Solids-Free Loss-Control System Enables Efficient Coiled Tubing Workover Operations: Case Studies in Ukraine

Abstract This paper describes three coiled tubing (CT) applications in depleted reservoir wells, where full circulation and precise fluid placement were achievable only by using a novel solids-free loss-control system, such as abrasive perforating applications. It also describes the preparation work, such as laboratory results and mixing procedure performed to ensure successful implementation. The analysis of Ukrainian reservoir conditions by local and global engineering teams showed that in a highly depleted well, abrasive jetting through CT was the best option to efficiently perforate the wellbore. However, this approach could lead to later impairment of the gas production if the abrasive material (sand) could not be entirely recovered. Such a risk was even higher as wells were depleted and significant losses to the formation occurred. The use of solids-free fluid-loss material that was easy to mix, pump, and remove after the operation, was, therefore, critical to the success of that approach. In Ukraine, most of the brownfields have a reservoir pressure that varies between 50% and 20% of the original reservoir pressure. This is a challenge for CT operations in general and especially for abrasive jetting, which requires full circulation to remove solids. It also complicates intervention when precise fluid placement control is required, such as spotting cement to avoid its being lost into the formation. The perforation solids-free loss-control system is a highly crosslinked Hydroxy-Ethyl Cellulose (HEC) system designed for use after perforating when high-loss situations require a low-viscosity, nondamaging, bridging agent as is normally required in sand control applications. It is supplied as gel particles that are readily dispersed in most completion brines. The particles form a low-permeability filter cake that is pliable, conforms to the formation surface, and limits fluid loss. The system produces low friction pressures, which enable its placement using CT. Introduction of that system in Ukraine allowed the full circulation of sand or cuttings to surface without inducing significant damage to the formation for first time; it was also used for balanced cement plug placements. This project was the first application of the solids-free loss-control system in combination with CT operations. It previously was used only for loss control material during the well completion phase in sand formations with the use of drilling rigs.