Evaluation of Navigation Quality of Non-Survey Logging or MWD Tools andMethods of Navigation Sensor Correction and Reconstruction

摘要

The techniques used to process open- and cased-hole image logging and other oriented tools that are run in offshore explorationand development wells require highly accurate navigation logs. Navigation packages usually consist of triaxial accelerometers andmagnetometers that require calibration of the offset (bias), gain, and tool axis alignment for each sensor. Despite pre- or post-acquisition surface calibrations, the downhole environment will alter calibrations of offset and gain as a result of many factors,such as temperature and electrical noise. These factors are impractical to characterize or predict, and it is impossible to completelyisolate the navigation package from these factors. Experience shows that the offsets and gains of the sensors may changeappreciably over time scales much shorter than the total logging time. This paper provides four detailed examples of possible navigation logs with various problems to develop a methodology forevaluating their quality and proposing corrective action. The goals of this study are to determine which navigation sensors to adjustor which sensors are not responding to the appropriate Earth field and need reconstruction. The paper proposes several qualitymeasures for identifying, from the navigation log, drift in sensor response, including measured total field variations andcorrelations of tool deviation to tool rotation. When the methods described in this paper are used to align images with respect to a geographical reference (in cases where thenavigation was affected by sensor failure or external factors, e.g., permanently magnetized formation layers, that alter the sensorcalibrations), they significantly enhance the navigation quality. For example, image logs that originally wobbled erratically inferrous formations can be oriented to the north allowing dipping-bed orientation to be measured. In addition, magnetometerreconstruction and accelerometer correction can easily manage oriented navigation in metal casing. In general, the algorithmsdeveloped in this study assume that a rough surface calibration is available to obtain an initial solution, but time is no longerrequired for meticulous wellsite surface calibration.