A high-strength, alumina-based solid scale inhibitor was developed for subsea applications. Laboratory elution tests show that this solid inhibitor has a nearly constant inhibitor release-rate profile in the long term. The strength of this solid inhibitor is comparable with high strength proppant, and the smallest particles generated were 135 µm as the mixture of 20 wt. %.
This material was applied to four zones in two deepwater Gulf of Mexico wells. After 17 months of production, the wells were still producing inhibitor above the MEC, and the cumulated water/oil production was approximately 1.1MM bbl and 7.8MM bbl respectively.
Conventional inhibitor treatment in deepwater wells with wet trees have technical challenges 1-4. Continuous injection treatment may have delivery issues and protection against scale deposition is only for the tubulars; there is no protection in the formation or perforations. Inhibitor squeeze treatments require the use of Remotely Operated Vehicles (ROVs), which affects operating cost. The cost of subsea well inhibitor squeeze treatment during the production phase is reported to be 10 to 50 times higher than the platform well bullhead treatment 5. Moreover, the complexity of horizontal and multi-stage wells completion makes the scale inhibitor placement during squeeze treatment challenging. The pressure gradients between each stage in long reach horizontal wells can result in uneven placement of inhibitor chemicals in reservoir after a squeeze treatment. As a result, most of scale inhibitors may be placed in inappropriate zone and lead to significant reduction in inhibitor treatment lifetime and inadequate protection in target zones.
The alternative to conventional treatment is a solid inhibitor delivered through hydraulic fracturing. Earlier attempts to add solid inhibitors into fracturing fluid is reported by Norris et al. 6 and Selle et al. 7. These solid inhibitors either have high strength but lower chemical loading, as described by Norris (scale inhibitor impregnated proppant -SIIP), or a lower strength but higher chemical loading as described by Selle (scale inhibitor impregnated gravel- SIIG). The SIIG is not suitable for deepwater application because the poor strength that could not hold the encountered high closure pressure. The closure pressure of SIIP seemed to reach 10,000 psi, but the inhibitor released too fast, and the treatment was short-lived. Norris 6 reports on SIIP addition to a well in the British sector and a well in the Norwegian sector of the North Sea. They reported 55 days of acceptable inhibitor residuals. Later on, they report 33,000 bbl of cumulative water production with acceptable residuals.