Gyro Tool Centralization or Decentralization within the cased borehole remains a fundamental and important aspect of all gyro surveys performed. This applies equally to the limited number of surveys still carried out using older technologies as well as those performed with the latest systems inclusive of full inertial applications.
As noted above, earlier forms of Spring Bow (Weatherford Type) centralizers could contribute significantly to errors in true borehole axis representation. Early attempts at offset calculation and correction did provide a partial solution but this improvement was also dependent on the centralizer integrity with respect to uniform vane wear and varying borehole inclination. As now, the main dilemma was to use centralization which adequately supported the tool in the central axis whilst allowing the system to smoothly progress down hole, keeping in mind the loss of effective mass (α Cos incl.) in the borehole axis to aid transport and the simultaneous increasing mass supported by the centralizers.
Ultimately, the most effective and practical solution was to run full centralization until the tool could be guaranteed to run low-side. However, dependent on the casing/survey program deployed, this procedure could require a minimum of two separate runs in hole in which the second run was performed with very stiff (rigid) under gage centralization which basically operated as skids and supported the tool at a fixed constant distance from the contact low-side and hence parallel to the borehole axis.
As wellbore inclination increased, various forms of Sinker Bar were used to aid tool transport but these too could generate off-axis problems where the Sinker Bar is screwed directly to the survey tool but not adequately supported (centralized), with a tendency to bias the overall tool alignment. Ideally, Sinker Bar should be attached to the survey tool using a universal joint or more preferably a connecting rod with universal joints at each end. This hook-up predominantly isolates any off-axis interaction between tool and weight bar.
These basic criteria still hold true today where the use of Centrollers (Precision Wheeled Centralizers) and Roller Bearing De-centralizers are used with North Seeking and full Inertial Gyro Tools in cased boreholes up to ≈ 70° inclination.
Ideally the use of De-centralizers should be avoided where possible, in large surface casings near vertical as tool alignment can be disturbed by the effects of off-axis cable tension. Similar problems can exist in the early build or high dogleg sections within smaller casings.
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