With many horizontal open hole completions beyond the reach of coiled tubing, the need arises for a reliable conveyance system to access the wells. Several types of tractors are available for cased-hole well completions. These tractors can be run in specific hard formation open hole horizontal wells, however all types failed in accessing openhole wells in softer formation.
Open-hole completions in carbonate formations have been quite challenging for tractors. Initially designed for cased-hole applications the major technical challenge is gripping in a soft formation with sufficient radial force to provide a required tractoring force whilst not slipping. The major gripping/slipping issues have been faced in formations with an Unconfined Compressive Strength (UCS) of less than 5000 PSI.
A new innovative design has been introduced that addresses the needs for logging these openhole horizontal wells in soft formations. Several phases took place in the development; such as design, laboratory testing, field testing, evaluation, upgrades and modifications.
The latest design was first tested in hard formations prior to the unique challenge of logging horizontal open hole wells in relatively soft formations. Fields where the new design was tested were considered a real challenge for all logging conveyance methods. Successful operational results have been achieved for the first time worldwide.
This paper describes the whole process history from challenge to success, giving clear steps towards successful logging of openhole soft formation horizontal wells. Discussed are the evaluation that led to the new tool design, tool design overview, laboratory and field tests, selection criteria of the candidate wells, review and screening of the well candidates and operational results.
In December 2002, a successful testing of a cased-hole reciprocating tractor was performed in openhole hard formation horizontal wells in Saudi Arabia. The engineering design of this tractor was based on cams that grip the inner wall of the casing or openhole formation.
The same design failed in a soft formation test due to the low Unconfined Compressive Strength (UCS) of the formation. The low strength allowed the cams to dig into the formation and lose the gripping force. After several tests, it was decided that any formation with UCS below 5000 psi would be considered as soft formation for this purpose, Figure 1 shows the UCS of the two different types of formations. The same tool design also faced problems in nonuniform shape of openhole boreholes. A complete redesign of the Tractor concept was necessary.