Use of flow-back and pressure rebound data improves the minimum stress estimation in a tight claystone formation

Abstract

Andra is in charge of the long-term management of all radioactive waste in France. It has acquired between 2000 and 2008 a vast amount of stress data around the area selected to host the French deep geological repository for high activity and long-lived radioactive waste. The minimum principal stress was estimated from hydraulic fracturing tests and the fracture closure pressure was interpreted during the shut-in phase. However, in the Callovo-Oxfordian claystone, this method results in a minimum horizontal stress that is much larger than the vertical stress even when vertical fractures were clearly generated. As the uncertainty on the weight of the overlying sediments is low, the validity of this technique in a tight claystone formation with very limited leak-off is questioned. Several tests were reinterpreted using the flow-back data and the pressure rebounds when they were recorded. The new interpretation gives consistent results between the wells drilled around the Meuse/Haute-Marne underground research laboratory and the magnitude of the minimum principal stress matches well the actual orientation of the induced fractures. These data support the use of flowback tests as the preferred stress determination technique in tight formations.