Present-day stress of the central Persian Gulf: Implications for drilling and well performance

摘要

The present-day state of stress in the Persian Gulf is poorly understood but has significant impacts on well drilling and performance. The upper Permian to lower Triassic formation of Kangan/Dalan, Persian Gulf, exhibits a complex structural context in the neighborhood of the Oman Mountains and the Zagros orogenies. This formation is divided into four reservoir layers (K1 to K4) where three main lithologies (limestone, dolomite and anhydrite) are alternating. We conduct an analysis of the present-day stress and natural fractures at the wellbore using full-bore FMI logs, leak off test and density logs. For this purpose, borehole breakout and tensile fracture data are used to determine orientation of S_H. Furthermore, density log, leak-off test and Kirsch equation for tensile fracture formation in the wellbores are used to calculate the magnitude of S_v, S_h and S_H, respectively. Vertical stress (S_v) gradient at 3100m depth approximates 20MPa/km (2.9psi/m), indicating a bulk density of 2.04g/cm~3. A total of 131 drilling induced tensile fractures and 21 breakouts with an overall length of 262m are observed in two wells, indicating a mean maximum horizontal stress (SH) orientation of N53° (±18.45°) for drilling-induced tensile fracture (DITF) data and N50° (±10.79°) for breakout data. The mean orientation of S_H rotates counterclockwise with depth from K2 (N70°±4.2°) to K4 (N40°±5.1°) reservoirs. Noticed correlation between these data and stress orientations from earthquake focal mechanism solution, first of all, indicates that the stresses are linked to the resistance forces generated by the Arabia-Eurasia collision at the Zagros orogeny and secondly confirms the reliability of focal mechanism solution data near continental collision zones. In the Kangan/Dalan Formation, the NW-SE main open fracture direction is found as a common regional direction which is sub-perpendicular to the present-day maximum horizontal stress. Minimum horizontal stress (S_h) gradient in reservoir sections is estimated to be equal to 17MPa/km (2.5psi/m). The concluded strike-slip stress regime (S_H>S_v>S_h) in the study area is consistent with the compressive regime in the Zagros thrust-fold belt. The present-day stress in the Kangan/Dalan Formation has implications for wellbore stability, lost circulation and well Inflow Performance Relationship (IPR). Wells are more unstable if deviated toward the Sv direction, whereas well productivity and mud loss increase in wells deviated toward S_H, which conveys the idea of a strike-slip faulting effect tends to keep the natural fractures open in that direction. copy; 2013 Elsevier B.V.