The Molasse Basin in Southern Germany has many geothermal wells targeting the Malm carbonate formation. However, drilling in the final section of this formation often leads to substantial drilling losses due to the Malm's inherent characteristics, such as abundant natural fractures and slightly subhydrostatic conditions. Consequently, a portion of the generated drilling cuttings and fines can plug the natural fractures, leading to reduced well productivity.
To minimize or eliminate this risk, operators have traditionally used acid stimulation treatments, which are applied to re-establish injectivity and formation productivity by dissolving the drilling cuttings. However, an environmentally favorable and compliant corrosion inhibitor is necessary to protect the expensive drilling strings and well casings from corrosion in the high-temperature environment. Therefore, a new solution was developed.
The new stimulation system with corrosion inhibitors and diversion systems that were rated German Water Hazard (WGK) Classification 1 exceeded the minimum requirements for acid stimulation additives in Germany. This system, along with the state-of-the-art stimulation design software, provided the operator with the capability to model acid and diverter placement in dynamic well conditions to optimize the stimulation treatment and zone coverage. This ensured efficient stimulation of multiple zones in an openhole well without bullheading via the drillstring and successful delivery of stimulation fluid to zones with the lowest resistance to flow.
Multiple acid stimulation treatments were performed successfully in the state of Bavaria, providing 100% corrosion protection in wells with up to 165 degC downhole temperatures. Well productivity increased to more than 100 L/s, enabling the operator to exploit the full potential of each well.
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