The use of coiled-tubing systems has proven practical and cost effective for many well intervention procedures. With the development of better quality, longer, and larger-sized coiled-tubing systems, the variety of processes available for using coiled-tubing systems continues to rise rapidly. One application is the removal of heavy solids through a deviated wellbore section, which causes immense difficulties to the operator. Even when the particulate drop rate through the carrier fluid is slow, this velocity is increased by up to ten times in deviated wellbore sections; which can cause failure, and even worse, sticking of tubing. A method that is successful in counteracting this phenomenon is discussed in this paper. The need for increased flow rates, meaning the use of larger coiled-tubing systems, is not necessary for this method; thus allowing effective cleanup through smaller production tubing.
Since the first use of coiled tubing in 1963, its usehas proliferated into many sectors of the oilfield services industry. These sectors include pipe cleaning, perforating, placement of various tools, logging, fishing, chemical placement or matrix treatment, and drilling, just to mention a few.[1–5] In most cases, coiled tubing attracts users who want rapid deployment without the use of drilling rigs, or better yet, to go through the small production tubing in their wells. Additionally, the inherent safety backup feature of annular blowout preventers (BOPs) included with coiled-tubing systems makes the use of these systems increasingly popular.
In the area of well cleaning, the use of coiled tubing quickly caught on because removal of production tubulars was not required. Removal of scale and deposits from tubulars became commonplace, and coiled-tubing tools that perform such services have continually been improved by the invention of various rotary-blast and jetting tools. Unfortunately, transporting sand, fill, and removed scale out of the well involves a totally different technology. Fluid-transport technology involves the capability of the fluid to carry the sand, fill, or cuttings upwards out of the hole. As realized by many, transporting material in a horizontal or vertical conduit generally does not pose a significant problem. It is, however, a very serious situation when transitioning from the horizontal into the vertical. Although many have claimed that this situation can be alleviated using the correct fluid systems, the operator may eventually have to face the harsh reality that high fluid velocities or very high-viscosity fluids must be used. As discussed later in this paper, an understanding of the phenomenenon and a specialized tool with rotational angle are required to resolve this problem.
This paper also discusses two issues that have recently been introduced into the realm of coiled-tubing systems. Use in fracture-stimulation technology is a logical development. The other area being approached is wellbore sweeping, which is often related to fracture stimulation as after-the-job cleanup service. Although wellbore cleaning has been a common use of coiled tubing, its transport mechanics have often been misconstrued and therefore are also discussed in this paper.