In-situ Stress Orientation and the Effects of Local Structure – Scott Field, North Sea

Abstract

In-situ stress orientation was measured in 17 wells throughout the Scott field in order to predict the orientation of waterflood induced fractures. The Scott field is heavily faulted and significant variations in in-situ stress orientation were found between different fault blocks. The in-situ stress orientations appear to be strongly controlled by the orientation of the local faults and tectonics. The direction of maximum horizontal stress appears rotated 30° to 50° on either side of the NNW-SSE regional compression trend generally found in the North Sea. In most cases, the maximum horizontal stress is parallel to the orientation of nearby normal and wrench faults, as would be predicted by a tectonic model of such faulting. However, the tectonics of the region suggest that the rotation of the regional stress field from fault block to fault block is due to the presence of the faults rather than active faulting. The orientation of the in-situ stress was determined from shear acoustic anisotropy measurements on cores from 5 wells and from wellbore elongation measurements in 13 wells. The two methods show very similar orientations within the same fault block. No statistical variability in stress orientation was noted between different formations or with depth. Shear acoustic anisotropy utilizes the polarization of shear acoustic waves propagating through oriented core samples and has proven to be very reliable in determining stress orientation. The observed wellbore elongations in the field do not appear due to breakouts. The character, magnitude, placement, and orientation of the wellbore elongations strongly support the premise that the measured elongations were due to drilling or coring induced wellbore fractures. Previous experience by the authors and other recently published observations support this conclusion and the reliability of these elongations for the determination of stress orientation.

Present‐day stress orientations and tectonic provinces of the NW Borneo collisional margin

“…The presence of preexisting faults may also deflect the stress field so that σ H orientations are perpendicular or parallel to normal or reverse faults [ Yale , 2003; Yale et al , 1994; Townend and Zoback , 2004]. Therefore, the presence of the preexisting late Miocene–early Pliocene compressional and wrench faults and associated local structures may deflect the present‐day stress field, as is suggested by the almost perpendicular σ H orientations in wells S4 and S5 (white square, Figure 1b, see Table 1).…”

Maximum horizontal stress orientations in the Cooper Basin, Australia: implications for plate-scale tectonics and local stress sources

“…The S Hmax orientation determined for the three wells is rotated 20°–30° in a clockwise direction compared to the S Hmax orientation in the nearest wells to the east. Fault parallel stress orientations have been observed in a number of studies (Aleksandroski et al 1992; Yale et al 1994). The stress rotation observed in the Challum wells is the same as that predicted by 2‐D distinct element modelling of a discontinuity previously conducted by Homberg et al (2004) and Homberg et al (1997).…”

“…Numerous studies have explained perturbations in the regional stress field resulting from the presence of geological structures (e.g. Aleksandroski et al 1992; Yale et al 1994; Dart et al 1995) and crustal density heterogeneities (e.g. Mareschal & Kuang 1986; Assameur & Mareschal 1995; Mandal et al 1997).…”

Stress Field Evolution of the Southernmost Andean Cordillera From Paleostress Analysis (Argentine Tierra del Fuego)

“…Therefore, we can underestimate the anchoring effect at the fault tips and attend to how the stress field deflection occurs in the central zones of the faults. Regarding this, there are numerous studies based on mathematical models and data obtained from the fault populations analysis and breakout wells (e.g., Aleksandroski et al, ; Homberg et al, , ; Reynolds et al, ; Yale et al, ). The 2‐D modeling of these studies shows that the orientation of the local stress perturbation closely parallels the fault orientation along the midsection of the fault in situations where there is a small angle between the maximum horizontal compressive stress orientation and the fault.…”