When drilling a well, understanding the orientation of the principal stresses acting on the rock is crucial. In most cases, these stresses are deviatoric, meaning they are not equal in all three dimensions. Typically, the stresses are unequal (anisotropic) and can be broken down into three orthogonal components: σ1, σ2, and σ3, where σ1 is the largest stress, σ3 is the smallest, and σ2 is intermediate. Alternatively, in sedimentary basins, stress can be described using a vertical stress component, σV, along with two horizontal stress components: the maximum horizontal compressive stress (SHMax or σH) and the minimum horizontal compressive stress (Shmin or σh).
As a well is drilled into a formation, the stressed rock is removed and replaced by drilling fluid. Because the pressure of the well fluid does not perfectly match the stress exerted by the removed solids, the stress state of the surrounding formation is altered. This alteration can cause the circular cross-section of the drilled wellbore to become oval-shaped, depending on the conditions and trajectory of the borehole. Stress anisotropy in the wellbore can produce indicators showing the maximum and minimum in-situ stress directions.
Borehole breakouts form in the compressive region of the wellbore (red on the diagram below), where compression causes the rock to spall off into the open wellbore. Breakouts appear 180° apart and align with the orientation of the minimum horizontal compressive stress (Shmin or σh). Conversely, drilling-induced tensile fractures (DITFs) form in the tensile region of the borehole when the weight of the circulating fluid exceeds the strength of the surrounding rock. DITFs manifest as vertical cracks, also 180° apart, aligned with the orientation of the maximum horizontal compressive stress (SHMax or σH).
In the diagram below, red is used to identify the zone of compression (borehole breakouts), and blue is used to indicate the tensile region (DITFs). Borehole breakouts are frequently observed when the circulating fluid is underbalanced, whereas DITFs generally form when the drilling fluid is overbalanced.
The orientations of SHMax and Shmin are defined to be 90° apart. In some boreholes, both types of present-day stress indicators may be observed, indicating a two-stage loading history. When the orientations of SHMax or Shmin from different wells are plotted on a map, they illustrate the regional state of stress. This information is displayed on the World Stress Map Project website. Generally, the orientations of SHMax and Shmin are uniform across large regions. However, in many cases, these orientations can vary and even rotate over relatively short distances. Such stress rotation suggests the influence of tectonic activity.
