Sulfate Scale Inhibition via Stimulation Programs Carried Out During Well Completion, Deepwater, West Africa

The injection of seawater into oil-bearing reservoirs in order to maintain reservoir pressure and improve secondary recovery is a well-established, mature operation.  Moreover, the degree of risk posed by deposition of mineral scales (carbonate/sulphate) to the injection and production wells during such operations has been much studied.  The current deep-water subsea developments offshore West Africa and Brazil have brought into sharp focus the need to manage scale in an effective way.  

In a deepwater West African field the relatively small number of high-cost, highly productive wells, coupled with a high barium sulphate scaling tendency upon breakthrough of injection seawater meant not only was effective scale management critical to achieve high hydrocarbon recovery, but even wells at low water cuts have proven to be at sufficient risk to require early squeeze application.

Use of conventional, water-based squeezes have been reported to cause significant damage to productivity in low watercut wells without suitable mutual solvent preflush chemicals. To provide effective scale control in these wells at low water cuts, phosphonate-based inhibitors were applied as part of the acid perforation wash and overflush stages prior to frac packing operations.  The deployment of this inhibitor proved effective in controlling barium sulphate scale formation during initial water production eliminating the need to scale squeeze the wells at low water cuts (<10% BS&W).

This paper outlines the selection methods for the inhibitor chemical deployed and present the chemical returns profile from the 3 wells treated (some treatments lasting > 450 days) along with monitoring methods utilised to confirm scale control in the wells treated.  

The paper also explores in detail the issues associated with inhibitor squeeze vs. inhibitor stimulation deployment in deepwater, subsea fields, many of which are currently being developed in the Campos basin, Gulf of Mexico and West Africa, and is a good example of best-practice sharing from another oil basin.