A casing well cement ring thickness calculation method based on density logging data

Abstract

The application discloses a casing well cement ring thickness calculation method based on density logging data and discloses a stratum density calculation method in a casing well 137 The density logging instrument composed of a Cs gamma source and two gamma detectors collects data, calculates the length of a connecting line of a measured near detector data intersection point and a "cement point" on a "ridge line" of the casing well on a log cross plot of near and far detector count rates of the density logging, and determines the stratum density after casing according to a response relation between the length in the calibration well and the stratum density, so that a new thought is provided for casing density evaluation.

Description

Technical Field

[0001] This invention relates to the field of oil and gas well logging systems, and in particular to a method for calculating the thickness of the cement sheath in a casing well using density logging data. Background Technology

[0002] Density logging refers to gamma-gamma density logging, primarily used in open-hole wells. 137 The system consists of a Cs gamma source and two near- and far-field gamma detectors. By recording the scattered gamma counts of the gamma rays generated by the gamma source after Compton scattering in the formation and entering the two detectors, the system compensates for the influence of mud cake on the wellbore wall and obtains the original formation density information.

[0003] In recent years, a logging method using gamma-gamma density logging to measure formation density has emerged in casing wells. Compared with open hole wells, traditional density correction methods cannot obtain the original formation density in casing wells due to the absorption of gamma rays by the casing and cement sheath. One of the main problems is how to accurately calculate the thickness of the cement sheath. Summary of the Invention

[0004] The purpose of this invention is to design a method for calculating cement sheath thickness in cased wells using density logging data.

[0005] The technical solution adopted in this invention is as follows: on the logarithmic intersection plot of the near and far detector count rates of density logging, the slope of the line connecting the intersection point of the measured near and far detector data and the "cement point" on the "ridge line" of the casing well is calculated, and the cement sheath thickness is calculated based on the response relationship between the slope in the calibrated well and the cement sheath thickness.

[0006] The aforementioned density logging near- and far-field detector count rate log cross plot, also known as a "ridge plot," is shown in the attached image. Figure 1 As shown, the horizontal axis represents the logarithm of the near detector count rate, and the vertical axis represents the logarithm of the far detector count rate. The near and far detector count rates are the sum of the count rates within a certain energy range in the scattered gamma energy spectrum recorded by the near and far detectors, preferably within the energy range of 187keV to 537keV.

[0007] The intersection point of near-field and far-field detector data is defined in the logarithmic cross plot of near-field and far-field detector count rates, using the logarithm of the near-field detector count rate lnN. S The x-axis represents the logarithm of the remote detector count rate, lnN. L The data point is represented by the ordinate, and its coordinates are (lnN). S ,lnN L ).

[0008] The aforementioned "ridge line" in a casing well is a straight line formed on the logarithmic cross plot of near- and far-field detector count rates when only the casing exists between the density logging instrument and the detected formation, as shown in the attached figure. Figure 2 As shown.

[0009] The "cement point" mentioned above refers to the data point on the ridge line of the casing well when the formation density equals the cement density (as shown in the attached figure). Figure 3 Midpoint 2), its coordinates are (lnN) cemS ,lnN cemL )express.

[0010] The slope mentioned is the point (lnN) in the logarithmic cross plot of the near and far detector count rates. S ,lnN L ) and point (lnN) cemS ,lnN cemL The slope of the connecting line (as shown in the attached figure) Figure 3 As shown), represented by SLP:

[0011]

[0012] The aforementioned calibration well is a well for which the casing thickness, cement sheath density, cement sheath thickness, and formation density are known.

[0013] The beneficial effects of this invention are as follows: This invention proposes a method for calculating cement sheath thickness in casing wells using scattered gamma count rate, which solves the problem of inaccurate calculation of cement sheath thickness in casing wells and further provides a new approach for evaluating post-casing cementing quality. Attached Figure Description

[0014] Figure 1 This is a logarithmic intersection plot of near- and far-field detector count rates in density logging, also known as a "ridge-rib plot". The ridge line is the logarithmic intersection line of near- and far-field detector count rates when there is no mud cake on the open-hole well wall; the rib line is the curve formed by the logarithmic intersection of near- and far-field detector count rates when there is mud cake on the open-hole well wall.

[0015] Figure 2 This is a casing well ridge line diagram, which is a straight line formed on the logarithmic intersection of the near-field and far-field detector count rates when there is only casing between the density logging instrument and the detected formation, and the formation density changes as the casing thickness is constant. In the diagram: 1 is the ridge line of the open hole well when the casing thickness is 0; 2, 3, and 4 are the ridge lines of the casing well when the casing thickness gradually increases.

[0016] Figure 3 This diagram illustrates the calculation of cement sheath thickness using SLP in a casing well. In the diagram: 1 represents the intersection of formation density on the casing well ridge line under the current conditions; 2 represents the intersection of cement sheath density on the casing ridge line; and 3 represents the intersection of the logarithms of near-field and far-field detector count rates under the current conditions. Detailed Implementation

[0017] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0018] The method for calculating the thickness of the cement ring is characterized by the following steps:

[0019] Step 1: In a calibrated well with known casing thickness and formation density, establish the relationship between near- and far-field detector count rates and casing thickness and formation density under different casing thickness conditions.

[0020] Preferably, the relationship between near-field and far-field detector count rates and casing thickness and formation density is expressed as follows:

[0021]

[0022]

[0023] Where, lnN S 0 lnN L 0 For near-field and far-field detector count rates; T cas ρ is the casing thickness; a1, a2, a3, a4, b1, b2, b3, b4 are scale coefficients.

[0024] Step 2: In a calibrated well with known casing thickness, cement density, and cement sheath thickness, establish the response relationship between cement sheath thickness, cement density, and slope SLP.

[0025] Preferably, the response relationship between cement ring thickness and cement density, and slope SLP is as follows:

[0026] T cem =c1·SLP·ρ cem +c2·ρ cem +c3·SLP+c4 (3)

[0027] Among them, T cem ρ is the thickness of the cement ring; cem C is the density of cement, and c1, c2, c3, and c4 are scale coefficients.

[0028] Step 3: In wells with unknown formations, collect the count rates of scattered gamma energy spectra from near- and far-source gamma detectors, respectively, N. S N L ; Calculate the coordinates of the "cement point" corresponding to the cement density at this time.

[0029]

[0030]

[0031] Step 4, calculate the slope SLP at this point:

[0032]

[0033] Step 5: Based on the cement sheath thickness T obtained by the instrument in the calibration well. cem The thickness of the cement ring is calculated by using the response relationship between cement density and slope SLP (3).

[0034] This invention provides a method for calculating the thickness of the cement sheath under casing well conditions by using the slope of the line connecting the intersection of near-field and far-field data from density logging and the cement point on the "ridge line" of the casing well. This provides a new approach for evaluating formation density and post-casing cementing quality in casing wells.

[0035] The above description is not intended to limit the invention. Any changes, modifications, additions, or substitutions made by those skilled in the art within the scope of the invention should also be considered within the protection scope of the invention.

Claims

1. A method for calculating the thickness of the cement sheath in a cased well based on density logging data, utilizing... 137 The density logging instrument, consisting of a Cs gamma source, near-field detector, and far-field detector, performs measurements. Its characteristic is... Includes the following steps: Step 1: In a calibrated well with known casing thickness and formation density, establish the relationship between near- and far-field detector count rates and casing thickness and formation density under different casing thickness conditions, expressed as: （1） （2） Where, lnN S 0 lnN L 0 T represents the logarithm of the count rate for near-field and far-field detectors. cas ρ is the casing thickness; a1, a2, a3, a4, b1, b2, b3, b4 are scale coefficients; Step 2: In a calibrated well with known casing thickness, cement density, and cement sheath thickness, establish the response relationship between cement sheath thickness, cement density, and slope SLP: （3） Among them, T cem ρ is the thickness of the cement ring; cem C is the density of cement, and c1, c2, c3, and c4 are scale coefficients. The slope mentioned is the point (lnN) in the logarithmic cross plot of the near and far detector count rates. S ,lnN L ) and cement point (lnN) cemS ,lnN cemL The slope of the line connecting the two points is denoted by SLP; the "cement point" mentioned above is the data point on the "ridge" of the casing well when the formation density equals the cement density, and its coordinates are denoted by (lnN). cemS ,lnN cemL The "ridge line" of the casing well is a straight line formed on the logarithmic cross plot of the near and far detector count rates when only the casing exists between the density logging instrument and the detected formation. Step 3: In wells with unknown formations, collect the count rates of scattered gamma energy spectra from near- and far-source gamma detectors, denoted as N. S N L ; Calculate the coordinates of the "cement point" corresponding to the cement density at this time ( , ): （4） （5） Step 4: Based on the coordinates of the "cement point" obtained in Step 3 and the measured count rate, calculate the slope SLP: （6） Step 5: Substitute the slope SLP obtained in Step 4 and the known cement density into the response relationship in Step 2 to calculate the cement ring thickness.

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