Annular fracturing of multiple zones with coiled tubing (CT) has been practiced for more than a decade. A typical treatment involves sand jetting perforating, cleaning the well, fracturing and isolation. It is economically desirable to perforate, fracture and isolate each interval quickly, allowing all intervals to be treated in the shortest time possible. However, CT lying on the bottom of horizontal wells creates a solids bed during the perforation and fracturing stage, this introduces additional complexities into the reliable execution of the fracturing process. For tight formations, proppant beds have to be completely or partially removed, otherwise the pad can entrain the proppant which creates a high potential for premature screen out. In this paper, a new approach to cleaning the hole between multiple annular fractures is proposed. This hole cleaning process involves circulating the clean fluid through the annulus to the bottom of the wellbore and bringing the sands back to the surface through the CT; at the same time the CT pulls out of the hole (POOH). The sand particles at the top section of the wellbore are swept toward to the end of CT, while the solids near the end of the bottom hole assembly (BHA) are sucked into the CT as it is pulled out of hole. Eventually, all solids are removed with the CT partially pulled through the initial bed section. As a wiper trip to surface is not required, the efficiency of this new process is higher than other cleaning processes. A full scale flow loop was used to simulate the process. Maximum POOH speed and the hole cleaning time were determined under various conditions. Empirical correlations were developed to allow for optimization of the process under the down hole conditions.

In the past four decades, several computer programs have been developed to predict steady-state fluid flow in coiled tubing (CT) applications. They are useful in designing the ultimate treatment phase of an intervention but questions remain concerning how their results are affected by the transient nature of the overall job. Any change in the operating conditions on surface or down hole will introduce a transient response. Common treatments such as liquid unloading, under balanced drilling with nitrified fluid, well control, cementing and hole cleaning may never reach a steady state. Only a transient simulation can determine whether a steady state is achieved. If it is not, the simulation can be used by job programmers to develop procedures that will deal with transient phenomena. This paper describes a transient multi-phase flow simulation that has been developed based on the correlations used to describe steady-state multi-phase flow. The software was extensively validated against data collected on a full scale flow loop, field operations, and other transient software. Several field cases are used to illustrate the transient nature of CT applications and the benefits of using the transient simulation for the operational design, training and job execution.

During the last three decades a coiled tubing (CT) modeling software package has been continuously developed to assist in the planning and executing of global CT operations. The first models were steady-state. In the past decade these models have been extended to consider transient effects of operations as they are executed. These models will also be used in the not-so-distant future to automate CT operations. In this paper, a review of all these models is presented for the first time. The development of a computer program was initiated in the early 1980s to help understand the downhole flow and pressure conditions during CT operations. Utilizing multi-phase rheological and frictional correlations obtained from laboratory flow loop testing, the CT flow model was extensively validated against global field data. Later, CT force and stress analysis models, considering such effects as the specific well geometry, mechanical friction, CT size, shape and material strength, were developed to predict lateral reach and assist in preventing downhole CT failures. While the most common and simplest approach within the industry is still to use steady-state models, in practice, the downhole conditions during CT operations, such as well cleaning, well unloading, well control, stimulation, cementing, underbalanced drilling with nitrified fluid, etc., are transient. Consequently, the steady-state models have been extended to account for downhole transient effects at the pre-planning and execution stages of CT operations. In addition, with the advent of the state-of-the-art CT telemetry systems, it is possible to acquire the downhole data in real time and use the transient CT software model to automate and optimize CT operations, increasing their safety and efficiency. A review is presented for the first time about the steady-state and transient models included in the CT software model, with details about each model and how they performed during 30 years of operations. Results and discussions regarding the extensive validation of the software against laboratory and field data are also presented. Several field cases from around the world help illustrate the transient nature of CT operations and the benefits of using the transient simulation in the pre-planning and execution stages of these operations. The paper presents the results from 30 years of global experience with the CT modeling software program. The mathematical models, validation against laboratory and field data, verification against other models available in literature, and case histories are described. The current trends within the industry are leading to a shortage of experienced CT field engineers, so the use of CT software models to increase the efficiency, compliance and safety of CT operations has never been as important as now.