theoretical pressure and pressure difference values ​​are relatively easy to calculate, and the relationship with the measured values ​​is relatively stable over a certain period of time. Therefore, using the pressure measured by the two measuring subs as a basis for leak detection has good reliability and accuracy.

[0092] When performing well leakage detection using dual pressure, a crucial step is the actual well leakage index in step S3. The calculation process for is as follows:

[0093] S3.1, Calculate the drilling fluid gravity correction factor f g (Dimensionless value), specifically, the drill string assembly is lowered to the bottom of the well, the pump is kept off, and after the annulus stabilizes, the pressure data measured by n sets of dual PWD subs (referred to as dual PWD pressure data) is recorded, and the drilling fluid gravity correction coefficient is calculated according to the following formula (6). The dual PWD pressure data can be sent to the surface for calculation and detection in real time in an alternating manner, or it can be stored in the tool first and then exported for reading and calculation after the drill string is pulled out.

[0094]

[0095] In the formula, and These are measured pressure data from the PWD sub section when the drilling fluid is at rest. ρ represents the density of the drilling fluid in the annulus, g represents the acceleration due to gravity, and H represents the pressure of the drilling fluid. B H represents the vertical depth at point B. A This represents the vertical depth at point A.

[0096] After the calculation is completed, the pump is started to carry out normal drilling, and real-time data such as well depth, drill bit position, well trajectory, drilling fluid arrangement and downhole pressure are received and recorded.

[0097] S3.2 Calculate the theoretical value of real-time annular pressure loss between the two measuring sections. This calculation is mainly based on basic data and real-time data, and the calculation method is as follows:

[0098] P gAB =f g ρgH AB (7)

[0099]

[0100] P gB =f g ρgH B (9)

[0101]

[0102] P gA =P gB +P gAB (11)

[0103] ΔP fA =ΔP fB +ΔP fAB (12)

[0104] In the formula, P gAB P gB P gA ΔP represents the annular drilling fluid static pressure between points A and B, above point B, and above point A, respectively. fAB ΔP fB ΔP fA These represent the annular drilling fluid flow pressure loss between points A and B, above point B, and above point A, respectively; ρ represents the annular drilling fluid density; g represents the acceleration due to gravity; and H represents the pressure loss due to gravity. B H represents the vertical depth at point B. AB L represents the vertical distance between A and B. AB L represents the distance between A and B. B This indicates the depth of the well at point B (which varies with well depth), L C L indicates the length of the bottom drill string assembly. Ci D represents the length of the i-th drill string in the bottom sprue assembly (bottom sprue assembly is a drilling-specific concept, generally referring to a string of non-drill pipe drill strings that performs a certain function about 200 meters above the drill bit), D w D represents the wellbore inner diameter. AB D represents the outer diameter of the drill string between A and B. P D represents the outer diameter of the upper drill pipe. ci Let f represent the outer diameter of the i-th drill string in the bottom drill assembly, Q represent the drilling fluid displacement, and f represent the drilling fluid displacement. AB f p f ci represents the annular flow friction coefficient between AB, at the upper drill pipe, and at the i-th type of drill string in the bottom drill string assembly, respectively.

[0105] S3.3, calculate the actual real-time annular pressure loss between the two measuring sections, assuming... and Given the measured pressure data for the PWD short section, the measured pressure loss is:

[0106]

[0107]

[0108]

[0109] S3.4, calculate the real-time pressure loss coefficient. The pressure loss coefficient k is the ratio of the calculated value of the actual measured annular pressure loss to the theoretical value of the annular pressure loss. The subscript j refers to AB, A, and B, which represent the annular pressure loss coefficient between the two measurement sections, and the annular pressure loss coefficient above points A and B, respectively. Then:

[0110]

[0111] S3.5 Calculate the real-time pressure factor δ. Define the pressure factor δ as the ratio of the pressure loss coefficient k at time t to the baseline pressure loss coefficient (normal drilling pressure loss coefficient) k'. Theoretically, during a period of normal drilling (without well leakage), the pressure factor δ should remain within a small range of fluctuation around 1. When the pressure factor drops abnormally, it indicates that well leakage may occur.

[0112]

[0113] S3.6 Calculate the baseline pressure loss coefficient k′. The baseline pressure loss coefficient is the average value of the pressure loss coefficients k corresponding to multiple detection points within the sliding detection window. Set the length of the sliding detection window to x (x can be a number, such as 10-30, or a duration, such as 10-30 seconds). Calculate the pressure loss coefficients at each time point within the window in chronological order. and its average value Set this average value as the baseline pressure loss coefficient k j The meaning of ′ and j is the same as above. It should be noted that the data in the first window of the detection must be normal drilling or circulation data without well leakage.

[0114]

[0115] After the baseline pressure loss coefficient k′ is calculated, the pressure factor at each time point is calculated according to formula (17). and its average value

[0116]

[0117] S3.7, use the Bayesian online change point detection method to detect whether the pressure factor calculated in formula (17) at this point changes abruptly. If all x points are normal, update the benchmark pressure loss coefficient k′ according to formula (17) in S3.6. If an abnormal change is detected, calculate the actual well leakage index at this time according to formula (19).

[0118]

[0119] Where α is the weighting coefficient, which is arbitrarily assigned a value of 0.1-0.2; and Let represent the pressure factors between A and B, and at point A and point B, respectively, at time t.

[0120] The specific process for detecting whether the stress factor has abruptly changed is as follows:

[0121] S3.7.1 Given initial values ​​for hyperparameters It can take values ​​between 0 and 1.

[0122]

[0123]

[0124]

[0125] P(r0=0)=1

[0126] S3.7.2 Calculate the predicted probability of the current point.

[0127]

[0128] In the formula, x t represent That is, to represent respectively and

[0129] S3.7.3 Calculate the prior probability that the current point is normal.

[0130]

[0131] In the formula, H is the danger function, which is usually taken as a relatively small constant, such as 0.002.

[0132] S3.7.4 Calculate the prior probability of an anomaly at the current point.

[0133]

[0134] S3.7.5 Calculate the posterior probability of the current point being normal or abnormal.

[0135]

[0136] S3.7.6 Determine if there is an anomaly.

[0137] if If the current point is an anomaly, then it is an anomaly; otherwise, it is a normal point.

[0138] S3.7.7 Update hyperparameter values, receive new data, repeat steps (3.7.2)-(3.7.7) to continuously detect.

[0139]

[0140]

[0141]

[0142]

[0143]

[0144] To further improve the real-time monitoring effect, in this detection method, steps S3 and S4 are both calculated by the monitoring system. The monitoring system includes a processor with the above calculation process programmed in it. The processor has a data receiving end, a data calculation module, a comparison module, and an execution module. It is also connected to a data input panel or input terminal. During real-time monitoring, real-time data such as well depth and pressure can be received by the receiving end through the corresponding sensor line. Other basic drilling data can be directly input through the panel. The manually set well leakage index threshold value can be set according to the block conditions, usually between 0.05 and 0.1.

[0145] During drilling, the monitoring system uses real-time ground data, measurement sub data, and other input basic data to calculate parameters such as pressure factor and actual well leakage index. This data is then compared with a manually set well leakage index threshold by a comparison module. If well leakage is detected, the system sends a warning signal, such as an audible or visual alarm, to the connected execution device via the execution module. During the monitoring and calculation process, the calculated pressure factor, well leakage index, time, and drill bit position are graphically displayed. Figure 3 (This can be accomplished by a graphics conversion module connected to the data output terminal of the monitoring system processor.) As shown, this allows for a quick determination of the specific well depth (drill bit position) and the moment when the pressure factor t became abnormal, thereby identifying whether well leakage has occurred.

[0146] The actual well leakage index calculated according to formula (19) given in S3.7 If a well leak is determined to have occurred, step S4 is performed as follows: assuming there is only one leak in the well (if there are multiple leaks, the error of this detection method will increase relatively), and based on... and The changes can be used to preliminarily infer well leakage parameters, such as and Both decreased, while If it remains unchanged, the leak point is above point B; for example... and If both decrease, the leak point is below point B. Simultaneously establish a formula relating leakage velocity (leakage discharge rate) and leakage location. Knowing one of these two parameters allows you to determine the other. Then, use this formula to plot a well leakage parameter analysis chart.

[0147]

[0148] in, Q represents the actual annular pressure loss between points A and B, f(Q) represents the functional relationship between the annular pressure loss and displacement between A and B, and m represents other influencing factors besides Q; L Indicates leakage discharge, L LL represents the distance between the leak point and point A. If the leak point is below point A, then L... L =0,k A ′ B This is the baseline pressure loss coefficient between A and B.

[0149] Well leakage parameter analysis chart is roughly as follows Figure 4 As shown, mainly based on Figure 3 As shown, after a sudden change in the pressure factor, a graph is plotted at a specific measurement point (or at a specific location at a specific time), assuming the leakage rate remains constant. The annular pressure drop between the two measurement sections (PWD1 at the bottom and PWD2 at the top) differs depending on the location of the leakage point. For example, if the leakage point is below PWD1, the reduction in annular pressure drop between the two measurement sections has already reached its minimum and will remain constant. When the leakage point is between the two measurement sections, the lower the leakage point, the greater its impact on the reduction in annular pressure drop; conversely, the higher the leakage point, the smaller the impact, until it is above PWD2, at which point the reduction in annular pressure drop between the two measurement sections is zero. Of course, under the same conditions, the greater the leakage rate, the greater the reduction in annular pressure drop between the two measurement sections. Therefore, reference lines for the reduction in annular pressure drop corresponding to different leakage rates can be pre-done on the graph as needed.

[0150] When the leakage rate can be accurately obtained from the monitoring of the ground drilling fluid tank, the actual leakage rate indicator line can be drawn on the graph. The location where the actual leakage rate indicator line intersects with the indicator line of the current pressure drop value of the annular pressure loss between the two measuring sub-sections is determined as the location of the leak.

[0151] Once the location of the leak is known through other means, a vertical line is drawn corresponding to the depth of the leak. The leakage velocity corresponding to the intersection of this line and the current pressure reduction value is the actual leakage velocity. A more accurate value can be obtained by comparing the intersection with the reference line.

[0152] In a specific embodiment, the above detection method was used to monitor real-time well leakage during drilling of a well in a certain block. During drilling, the drill bit diameter was 215.9 mm, PWD1 was 28 m from the drill bit, PWD2 was 86 m from the drill bit, the distance between the two PWDs was 58 m, and there was a 127 mm weighted drill pipe between the two PWDs. The drilling fluid density was 1.25 g / cm³. 3 The displacement is 33L / s. Leakage was observed on the ground at 16:16 in the afternoon, with an average leakage rate of about 11L / s.

[0153] The data before and after the leakage were detected by the well leakage monitoring system as follows: Figure 3As shown in the figure and display, it can be quickly determined that the well leakage started at 16:11:43 and the drilling reached a depth of 2794.78m. This time is significantly earlier than the time when the leakage was observed on the ground. Therefore, it is evident that the monitoring method of this application can detect well leakage earlier than the ground observation method. The well leakage identification accuracy is higher and the reliability is better. This is conducive to quickly taking corresponding plugging or other construction measures on site to ensure downhole safety.

[0154] Based on experience from adjacent wells and a comprehensive assessment of drilling conditions, the leak is preliminarily suspected to be at the bottom of the well. The calculated maximum leakage velocity is 11.6 L / s, and the minimum leakage velocity is 8.9 L / s, with the leakage velocity varying at different times. The well leakage parameter analysis results at a drilling depth of 2795.5m are as follows: Figure 4 As shown, if the leak point is between the lower PWD1 and the drill bit (2767.5-2795.5m), the leakage rate can be calculated to be 11.3L / s (this can also be obtained by drawing a line (not shown) and comparing it with the reference line), which is close to the leakage rate observed on the ground, further confirming the feasibility of this detection method in quantifying the leakage rate.

[0155] Combination Figure 1 , Figures 5 to 9 As shown, another short-distance differential pressure detection method proposed in this application is as follows: when it is determined that well leakage has occurred downhole in step S3 based on the ground pressure or the level of the drilling fluid tank, the location of the leak and the leakage discharge volume are determined in step S4 according to the following steps.

[0156] S4.1 After a leak occurs, the location of the leak is initially determined on-site, and the drill string is raised or lowered to a range of 50-100 mm above the leak location. Basic data such as well structure and drill string assembly are recorded at this time.

[0157] S4.2, Fixed-point circulation test: Circulate at normal drilling displacement for 1-3 minutes, recording the lowering position, start time, and end time of the circulation. After the circulation displacement stabilizes, calculate the circulation pressure difference ΔP using the dual pressure or differential pressure data obtained from n measuring subs (a dual PWD sub or other similar pressure measuring sub can be used; pressure data can be sent to the ground in real-time in an alternating manner for calculation and analysis. A differential pressure gauge can also be used, and differential pressure data can be directly sent to the ground in real-time for calculation and analysis. Alternatively, a stored pressure or differential pressure tool can be used; after the test is completed and the sub is brought to the ground, the data can be exported for analysis).

[0158] or

[0159] In the formula, These represent the pressure values ​​measured by the i-th lower PWD1 and upper PWD2 when using dual PWDs; ΔP i This represents the differential pressure value measured using a differential pressure timer for the i-th time.

[0160] S4.3, Pump Shutdown Observation: Record the start and end times of the pump shutdown observation, and take the minimum pressure difference recorded within a few seconds immediately after pump shutdown (usually the next 5-10 seconds after complete pump shutdown) as the measured drilling fluid gravity pressure drop ΔP. g Complete a fixed-point test.

[0161] S4.4, Conduct a lowering test. Repeat steps S4.2 and S4.3 every 10-30 meters and record the data until the bottom of the drill string is within 50-100 meters below the suspected leak point, or has reached the bottom of the well.

[0162] S4.5, Based on the data recorded in S4.4, perform data processing. The data processing can be carried out according to Table 2 below. Table 2 Data Recording and Processing Table

[0163]

[0164] In the table, MD, MD2, and MD1 represent the depth measurements at the bottom of the drill string, PWD2, and PWD1, respectively; V d This represents the vertical distance between two PWDs, which can be calculated from wellbore trajectory data. This represents the theoretical gravity pressure drop of the drilling fluid between the two PWDs; ΔP is the known recorded data. Indicates the actual annular pressure loss between the two PWDs; ΔP fd This indicates the annular pressure drop between the two PWDs when there is no leakage near the location. It is not mandatory, but its presence can improve the accuracy of determining the location or volume of well leakage. If there is a significant difference between the actual gravity pressure drop and the theoretical gravity pressure drop, the test data may be problematic, or there may be unknown conditions downhole.

[0165] According to fluid mechanics principles, the annular pressure difference between two PWDs includes the drilling fluid gravity pressure drop and the annular drilling fluid flow pressure loss (referred to as annular pressure loss). The former is only related to the drilling fluid density and vertical distance, while the latter is related to the drilling fluid density, viscosity, displacement, and annular geometric parameters. When other parameters remain unchanged, the annular pressure loss is only affected by the displacement; a larger displacement results in a larger pressure loss, and a smaller displacement results in a smaller pressure loss.

[0166] Therefore, the actual annular pressure loss ΔP between the leak point and the dual PWD at different locations is... f The changes can occur in the following ways:

[0167] When leak point C is below point A, the corresponding annular pressure loss ΔP f1 Minimum,

[0168] ΔP f1 =mf(QQ) L )L AB ; (twenty one)

[0169] When the leak point C is between points A and B, the closer the leak point is to point A, the greater the corresponding annular pressure loss ΔP. f2 The smaller,

[0170] ΔP f2 =mf(Q)L L +mf(QQ L (L) AB -L L ) (twenty two)

[0171] When leak point C is above point B, the corresponding annular pressure loss ΔP f3 Maximum, equal to normal annular pressure loss.

[0172] ΔP f3 =mf(Q)L AB (twenty three)

[0173] The formula relating annular pressure loss to leak location is:

[0174]

[0175] Where A is the measurement position of the lower measuring section, B is the measurement position of the upper measuring section, and Q represents the normal drilling displacement. L Let f(Q) represent the leakage discharge rate, f(Q) represent the functional relationship between annular pressure loss and Q, m represent other influencing factors besides Q, and L represent the leakage discharge rate. AB L represents the distance between the measurement points of two measurement sections. L Indicates the distance between the leak point and the lower measuring section;

[0176] After completing the data processing using Table 2 above, the real-time relationship formula (25) between annular pressure loss and leak location can be obtained by combining formula (24). Multiple results calculated according to formula (25) can be used to plot the annular pressure loss variation between the two measurement sections (e.g., ...). Figures 6 to 8 (As shown), to determine the location of the leak.

[0177]

[0178] In the formula, ΔP f1 ΔP represents the actual pressure loss measured at the measuring point above the i-th leak point. f2 ΔP represents the actual pressure loss measured at the i-th leak point location. f3 L represents the actual pressure loss measured at the measuring point below the i-th leak point. AB L represents the distance between two PWDs. Li This represents the distance between the leak point and PWD1 during the i-th leak point test.

[0179] For example, drawing a graphic Figure 8As shown in the figure (the upper PWD is PWD2 and the lower PWD is PWD1), and if leakage has been clearly detected on the ground, or if the test pressure loss is significantly less than the normal pressure loss, it indicates that the leak point is at the bottom of the well. However, since there are other drilling tools between the lower PWD and the bottom of the well, they cannot reach the bottom of the well. Therefore, the pressure loss above the leak point is close to and significantly less than the normal pressure loss without leakage.

[0180] For example, drawing a graphic Figure 6 As shown, the tested annular pressure loss exhibits two relatively stable regions, with a significant difference between them, indicating a leak point between these two stable regions; similarly, referring to... Figure 7 The tested annular pressure loss has three stable regions. The annular pressure loss values ​​between adjacent stable regions from top to bottom show an increasing trend. Therefore, it can be inferred that there are leaks between adjacent stable regions. Based on the measurement points or by increasing the number of measurement points, the location of the leaks can be obtained more accurately.

[0181] S4.6, Determination of leakage discharge volume;

[0182] If no annular pressure loss data is recorded under normal circumstances without leakage, according to drilling hydraulics theory, f(Q) can be approximately expressed as Q y y is related to the rheological mode and flow regime of the drilling fluid. For example, for Bingham fluid, y can be taken as 1 for laminar flow and 1.8 for turbulent flow, but there may be errors.

[0183] If Table 2 contains normal annular pressure loss (no leakage) data, the functional relationship of f(Q) can be determined using the annular pressure loss data. Once f(Q) is determined, the formula for calculating the total leakage discharge is:

[0184]

[0185] If the total leakage rate is significantly less than the leakage rate detected by the surface drilling fluid tank, it indicates that there is still a leak below the last measuring point, and further downward detection and calculation analysis are required. If the calculated total leakage rate is still too small even though the well bottom is approaching, it indicates that there is still a leak at a location that cannot be detected between PWD1 and the well bottom, and its leakage rate is equal to the surface leakage rate minus the total leakage rate calculated above.

[0186] If the total leakage discharge is close to the leakage discharge detected by the surface drilling fluid tank, the leakage discharge at each leak point location can be calculated from bottom to top using the following formula.

[0187]

[0188] In the formula, Q i Q represents the annular displacement below the i-th leak point; for the lowest leak point, it is equal to the zero-leakage displacement. LiThis represents the leakage rate at the i-th leakage point.

[0189] If there is normal pressure loss data, it is also possible to determine whether there are any leaks below the lowest measuring point by checking whether the pressure loss at the lowest measuring point is close to the normal pressure loss.

[0190] In a specific embodiment, a short-distance differential pressure method was used to detect well leakage parameters in a certain well. During drilling, well leakage was determined to have occurred downhole based on observations of surface parameters. The wellbore size was 215.9 mm, with PWD1 28 m from the drill bit. Between the two PWDs were six 127 mm weighted drill pipes spaced 58 m apart (the drill string assembly could be a newly assembled one for tripping out of the well or the one used during drilling, meaning the two PWD short sections were already being carried during drilling, saving tripping time and facilitating faster well leakage detection). Based on the operating parameters, the leakage point was initially estimated to be between 2500-2600 m. Therefore, the drill string was raised to approximately 2470 m, and from top to bottom, a fixed-point circulation test and pump stop test were conducted approximately every 50 m. The circulating mud density was 1.45 g / cm³. 3 The displacement was 29L / s, up to 2750m, and a total of 6 sets of data were measured. The data processing results are shown in Table 3 below.

[0191] Table 3. Data Recording Table for Dual PWD Detection of Well Leakage Location

[0192]

[0193] Because no double PWD subs were installed during normal drilling of this well, the table above does not contain annular pressure loss data for double PWD during normal drilling. The drill string assembly was a newly installed assembly after tripping out of the well. Based on the data, a schematic diagram of the annular pressure loss between the two measuring subs at different measuring points was drawn as follows: Figure 9 As shown in the figure (the annular pressure loss in the figure is the actual annular pressure loss in Table 3), combined with the data, it can be seen from the figure that the annular pressure loss of Test 1, Test 2 and Test 3 tends to be stable at around 0.5MPa, while Test 5 and Test 6 are similar at around 0.7MPa. Therefore, it can be inferred that the actual leakage point is between Test 3 and Test 5. Specifically, it is between the PWD1 position when Test 3 is performed and the PWD2 position when Test 5 is performed, that is, between 2551-2603m. Further calculation using formulas (25) and (27) can be performed to obtain the well leakage location depth as 2584m and the leakage discharge rate as approximately 8.4L / s.

[0194] All formulas in this application use international standard units, such as meters (m) for length, Pascals (Pa) for pressure, and kilograms per cubic meter (kg / cm³) for density. 3 The flow rate (discharge, leakage, etc.) is expressed in cubic meters per second (m³). 3 / s), with time units in seconds (s), which will not be elaborated further here. For the convenience of professionals, commonly used engineering units have been used when specific values ​​are involved in the examples, and these have been clearly indicated in the relevant locations.

[0195] refer to Figures 1 to 9 The oil and gas well leakage monitoring and location detection method of this application, compared with the detection method using dual pressure data, requires knowledge of either parameter when detecting the location and rate of leakage, and can only be calculated for a single leakage point. In the existing technology, there is no relatively accurate method to directly obtain the location and / or rate of leakage. Therefore, its detection method is mainly used to monitor whether leakage has occurred in real time during drilling, and to judge the leakage situation by the sudden pressure factor. The leakage identification is more timely and the monitoring results are more accurate.

[0196] The second method, short-distance differential pressure detection, only requires deploying a measuring sub in the suspected well leakage section (current technology relies on surface parameters to determine well leakage, although the response time is slightly long). (If a measuring sub is already in place during drilling, the measurement can be completed faster). The leak point and leakage rate can be calculated using the above testing method, and the calculation results are relatively accurate and reliable. Therefore, in actual production applications, the two methods can be combined. The first method can be used for real-time monitoring during drilling, and when well leakage is detected, the second detection method can be used to confirm parameters such as the number of leak points, the location of the leak points, and the leakage rate.

[0197] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention. Those skilled in the art, under the guidance of the present invention, can make various similar representations without departing from the spirit and claims of the present invention, and such modifications all fall within the protection scope of the present invention.

Claims

1. A method for monitoring and detecting well leakage in oil and gas wells, characterized in that, Includes the following steps: S1, Two measuring subs are installed in the drill string assembly. The measuring subs are used to measure the pressure or differential pressure at the corresponding well depth, and one of the measuring subs is located near the lower end of the drill string. S2, record basic drilling data for later use. The basic data includes at least the wellbore structure, drill string assembly, drilled wellbore trajectory, and drilling fluid density and rheological performance parameters. S3, determine whether well leakage has occurred based on ground pressure and / or the level of drilling fluid in the tank; Alternatively, real-time drilling parameters can be used to calculate the actual well leakage index at the anomaly point of the pressure factor δ. It compares the actual well leakage index with the manually set critical value to determine whether well leakage has occurred. If the actual well leakage index is greater than the critical value, well leakage is determined to have occurred. Well leakage parameters are inferred based on the pressure factor change status, where the pressure factor δ is the ratio of the real-time pressure loss coefficient at time t to the baseline pressure loss coefficient. S4. When well leakage is determined to have occurred in step S3, the location and rate of well leakage are determined based on the annular pressure loss between the two measuring sections at the multi-measuring point location.

2. The method for monitoring and detecting well leakage in oil and gas wells according to claim 1, characterized in that: The distance between the two measuring sections is 30m-60m, and the two measuring sections are drilled with the same size tool.

3. The method for monitoring and detecting well leakage in oil and gas wells according to claim 1 or 2, characterized in that: The actual well leakage index mentioned in step S3 The calculation process includes the following steps: S3.1, Calculate the drilling fluid gravity correction factor; S3.2, Calculate the theoretical value of real-time annular pressure loss between the two measured short sections; S3.3, Calculate the actual real-time annular pressure loss between the two measurement sections; S3.4 Calculate the real-time pressure loss coefficient, which is the ratio of the actual value of the annular pressure loss to the theoretical value of the annular pressure loss; S3.5, Calculate the pressure factor δ in real time; S3.6, Calculate the baseline pressure loss coefficient and the average pressure factor, wherein the baseline pressure loss coefficient is the average value of the pressure loss coefficients corresponding to multiple detection points within the sliding detection window; S3.7, use the Bayesian online change point detection method to detect whether the pressure factor calculated in S3.5 changes abruptly. If an anomaly is detected, calculate the actual well leakage index at this time. Where α is the weighting coefficient, which is arbitrarily assigned a value of 0.1-0.2; and Let A and B represent the pressure factors at time t, between A and B, and at points A and B, respectively. Point A is the measurement position of the lower measuring section, and point B is the measurement position of the upper measuring section.

4. The method for monitoring and detecting well leakage in oil and gas wells according to claim 3, characterized in that: The actual well leakage index calculated according to S3.7 If well leakage is detected, step S4 is performed as follows: assuming there is one and only one leakage point downhole, and based on... and The changes can be used to preliminarily infer well leakage parameters, such as and Both decreased, while If it remains unchanged, the leak point is above point B; for example... and If both decrease, the leakage point is below point B. Simultaneously establish the relationship formula between leakage velocity and leakage location, and use this formula to draw a well leakage parameter analysis chart: in, Q represents the actual annular pressure loss between points A and B, f(Q) represents the functional relationship between the annular pressure loss and displacement between A and B, and m represents other influencing factors besides Q; L Indicates leakage discharge, L L L represents the distance between the leak point and point A. If the leak point is below point A, then L... L =0,k A ′ B This is the baseline pressure loss coefficient between A and B.

5. The method for monitoring and detecting well leakage in oil and gas wells according to claim 4, characterized in that: Steps S3 and S4 are both calculated by the monitoring system. If well leakage is detected, an alarm signal is issued through the execution device that is connected to the monitoring system.

6. The method for monitoring and detecting well leakage in oil and gas wells according to claim 1, characterized in that: In step S3, if it is determined that a well leak has occurred downhole based on the ground pressure or the fluid level in the drilling fluid tank, then in step S4, the location of the leak and the amount of leakage are determined according to the following steps. S4.1, make a preliminary on-site assessment of the leak location, and raise or lower the drill string to a position 50-100 mm above the leak location; S4.2, Fixed-point circulation test, with normal drilling displacement circulation, calculate the circulation pressure difference ΔP from the pressure or pressure difference data obtained from the measurement sub; S4.3, Pump shutdown observation: Record the start and end times of the pump shutdown observation, and take the minimum pressure difference recorded within a few seconds immediately after pump shutdown as the measured drilling fluid gravity pressure drop ΔP. g ; S4.4, Conduct a lowering test, repeating steps S4.2 and S4.3 every 10-30 meters, and record the data. S4.5, Based on the data recorded in S4.4, perform data processing, plot the actual annular pressure loss variation between the two measurement sections, and based on the actual annular pressure loss ΔP f Formulas relating leak location to other factors are used to determine the location of leaks. When leak point C is below point A, the corresponding annular pressure loss ΔP f1 Minimum, ΔP f1 =mf(Q-Q L )L AB ; When the leak point C is between points A and B, the closer the leak point is to point A, the greater the corresponding annular pressure loss ΔP. f2 The smaller, ΔP f2 =mf(Q)L L +mf(Q-Q L )(L AB -L L ); When leak point C is above point B, the corresponding annular pressure loss ΔP f3 Maximum, equal to normal annular pressure loss. ΔP f3 =mf(Q)L AB ; The formula relating the annular pressure loss to the location of the leak is: Where A is the measurement position of the lower measuring section, B is the measurement position of the upper measuring section, and Q represents the normal drilling displacement. L Let f(Q) represent the leakage discharge rate, f(Q) represent the functional relationship between annular pressure loss and Q, m represent other influencing factors besides Q, and L represent the leakage discharge rate. AB L represents the distance between the measurement points of two measurement sections. L Indicates the distance between the leak point and the lower measuring section; S4.6, Determination of leakage discharge volume; If no annular pressure loss data is recorded under normal circumstances without leakage, according to drilling hydraulics theory, f(Q) can be approximately expressed as Q y , where y is related to the rheological mode and flow regime of the drilling fluid; If normal, leak-free annular pressure loss data is recorded, the functional relationship of f(Q) can be determined through the annular pressure loss data. Once f(Q) is determined, the formula for calculating the total leakage discharge is:

7. The method for monitoring and detecting well leakage in oil and gas wells according to claim 6, characterized in that: Based on the comparison between the leakage amount obtained in step S4.6 and the leakage amount detected on the surface, the number of downhole leakage points and the corresponding leakage amount are determined. If the calculated total leakage discharge is significantly less than the leakage discharge detected by the surface drilling fluid tank, it is inferred that there is a leak below the last measuring point. If the calculated total leakage discharge is close to the bottom of the well but still too small, it is inferred that there is a leak between the lower measuring section and the bottom of the well, and its leakage discharge is equal to the surface leakage discharge minus the calculated total leakage discharge. If the calculated total leakage discharge is close to the leakage discharge detected by the surface drilling fluid tank, then use the following formula: Calculate the leakage rate at each leak point from bottom to top; In the formula, Q i Q represents the annular displacement below the i-th leak point; for the bottommost leak point, it equals the zero-leakage displacement. Li This represents the leakage rate at the i-th leakage point.

8. The method for monitoring and detecting well leakage in oil and gas wells according to claim 7, characterized in that: If normal annular pressure loss data is available, when determining the number of leaks, the pressure loss at the lowest measuring point can be compared with the normal pressure loss to determine whether there are any leaks below that measuring point.

9. The method for monitoring and detecting well leakage in oil and gas wells according to any one of claims 1, 2, or 6 to 8, characterized in that: The measurement section is a dual PWD measurement section.

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