Benefits of HT-Hostile Environments on Wellbore Stability: a Case Study from geothermal Fields in Central Australia
Proc. Indon Petrol. Assoc., 33rd Ann. Conv.
In high temperature (HT) environments additional stresses are supplied to the rock at the borehole wall due to the differences in temperature between the drilling fluids and the rock (∆T). Cooling of the hole (the usual case) increases the tensile stresses (and decreases the compressive stresses) at the wellbore wall. As the hole warms up, compressive stresses will be increased and breakouts will grow in severity. However, drilling in HT-environments may benefit wellbore stability if properly
... ility if properly planned. The Cooper-Eromanga basin is characterized by high heat flow that has been related to the presence of high radiogenic heat producing granites. Several wells have been drilled in the area to exploit the heat from the fractured granitic rocks of the basement. Drilling through the hot formations in the Cooper basin (max. temperature ca. 250 °C) with relatively cool drilling fluids induces an almost instantaneous cooling of the wellbore wallrock. LWD temperature data from these wells suggests a maximum temperature contrast between the mud and the basement of 100 °C. This temperature contrast is enough to initiate drilling induced tensile fractures, as it is seen in acoustic image data. Modeling of the in situ stress tensor and mechanical properties of the wellbore rocks have revealed the time-dependent effect that the borehole collapse pressure has on the stability of the wells. Narrow breakouts form at the time of drilling. Afterwards, the temperature difference (∆T) decays with time, and as the hole warms up compressive stresses increase and breakouts become enhanced. This analysis suggests that the first 10 days seem to be critical for breakout development. Therefore, if a