The contemporary stress field of the Barrow-Dampier Sub-Basin and its implications for horizontal drilling

Abstract

The interpretation of borehole breakouts suggests that minimum horizontal stress (sh) is regionally oriented approximately N-S in the Barrow-Dampier Sub-Basin. More locally, breakout interpretation suggests that sh orientation is 005°?010° in the Wanaea/Cossack area, and 012°?022° in the Griffin/Chinook/Scindian area. Horizontal stress magnitudes in the Wanaea/Cossack area, derived from modified leak-off tests, together with vertical stress magnitudes derived from density and sonic log data, suggest that the stress (fault) regime in the Wanaea/Cossack area is on the boundary of extension (normal faulting) and strike-slip. The contemporary stress field impacts on planning the drilling direction of deviated and horizontal wells through the issues of mechanical wellbore stability and fracture intersection. In order to maximise intersection with any open, natural fractures, and to optimise any hydraulic fracturing program, wellbores should be horizontal and parallel to ah (minimum horizontal compressive stress) in the extensional (normal fault) and strike-slip stress regimes, and vertical in the compres-sional (reverse fault) regime. In order to minimise the tendency for borehole breakout, the stress anisotropy around the wellbore should be minimised ? i.e. horizontal wells should be drilled in the direction of sh in the extensional regime, and in the direction of sH (maximum horizontal compressive stress) in the compressional regime. In the strike-slip regime, horizontal wells can be oriented such that they are not subject to any stess anisotropy by progressively changing from the sh direction towards the sH direction as the horizontal stresses increase with respect to the vertical. In the Wanaea/Cossack area, wells drilled towards 005°-010° or 185°?190° will maximise the potential for intersection with open, natural fractures (should they be present), and recovery from any induced, hydraulic fracturing will be optimised. Furthermore, wells in this direction will be subject to the minimum stress anisotropy, and are thus least likely to be subject to breakout.