Drilling conditions involving high mean and deviatory stresses and natural fractures in the Cooper Basin pose difficulties in drilling and introduce wellbore rugosities, leaving a damaged wellbore subject to a stress cage effect. Fracture initiations have been problematic in vertical Cooper Basin wells, exhibiting high initiation and treating pressure frac treatments, and high stress conditions pose greater risks in non-vertical completions. Whilst far-field fracture complexity should simplify, the near wellbore complexity results in reduced fracture conductivity.  We believe that current drilling practices and wellbore azimuths may be contributing to sub-optimal hydraulic fracture initiations and complexities.

Current analytical modelling methodologies can derive initiation pressures for circular wellbores, but require more complex numerical models to include flaws and ellipticity to represent natural fractures and wellbore rugosities.  This study compares initiation pressures and presents graphical results comparing circular and elliptical wellbore cases with flaws. We will outline the criteria used in these models and remark on areas for further research and model development.

Finally, we propose improved drilling techniques to achieve more stable, smoother wellbores, potentially reducing some rugosity and drilling induced fractures. Then, using data from recent research and other cases with complex stress environments, it is proposed that initiation pressures might be reduced by inclining wells for hydraulic fracturing treatments in a favourable alignment to the maximum horizontal stress direction (σH-Max) and implementing completion techniques that aid better fracture initiation.

 Presented at APPEA 2015.

Author companies:
1. Unconventional Reservoir Solutions
2. Australian School of Petroleum, University of Adelaide
3. CSIRO Energy, CSIRO
4. Ikon Science
5. Petro-king Australia Pty Ltd