Abstract
Long-term well shut-ins have become a common practice for operators to defer oil production in an oversupplied market. This paper discusses several scale challenges that are expected under such circumstances. Two specific cases were explored in detail in terms of product development and performance validation under various scenarios. The first category of wells had high-water production, featuring extreme levels of calcium and iron, as well as an ultra-high, bottom-hole temperature of 350°F. These wells began experiencing significant calcium carbonate scale precipitation when bottom-hole conditions reached extreme temperatures of around 400°F. To solve this issue, this study utilized laboratory examination to determine the most effective scale inhibitor chemistries and evaluate the thermal stability and restartability in a capillary injection system, before and after an extended period of heat stress. The second group of wells had a moderate bottom-hole temperature but were not equipped with capillary tubing. Scale inhibitor was historically dripped in neat forms through the annulus and had caused significant corrosion issues on the production-tubing, which was composed of carbon steel. The corrosion induced by scale inhibitors was expected to become more severe when the well was shut in, during which little to no water would be produced to flush the concentrated product off the pipe and equipment surfaces. Through literature review and a pre-screening of common scale inhibitor chemistries, it was confirmed that essentially all scale inhibitors had very high corrosion rates on carbon steel (5-100 mpy), heavily dependent on the temperature, pH, and the product formula in the testing. Therefore, an approach using a small percent additive was selected to address this obstacle. The final candidate was able to effectively lower the corrosivity to negligible levels of far less than 1 mils per year (mpy) at 180°F but could also be used in an ultra-high temperature application. It is a stable standalone formulation that could be delivered to the location separately and convert the corrosive product into a noncorrosive combination through simple onsite mixing. The combination package retained high effectiveness over iron and calcium carbonate inhibition at very low treatment rates.
This study utilized a systematic approach to solve various scaling issues for wells selected for extended shut-ins. A highly effective scale inhibitor was developed for use in capillary injection systems exposed to ultra-high bottom-hole temperatures. It had validated formulation integrity and was considered least likely to cause capillary tubing plugging during the shut-ins. With the incorporation of 1% or less additive, it could even be used as a non-corrosive approach for direct annulus injection.
