Today, one of the most important key for hydrocarbon production in the oil & gas industry is economic justification. In the later state of the well’s production, the aging wells may introduce many problems such as scale, sand and liquid load-up. These impede the hydrocarbon production, preventing all the reserve to be fully produced. In order to fight this impedance, the use of small size coiled tubing is a promising practice, although has its own limit. The objective of this paper is to evaluate the limit of well services applications of small size Coiled Tubing (1" and 1.25") in various oil & gas well scenarios. The evaluation utilizes the computer modeling which is used to determine the hydraulic and mechanic operating conditions. Our study found the viability of 1” CT in the low inclination well scenarios with the lower pumping rate requirement, while 1.25” CT can be used in all of well services applications and scenarios.