Saudi Aramco has long faced significant challenges to remove scales from the wells due to the high H2S contents and the sub hydrostatics reservoir pressure. Conventional techniques often fell short of expectations in the past, there had been cases of uncontrolled H2S releases at surface, which caused major HSE issues to the personnel; also coiled tubing (CT) got stuck due to sudden loss of circulation stemming from the inability to control the bottom-hole pressure and the instability of the fluid system during the scale removal treatment. As a result of those repeated issues, scale removal treatments were suspended for some time waiting for a safe technique to be devised. As a first step, the service company proposed to use a non-damaging chemical plug technique to temporarily plug the open perforations during the scale removal treatment, in an effort to avoid H2S release and to maintain circulation. The first results of this technique showed a marked improvement, which led the operator to resume the descaling operations, but in some operations the isolation process was extended several days and a large amount of fluid were injected into the formation, leading to induced damage, requiring high volume acid stimulation treatment and longtime flow back operation, to get the well back in production.

To further optimize descaling operations via CT in Saudi Aramco, a novel scale removal technique was introduced that leverages the real-time downhole monitoring capabilities of CT equipped with fiber optics, to obtain a constant feedback on downhole conditions and allow swift adjustments to ensure safe operations. It also uses a redesigned foam system and implements a new pressure and fluid management system (PFMS), to eliminate the use of the temporary plug across the formation.

With CT fiber-optic real-time telemetry, engineers can control the bottom-hole pressure throughout the intervention, to maintain the well slightly over-balanced and to prevent H2S from being released during fluid circulation. This system counts with a bottom-hole assembly (BHA) that gathers a full array of real-time sensors (pressure, temperature, casing collar locator, gamma ray, load measurements), and is compatible with downhole tools that require high flow rate to operate — in this case, a 2-7/8-in. turbine with a nominal flow rate of 2.8 bpm). This BHA can withstand a high level of shocks, vibrations and bottom-hole temperatures in excess of 300°F. As for the foam system, it ensures stable solid transport from downhole to surface conditions minimizing leak off into the formation, while the pressure flow management system (PFMS) is used to accurately control wellhead pressures, thanks to an array of auto chokes to control solid returns, and to remove entrained gasses (including H2S) from the returning fluid.

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