Downhole tool measurement device mounting system and method

A downhole tool measurement device mounting system and method are provided. The mounting system may include a chassis and a first sensor area and a second sensor area in the chassis capable of housing a first sensor and a second sensor that measure a characteristic along a first axis and a second axis, respectively. The system may also have a sensor assembly mounted in a cavity of the chassis in a third sensor area wherein a sensing axis of the sensor assembly is orthogonal to the first and second axes, the sensor assembly having a block and an orthogonal sensor mounted in the block to form a monolithic assembly that is secured into the cavity.

The disclosure relates generally to downhole tools and in particular to downhole survey sensors.

BACKGROUND

A downhole survey instrument typically consists of electronics and various sensors packaged within a chassis usually machined from a cylindrical piece of non-magnetic material. The downhole survey instrument is usually a long, slender, assembly, due to the form factor of drilling sensor apparatus in which it is used. These typical instruments also have features that retain and ruggedize the sensors and electronics due to the harsh downhole environment. The survey assemblies typically include an inclinometer which sometimes consists of three or more, typically orthogonally positioned, accelerometers. An accelerometer pair is usually mounted normal to the elongated chassis axis, and a third accelerometer, usually called the Z axis accelerometer, is typically coaxial to the instrument chassis long axis.

In most typical systems, the ‘Z’ axis accelerometer for a downhole survey sensor is installed at one end of the elongated chassis since the cross-section of the accelerometer consumes a large percentage of the available chassis cross section. Furthermore, the chassis material cross-section must provide the structural integrity needed to maintain the relative positions of the sensors in the assembly through handling and use in the harsh downhole environment.

The typical configuration consists of installing an accelerometer into a coaxial pocket machined into one end of the instrument chassis and then retaining it with a threaded member or members that clamp the accelerometer in place. The disadvantage of this scheme is that it 1) limits placement of the ‘Z’ axis accelerometer near one end of the assembly; 2) If the ‘Z’ axis accelerometer is not located at the end of the chassis, additional members may need to be attached which complicates the sensor; 3) it makes routing of conductors difficult as not much cross-section remains for thru-wireways in the chassis; and 4) it takes more time to replace accelerometer due to the complexity of the assembly so that, for example, end components would need to be removed/un-wired to access the accelerometer.

Another configuration simply uses an elastomeric compound to ‘glue’ the accelerometer in place. This has the disadvantage of poor stability (since the accelerometer may be able to move) and difficult replacement/rework since the “glue” must be removed.

Thus, it is desirable to provide a mount for downhole survey sensor that overcomes the limitations and drawbacks of typical systems and it is to this end that the disclosure is directed.

DETAILED DESCRIPTION OF ONE OR MORE EMBODIMENTS

The disclosure is particularly applicable to a downhole survey sensor with accelerometers and it is in this context that the disclosure will be described. It will be appreciated, however, that the mounting system and method has greater utility since it can be used to mount other types of sensors and may be used for a variety of different industries in addition to the drilling/boring examples described below.

In one implementation, an accelerometer may be mounted, but the mounting device and mounting method may be used for other sensors and the mounting of other sensors is within the scope of this disclosure. In one implementation, an accelerometer subassembly may be mounted into/onto features machined on/in a chassis body12as shown inFIGS. 1 and 2. However, using the mounting device and method, the accelerometer assembly may be installed along axes (the x axis and/or they axis) other than the typical z axis. In general terms, the accelerometer may be mounted into a ‘block’ that contains features for rigidly attaching the accelerometer. For example, the “block” may be a configuration in which a flange of the accelerometer is captured between a shoulder within the block and the shoulder of a threaded member or intermediate member that is captured by a threaded member. The block may be fastened to the chassis, such that the accelerometer has its sensing axis parallel to the chassis axis. The block and chassis may both have machined features, in this case shoulders/ledges, that, when the sensor is fully assembled, ensures a stable relative position between them and hence between other sensors and the Z accelerometer.

FIG. 1illustrates an example of an implementation of a mounting10for a survey sensor which is also shown in more detail inFIG. 2. As shown inFIGS. 1 and 2, an implementation of the mounting10may include the chassis12into which a sensor, such as an accelerometer, may be mounted. The chassis12of the sensor body may have one or more circuits or printed circuit boards14attached to the chassis body as shown adjacent to the sensors. In one example, the circuits/printed circuit boards14may be mounted on a side of the chassis that is the same side on which at least one of the sensors are installed. As shown inFIG. 1, a tool9into which the mount10is connected/attached/affixed/part of may be significantly larger than the mount and the mount10may be used with various different types of tools. In the example inFIGS. 1 and 2, the mount12may house a first sensor16and a second sensor18, such as an X axis accelerometer and a Y axis accelerometer when the two accelerometers are mounted on different sides of the chassis10as shown. As shown inFIG. 2, each of the sensors16,18may be mounted and secured into a cavity30,31in the chassis. The mount may be used for various downhole sensors. For example, as described below in more detail, the sensor may be a magnetometer or gyroscope.

In the example inFIGS. 1 and 2, the mount12also may house a third sensor assembly20, such as a Z axis accelerometer assembly, that mounts in and is secured in a third cavity32in the chassis12. The third accelerometer assembly20may further comprise a block22into which an accelerometer24is mounted. The third accelerometer assembly20may further comprise a retaining cap26that sits on top of the accelerometer24inside of the block22. The third accelerometer assembly20may further comprise a retainer member28, that may be threaded or may use another mechanism to secure the accelerometer in the block that releasably secures the accelerometer24and the retaining cap26into the block22to create a monolithic accelerometer assembly. Alternatively, the block22may be a configuration in which a flange of the accelerometer24is captured between a shoulder within the block (not shown) and a shoulder of a retainer member28of the retainer cap26that is captured by the retainer member28. The monolithic accelerometer assembly may then be inserted into the cavity32and secured or fastened into the cavity so that, for example, the accelerometer24has its sensing axis parallel to the chassis axis. The monolithic accelerometer assembly may be secured to/fastened to the cavity/chassis by various mechanisms. For example, the block22and chassis cavity32may both have machined features, such as shoulders/ledges30(one of which is shown inFIG. 4), that, when the sensor is fully assembled, ensures a stable relative position between them and hence between other sensors and the accelerometer24. Alternatively, as shown inFIGS. 1 and 3, the mount may have one or more securing mechanisms29, such as screws, that secure the block22to the chassis.

FIG. 3is a sectional side view of the mounting10for a survey sensor. In this example, the sensors16,18,20are mounted on sides of the chassis12so that acceleration in the X axis, acceleration in the Y axis and acceleration in the Z axis can be measured. It is important to note that the third sensor assembly20does not need to be coaxial to the chassis12or the other sensors16,18. In particular, the ability to measure each different axis signal is based on the sensor axes being orthogonal and the configuration shown inFIGS. 1-4satisfies this requirement while having the third sensor assembly20that is not coaxial to the chassis12or the other sensors16,18. The configuration shown inFIG. 1-4is possible due to an offset that allows the mount to utilize a typically sized mini accelerometer. The configuration allows for the close grouping of the sensors (the inclinometer or accelerometers) if desired. In addition, the configuration enables and permits the placement of the third sensor at any location along the length of the survey sensor while maintaining a rugged, rigid, one-piece, chassis. For example, locating the sensors closer to the magnetometer and having a more rigid structure between them is important to survey accuracy and stability.

Returning toFIG. 3, the figure also shows the securing mechanisms29passing through the passing through the chassis12and securing the third sensor assembly20to the chassis12.FIG. 4is a sectional end view of the mounting20for a survey sensor in which the securing mechanisms29passing through the chassis12can be seen.

The mount, as shown inFIGS. 3 and 4, has a passageway32underneath the sensor assembly20. The passageway may be used as a wireway for conductors that need to pass through this region of the assembly. Using this configuration, the sensor assembly20can be removed and re-installed without needing to remove these conductors.

In an alternative embodiment, the mount may mount a magnetometer sensor or a fluxgate magnetometer sensor. The magnetometer sensor or a fluxgate magnetometer sensor embodiment may also have an accelerometer or a group of accelerometers that are mounted on the downhole assembly. They magnetometer sensor or a fluxgate magnetometer sensor is an independent sensor commonly used in downhole assemblies. A magnetometer is a sensor system that measures a magnetic field. In a guidance system used in downhole assemblies, the common magnetic fields are from the earth's field or from other sources of magnetic fields such as interfering magnetic assemblies or adjacent wells with magnetic material in the well bores. Flux gate magnetometers are commonly used in downhole applications and all known downhole applications of flux gates use Permalloy material. Further details of magnetometers and flux gate magnetometers may be found in U.S. Pat. No. 6,972,563 which is incorporated herein by reference.

For a fluxgate magnetometer sensor, most modern fluxgate designs rely on a tape wound ring core approach which typically allows the combination of two orthogonal measurement axes on the same structure. The traditional material used is commonly known as Supermalloy, with a fairly high squareness BH loop index of some 0.75. Amorphous materials can offer improvements in squareness BH loop index to exceed 0.85 and much higher initial permeabilities, coupled with lower excitation current. All these qualities allow for a fluxgate structure which is smaller in size for equivalent sensitivity when compared with the traditional approach, plus lower drive current needs, which is highly beneficial since the typical measurement while drilling (MWD) string is battery operated. Amorphous material also exhibits lower magnetostriction effects, which produces better signal to noise ratios. The lower core losses verses temperature also allow the use of thicker ribbon, leading to a larger effective cross sectional area for a given OD/ID, which further improves the volumetric efficiency. An example of improved amorphous materials include Magnetic Metals DC annealed 9001.