Interpretation Challenges for In Situ Stress from Mini-Frac Tests in Soft Rocks/Hard Soils

The importance of measuring in-situ stresses as an essential input factor to caprock integrity analysis is realized for both the technical management of SAGD and CCS projects (design of optimal operating pressure) and environmental reasons (loss of caprock containment). A common technique to carry out stress tests in impermeable and weak rocks, like clay shale, is to combine sleeve fracturing with micro-hydraulic fracturing test for which premature initiation of the fracture at the packer level can be avoided. Following a brief introduction of the micro-hydraulic fracture technique, field data obtained from a micro-hydraulic fracturing test program completed to assess the far field in-situ stress state for a proposed low pressure Steam-Assisted Gravity Drainage (LP-SAGD) pilot project located southeast of Fort McMurray, Alberta, Canada has been analyzed. Pressure analyses during and after micro-hydraulic fracturing are used to compute the minimum in situ stress at depth and illustrate the wide variability in the results. Issues surrounding stress alterations in the near wellbore region, low permeability and low injection volumes are shown to contribute to incorrect estimates of the far field minimum in situ stress.