Abstract
Borehole instability, in most of the cases, is a direct reflection of earth's in situ stress state. It is well known that the stress distribution around the wellbore induces deformation depending on many factors ranging from wellbore pressure history and rock strength to the trajectory orientation.
A stress direction map is generated for the GoS from observations of borehole breakout detected in multi-arm-caliper logs and other log data base, viz., electrical Images and sonic logs. In vertical wells, the maximum tangential stress around borehole can produce breakouts and their orientation indicates the direction of minimum in situ horizontal stress (Sh). In the case of deviated wells, a stress-tensor diagram defines Sh direction with reasonable accuracy, provided wells cover wide range of deviation angle and azimuth
The current study indicates that Sh in GoS is aligned along two major trends. The main NNE – SSW trend, with average orientation of N10degE, exists in most of the region. The second trend is aligned NE – SW and observed locally at the central eastern and south-western part of GoS, with an average orientation of N50degE. Most studies of the structural and tectonic history of the GoS have identified two age significant orientations for this extensional rift. The early to middle Miocene rifting, responded to a Sh direction of N55-60degE (rift-climax). The younger stress fields of the Late Miocene and Pliocene times rotated progressively counterclockwise to a N15–25degE direction that persisted into early-late Pleistocene time. The dominant in situ stress orientation trend, identified in this study, therefore, is mainly controlled by this younger stress field of the GoS rifting.
In situ stress directions have strong impact in drilling high angle wells in GoS. Proper placement of well trajectory with respect to in situ stress reduces instability in drilling. The paper exhibits example of directional sensitivity of well trajectory and successful drilling campaign based on the developed stress map.
