This invention relates generally to the field of production of oil or gas, and more particularly to well drilling. Specifically, the invention is a method for evaluating cement density as a function of azimuth angle around well casings.
When a well is drilled and steel casing is placed, cement slurry is pumped into the annular space between casing and formations. The primary objectives of cementing are to provide mechanical support for the steel casing string and zonal isolation between earth strata or formations. Multiple-stage casing and cementing operations are common procedures to establish pressure barriers during drilling a well. It allows the use of heavier drilling muds in drilling deeper sections without damaging or fracturing the shallower formations due to hydrostatic pressure gradient. An ideal cementing job would fill the casing and formation annulus completely with cement. Potential issues encountered in cementing operations are fluid filled channels within the cement sheath and fluid contaminated cement due to incomplete replacement or sweep of drilling mud with cement slurry. Zonal isolation assessment is a critical aspect of well integrity tests to ensure hydrocarbon production in a safe manner. Cement evaluation measurements are relied upon to demonstrate fluid cross flow is not expected from unwanted zones, i.e. zones other than the producing intervals. This invention relates to in situ evaluation of cement quality between steel casing and formations in a wellbore.
Gamma ray density logging technology is a well-known art to provide formation density and porosity data in petrophysical analysis and formation evaluation. A nuclear density log provides volumetric density measurements of the wellbore and surrounding media. In theory, a nuclear density tool may be constructed with a gamma ray source and one gamma ray detector. The bulk density response in a homogeneous and infinite medium is calibrated as a function of detector count rates. The calibration is normally performed in rock standards with known density value in laboratories. A unique calibration converting count rates to density values is assigned to every density logging tool. Density tools, in practice, have multiple detectors in place in order to provide accurate density measurements in layered media, such as mud cake, casing, and cement.
Existing density tools are designed to optimize sensitivity to the density in the formation rather than the density in the mud cake, casing, or cement. Interpretation methods are devised to remove mud cake, standoff, casing and cement effects as unwanted environmental effects. Shown in
To use
There are primarily two types of gamma ray density tools. Shown in
ρb=ρLS+Δρ
Where ρb is formation density, ρLS is long-spaced density density, and Δρ is the mud cake density correction which is a function of long-spaced and shorted spaced density measurements:
Δρ=f(ρLS,ρSS)
where ρLS and ρSS are long-spaced and short-spaced densities.
It is most commonly run in open holes before steel casing and cement are placed in a wellbore. However, it is also used occasionally to log cased wells to acquire formation density logs, with much reduced data precision and accuracy.
Shown in
Following is a partial list of publications on density logging techniques.
U.S. Pat. No. 3,815,677 (“Method for operating in wells”) describes running an open hole sonic log and a cased hole neutron log to detect fluid channels in cement. Azimuthally oriented nuclear density logs are also run to detect fluid-filled channels. The oriented density scan is plotted and the density variations in the plot provide indications of channels in the cement. The disclosed method of fluid-filled channel detection is qualitative in nature.
U.S. Pat. No. 5,627,368 to Moake (“Four-detector formation-density tool for use in cased and open holes”) discloses a nuclear density tool design with four detectors to provide measurements of casing and cement weight and thickness as well as formation density. It teaches a method for measuring the thickness of the cement, and a method for measuring the density of the cement. There is no disclosure of a tool design with azimuthal capability for measurement around the wellbore to generate cement density and thickness maps.
U.S. Pat. No. 6,781,115 (“Subsurface radiation phenomena detection with combined and azimuthally sensitive detectors”) discloses a nuclear tool design with a detector system that has azimuthal sensitivity by placing multiple detectors at different azimuths. Unlike the present invention, these detectors are not placed on articulated pads to accommodate varying casing size.
U.S. Patent Application Publication No. 2008/0061225, “Logging tool for determination of formation density (Embodiments),” by Orban et al., discloses a logging tool design with one or more detectors and a rotating source. Azimuthal formation density measurements are provided by, for example, collimated shielding placed around the rotating source. The multiple sensor pad configuration of
Patent application publication WO 2011/152924, “System and method for generating density in a cased-hole,” describes a method and device for cased hole density logging. The objective is to find formation density, and interpretation algorithms are disclosed for doing that, and for making corrections for the effects of casing and cement. No disclosures are made regarding azimuthal measurements.
Commercially available logging tools/products offered by oil service companies include:
Cased Hole Analysis Tools (CHAT) by Voltage Wireline Inc., and the Three detector Litho-Density tool (TLD) by Schlumberger. With adaptations as taught by the present disclosure, these tools could provide qualitative cement density and thickness measurements, but their design objectives are for formation rock density measurements and not for accurate measurements of cement properties. Nor do they have azimuthal sensitivity to scan the wellbore.
In one embodiment, the invention is a method for generating a cement density image in a cemented borehole, comprising positioning a logging tool inside the borehole at a selected depth, wherein the logging tool comprises a gamma ray source, a short-spaced detector positioned ≦7 inches from the source, and a long-spaced detector positioned less than 12 inches but farther than the short-spaced detector from the source; and measuring gamma ray count rates at each detector and using them to calculate an estimated cement thickness or density.
In another embodiment, the invention is a nuclear density logging tool for cemented borehole use, for estimating cement density or thickness, comprising a tool body adapted to be lowered longitudinally into a casing in a cylindrical borehole along the borehole axis, said tool body housing a gamma ray source, a short-spaced detector and a long-spaced detector, all fixed to the tool body and spaced in the longitudinal direction such that the short-spaced detector is positioned ≦7 inches from the source and the long-spaced detector is positioned less than 12 inches from the source but farther than the short-spaced detector.
The present invention will be better understood by referring to the following detailed description and the attached drawings in which:
The invention will be described in connection with example embodiments. To the extent that the following description is specific to a particular embodiment or a particular use of the invention, this is intended to be illustrative only, and is not to be construed as limiting the scope of the invention. On the contrary, it is intended to cover all alternatives, modifications and equivalents that may be included within the scope of the invention, as defined by the appended claims.
A method and apparatus are disclosed using nuclear density logs to evaluate cement density and thickness in a wellbore completed with casing and cement. A key feature of the invention is that the logging tool configurations have much smaller than typical spacing between gamma ray source and detectors, in order to optimize the measurement sensitivity to cement properties as shown in
A second key feature of the present invention is that it scans all azimuth angles, providing azimuth-dependent density (count rate) data. The new nuclear density tool can be built either on a single pad-mounted a rotating device to scan the wellbore circumferentially (
The gamma ray detectors record gamma ray count rates as a function of azimuthal bins or sectors at each logging depth. Ideally, the detectors are housed in a cylindrical tool housing made of shielding materials such as tungsten. If the entire tool housing cannot be made of shielding material due to mechanical strength requirements, it should be designed such that the detectors are surrounded with shielding materials to prevent detection of undesirable radiation influences. The collimation is achieved by carving out windows in front of detectors and filling the windows with materials of low atomic numbers such as beryllium to achieve a pressure seal in the tool body. Tool integrity is an important aspect in tool design so that a logging tool remains functional in hostile downhole conditions at elevated pressure and temperature. These window openings can be designed with various azimuthal apertures depending upon the azimuthal resolution. A window with small azimuthal aperture has a higher azimuthal resolution than a large azimuthal window. Commercial density logging tools typically have 45-deg azimuthal windows. The window apertures on the cement image density tool should not be more than 10-15 degrees.
The cement density image tool with a single pad mounted on a rotating chassis has advantages over the multiple-pad tool in that it is simple to build and provides a continuous image around the wellbore. It constantly rotates and acquires count rate data while being pulled out of or lowered in the wellbore. The count rates are then binned into azimuthal sectors at each logging depth. The number of angular sectors can be 16, or 32, or even more to provide high resolution image data. The logging speed or the cable velocity is controlled so as not to compromise the vertical resolution. For example, a 600 ft/hour logging speed with 20 rpm (revolutions per minute) of scanning speed allows acquisition of image data with 6 in vertical resolution.
A nuclear density interpretation algorithm is built to carry out mathematical inversions using detector count rates and calculate casing and cement density and thickness distributions azimuthally. This interpretation algorithm can be constructed using similar physics and computational principles as are used for existing gamma ray tools that are optimized to give the formation density.
A cement density image around the wellbore is obtained in this manner. With known well completion information such as casing size and weight as well as cementing job information indicating cement slurry type and weight, and open hole caliper data, the number of unknowns in the interpretation algorithm can be reduced such that the cement density image provides a cement quality map indicating potential presence of fluid contaminations and/or fluid-filled channels in cement sheath between casing and formation. Together with traditional cement bond logs, cement density image logs can significantly reduce the uncertainty of cement bond interpretation.
One embodiment of this invention involves a wireline tool design with a rotational tool body capable of providing circumferential cement density image around a wellbore.
An alternative embodiment of this invention involves a wireline tool design with multiple measurement pads mounted on a stationary tool body for circumferential density measurement within a wellbore.
The collimators and windows can be seen in
Another aspect of the invention involves an interpretation algorithm 93 to derive cement density from gamma ray count rate. The algorithm can be built using either tool characterization data acquired in standard rock formations in laboratories or computing modeling data, or a combination of both types of data. Methods are known for doing this, and they do not vary in principle from the methods used in the interpretation algorithms in existing gamma ray density tools.
The primary input of the algorithm is gamma ray energy spectra which are detector count rates acquired as a function of gamma ray energy. A typical commercial gamma source, Cs-137, may emit as many as 7.4×1010 gammas per second with a half-life of 30 years. Other gamma sources may have different intensities and half-lives. The spectral count rates are recorded and sorted azimuthally in angular bins at each logging depth. Well completion information and cementing job information such as borehole caliper, casing size and weight, cement slurry weight, are also needed to achieve an optimal solution. A general embodiment of the interpretation workflow is illustrated in the flow chart of
A more specific embodiment of the method of
Step 1: Density measurements in kth azimuthal bin are obtained using individual detector calibration functions. These are also called apparent density measurements.
From detector 1: ρ1k=a1 ln(C1k)+b1
From detector 2: ρ2k=a2 ln(C2k)+b2
From detector 3: ρ3k=a3 ln(C3k)+b3
where the ai and bi are detector specific count rate-to-density calibration coefficients, and k=1 . . . K where K is the total number of azimuthal bins (4, 8 or 16, etc.).
Step 2: The apparent density from each detector is a function of casing density and thickness, cement density and thickness, and formation rock density as shown in
ρ1k=f1(ρcasing,hcasing,ρcement,hcement,ρformation)
ρ2k=f2(ρcasing,hcasing,ρcement,hcement,ρformation)
ρ3k=f3(ρcasing,hcasing,ρcement,hcement,ρformation)
Step 3: Several of the parameters in these equations are known. For instance, casing density and thickness are determined by casing weights for a specific casing size in a casing specs table. Formation density is known if an open hole density log is run. The remaining unknowns in the system of equations in step 2 are cement density ρcement; cement thickness hcement; and formation density ρformation which can be easily solved with mathematical inversion methods in which theoretically predicted (using initial models for the inversion unknowns) count rates are compared to the measured count rates, the models for the inversion unknowns are adjusted to reduce the misfit, and the process is repeated until a predetermined convergence criterion is satisfied or other stopping condition is met.
In summary, key features of the present invention include a tool design with shortened source-to-detector distances to maximize the measurement sensitivity to cement properties, an interpretation method to solve for cement density and thickness, and a scanning mechanism to provide azimuthal sensitivity. Existing density tools have larger distances between source and detectors, which makes them unsuitable for analyzing cement in a cased well.
The foregoing application is directed to particular embodiments of the present invention for the purpose of illustrating it. It will be apparent, however, to one skilled in the art, that many modifications and variations to the embodiments described herein are possible. All such modifications and variations are intended to be within the scope of the present invention, as defined in the appended claims.
This application claims the benefit of U.S. Provisional Patent Application 61/838,072, filed Jun. 21, 2013, entitled Azimuthal Cement Density Image Measurements, the entirety of which is incorporated by reference herein.
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