Scale Inhibitor Dosage In Seawater Injection: Insights Into Long Term Advantages Of Continuous Dosage and Implications Of Inadequate Dosage

Abstract

Abstract This paper aims to elucidate the various benefits of continuous dosage of suitable scale inhibitor into the reservoir through injection water to prevent down hole deposition of scale in offshore development wells. Various consequences of inadequate chemical dosing have been deliberated. Detailed description of methodology adopted for chemical selection has also been elaborated in the paper. Most oilfields in India's Western Offshore region are brown/matured fields which have been in production for more than three decades. Reservoir pressure management system, which includes seawater injection as one of the primary techniques, has been in place since the 1990s. Seawater has a high concentration of inorganic minerals which are not always compatible with formation water. The chemical injection plan, part of the larger seawater injection system, had initially included provision for anti-scalant injection. The same was however discontinued at a later stage based on a separate study. The study took into account the water chemistry at the time (late 1990s) and stated that the water was unlikely to scale in the near future. Two decades on and inorganic scaling has plagued majority of the development wells in the field. This has led to a rampant rise in the requirement of scale removal jobs in the field over the past decade or so. Scale prevention is a superior and economical alternative to scale removal due to reduced consumption of chemicals and prolonged life. As part of this study, the feasibility of continuous injection of a suitable scale inhibitor into the reservoir has been re-examined and subsequently, extensive laboratory experiments were performed to identify suitable chemicals. Additionally, the repercussions of not dosing a suitable chemical/ inadequate dosage have been discussed. As part of the study, injection water samples were collected from strategic locations all over the western offshore injection water network and analysed for scaling tendencies. The results show beyond doubt that decades of production has led to injection water breakthrough/encroachment and with it, the problem of inorganic scaling. Based on a comprehensive literature review and market scouting, six inhibitors were evaluated for their inhibition efficiency, thermal stability and compatibility with the rock formation through laboratory experiments under simulated reservoir conditions. Based on the results, chemical X was identified as being most suitable for continuous injection into the reservoir to combat scaling issues in both injection and development wells. The study discusses the various pros and cons of continuous scale inhibitor dosing thru seawater injection for an offshore carbonate reservoir. It analyses the feasibility of starting afresh a scale inhibitor injection program, after the field has been in production for more than 3 decades. The entire chemical selection process has also been detailed.