First Field Implementation of the Short-Trip Module: A Coiled Tubing Fatigue Reduction System

Abstract

Prudhoe Bay Alaska is a mature oilfield where coiled tubing (CT) drilled sidetracks are the preferred method for gaining access to small reserves. For the CT drilling provider, and ultimately the client, the cost of the CT is a significant component of the overall operational cost and increasing the life of the working CT directly impacts operational economics. A primary contributor to CT fatigue is plastic deformation. This occurs when the CT is spooled on and off the reel and also when it passes through the injector head. Both these events subject the CT to bending stress and any attempt to reduce the number of such cycles will increase the CT life. Particular operations such as window milling, stuck coil, drilling in open hole require the CT to be moved short distances in and out of the well which increase the number of CT fatigue cycles. The Short-Trip Module (STM) has been developed to minimize fatigue peaks during these short movements. The system allows small (maximum 20 ft) CT movements in and out of the well without the need to turn the CT reel. This is done by keeping the CT reel stationary and letting the CT, between the reel and injector head, form a natural parabolic arch. Depending on the circulating pressure and CT movement schedule the fatigue can be reduced over 50% in CT fatigue life prediction model. This paper will detail the theory and design of the STM as well as the issues of its integration onto a CT drilling unit. Introduction A significant variable cost when operating a CT drilling unit is the cost of the CT itself. Any attempt to expand its life expectancy directly impacts the operational economics. A typical CT drilling unit operating in Prudhoe Bay used a total of nine strings of CT in the year 2000! The following is a list of CT drilling constraints that must be considered when endeavoring to change any component part of the system:Large diameter, thick walled CT is expensiveLarge diameter CT has a lower fatigue lifeGooseneck and reel radii are at the maximum limits and can not be increased because of logistical limitationsCT drilling requires continuous pumping while moving CT in or out of the hole.CT drilling operations require a large number of short trips when performing window milling and open hole drilling operationsTransportation, crane use and labor are costly The distribution of fatigue over the length of a CT string shows that it is localized to certain areas over the CT length. Areas exhibiting high fatigue are typically experienced at the CT reel and injector head during specific repetitive operations. Multiple short-trips during window milling and the drilling of hard formations translate to fatigue being caused by cycling the CT in and out of the well at or around a specific depth. This fatigue though is taking place at the CT reel and injector head area where the CT is being repeatedly subject to plastic deformation. In order to reduce these fatigue peak points and extend the usage life of whole pipe, an innovative system, the STM, has been developed and is in the process of being installed on a CT drilling unit in Prudhoe Bay.