In geothermal projects, the main goal of acidizing injector wells in sandstones is scale removal and consequently injectivity increase. At low temperature, hydrochloric-based treatment fluids are commonly employed for this purpose. In this context, employing diverting agents to prevent acids from leaking into the most permeable sub-layer of the target zone is recommendable.
This paper presents the surfactant-based product SDA-550 which shows a tendency of forming rodlike micelles in acidic solutions. Here, a chaotic worm-like arrangement of dissolved molecules leads to an increase in viscosity. This behavior creates a temporary blocking effect which causes fluid diversion and facilitates successful acidizing.
Preparing the first field trial in a Dutch geothermal well, we performed extensive lab experiments regarding solubility of solid samples, corrosion of metal coupons, and rheology of acidic recipes containing different concentrations of SDA-550. For dissolving carbonates, as well as silicates in the Slochteren sandstone formation (BHST at approximately 75°C), HCl- and HCl/HF-based recipes were pumped in a stepwise approach. The addition of SDA-550 and hence the viscosity of the diverter step was adjusted based upon lab experience and in accordance with the client.
The subsequent injection test with brine revealed a significant improvement in injectivity of the formation. While keeping the well head pressure at a constant level, we could increase the pumping rate by a factor of four. This great result was achieved due to the superior chemical properties of our innovative diverter agent combined with the great effectiveness of the tailor-made treatment fluids. Thus, laboratory and field results impressively proof that we have reached the next level of acidizing sandstones.