In many wells it is advantageous, both economically and operationally to perform stimulation techniques using coiled tubing. More often than not this process will require that the zone of interest be isolated for the treatment to be effective. Several basic means of isolation are available depending on whether the application requires intervention through tubing or into a monobore, or "tubingless", completion. These basic tools require some form of pipe manipulation to set and retrieve, which in straight holes presents little difficulty, but in deviated wells becomes problematic. As the deviation from vertical increases and eventually reaches horizontal, tool manipulation becomes increasingly difficult and eventually impossible.
To address this problem in highly deviated wells a new generation of downhole straddle tools has been developed which requires no pipe manipulation to set and retrieve. These tools, called Fluid Velocity Set devices, use fluid pressure build up created when pumping through a nozzle to activate and relaxation of that pressure to deactivate a tool. Two distinct types of tool have been developed:
Inflatable straddle packers for through tubing applications, which can be inflated to seal in an I.D. up to 2.5 times larger than the running O.D.
Mechanical straddle packers for monobore applications, which have a running O.D. small enough to pass through standard tubing mounted accessories, such as landing nipples and sliding sleeves, and set in the tubing I.D.
This paper will discuss the advantages and disadvantages of commonly used isolation methods and will detail the design, development and testing of these new tools. Using recent field tests the authors will illustrate that this type of tool provides a functional and cost effective method of isolating zones in highly deviated and/or horizontal well sections.
In recent years, the oil industry has been utilizing more highly deviated and horizontal wells, in order to more adequately and economically produce formations. This has in turn presented the Service Industry with many new challenges, not the least of which is the operationally and cost effective stimulation of the resultant well sections. One of the most effective and economical methods employed is the use of coiled tubing, which can be run in a live well, and, with the proper tools, can be used to isolate and treat zones.
The original tools developed for zonal isolation were designed to meet the requirements of two distinct types of completion:
the monobore completion, in which the productive zones are in well sections which have the same I.D. as the production tubing.
The standard completion, in which the well is completed in casing and production is "thru-tubing" in nature.
These requirements resulted in the development of several distinct tool alternatives to suit the two requirements:
The opposed cup type tools (see fig. 1) required the same ID from the start of the tubing to the zone to be treated. The primary advantage of this type of tool was that it did not require manipulation of the coiled tubing to set and seal.However the cup type tool was also limited in the distance it could effectively travel in the tubing ID before resultant cup damage, due to the drag caused by the interference fit, resulted in impairment of its ability to seal. This situation was exaggerated by having to pass through tubing restrictions such as landing nipples and sliding sleeves and by the additional drag associated with highly deviated wells.