--> New Biodegradable Stimulation System – Field Trial In A Sandstone/Granite Formation

AAPG European Region, Geothermal Cross Over Technology Workshop, Part II

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New Biodegradable Stimulation System – Field Trial In A Sandstone/Granite Formation

Abstract

The primary objective of acidizing geothermal wells in fractured sandstones and granite is to remove scales blocking the pathway of water and hence increase the productivity or injectivity of the formation. For this purpose, Hydrochloric acid (HCl) is commonly used. At high temperature, however, the fierce reactivity of this fluid often hinders deep penetration of the formation making stimulation of removed damaged zones difficult to attain. Selection of chemicals to reduce reactivity adds to the difficulties encountered in the acceptance by mining and water authorities. Furthermore, the extreme corrosion tendency of HCl requires high concentrations of corrosion inhibitors and intensifiers. Another problem often encountered during acidizing sandstones with HCl-based formulations is their incompatibility with clay minerals. Core flood experiments and field results indicate that high temperature illitic sandstone is sensitive to conventional mud acid treatments. Here, the commonly employed HCl pre-flush degrades illite and chlorite leading to fines migration and formation damage. Considering environmental constraints, pumping HCl/HF-based systems may not be an appropriate option. An alternative stimulation fluid should ideally lack such drawbacks and additionally be environmentally friendly. This paper introduces a new naturally 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 compatibility (with clays / elastomers / formation water) were conducted preparing the first field trial. 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. For the first field trial, three target zones in the openhole section were selected for chemical injection via coiled tubing. 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 performance of the well. Due to its naturally retarded reactivity, this fluid can be pumped at much lower rates, compared to most HCl-based formulations. Laboratory and field results impressively proved that this new biodegradable stimulation fluid is an excellent alternative to commonly used HCl-based systems.