Effect of stress on fracture development in the Asmari reservoir in the Zagros Thrust Belt

Abstract

Development and production from fractured reservoirs require extensive knowledge about the reservoir structures and in situ stress regimes. For this, this paper investigates fractures and the parameters (aperture and density) through a combination of wellbore data and geomechanical laboratory testing in three separate wells in the Asmari reservoir, Zagros Belt, Iran. The Asmari reservoir (Oligo-Miocene) consists mainly of calcitic and dolomitic rocks in depths of 2000-3000 m. Based on the observation of features in several wellbores, the orientation and magnitude of the in situ stresses along with their influence on reservoir-scale geological structures and neotectonics were determined. The study identifies two regional tectonic fracture settings in the reservoir: one set associated with longitudinal and diagonal wrinkling, and the other related to faulting. The former, which is mainly of open fractures with a large aperture, is dominant and generally oriented in the N45°-90°W direction while the latter is obliquely oriented relative to the bedding and characterized by N45°-90°E. The largest aperture is found in open fractures that are longitudinal and developed in the dolomitic zones within a complex stress regime. Moreover, analysis of drilling-induced fractures (DIFs) and borehole breakouts (BBs) from the image logs revealed that the maximum horizontal stress (SHmax) orientation in these three wells is consistent with the NE-SW regional trend of the SHmax (maximum principal horizontal stress) in the Zagros Belt. Likewise, the stress magnitude obtained from geomechanical testing and poroelastic equations confirmed a variation in stress regime from normal to reverse, which changes in regard to active faults in the study area. Finally, a relationship between the development degree of open fractures and in situ stress regime was found. This means that in areas where the stress regime is complex and reverse, fractures would exhibit higher density, dip angle, and larger apertures.