The In-Situ Stress Characterisation and “Sweet Spot” Discrimination of Egilabria 2 and Egilabria 4, Isa Superbasin, Queensland
Georgia M. Matthews - 2014
Honours Degree of Bachelor of Science (Petroleum Geology and Geophysics)
Australian School of Petroleum
The University of Adelaide
In unconventional resource development it is key to understand the stresses acting on the wellbore in order to design and execute an effective hydraulic fracture operation and to facilitate predictions about wellbore failure. Additionally, a good understanding of natural fracture characteristics and targeting zones of hydrocarbon potential in the wellbore will contribute to achieving optimal production.
Armour Energy drilled wells Egilabria 2, Egilabria 2 DW1, and Egilabria 4, located approximately 300Km north of Mt Isa in the Isa Superbasin. Egilabria 2 is a vertical pilot well for the lateral addition Egilabria 2 DW1, drilled with the intent to exploit the gas potential of the mid-Proterozoic, unconventional resource, Lawn Hill Formation. However, during drilling Egilabria 2 DW1 experienced well failure, exhibiting roof collapse. Egilabria 4 was drilled as an appraisal well to assess hydrocarbon potential, where a lesser quantity of natural fractures was observed than in Egilabria 2.
The in-situ stress regime was calculated for Egilabria 2 and Egilabria 4 using published methods and resistivity image logs were interpreted to identify wellbore failure and natural fracture distributions. These results ultimately elucidated the cause for well failure and enabled a comparison of stress regimes in Egilabria 2 and Egilabria 4 to account for natural fracture distributions. "Sweet spots", areas with potential for optimal gas production, in Egilabria 2 and Egilabria 4 were also discriminated to identify potential target zones for optimal hydrocarbon production.
Both Egilabria 2 and Egilabria 4 are characterised by strike-slip faulting regimes. In this regime, horizontal stress orientations defined a mean minimum horizontal stress orientation of NE-SW (37.9° in Egilabria 2 and 24.7° in Egilabria 4) and a mean maximum horizontal stress orientation of NW-SE (120.9° in Egilabria 2 and 133.8° in Egilabria 4).
It was also determined that fractures were more numerous where wells were located ‘on-structure' i.e. Egilabria 2 and that wellbore failure in Egilabria 2 DW1 was a complex interplay of several factors including drilling direction, rock cohesion, and drill style.
"Sweet spots" were identified at 1520m in Egilabria 2 and at 1040m, 1500m, 1550m, and 1630m in Egilabria 4. The "sweet spot" identified in the Lawn Hill Formation of Egilabria 2 had already been exploited during the drilling of Egilabria 2 DW1, however "sweet spots" identified in the Lawn Hill Formation and the Riversleigh Siltstone in Egilabria 4 remain potential targets for exploitation.