Patent Application
20080061225
The present invention provides techniques for use in imaging tools. An imaging tool has to ensure a proper coverage of the well-bore with maximum uniformity. The imaging process requires the use of multiple paths (signal source, receiver) of measurements: each measurement representing one pixel of the image, as it is affected by the properties of the local material of the bore-hole.
To limit the complexity of the system, most imaging system share the source between multiple measurements. It is typical to install the common source and the arrays of receivers at a given position: then the measurements on all receivers are performed in a quasi-simultaneous fashion. This general concept also applies in the present invention. The nuclear source generates gamma rays in random time and random direction. However, if the radioactive source has a relatively high activity, it can be considered that gamma rays are transmitted in all directions with a nearly uniform probability at any time of measurement.
For proper imaging of the bore-hole, “quasi” uniform gamma ray emission around the bore-hole is required. This can be achieved either with an instantaneous emission all around the bore hole or with a rotating radial source. Different implementations can be used for this objective.
In one embodiment, the source is installed at the center of the bore-hole. A mechanical implementation ensures that the source is at the center of the wireline tool body which can itself be centralized in the bore-hole.
One example has a fixed central source. The source comprises a small radioactive element generating the gamma radiation directly. The source is located at the center of the tool, contained inside a housing defined by a chamber of heavy metal with a circumferential slit at the periphery of the wireline tool. This slit allows radiation to exit the tool in an axi-symmetrical fashion. The chamber may be under vacuum to limit the ray scattering and absorption within the chamber. A thin wall may be provided to ensure hydraulic isolation from the well bore fluid. This thin wall can be wrapped around the heavy metal with the slit.
In another example, a rotating source is used. The uniform emission versus azimuth of gamma-ray towards the formation can be achieved by rotating a focused source inside the well-bore. After one rotation at constant speed, the energy distribution is uniform for all azimuths. For high radiation energy and better use of the receivers, multiple sources installed at different azimuths of the rotating mechanism can be used.
Various designs of rotation systems are possible:
2) In the embodiment of
The standoff from the formation is reduced from solutions 1 to 5, improving the radiation level into the formation to be characterized: with the source close to the formation, less energy spreading by spherical divergence affects the radiation before reaching the formation with less attenuation by the wellbore fluid.
The rotating focusing imposes the condition that information for imaging can only be acquired from the detectors aligned azimuthally with the source. In practical terms, this means that the logging speed should be low enough for proper coverage of the full well-bore. Data acquisition can then be synchronized to the rotation angular position. Detectors within a azimuthal angle of +/−25 degrees can typically be used for proper density imaging.
Improved usage of the detectors can be achieved with multiple rotating source points. Four source points can be installed at 90 degrees from each other. Solution 2 discussed above in relation to
It is also possible to use detectors not azimuthally aligned with the source for imaging of dipping event as will be described below.
One way to provide axi-symmetrical gamma-ray emission around the logging toot is to use a long distributed source which is wound around the tool body. One implementation of this approach uses a small diameter tubing and with proper distribution of the radioactive material inside the tubing. The tubing plays the role of protector for the radioactive element. During the source installation, the small tubing is forced into a circumferential groove in the tool: this groove is near the periphery of the tool, and is accessible via a tangential hole. This tangential hole can be used for source loading. This hole is plugged with a proper retainer, so that the source cannot be lost in the hole.
In conventional tool design, the directivity of the gamma-ray emission towards the formation is obtained by shielding the source so that the gamma-rays propagating in unwanted directions are absorbed. This technique is adequate for the ensuring the proper source directivity. However most of the emitted photons are absorbed and in the case of an imaging tool, this approach can make the design inefficient, as high energy sources or multiple sources are required.
To counteract this difficulty, a different source concept can be used so that most of the radioactive process generates gamma photons towards the formation. This increases of efficiency makes the system more adequate for imaging.
This technique is based on the following concept (see
The approach offers a number of potential advantages:
The bending of the particle path can be achieved by the use of electrostatic fields (see
The bending of the path of the charged particles can also be achieved by the used of magnetic fields. The force to bend the trajectory of the particles is obtained from the vector product of magnetic field and particle velocity x charge. This means that the particle path is bent due to the acceleration perpendicular to the plane of the two other vectors (field and velocity).
In one embodiment of this invention, the magnetic flux is arranged to be perpendicular to the radial plane: it should in theory be following a circle. It could also be approximated by series of chords. Also, the flux should be directed in one direction near the bottom plate, and to the other direction near the top plate, while being null in the plane at mid distance from both plates. This means that the flux amplitude depends on the Z coordinate while increasing towards the plates but heaving the opposite rotation direction.
The radioactive source is installed at coordinates Z=0/R=0 (R, α being cylindrical coordinate in the plane perpendicular to the tool/hole axis).
With this field distribution, the following acceleration is applied to the charged particles:
The amplitude of the circumferential field is optimized following a law depending on (Z,R):
The circumferential magnetic field can be generated by radial electrical current in the plates. A practical realization is based on winding of wire around a ring of non-magnetic material (toroid). The ring has sufficient thickness to ensure a relatively large distance between the two “flat” surfaces of wire.
Each radial wire generates a circular magnetic field which decays as 1/L with “L” being the distance from the considered point to the wire. Due to the combination of the multiple radial wires, the magnetic field appears to be a nearly continuous circumferential line.
With the proposed toroid wiring, the apparent radial current density reduces with R (R=distance from the tool center): So the magnetic field reduces with R.
As a toroid, perfect winding is used at each cavity plate. The combined field in the cavity meets the (approximate) requirements:
If particles are not deflected enough and enter inside the ring, they are strongly influenced by the high circumferential flux and are redirected towards the central plane of the system (outside the winding). Making the winding as light as possible with minimum cross-section avoids the particles being absorbed by the winding material. The core of the winding can be a vacuum for limiting particle absorption.
More complex toroid winding can be used to impose a predetermined distribution of the radial current average density in the winding plane. This allows to control the distribution of the flux versus R. This can be useful for optimum guidance of the particles towards the circumferential target and the slit.
Ensuring that the fields from both toroid windings are properly balanced ensures the proper field distribution. In theory with perfect geometrical system and uniform material properties, the current should be equal in both windings. In practical applications, it may be necessary to adjust the current in the windings for the perfect balance.
It is important to ensure that the electrical power transmission from one side of the cavity to the other side is performed while providing perfect field cancellation of the currents (in and out). Without perfect cancellation, charged particles will be submitted to circumferential acceleration which is not optimum for the present device operation. In the ideal case, a coaxial cable could be used at the axis of the tool. However the source is also at the center of the cavity; so that other approaches may have to be used. One is to install the coaxial cable at the periphery of the chamber, supposing that it magnetic radiation is nearly null. Some slight improvement can be achieved by installing several coaxial cables at the periphery at uniform angular positions.
The thickness of the toroid winding should be large enough to limit the influence of the wires on the remote flat face of the ring. For large spacings, the shielding material can be contained within the toroid itself: This shield may fill only part of the toroid cross-section.
By operating the system such that the guidance is not constant (and uniform) in the chamber, the high energy particle flux can be made to rotate. As a result of this rotating flux, the gamma-ray emission outside the tool can also be caused to rotate. Multi-pole energization (a quadri-pole gamma-ray emission) is preferred.
With an electrostatic guidance system, one possible implementation of a quasi rotating guidance can be obtained by splitting the electrode at the circumferential wall into multiple segments. The electrical system applies the guidance voltage only to specific segments of electrode to attract the charges particles towards them. If the electrical field is successively applied to the successive segments, a quasi rotating guidance is obtained. The un-used segments can be charged at the reversed potential to deflect any particles towards the desired direction.
With a magnetic guidance system, the rotary effect can be obtained by applying an axial magnetic field: this forces the radially moving charges to deflect their trajectory in the plane of the focalization slit. This deflection stops (or at least reduces the particles reaching the circumferential target in that zone.
When standoff is present between the tool and the formation, gamma-rays must pass through mud/borehole fluid before reaching the formation, leading to gamma-ray absorption inside the bore hole. This absorption is a limitation for the measurement quality, as the number of photons transmitted to the formation is drastically reduced. This absorption depends on the hole size (caliper) which may not be constant over the length of the hole, as well as on the mud properties (in particular mud density and the presence of special absorbing (high density) materials such as barite).
For proper imaging with a central radioactive source, it is desirable to either provide compensation for bore-hole effects (absorption), or to modify the tool design to limit this bore-hole effect. The best performance may be obtained by combining both approaches.
One compensation scheme is based on a direct measurement of gamma-ray attenuation across the fluid in the bore-hole. This measurement, at least one detector is placed at a fixed distance (a few centimeters) from the gamma-ray source, so that the gamma-ray path from the source to the detector is mainly through the well-bore fluid. Using this measurement allows to determine the attenuation through the mud.
Full compensation requires the determination of the length of the attenuation path in the bore-hole. If the tool is well centralized, this path may be considered to be the same for all azimuths at this depth. In this case, a single hole size measurement for each depth (single diameter caliper) may be appropriate. For better imaging performance, a measure the source standoff versus azimuth can be used. This can be a direct measurement of the attenuation path for all directions. By taking care to ensure that the standoff is detected at the proper depth, proper estimation of the gamma-ray path for imaging purpose can be obtained. This standoff (or diameter) measurement can obtained by various methods, for example:
The tool design for use with a central source can be adapted to limit the attenuation effect within the borehole. One solution is to equip the tool with a mud excluder. In practical terms, this comprises a nearly cylindrical solid body around the source section of the tool to fill a substantial part of the bore-hole section with this body. This body is designed for low gamma-ray attenuation and is preferably made of light material:
This use of a mud excluder works well with imaging tools having the imaging detectors within the main body as is shown in
It should be noted that the mud excluder can have a ‘crown’ cross-section so that the bore-hole fluid can flow around the excluder as well through in the inside.
In any case, excluder cannot fill the whole wellbore: it cannot replace all bore-all fluids, as the fluid has to pass from one side of the tool to the side during tool displacement in the well. Therefore, attenuation correction is still required for proper imaging. Furthermore, the use of en excluder of this type means that the source is held at some distance from the formation. This effect reduces the radiation level reaching the formation within the volume of rock which influences the measurements.
A preferred form of imaging tool is shown in
For imaging purpose, multiple detectors are typically used to speed-up the global process, while ensuring sufficient azimuthal coverage. This general also concept applies for density imaging. The bank of detectors can be installed either in the tool body itself, or in pads which are applied against the formation (see above). The detectors can be, for example:
Where the detectors are in the main tool body (see for example
Another embodiment of a tool according to the invention includes rotating detectors. This may be particularly applicable when a rotating source is used. In one example, the tool contains a section with focused source and detectors. This whole section can be rotated, so that the tool is physically facing the whole well-bore within one rotation. The imaging process of this tool is similar to the process used by LWD density tools which provide a density image.
A number of factors affect imaging resolution including the tool design and the bore-hole effect:
Detector performances differ from detectors to detectors. The performances depend also on various external parameters varying with age and temperature. It is then critical to have a method to normalize these effects.
In conventional density tool using scintillation crystal and photo-multiplier, gain adjustment is performed by using direct emission of photons into the crystal from a stable micro source. Typically this micro source is installed directly in the vicinity of the crystal so that direct radiation affects the crystal with minimum scattering effect. This amplitude of the energy ray (which is the source energy level) in the energy spectrum allows adjustment the gain of the measurement chain: typically, the adjustment is performed by automatic adjustment of the high voltage of the photo-multiplier. A similar concept can be used in the imaging tool according to the invention. However, with one stabilization source per detector the total radiation energy will be high and this may become as source of noise for the imaging system.
Suitable gain stabilization for the imaging system according to the invention can be based on one of the following concepts:
In density logging, it is common to use two detector spacings to allow the cancellation of the mud cake. This is typically done by processing called “spine & ribs” using the density measured by the short spacing detector, as well as the difference of density between both detectors.
This can also apply for imaging purposes. One particular issue with the imaging process is the typical low gamma-ray count reaching the far detectors (as the imaging requires most of the coverage of the well-bore). Proper care needs to be applied for the far spacing detector processing. Multiple approaches are possible:
The imaging tool can deliver a log of formation density versus depth (average density). For this purpose, some azimuthal averaging is required. Since measurement corrections for standoff and mud cake are not linear. So for optimum accuracy, processing in accordance with the invention processes the density information for each azimuth first (to included all corrections). Then, the averaging of the azimuthal density is performed.
A simple solution to produce the image of formation density is to compute the variation of density from the near detector. This variation is the added with the average density.
Tools equipped with rotary source (either by mechanical rotation or field guidance with secondary emission), and equipped of detector bank permanently in acquisition may require a particular approach. For a conventional process of density measurement, the acquisition at the detectors should be synchronized with the emission: the detector and the source should be on the generatrix line of the hole. With a single point of emission, this makes detectors utilization low, as most of the time most detectors will be in an inappropriate position for acquisition. The utilization of the detectors can be improved by using rotating source with multiple emission points. It should be noted that four emission points at 90 degrees is particularly preferred. However, detectors on an azimuth between two source points may be affected by two nearest sources, making their direct use difficult for imaging.
For detectors of limited azimuthal offset from the source (and with source spacing large enough), the imaging path is inclined relative to the well-bore axis. This inclination can be beneficial for imaging dipping events:
This type of irradiation allows visualization of the same volume of formation several times (at least three times). This is similar to the acquisition process of modern surface seismic with multiple coverage (multiple offsets with 2D seismic). Specific seismic-type processing can then be used to reduce the image noise and even improve its resolution.
The noise from the imaging process could be directly be achieved by averaging the density for the same mid point.
The imaging tool can be equipped with detectors at multiple spacings. These detectors can typically be on the same azimuth in the pad, but at different distances (spacing) from the source. As discussed above, most conventional logging tools are equipped with two detectors at two different spacings. The tool according to the invention can have more than two detectors, allowing more measurements for each position of the tool in the well-bore. As the spacing is different for each of them, the measurement is affected by different parts of the formation: typically, the longer the spacing, the deeper is the measurement. The depth of a measurement is typically defined by the zone which influences by 50% the response of the detectors. Appropriate processing allows separation of the effect for each depth of formation.
The use of small detectors allows the combination of two techniques of imaging by having multiple rows of detectors covering both axial extent of the tool or pad and substantially all azimuths of the bore hole: this allows for provision of images for all well-bore azimuths as well as multiple depths of measurement inside the formation.
Density imaging is obtained via a complex back-scattering process. The photons reach the detectors via complex paths. Use of the concept of migration (such as used in seismic processing) allows the origin of all scattered energy to be accurately located. The purpose of this process is improved the resolution following depth and azimuth. The correlation process includes the effect of scattering as well as absorption to allow location of the dense material. The migration process can be performed either for azimuth only or for azimuth and depth.
Another technique to improve the resolution of the image is to verify geometrical consistency between all the measurements performed at the same location. This applies particularly well with tool equipped with a rotary head, so that each element is measured three times (axial and two opposed dipping propagations) for determination of the mud cake effect. The mud cake should have the same properties (attenuation effect or thickness) independently from the propagation direction. Again this type of processing is similar to processing applied in surface seismic, especially involving a point sensor/source concept.
Forward modeling of the formation can be done to verify if the measurement and its estimated image are correct. Various elements for modeling can be considered, including:
The purpose of the modeling is close the loop for measurement to image, as well as from formation proposition to model tool measurement and can improve the quality of the image.
The present invention finds particular use in cased-hole applications. One such application is density imaging of the annulus behind the casing. This can be used to evaluate cement quality issues, including:
This technique is complementary to acoustic imaging techniques:
An output of this technique can be to provide the proper correction for the log of “density behind casing”.
Another eased hole application is gravel pack evaluation. It is typically difficult to determine the proper placement of gravel in the annulus during screen packing. The density image provided by this invention can directly image it in the same way than the cement behind the casing. The metal correction has to be average out based on the type of cut and shape of screen.
A further application is the evaluation of the state of tubulars in the well, including assessing the presence of scale (type and quantity) in the production tubing and local damage to the tubing such as loss of thickness due to erosion or corrosion, cracks.
The invention also allows for inspection of a second tubing layer, for example the casing behind the tubing, or a larger string of casing hidden behind a smaller casing. For this application, the correction for the measurements should be similar to the correction for LWD density:
Other uses are also possible. The particular benefit provided by this invention is that it is capable of providing density data that can be represented as a two-dimensional image in a similar way to electrical or acoustic measurement leading to improved capability in evaluation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006132312 | Sep 2006 | RU | national |
