Buried wellbore location from surface magnetic measurements

A method for locating a buried casing stub may comprise a) identifying a target region, b) providing at each of a plurality of survey points in the target region a casing stub locator that includes a vector magnetometer, c) measuring the magnetic field at each of the survey points using the vector magnetometer so as to generate a plurality of magnetic field measurements, d) using the magnetic field measurements to generate a model of the magnetic field of the target region, e) fitting the model generated in step d) to a selected model of a magnetic anomaly created by the casing stub so as to generate model fit information (MFI), and f) locating the casing stub using the MFI. At each survey point, an expected Earth magnetic field can be subtracted from the measured magnetic field. A total station can measure the position and/or the azimuth of the package.

TECHNICAL FIELD/FIELD OF THE DISCLOSURE

The present disclosure relates generally to wellsite operations and specifically to locating buried casing stubs.

BACKGROUND OF THE DISCLOSURE

When abandoning a well, wells are often plugged below ground level. Subsequently, the wellhead is cut off and the remaining casing stub is buried. In the event that intervention of the well is necessary, such as where a well is improperly plugged, the casing stub must be located so a proper abandonment can take place. However, records of the well may not include surface location data, requiring an operator to find the buried casing stub before attempting the intervention operation.

Typically, buried casing stubs are located using a scalar magnetometer on the surface to map the target region where the buried casing stub is located for magnetic anomalies. Mapping magnetometer readings to the location each is taken within the region may generate a “hot spot” map that indicates the distribution of magnetic perturbation across the target region. The area having the highest magnetic perturbation may generally indicate the location of the buried casing stub. However, any other magnetic material in the target region such as ferrous debris, well equipment, other wells, or other magnetic materials may also be measured during the mapping operation by the scalar magnetometer. Additionally, scalar magnetometer readings may not indicate the depth to which the target casing stub is buried.

SUMMARY

In some embodiments, a method for locating a buried casing stub may comprise a) identifying a target region for the buried casing stub; b) positioning a casing stub locator at a survey point in the target region, the casing stub locator including a vector magnetometer; c) measuring the magnetic field at the survey point with the vector magnetometer; d) moving the casing stub locator to a second survey point; e) measuring the magnetic field at the second survey point with the vector magnetometer; f) generating a model of the magnetic field of the target region using the magnetic field measurements from steps c) and e); g) fitting the model generated in step f) to a selected model of a magnetic anomaly created by the casing stub so as to generate model fit information; and h) locating the casing stub using the model fit information.

The method may further comprise defining a survey grid within the target region; moving the casing stub locator to each point of the survey grid; and measuring the magnetic field with the vector magnetometer at each point of the survey grid. The method may further comprise, for each measurement location: determining an azimuth relative to true north of the casing stub locator; using the azimuth and position of the casing stub locator to determine expected Earth magnetic field components of the measured magnetic field at that measurement location; and subtracting the Earth magnetic field components from the measured magnetic field at that measurement location. The method may further comprise positioning a total station at a location in the target region and measuring at least one of the position of the casing stub locator within the target region or the azimuth relative to true north of the casing stub locator with the total station. The method may further comprise determining the location of the total station in the target region using a positioning device. In some embodiments, the positioning device may be selected from the group consisting of differential GPS units, Global Navigation Satellite System units, or satellite navigation system receivers.

The azimuth relative to true north of the casing stub locator may be determined by visually aligning the casing stub locator to an external reference. The casing stub locator may further comprise one or more positioning devices adapted to locate the casing stub locating package within target region and determine the azimuth of casing stub locator and the positioning device may be a differential GPS unit. The casing stub locator may further comprise an accelerometer, and the method may further comprise determining the inclination of the casing stub locator with the accelerometer.

The method may further comprise identifying a ferrous object in proximity to the casing stub locator by comparing the measured magnetic field to an estimated magnetic field of the casing stub; and removing the ferrous object from the target region. The method may further comprise identifying a ferrous object in proximity to the casing stub locator by comparing the measured magnetic field to an estimated magnetic field of the casing stub; wrapping a wire around the ferrous object; connecting the wire to a current source; and demagnetizing the ferrous object by passing a current through the wire. The method may further comprise the steps of e1) measuring the magnetic field with the magnetometer in proximity to the demagnetized ferrous object and e2) including the measurement made in step e1) in the model generated in step f).

In some embodiments, a casing stub locating package for use in identifying a buried casing stub may comprise a tool body, a vector magnetometer, the vector magnetometer adapted to detect a magnetic anomaly created by the buried casing stub, an accelerometer, the accelerometer adapted to determine the inclination of the casing stub locating package, and a positioning device, the positioning device adapted to determine the position of the casing stub locator package in a target region.

DETAILED DESCRIPTION

FIG. 1depicts buried casing stub100, represented as an X located in target region10. Buried casing stub100may, for example and without limitation, be the upper end of a length of casing of a previously abandoned wellbore. Buried casing stub100may therefore be formed from a ferromagnetic material such as steel and may create a local magnetic anomaly in the magnetic field within target region10about buried casing stub100. Target region10may, in some embodiments, be identified as the general known location of buried casing stub100to be surveyed in order to locate buried casing stub100.

In some embodiments, casing stub locator101may be brought to target region10and may be used in an operation to locate buried casing stub100. As discussed further herein below, casing stub locator101may include one or more magnetometers adapted to measure the local magnetic field at the position in target region10at which casing stub locator101is located. Each such magnetic field measurement is referred to herein as a survey operation. Casing stub locator101may be repositionable within target region10. In some embodiments, casing stub locator101may be iteratively moved from position to position such that a survey is taken at each point of a grid defined within target region10, depicted inFIG. 1as survey grid12.

In some embodiments, a total station50or equivalent equipment may be positioned at a known location in target region10with a line of sight to casing stub locator101. Total station50may be used, for example and without limitation, to define survey grid12and to position casing stub locator101within target region10. In some embodiments, total station50may include one or more positioning devices including, for example and without limitation, a differential GPS unit, Global Navigation Satellite System unit, or satellite navigation system receiver to determine the precise location of total station50within target region10. In some embodiments, as discussed further below, total station50may be used to measure the azimuth17of the long axis of casing stub locator101relative to true north before a survey operation is undertaken. In some embodiments, for example, total station50may include a laser-reflector device to locate casing stub locator101that uses one or more laser beams51to determine the relative distance between total station50and selected portions of casing stub locator101, from which the azimuth17of casing stub locator101can be calculated.

In some cases, one or more magnetic or ferromagnetic objects may be located within target region10. For illustration's sake,FIG. 1depicts a movable ferrous object15and an unmovable ferrous object20located within target region10. As an example, movable ferrous objects15may include, for example and without limitation, metal waste such as buried trash or garbage. Unmovable ferrous objects20may include, for example and without limitation, metal culverts, existing wellheads or wells, fences, or other such structures. Ferrous objects may cause anomalous magnetic field perturbations that may, for example and without limitation, interfere with the ability of casing stub locator101to locate buried casing stub100when a survey is taken in proximity to the ferrous objects.

In some embodiments, as depicted inFIG. 2, casing stub locator101may include tool body103and stands105. Tool body103may house components of casing stub locator101. Stands105may, for example and without limitation, support tool body103of casing stub locator101as it is used in survey operations. Casing stub locator101may include a vector magnetometer107. Vector magnetometer107may measure the magnitude and direction of the magnetic field passing through casing stub locator101. In some embodiments, vector magnetometer107may be a three-axis magnetometer. In some embodiments, casing stub locator101may also include one or more accelerometers109. Accelerometers109may detect local acceleration due to gravity and may be used, for example and without limitation, to directly measure the inclination of casing stub locator101relative to a horizontal plane. In some embodiments, accelerometers109may be single or multi-axis accelerometers. In some embodiments, accelerometers109may include one or more three-axis accelerometers.

In some embodiments, casing stub locator101may include one or more positioning devices111positioned to accurately locate casing stub locator101in target region10. In some embodiments, positioning devices111may be positioned at each end of tool body103such that the azimuth17of casing stub locator101can be determined. In some embodiments, positioning devices111may each include, for example and without limitation, one or more differential GPS units, Global Navigation Satellite System units, or satellite navigation system receivers. In such an embodiment, the azimuth17and position of casing stub locator101may be determined without need for other tools. Additionally or alternatively, positioning devices111may include targets or reflectors for use with a laser-reflector device on total station50as described herein above.

In some embodiments, casing stub locator101may be a measurement-while-drilling (MWD) tool supported by stands105.

In operation, with reference toFIG. 3, when an intervention into a wellbore associated with buried casing stub100is desired, target region10may first be identified (200). Target region10may be identified using, for example and without limitation, historical drilling data associated with the wellbore associated with buried casing stub100. Survey grid12of target region10may then be defined (210). In some embodiments, survey grid12may be defined using total station50. In some such embodiments, total station50may be located within target region10(211) using, for example and without limitation, a differential GPS unit, Global Navigation Satellite System unit, or satellite navigation system receiver.

Casing stub locator101may then be positioned within target region10at a first survey point on survey grid12(220). In some embodiments, the location of casing stub locator101may be determined using total station50. In some such embodiments, the azimuth17of casing stub locator101may be determined using total station50(221). In some embodiments, the inclination of casing stub locator101may be measured using accelerometers109(222). Casing stub locator101may then take a survey by measuring the local magnetic field at the survey point using vector magnetometer107(230). In some embodiments, casing stub locator101may then be moved to a different survey point on survey grid12(240). These operations may be repeated until a desired number of surveys are taken corresponding to survey grid12. It will be understood that, whileFIG. 4depicts survey points arranged in a line for purposes of illustration, the survey points do not need to be arranged in a line and may be arrayed in a grid or any other desired configuration. It will further be understood that, rather than moving equipment to each desired survey point in sequence, surveys at two or more survey points may be taken simultaneously using multiple surveying devices, or surveys may be taken intermittently or continuously as the surveying device(s) is (are) moved continuously.

In some embodiments, during the surveying process, the magnetometer data collected by casing stub locator101may be affected by identify ferrous objects (250) other than buried casing stub100. For example, as depicted inFIG. 4, a ferrous object, depicted as movable ferrous object15, may be positioned on or under the surface in a position such that the magnetic perturbations Ba of movable ferrous object15may interfere with the detection of the magnetic anomaly B created by buried casing stub100. Similarly, in some embodiments, the magnetic anomaly created by buried casing stub100may propagate as if buried casings tub100is a magnetic monopole or magnetic dipole as shown inFIG. 4. By reviewing the magnetometer data collected by casing stub locator101collected at multiple locations with the survey grid12and comparing each set of collected magnetometer data to the expected model, the perturbations in the magnetic field attributable to the movable ferrous object15may be identified. For example, as depicted inFIG. 4A, the vector magnetometer readings13a-f(shown as 2-dimensional vectors) taken at survey points12a-f(shown inFIG. 4) do not correspond with the expected vector magnetometer readings for a magnetic dipole.

In such an instance, movable ferrous object15may be identified and, if possible, removed from target region10(251) as shown inFIG. 5. In embodiments in which movable ferrous object15can be removed, subsequent surveys taken at survey points12a-fmay result in magnetometer readings13a-fcollected by casing stub locator101that correspond more closely with those expected of a magnetic dipole as shown inFIG. 5A.

In the case that the interference is caused by an object that cannot be moved, such as, for example and without limitation, where the object is unmovable ferrous object20, the object may be demagnetized (252). In such a demagnetizing operation, as shown inFIG. 6, a wire61may be wrapped a number of times about unmovable ferrous object20, here depicted as a steel wellhead. Current may be supplied to wire61by power source63. In some embodiments, the current may be supplied at various frequencies and current levels at predetermined intervals for predetermined periods of time such that, without being bound to theory, the magnetic domains of the ferrous material are rearranged, resulting in a net lower magnetic field of unmovable ferrous object20.

The magnetic field from unmovable ferrous object20can be mapped with the magnetometer/accelerometer package and referenced with the total station. This allows the magnetic field associated with unmovable ferrous object20to be characterized in 3D space. The magnetic field from unmovable ferrous objects20may then be mapped (253) using additional surveys taken by casing stub locator101such that the magnetic field of unmovable ferrous objects20may be characterized and taken into consideration when locating buried casing stub100. In the case that additional ferrous objects are identified, such movable ferrous objects15or unmovable ferrous objects20may each in turn be handled as described above.

The grid collection of data from the 3 axis magnetometer can then be repeated over the expected buried casing stub of interest. The grid can change in latitude and departure and be repeated at different elevations to develop a “cube” of data.

During or after this surveying process, the magnetometer data collected by casing stub locator101may be used to generate a model of the magnetic field of target region10(260). The model may include the expected magnetic anomaly created by buried casing stub100as well as, for example and without limitation, the Earth magnetic field, crustal anomalies, and other identified magnetic sources as discussed above. In some embodiments, additional processing may be undertaken on the magnetic field data (270).

For example, in some embodiments, the knowledge of the position of casing stub locator101at each survey point as well as the inclination and azimuth thereof may allow the expected Earth magnetic field components of the measured magnetic field to be determined based on the expected magnetic field at the position, inclination, and azimuth of casing stub locator101, and therefore may be subtracted from the magnetic field data. The model of the magnetic anomaly—depicted two-dimensionally from the top inFIG. 7as model301—created by buried casing stub100may then be generated and may be used to locate buried casing stub100(280). In some such embodiments, the model of the magnetic field of target region10may be fit to an expected model of the magnetic anomaly created by buried casing stub100, such as a magnetic monopole or magnetic dipole. In some embodiments, because casing stub locator101uses vector magnetometer107, both the position and the depth of buried casing stub100may be determined.

In some embodiments, a model of buried casing stub100may be generated and fit using scalar magnetometer readings.

In some embodiments, buried casing stub100may be located without the use of total station50. In such an embodiment, a fit based on horizontal and vertical magnetometer readings alone may be used to identify a difference between the observed magnetic fields and the expected background reference model. In other such embodiments, casing stub locator101may be aligned visually to an external reference to provide a full three-dimensional model, though error may be introduced due to inaccuracy of the positioning of casing stub locator101.

The foregoing outlines features of several embodiments so that a person of ordinary skill in the art may better understand the aspects of the present disclosure. Such features may be replaced by any one of numerous equivalent alternatives, only some of which are disclosed herein. One of ordinary skill in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. One of ordinary skill in the art should also realize that such equivalent constructions do not depart from the scope of the present disclosure and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. Likewise, unless explicitly so indicated, the sequential recitation of steps in the claims that follow is not intended to be a requirement that the steps be performed sequentially.