This paper introduces a new retarded stimulation system which is based upon an environmentally friendly, biodegradable acid and highlights its first field trial in a sandstone/granite formation. Here, demanding borehole conditions (165°C, anhydrite and carbonate scales, highly fractured sandstone and granite) as well as strict regulations regarding environmental impact of stimulation fluids set forth by mining and water authorities were encountered.

Extensive laboratory tests regarding reactivity, acid corrosion tendency, and fluid compatibility were conducted preparing the first application in a geothermal well. In contrast to HCl-based fluids, this state of the art acid system has a low corrosion tendency and an excellent compatibility profile, even at high temperature.

The superior chemical properties of this stimulation system combined with an outstanding performance of inflatable packers in the openhole section lead to a greatly enhanced injectivity of the geothermal well. Laboratory and field results impressively proved that this new biodegradable acid system is an excellent alternative to commonly used HCl-based fluids.

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