Novel Coiled Tubing Post-Fracturing Cleanout of Unconventional Wells Without Downhole Motor: A Case Study from Argentina

Abstract

Unconventional wells that are completed with plug-and-perf fracturing techniques require coiled tubing (CT) to mill the plugs. In Argentina, CT must mill upwards of 60 plugs to bring the well into production. The CT downhole motors are leading contributors to service quality incidents because: 1) they mill challenging materials over extended periods of time, and 2) they limit the pump rate, which reduces cleanout effectiveness and increases the risk of stuck pipe. A completion campaign selected soluble plugs instead of composite ones to improve efficiency. Initial CT runs were still completed with a milling bottomhole assembly (BHA) to address the risk of low plug solubility. However, as evidenced during the initial campaign runs, the plugs dissolved nearly completely, leaving only the steel buttons inside the well. Those buttons required a high pump rate to carry them to surface. Given the excellent plug dissolution and the fluid velocity requirement to clean out buttons, the BHA was simplified to an extended reach tool (ERT) and a customized, high-rate jetting nozzle, thus eliminating the downhole motor. The new and simplified BHA extended the pump rate limit from an average of 3.8 to 4.5 bbl/min with no adverse effect on circulating pressure. The operator observed a tenfold increase in the number of plug buttons recovered at surface, which reduced the risk of stuck CT pipe during interventions in those wells. The cleanout speed was also increased, reducing total operating time by 25%, costs, and diesel consumption. A special procedure was developed for the cases when the BHA encountered consolidated sand and plug debris during the operations, a situation that occurred an average of three times per well. To clean those accumulations, the pump rate was increased to generate high jetting energy and stronger ERT agitation, and the CT was run in hole at slow speed until tagging the solids. This procedure allowed passing all the restrictions found during the execution of the CT cleanout jobs. This approach also made it possible to clean wells with known casing deformations, a common situation in Argentina unconventional wells. Over the course of 44 wells, 13 were cleaned with the simplified BHA. This new approach addressed the two main sources of nonproductive time before initiation of the project: BHA failures decreased from 16% to 8% and no CT stuck pipe event occurred. This is the first published case study of a post-fracturing cleanout in an unconventional well without the use of a downhole motor and which, instead, combined soluble plugs and a tailored BHA. Eliminating the downhole motor in this application significantly improved operational reliability and efficiency. The use of this simplified BHA to clean the well down to total depth is only possible with a good dissolution performance of the plugs present in the well.