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9.5.4 Tool Centralization 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 h loss of ff c m ss (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-c n l z s us d w h o h kng ndfull n lGyo ools nc s dbo hol sup o≈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.
9.6 ERD and Horizontal Transit
With the prevalence of ERD and Horizontal Well Profiles, gyro surveying became even more difficult to perform by conventional wireline operations. Various techniques were adopted with improvements made relative to technology advances in both assisted tool transport and survey tool development. Side-Entry Sub:– The Side-Entry Sub was first used with the Magnetic W/L Steering Tool Systems (Pre MWD) in order that drilling could proceed during deeper drilling operations without the requirement to trip out the W/L – Steering Tool from the drill pipe to make a connection (Add Pipe). Using this system enabled W/L Gyro Surveys to be performed in extended reach horizontal wells, with the survey tool latched within the BHA and surveys recorded at each planned depth interval. The Side Entry Sub consisted of a sub with a side wall orifice with associated clamp and stuffing gland arrangement which allowed the cable to pass through the sub wall with the clamp and gland seal applied as required. The BHA/drill pipe would be tripped in hole to the high angle survey start depth. The W/L and survey tool would then be run to bottom within the pipe entering via the side wall sub connected to the drill pipe at surface. The survey would then commence by tripping in hole to TD with pipe added as normal. The assembly was then tripped out (with outrun data also recorded) to retrieve the Sub at surface with the W/L Tool then pulled out of hole. The system was subsequently applied to W/L FE logging prior to equivalent MWD sensor development.
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