Pre-drilling prediction of the tectonic stress field with geomechanical models

Knowledge of the tectonic stress field in a reservoir is essential to optimize drilling and production. Borehole stability, orientations of natural and hydraulically induced fractures, fluid flow anisotropies, among others, all depend critically on the present-day stress distribution. Several techniques ranging from dipmeter analysis of borehole breakouts to anelastic strain recovery and shear acoustic anisotropy analysis of core samples (e.g., Yale, 2003; Sperner et al., 2003) can be used to determine the in-situ stress orientations and relative magnitudes, but obviously this valuable information will only become available after the well has already been drilled. However, there are also numerous cases where the stress orientation should be known prior to drilling. For example, if multiple fracs in a horizontal well are planned, the stress field orientation needs to be known beforehand because for optimal frac design the horizontal well path must be aligned parallel to the orientation of the least principal stress axis σ3. Similarly, the planning of well trajectories with respect to borehole stability as well as the design of secondary and tertiary recovery measures (e.g., water injection, hydraulic fracture treatments) are significantly improved by a pre-drilling knowledge of the subsurface stress field. Information on the regional stress orientations can be derived from large-scale data collections like, for example, the world stress map project (Zoback, 1992; Sperner et al., 2003). The orientation and magnitude of the stress field in sedimentary basins, however, can be highly variable and, particularly near faults, the local stress orientations can differ by up to 90° from the regional trend (e.g., Yale, 2003). In such cases, inference of reservoir-scale in-situ stress orientations from regional scale maps would inevitably lead to an incorrect pre-drilling prediction. This paper uses a numerical modelling approach to determine the magnitude and orientation of the tectonic stresses in a reservoir and, particularly, the local stress perturbations near faults. The model is based on reservoir and fault geometries taken from seismic data and boundary conditions representing the regional stress field. Thus, this tool is also applicable to cases where well data are absent. Following a brief outline of the modelling approach, a case study is presented to assess the practical value of such geomechanical models for the pre-drilling prediction of the tectonic stress field in fault-controlled reservoirs.