An Effective Scale Inhibitor and a Tailored Acid Cleanout Successfully Maintains Well Productivity After Severe Scale Formation in a Mature Gas Producing Field: Case History in a Sub-Andean South Field

Abstract This paper presents a successful study case of restoration and prolongation of a naturally fractured reservoir gas well deliverability in a Sub-Andean South field in Bolivia, using a suited chemical treatment applied with Coiled Tubing. In addition, with an integrated scale model the optimum operational production condition was determined. The project outlines the methodology followed, execution and results. Tubing restriction by scale in the latest drilled well "W#8" was confirmed for the first time in the field after 15 production years. To identify the scale, coiled tubing (CT) with a junk basket was run in hole (RIH) to bring a bottomhole sample to the surface for analysis. In an in-situ mobile laboratory, the scale samples were summited to an organic acid (13% acetic/9% formic) blend to compare the dissolving capacity with HCl acid. Approximately 90% dissolution of the scale sample was achieved using the organic acid mixture. Due to HCl sensitive formation mineralogy, reservoir temperature (+280°F) and to minimize corrosion exposure to the Super 13 Chrome completion, organic acid formulation was selected. CT was then RIH with a scale removal rotating jet to clean out the well, circulating and reciprocating through the affected area, and opened flow to surface to maintain a flowing gas column. Well gas productivity recovered but only for a 3-month period. Afterwards, a second organic acid pickling was repeated, but production recovered for less than 2 months. A technical study was conducted to identify well conditions and to select the appropriate treatment for scale removal and inhibition in following CT interventions, aiming to reestablish and maintain gas deliverability. Complementary, a scale assessment permitted to identify the origin and tendency of the scale and to determine the optimum production condition. Then, for the next performed intervention the pickling acid was repeated, the middle zone was isolated, a near wellbore acid in the upper zone was performed and a polyacrylate scale inhibitor was pumped. As a result, the well not only exhibited an immediate deliverability recovery to 75 Mscfd but also kept producing at a stable recommended rate of 66 Mscfd for more than 12 months. This paper offers tailored methodologies and solutions to existing well conditions that resulted in a well remediation that increased the time between clean-up interventions to +300%. These customized solutions can be applied in similar or neighboring mature fields as corrective and preventive techniques.