ASSESSMENT OF BOREHOLE BREAKOUTS FROM ACOUSTIC IMAGE LOG AND ITS GEOMECHANICAL IMPLICATIONS – A CASE STUDY FROM TRIASSIC-ORDOVICIAN INTERVAL OF BERKAOUI FIELD, SOUTHEASTERN ALGERIA

Abstract

In this study, we interpreted a cumulative 600m acoustic image log across the Triassic to Cambro-Ordovician interval in the Berkaoui oil field, Algeria. We interpreted 40 distinct breakout zones which have a combined length of 210m. These breakouts are aligned in the NNE-SSW direction indicating a mean maximum horizontal stress (SHmax) azimuth of 110°N. The observed breakouts are ranked as “A-Quality” following the World Stress Map ranking guidelines. The angular width of each breakout has been inferred from the image log analysis and the same has been utilized to infer the SHmax gradient by stress polygon approach following the frictional faulting mechanism. The stress polygon across all the breakout intervals provides a practical Shmax range between 24.7-31.1 MPa/km, with an average gradient of ~ 27 MPa/km. Considering the Shmin range across the studied intervals, we infer a SHmax/Shmin ratio dominantly between 1.40-1.65, which is a much narrower and better-constrained range when compared to the previously published ranges from nearby fields with the same stratigraphy. The relative magnitudes of the in-situ stresses indicate a strike-slip faulting regime in the Berkaoui field. This study presents the utility of image log analysis and integration of breakout interpretation to obtain a more robust geomechanical model with reduced SHmax uncertainty.