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.