The Economic Feasibility of Coiled Tubing Drilling Compared to Conventional Drilling in Mature Fields: An Integrated Decision Analysis
Authors: Alsharea Maher 
Published: September 2024

Abstract
As energy demand increases, oil and gas operators are drilling into more geologically challenging and economically marginal remaining reserves in depleted mature fields. CTD provides cost savings compared to conventional drilling and accessibility to these challenging reserves when using controlled pressure or underbalanced drilling. However, intrinsic nonproductive time, the risk of wellbore collapse, and pipe sticking significantly affect the economic viability. This study assesses the economic feasibility of CTD and offers a straightforward decision framework.
The decision analysis approach is used to evaluate the economic viability of CTD versus traditional drilling, and the Quantitative Risk Analysis (QRA) is utilized to measure the probabilities of collapse, overbalance, and unrecoverable stuck pipe. Cost savings and the value of reducing formation damage when drilling underbalanced are quantified through a thorough review of case histories. The risks of collapse, overbalance, and unrecoverable pipe sticking are calculated probabilistically. Uncertainties in the inputs are quantified, and the probabilities are computed using Monte Carlo simulation. The probabilities are then used for decision-making. The proposed approach is implemented in a case study.
Despite the uncertain Non-Productive Time (NPT) due to inherent drilling problems, Coiled Tubing Drilling (CTD) shows an average cost saving of 37% compared to conventional drilling, within a range of 22% to 48%. This saving is achieved at an average total depth of 11,700 ft, ranging from 2,270 ft to 17,500 ft, and an average lateral length of 2,976 ft, ranging from 576 ft to 5,237 ft, based on 507 onshore re-entries. In addition to cost savings, this paper shows that Underbalanced Coiled Tubing Drilling (UBCTD) improves production rate and recovery. Accessing depleted reservoirs through UBD adds an average recovery of 34.6%, and eliminating formation damage improves ultimate recovery by an average of 17% compared to overbalanced drilling, which ranges from 3% to 50%. This increase results from accessing previously inaccessible depleted reservoirs and identifying overlooked productive zones. However, there is a risk of wellbore collapse when drilling underbalanced and severe formation damage if underbalanced conditions are not maintained. This paper demonstrates that the economic feasibility of CTD in a mature field depends on factors including ultimate recovery, oil price, drilling costs, and the probability of reaching the target. Underestimating one of these factors significantly affects the economic feasibility.
The proposed decision framework enables oil and gas practitioners to make good decisions when using CTD as an exclusive option or comparing it with other drilling methods. This decision model simplifies the complex decision-making process in drilling operations into a systematic and straightforward process. We also provide a comprehensive range of value drivers to help practitioners better understand uncertainties, which is essential for decision-making.