Traditional measurement/logging while drilling (M/LWD) surveying starts with the measurement of the inclination and azimuth (essentially a vector direction or unit-vector) at a discrete number of locations in the well bore, usually parameterized by the measured depth. Typically, the inclination is determined from a three-axis accelerometer and the azimuth from a three-axis magnetometer. Care must be taken that the magnetometer be protected from magnetic interference. Such interference may come from improper BHA design, from proximity to casing, from the current well or nearby wells, or from magnetically active formations. In these cases the magnetometer does not provide valid readings. The traditional solution in such cases is to use a gyroscope. But a new tool, the Gravity MWD tool, is an alternative in these situations.

The tool consists of two rigidly connected three-axis accelerometers embedded in a bottomhole assembly (BHA). The three-axis accelerometer measurements at each of the two locations are made simultaneously. The tool measurements are dependent on standard, robust MWD technology that is immune to magnetic interference.

Figure 1 shows a typical BHA including the tool. Two standard directional packages, each consisting of a three-axis accelerometer and a three-axis magnetometer, are placed in a BHA string. The primary requirement is that the drill string connecting the two is rigid to relative rotation. For example, a motor may not be located between them because the motor bit box would allow relative rotation between the two directional packages. The lower directional package is thus usually positioned directly on top of the motor. The magnetometer at the lower location will never be used, so non-magnetic collars are not required.

The second package is located in the standard directional package position and is typically located 30 ft (9 m) above the first directional package. Therefore, in most cases, it may be used in standard survey procedures, switching to use Gravity MWD only when required. In this configuration Gravity MWD is always available when needed and has little impact on the BHA configuration.

The change in azimuth between the two directional packages may be determined from the accelerometer measurements only. The accelerometer from each directional package provides the information needed to calculate inclination and the tool's rotational orientation.

Using the two accelerometers, users can measure three tool quantities that, in fact, are the same as the hole quantities - the inclination at each location and the difference in toolface. Under the assumption that the well path between the two accelerometers may be accurately described as being minimum curvature, it turns out that these measurements will uniquely determine the azimuth change between the two locations. Figure 2 shows the contours (in red) of change in azimuth for the case where the inclination change between the two accelerometers is 1° for a range of inclinations in the upper accelerometer ranging from 5° to 85° and a range of holeface change for ±180°. The figure is also used to directly address the issue of sensitivity of the azimuth change measurement to the holeface change over a wide range of inclinations. Contours of the sensitivity, defined as sensitivity equaling the change in holeface divided by the change in azimuth, are displayed. Note that the contour spacing changes for values greater than three. Over a wide range of near vertical inclinations, the sensitivity to inclination is minor and the change in azimuth is approximately equal to the change in holeface.

Application of this method requires a "Tie-in Azimuth" as the azimuth change and not absolute azimuth. The quality of the original survey (or tie-in data) is an important factor that can produce azimuth errors that are carried through to each additional survey. An error model has been developed to determine the quality of the tie-in data allowing the tool to absorb the errors and complete the planned well. The error model is used when planning the well and running anti-collision surveys for the well plan. As the reference survey represents the start point for subsequent surveys, a reasonable level of confidence must be gained in the reference surveys before executing the method.

The azimuth measurement technique uses the results of a previous survey as a reference for the next. The consequence of this is that errors may accumulate over many survey points. Although this is only thought to be a significant problem in long sections of well, a practical limit of 1,000 ft (305 m) of survey section is in current use.

Inclination accuracies are the same as those made using standard MWD survey methods. But in this case, the inclinations from each accelerometer are independently determined. As the surveying methodology often calls for occupying the same well location with each of the accelerometers, the presence of independent measurements of the inclination increases confidence in the result.

Once free of magnetic interference, the hole is usually at a higher inclination. At this point the MWD tool lies more solidly against one side of the borehole wall, giving a more accurate representation of the hole's direction and inclination. At this point it is standard practice to re-reference the Gravity MWD displacement points to the magnetic (vector measurement) survey. The azimuth becomes a tie-in for a reverse azimuth calculation, resulting in a fresh determination of the initial azimuth and a more accurate determination of the referenced tie-in location than previously possible.

Conclusions

Gravity MWD is a technique for evaluating azimuth variation using standard accelerometers in regimes where magnetometers are not useful. A simplified discussion of the mathematical basis for this technique reviews the strengths, flexibility, limitations and errors inherent in this procedure. Finally, by understanding the practical limitations of executing this surveying method and developing procedures to mitigate systematic errors such as reference errors, sag errors and tie-in errors, a new method of surveying through zones of magnetic interference has been developed and proven. The procedure has been applied to more than 50 wells in more than five different countries around the world, providing a reliable method of positioning a well bore in the vicinity of casing where timely information on the near-bit behavior of a BHA is required.

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