A method for determining the deformation trajectory of a casing using distributed optical fiber

Abstract

A method for determining the deformation trajectory of a casing using distributed optical fiber, belonging to the field of oilfield development. First, set the wellbore direction as the Z-axis, and take the plane perpendicular to the Z-axis as the XY plane. Secondly, determine the deformation trajectory in the XZ plane, including the determination of the starting and ending points of deformation, the determination of the bending inflection points, and software drawing. Then, determine the deformation trajectory in the XY plane, including: reading the depth and bending degree values of the starting point A of the deformation of one of the optical fibers in the XZ plane, then reading the depth and bending degree values of the starting points of adjacent optical fibers, calculating the distance between the two points along the surface of the casing, and forming a network trajectory diagram after reading all the optical fibers. The present invention replaces the traditional measurement methods and processes of lowering an instrument string into the casing, and avoids the risks and losses brought by engineering accidents such as instrument string jamming and sticking caused by casing deformation.

Description

Technical Field

[0001] The present invention belongs to the field of oil and gas development, and particularly relates to a method for determining the deformation trajectory of a casing using a distributed optical fiber. Background Art

[0002] An oil casing is a steel pipe used to support the wellbore of an oil or gas well to ensure the smooth completion of drilling operations and the normal operation of the oil and gas well during production after completion. However, due to the combined effects of multiple factors such as cementing operations, perforating operations, fracturing operations, and changes in in-situ stress during the completion process, the casing often undergoes deformation. As Figure 1 shown. The originally intact casing as Figure 1 (a) will become Figure 1 (b) or Figure 1 (c) due to the aforementioned reasons, or even more complex situations. At this time, it is necessary to determine information such as the depth and trajectory of the casing deformation to provide reference information for casing repair and other subsequent downhole operations.

[0003] Currently, the commonly used method for measuring casing deformation is multi-arm caliper logging, that is, the instrument is lowered into the well through a logging cable or pumping. After reaching the target layer, the caliper legs are opened, and during the upward movement of the instrument, the casing radius in each direction is measured to obtain the deformation trajectory curve of the casing. The biggest challenge faced by this method is the problem of getting stuck and blocked. Once the instrument string gets stuck in the well, it will face great construction difficulties and economic losses.

[0004] If the distributed optical fiber sensor is arranged on the outer surface of the casing in a permanent manner, the measurement of casing deformation can be realized. When the casing deforms due to external forces, the optical fiber will bend along with the deformation of the casing, so that the propagation of light in the optical fiber sensor will have obvious losses at the bending points. By monitoring the loss of the optical signal, the depth range and trajectory of the casing deformation can be obtained. This method can fundamentally avoid the problem of the instrument string getting stuck and blocked. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for determining the deformation trajectory of a casing using a distributed optical fiber, replacing the traditional measurement methods and processes of lowering an instrument string into the casing, and avoiding the risks and losses brought by engineering accidents such as the instrument string getting stuck and blocked caused by casing deformation.

[0006] The technical solution adopted by the present invention is:

[0007] A method for determining the deformation trajectory of a casing using a distributed optical fiber. First, set the wellbore direction as the Z-axis and the plane perpendicular to the Z-axis as the XY plane. Second, determine the deformation trajectory in the XZ plane, and then determine the deformation trajectory in the XY plane. Finally, connect all the deformation trajectories in the XZ plane and the XY plane to form a net-like casing deformation trajectory diagram.

[0008] Among them, the method for determining the deformation trajectory in the XZ plane includes the following steps:

[0009] (1) Determination of the starting and ending points of deformation

[0010] Measure the optical loss when the optical fiber does not bend through an optical time domain reflectometer (OTDR), and calculate the bending degree value of each measurement point, that is, the background value, which is recorded as 0°±n° (n is a real number greater than 0 and close to 0).

[0011] When the casing does not deform, the optical fiber does not bend, and the measurement result under this condition is used as the bending degree background value. Different optical fibers are used, and the properties of the optical fiber and the optical wave wavelength are different, but they need to be consistent and stable throughout the length of the optical fiber.

[0012] Then, measure the optical loss of the optical fiber throughout the entire length through an optical time domain reflectometer, and calculate the bending degree at each depth sampling point (the depth interval is 0.5 - 1 meter). When it is found that the difference between the bending degree of a certain point A and the bending degree background value is greater than n, and starting from point A, the difference between the bending degrees of the next 3 - 10 sampling points and the bending degree background value is also greater than n, then point A is determined as the starting point of casing deformation; when it is found that the difference between the bending degree of a certain point B and the bending degree background value is less than n, and starting from point B, the difference between the bending degrees of the next 3 - 10 sampling points and the bending degree background value is getting closer and closer to 0, then point B is determined as the ending point of casing deformation.

[0013] The above judgment is based on the following characteristics of the trajectory graph when the casing deforms: The upper and lower parts of the deformation area are generally symmetrical; the deformation has continuity and gradual change.

[0014] (2) Determination of the bending inflection point

[0015] Within the range from point A to point B, calculate the difference between the bending degrees of adjacent two sampling points in turn, and use the difference as the ordinate. At the same time, use the depth of the sampling point as the abscissa to plot a graph, and the peak point in the graph is the inflection point.

[0016] (3) Software drawing

[0017] Starting from the starting point of the deformation, for every 0.5 - 1 meter change in depth, the bending degree of the curve changes by the corresponding value. After passing through several inflection points, finally reach the ending point of the deformation, thus completing the description of the casing deformation trajectory in the XZ plane.

[0018] Among them, the method for determining the deformation trajectory in the XY plane includes the following steps:

[0019] The XY plane is the development of the cylindrical surface of the casing. In this plane, the accuracy of characterizing the deformation trajectory of the casing depends on the number of optical fibers deployed on the outer side of the casing. All optical fibers are parallel to the Z axis and are equally spaced along the circumferential direction (the more the deployment quantity, the more accurate the description of the corresponding trajectory);

[0020] (1) Read the depth and curvature values of the starting point A of the deformation of one of the optical fibers K1 in the XZ plane;

[0021] (2) Read the depth and curvature values of the starting point A' of the deformation of the adjacent optical fiber K2 in the XZ plane;

[0022] (3) Obtain the distance along the casing surface from point A to point A'. Within this distance range, divide the difference in curvature between point A and point A' into m parts (m = 2, 3,...), and obtain the curvature value of each part. The larger the m value, the finer the trajectory description.

[0023] (4) From point A to point A', draw the change trajectory from point A to point A' according to the obtained distance and the curvature value of each part;

[0024] (5) Similarly, draw the change trajectory between the two termination points B and B' of the deformations of the two optical fibers K1 and K2 in the XZ plane;

[0025] (6) Divide the distance between A and B along the direction parallel to the Z axis and the distance between A' and B' into d parts (d = 10, 20,..., 50, 100, etc., depending on the specific situation), so as to obtain the depth points A1, A2,..., A d-1 ; B1, B2,..., B d-1 ;

[0026] (7) According to the above method, draw the trajectories between A1~B1, A2~B2,..., A d ~B d ;

[0027] (8) Then read the depth and curvature values of the starting point A" of the deformation of the optical fiber K3 adjacent to the optical fiber K2 in the XZ plane. According to the above method, obtain the trajectories of all points between the optical fiber K2 and the optical fiber K3; then read the corresponding values of the optical fiber K4 adjacent to the optical fiber K3 until all optical fibers are read.

[0028] Furthermore, the method for calculating the curvature includes the following steps:

[0029] (1) Fold the optical fiber with the optical fiber parameters (wavelength, refractive index, etc.) already determined into different curvatures, and use an optical time domain reflectometer to measure the optical loss values of the optical fibers with different curvatures;

[0030] (2) Substitute the optical loss values corresponding to different bending degrees in step (1) into the following formula to establish a functional relationship between the two, and obtain multiple sets of coefficient values of a and b:

[0031]

[0032] In the formula, a and b are constants related to the wavelength; is the bending degree (curvature), and α is the optical loss;

[0033] (3) Calculate the standard deviations of coefficients a and b respectively from the multiple sets of data in step (2), and then plot the normal distribution curves of coefficients a and b; through the normal distribution curves, check whether the values of data a and b conform to the 3σ principle of normal distribution. If they conform to the 3σ principle, it indicates that the multiple sets of data are valid. Select the arithmetic mean of a and the arithmetic mean of b as the final coefficients and substitute them into the above relationship formula to obtain the functional relationship between the optical loss and the bending degree; if they do not conform to the 3σ principle, repeat step (1) and step (2); if they still do not conform to the 3σ principle of normal distribution after repeated measurement and calculation, it is considered that the above functional relationship model cannot achieve a qualified match with the experimental data within the entire measurement range (bending degree from 3 to 120). Then, instead of using the arithmetic mean of a and the arithmetic mean of b to establish a unified applicable functional relationship within the entire measurement range, directly substitute the values of a and b in each set of data into the functional relationship formula in step (2) to obtain the respective functional relationships for each bending degree;

[0034] (4) According to the functional relationship established in step (3), after measuring the optical loss of a certain unit length of optical fiber by OTDR, the bending degree of this section of optical fiber can be calculated.

[0035] Optical intensity is one of the main parameters of an optical signal. After the optical fiber enters the well, the difference in optical intensity at two depth positions along the wellbore is the optical loss of the optical signal between these two points. The casing deformation causes the optical fiber attached to the casing surface to bend. The more severe the bending, the greater the optical loss. Define the change amount of the tangent inclination angle of the casing surface within a unit length (0.5 meters) as the bending degree (curvature) of the casing, with the unit of degree / meter.

[0036] It is known that the definition of optical loss is: α = (10 / L)log(P in / P out ), with the unit of dB / m. Among them, L is the length of the optical fiber, and P in and P out are the input and output optical powers respectively.

[0037] Furthermore, the determination method of the n value is as follows:

[0038] ① Measure the optical loss value of each depth sampling point of the unbent optical fiber using an OTDR;

[0039] ② Substitute the optical loss value into the relationship between curvature and optical loss function, and obtain the curvature value of each sampling point. This value is n.

[0040] Advantages of the present invention:

[0041] The present invention uses a distributed optical fiber externally mounted on the surface of the casing as a sensor. On the premise of maintaining the accuracy of the traditional casing deformation measurement method, it replaces the traditional measurement method and technology of lowering an instrument string into the casing, and avoids the risks and losses brought by engineering accidents such as instrument string blockage and sticking caused by casing deformation. Description of the drawings

[0042] Figure 1 shows the casing deformation situation;

[0043] Figure 2 is the normal distribution curve of coefficients a and b;

[0044] Figure 3 is a schematic diagram of the wellbore coordinate direction;

[0045] Figure 4 is the curvature difference between two adjacent sampling points and the depth diagram of the sampling points;

[0046] Figure 5 is a description diagram of the casing deformation trajectory in the XZ plane;

[0047] Figure 6 is the complete trajectory diagram. Specific embodiments

[0048] First, determine the function relationship between curvature and optical loss value through experiments in the laboratory, and then obtain the method for calculating curvature. After that, select an experimental simulation well in the laboratory for actual calculation and evaluation.

[0049] I. The method for calculating curvature includes the following steps:

[0050] (1) Fold the optical fiber with determined optical fiber parameters (wavelength, refractive index, etc.) into different curvatures, and measure the optical loss values of the optical fibers with different curvatures using an optical time domain reflectometer;

[0051] Take an optical fiber with a length of 1.0 m (model: OF-PEC, wavelength: 1550 nm, refractive index: 1.467), fold it into different curvatures in Table 1, and measure its optical loss value to obtain the following data:

[0052] Table 1 Experimental results of the relationship between curvature and optical loss

[0053]

[0054] (2) Substitute the optical loss values corresponding to different bending degrees in step (1) into the following formula to establish a functional relationship between the two, and obtain 40 sets of a and b coefficient values, as shown in Table 2:

[0055]

[0056] In the formula, a and b are constants related to the wavelength; is the bending degree (curvature), and α is the optical loss;

[0057] Table 2 Calculated a and b coefficient values

[0058]

[0059]

[0060] (3) Calculate the standard deviations of coefficients a and b respectively from the 39 groups of data in step (2), and then plot the normal distribution curves of coefficients a and b, as shown in Figure 2 ; Through the normal distribution curves, check whether the values of data a and b conform to the 3σ principle of normal distribution.

[0061] After inspection, it is found that the values of a and b conform to the 3σ principle of normal distribution. Therefore, the functional relationship between the bending degree and the optical loss is obtained as:

[0062]

[0063] Second, a method for determining the deformation trajectory of the casing using distributed optical fiber. First, set the wellbore direction as the Z-axis, and the plane perpendicular to the Z-axis as the XY plane; second, determine the deformation trajectory in the XZ plane, and then determine the deformation trajectory in the XY plane; finally, connect all the deformation trajectories in the XZ plane and the XY plane to form a net-like casing deformation trajectory map;

[0064] Among them, the method for determining the deformation trajectory in the XZ plane (as shown in Figure 3 ) includes the following steps:

[0065] (1) Determination of the start and end points of deformation

[0066] Measure the optical loss when the optical fiber is not bent through an optical time domain reflectometer (OTDR), and calculate the bending degree value of each measurement point, that is, the background value, and the value is recorded as 0°±n° (n is a real number greater than 0 and close to 0); among them, the determination method of the n value is:

[0067] ① Measure the optical loss values of each depth sampling point of the unbent optical fiber with OTDR, and the values are 0.083, 0.060, 0.089, 0.100, 0.073;

[0068] ② Substitute the optical loss value into the relationship between curvature and optical loss function (a) to obtain the curvature values at each sampling point \(n = 0.35, 0.20, 0.39, 0.46, 0.28\), and take their average value of 33.6 as the background \(n\) value;

[0069] When the casing does not deform, the optical fiber does not bend, and the measurement result under this condition is used as the background value of the curvature. Different optical fibers are used, and the properties of the optical fiber and the optical wave wavelength are different, but they need to be consistent and stable throughout the length of the optical fiber.

[0070] Then, measure the optical loss of the optical fiber throughout the entire length using an optical time domain reflectometer, and calculate the curvature at each depth sampling point (depth interval is 1 meter). When it is found that the difference between the curvature at a certain point A and the background curvature value is greater than \(n\), and starting from point A, the differences between the curvatures of the next 3 - 10 sampling points and the background curvature value are all greater than \(n\), then point A is determined as the starting point of the casing deformation; when it is found that the difference between the curvature at a certain point B and the background curvature value is less than \(n\), and starting from point B, the differences between the curvatures of the next 3 - 10 sampling points and the background curvature value are getting closer and closer to 0, then point B is determined as the ending point of the casing deformation;

[0071] The above judgment is based on the following characteristics of the trajectory graph when the casing deforms: The overall upper and lower parts of the deformation area are symmetric; the deformation is continuous and gradual, see Figure 1 .

[0072] In this example, at a depth of 3551 meters, the difference between the measured curvature value and the background starts to be greater than \(n\). After that, the curvature values of 5 consecutive points are 5.6, 9.3, 14.4, 20.1, 23.9 respectively. Therefore, it is determined that the depth of the starting point A of the deformation is 3551 meters; after that, the measured curvature values are 1.015, 0.943, 1.101, 0.869, 0.991, and the differences from the background curvature value are close to \(n\); when at a depth of 3489 meters, the difference between the measured curvature value and the background value starts to be less than \(n\), and after that, the curvature values of 5 consecutive points are 0.37, 0.43, 0.51, 0.29, 0.40 respectively. It can be seen that the depth of the ending point B is 3489 meters, and thus the range of the deformation is obtained.

[0073] (2) Determination of the bending inflection point

[0074] Within the range from point A to point B, calculate the differences between the curvatures of adjacent two sampling points in turn, and use the differences as the ordinate. At the same time, use the depth of the sampling point as the abscissa to plot a graph, and the peak point in the graph is the inflection point, see Figure 4 as shown;

[0075] (3) Software drawing

[0076] Starting from the starting point of the deformation, for every 1-meter change in depth, the curvature of the curve changes by a corresponding value. After passing through several inflection points, it finally reaches the deformation termination point, thus completing the description of the deformation trajectory of the casing in the XZ plane, as shown in Figure 5 the figure.

[0077] Among them, the method for determining the deformation trajectory in the XY plane includes the following steps:

[0078] The XY plane is the unfolding of the cylindrical surface of the casing. In this plane, the accuracy of characterizing the deformation trajectory of the casing depends on the number of optical fibers deployed on the outer side of the casing. All optical fibers are parallel to the Z axis and are equally spaced along the circumferential direction (the more the number of deployments, the more accurate the description of the corresponding trajectory); in this example, 3 optical fibers are deployed;

[0079] (1) Read the depth and curvature values of the starting point A of the deformation of one of the optical fibers K1 in the XZ plane, which are 3551 m and 0.882° / m respectively;

[0080] (2) Read the depth and curvature values of the starting point A' of the deformation of the adjacent optical fiber K2 in the XZ plane, which are 3551 m and 0.797° / m respectively;

[0081] (3) Obtain the distance along the casing surface from point A to point A' as 35.73 cm. Within this distance range, divide the difference in curvature between point A and point A' into 5 parts to obtain the curvature value of each part as 0.6° / m. The larger the part number, the finer the trajectory description.

[0082] (4) From point A to point A', draw the change trajectory from point A to point A' according to the obtained distance and the curvature value of each part;

[0083] (5) Similarly, make the change trajectory between the deformation termination points B and B' of the two optical fibers K1 and K2 in the XZ plane;

[0084] (6) Divide the distance along the Z-axis parallel between A and B and the distance between A' and B' into 62 parts, thereby obtaining the depth points 3551, 3552, 3553, 3554, 3555 m; 3493, 3492, 3491, 3490, 3489 m;

[0085] (7) According to the above method, make the trajectories between A1~B1, A2~B2, …, A d ~B d ;

[0086] (8) Read the depth and curvature values of the starting point A” of the deformation of the optical fiber K3 adjacent to the optical fiber K2 in the XZ plane. According to the above method, obtain the trajectories of all points between the optical fiber K2 and the optical fiber K3; then read the corresponding values of the optical fiber K4 adjacent to the optical fiber K3 until all the optical fibers are read. Finally, obtain a complete trajectory diagram, as shown in Figure 6 As shown, this figure contains all the graphic contents of steps (4) - (8).

[0087] Working process

[0088] (1). Lowering of the optical fiber

[0089] The optical fiber for casing deformation monitoring is deployed on the outer surface of the casing. Therefore, during the process of running in the production casing, it is deployed into the well along with the casing and is sealed between the casing and the cement sheath after cementing.

[0090] (2). Detection of the optical fiber

[0091] To master the state of the optical fiber downhole, it is necessary to regularly measure the optical loss of the optical fiber through an optical time domain reflectometer (once every half month or one month). In this process, the background value of the curvature of the optical fiber is obtained.

[0092] (3). Measurement of optical loss

[0093] When a casing deformation event occurs and the description of the casing deformation trajectory is required, measure the change in optical loss within the target depth range through an optical time domain reflectometer, and then obtain the data of the curvature of the optical fiber varying with depth.

[0094] (4). Description of the casing deformation trajectory

[0095] Use the above methods and steps to draw the casing deformation trajectory curve through software.

Claims

1. A method for determining the deformation trajectory of a casing using a distributed optical fiber, characterized in that First, set the wellbore direction as the Z-axis, and the plane perpendicular to the Z-axis as the XY plane; second, determine the deformation trajectory in the XZ plane, and then determine the deformation trajectory in the XY plane; finally, connect all the deformation trajectories in the XZ plane and the XY plane to form a net-shaped casing deformation trajectory diagram; Among them, the method for determining the deformation trajectory in the XZ plane includes the following steps: (1) Determination of the starting and ending points of deformation Measure the optical loss when the optical fiber is not bent by an optical time domain reflectometer, and calculate the bending degree value of each measurement point, that is, the background value, and the value is recorded as 0°±n°; Then measure the optical loss of the optical fiber in the entire length range by an optical time domain reflectometer, and calculate the bending degree at each depth sampling point. When it is found that the difference between the bending degree of a certain point A and the background value of the bending degree is greater than n, and starting from point A, the difference between the bending degrees of the next 3-10 sampling points and the background value of the bending degree is greater than n, then it is determined that point A is the starting point of the casing deformation; when it is found that the difference between the bending degree of a certain point B and the background value of the bending degree is less than n, and starting from point B, the difference between the bending degrees of the next 3-10 sampling points and the background value of the bending degree is getting closer and closer to 0, then it is determined that point B is the ending point of the casing deformation; (2) Determination of the bending inflection point In the range from point A to point B, calculate the difference between the bending degrees of adjacent two sampling points in turn, and use the difference as the ordinate. At the same time, use the depth of the sampling point as the abscissa to plot a graph, and the peak point in the graph is the inflection point; (3) Software drawing Starting from the starting point of the deformation, for every 0.5-1 meter change in depth, the bending degree of the curve changes by the corresponding value. After passing through several inflection points, finally reach the ending point of the deformation, and thus complete the description of the casing deformation trajectory in the XZ plane; Among them, the method for determining the deformation trajectory in the XY plane includes the following steps: The XY plane is the unfolding of the cylindrical surface of the casing. In this plane, the accuracy of characterizing the casing deformation trajectory depends on the number of optical fibers deployed outside the casing. All optical fibers are parallel to the Z-axis and are equally spaced along the circumferential direction; 1) Read the depth and bending degree value of the starting point A of the deformation of one of the optical fibers K1 in the XZ plane; 2) Read the depth and bending degree value of the starting point A' of the deformation of the adjacent optical fiber K2 in the XZ plane; 3) Obtain the distance along the casing surface from point A to point A'. In this distance range, divide the difference in bending degree between point A and point A' into m parts to obtain the bending degree value of each part; 4) From point A to point A', draw the change trajectory from point A to point A' according to the obtained distance and the bending degree value of each part; 5) Similarly, draw the change trajectory between the two ending points B and B' of the deformation of the two optical fibers K1 and K2 in the XZ plane; 6) Divide the distance between A and B along the Z-axis and the distance between A' and B' into d parts, thereby obtaining depth points A1, A2, …, A d-1 ; B1, B2, …, B d-1 ; 7) Create the trajectories between A1 to B1, A2 to B2, …, A d to B d ; 8) Then read the depth and bending degree value of the starting point A" of the deformation of the optical fiber K3 adjacent to the optical fiber K2 in the XZ plane. According to the above method, obtain the trajectories of all points between the optical fiber K2 and the optical fiber K3; then read the corresponding values of the optical fiber K4 adjacent to the optical fiber K3 until all the optical fibers are read.

2. The method for determining the deformation trajectory of a casing using a distributed optical fiber according to claim 1, characterized in that, The calculation method of the bending degree includes the following steps: S1: Fold the optical fiber with the determined optical fiber parameters into different curvatures, and use an optical time domain reflectometer to measure the optical loss values of the optical fibers with different curvatures; S2: Substitute the optical loss values corresponding to different curvatures in step S1 into the following formula to establish a functional relationship between the two, and obtain multiple sets of a and b coefficient values: where a and b are constants related to the wavelength; is the degree of curvature, and α is the optical loss; S3: Calculate the standard deviations of coefficients a and b respectively from the multiple sets of data in step S2, and then plot the normal distribution curves of coefficients a and b; Through the normal distribution curves, check whether the values of data a and b conform to the 3σ principle of normal distribution. If they conform to the 3σ principle, it means that the multiple sets of data are valid. Select the arithmetic mean values of a and b as the final coefficients and substitute them into the relationship formula in step S2, so as to obtain the functional relationship between optical loss and curvature; If it does not conform to the 3σ principle, repeat steps S1 and S2; If it still does not conform to the 3σ principle of normal distribution after repeated measurement and calculation, it is considered that the functional relationship model in step S2 cannot achieve a qualified match with the experimental data within the entire measurement range. Then, instead of using the arithmetic mean values of a and b to establish a unified applicable functional relationship within the entire measurement range, directly substitute the a and b values in each set of data into the functional relationship formula in step S2 to obtain the respective functional relationships for each curvature; S4: According to the functional relationship established in step S3, after measuring the optical loss of a certain unit length of optical fiber through OTDR, the curvature of this section of optical fiber can be calculated.

3. The method for determining the deformation trajectory of a casing using distributed optical fiber according to claim 2, wherein, The method for determining the value of n is as follows: ① Use OTDR to measure the optical loss values of each depth sampling point of the unbent optical fiber; ② Substitute the optical loss values into the curvature-optical loss functional relationship formula to obtain the curvature values of each sampling point, and this value is n.

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