Operations have been very challenging for the drilling and completion of Enhanced Geothermal Systems (EGS) wells in the Cooper Basin, South Australia. High temperature, greater than 4 km depths, and high reservoir pressure have combined to make this field one of the most difficult geothermal projects in the world. When a production well failed catastrophically the project was delayed for a number of years. A thorough review of the potential causes of this failure led to significant changes in the well design. To reduce the risk of caustic cracking, the most likely cause of the well failure, changes were made to the well design and cementing operations were altered significantly from standard practice. In addition to the well design and construction, there was the challenge of drilling through the main productive fault zone in the granite reservoir rock. Previous attempts to do so have led to large losses of weighted drilling fluid causing significant damage to the permeability of the fault zone. In one well, problems that occurred while drilling the main fault zone led to multiple operational problems and eventually the total loss of the well. This paper will provide a review of the key factors that led to the successful drilling and completion of a replacement well for the well that failed. An analysis of the unique casing failure mechanism will be explained along with changes that were made in the replacement well design. A description of why reverse circulation cementing was chosen as part of the strategy to ensure long term well integrity will be given along with how the job was executed and the ultimate results. Details will be provided of how the challenges of drilling the main fault zone were overcome. Finally, information on how the successful drilling operation was actually executed will be explained.

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