Application of piezoelectric sensors for stress-related measurements in wellbores

Abstract

Precise knowledge of hydraulic fracture orientation and closure pressure is important for successful well completions. Fracture orientation can be determined from various methods like active mapping, open borehole and predictive orientation core methods. However, active methods are impractical for offshore applications. Open borehole and predictive orientation core methods under inappropriate condition give less accurate results. Current methods of fracture closure-pressure determination are based on pressure fall-off analysis. This analysis becomes futile in high-fluid loss formations, and so is limited to formations having low fluid losses. This work puts forth a new approach to determine hydraulic fracture orientation and closure pressure. In this approach, polyvinylidine fluoride (PVDF) sensors attached to the casing measure direct stress changes around their outer surface. By analyzing stress changes around the casing during hydraulic fracturing, the fracture orientation and fracture closure pressure are determined. From experiments on hydrostone blocks conducted at Halliburton Technology Center, we found during hydraulic fracturing that only sensors parallel to the direction of fracture propagation experience a decreasing stress change. The negative signal from the sensor corresponding to this stress change is used to determine the fracture pressure, and by knowing the location of the sensor we can determine the fracture orientations. Similarly, during fracture closure only the sensor parallel to the fracture orientation gives a positive signal, indicating that it is experiencing increasing stress change. Wellbore pressure corresponding to this signal is the fracture closure pressure.