Abstract

Micro-hydraulic fractures have been widely and successfully used for characterisation of in situ stress for over 4 decades. Many field measurements, however, raise questions regarding the validity of the assumptions underlying the standard practice for analysis of field test data. Here we present a coupled numerical model that includes the compressibility of the injection system and the flow of a viscous fluid in a plane strain hydraulic fracture extending from a wellbore in the presence of a non-isotropic in situ stress field. Consistent with evidence from field data, this model predicts that with each injection cycle, the slope of the pressure versus time curve, before further crack extension takes place, will decrease. The pressure required for crack extension will also decrease with each injection cycle. Furthermore, the shut-in pressure is shown to essentially correspond to the pressure at which crack extension will commence on the next injection cycle, thus agreeing with the data compiled by Sano et al. (2005). Finally, the model demonstrates that the near-wellbore stresses, which also couple with the effects of compressibility and fluid viscous dissipation, lead to a difference of up to 50% between the peak wellbore pressure and the pressure at which the hydraulic fracture begins to grow. Ongoing work is aimed at quantitative re-interpretation of existing field data in light of these model results. The results of these simulations suggest that additional information could be extracted from the pressure-time record, if the characteristics of the injection cycles are properly chosen.