When drilling a well, a critical piece of information is the orientation of the principal stresses that are acting on the rock that is being excavated. In almost all cases, the stresses acting on a rock mass are deviatoric (directed) – that is, they are not equal (isotropic) in 3 dimensions. Most commonly, these stresses are not equal (anisotropic) and they can be resolved into the orthogonal components sigma 1, sigma 2, and sigma 3 (or σ1, σ2 and σ3)- where σ1 is the largest stress, σ3 is the smallest and σ2 is in-between. Yet another convention for describing the stress field in sedimentary basins is to identify a vertical stress component σV and 2 horizontal stress components: the maximum horizontal compressive stress (SHMax or σH) and the minimum horizontal compressive stress (Shmin or σh).
When a well is drilled in a formation, stressed solid materials are removed and replaced with drilling fluid. Since the well fluid pressure does not exactly match the stress exerted by the now removed solid, there will be an alteration in the stress state of the formation around the well. Depending on wellbore conditions and borehole trajectory, this alteration in stress state “squashes” the drilled wellbore, causing the circular cross-section to assume an oval shape. This stress anisotropy sometimes produces indicators of the maximum and minimum in-situ stress directions acting on the borehole. Borehole breakouts form in the compressive region of the wellbore (red on the diagram below) where the compression causes the rock to spall off into the open wellbore. Zones of borehole breakouts are found 180° apart and they are aligned with the minimum horizontal compressive stress (Shmin or σh) orientation. Drilling-induced tensile fractures form in the tensile region of the borehole when the weight of the circulation fluid exceeds the strength of the rock. DITFs appear as vertical cracks, also 180° apart, that are 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 and borehole breakouts and blue is used to indicate the tensile region and DITFs. Borehole breakouts are commonly observed when the circulation fluid is underbalanced; DITFs form mostly when the drilling mud is overbalanced.
The orientation of SHMax and Shmin are 90° apart, by definition. Some boreholes may show both kinds of present-day stress indicators, suggesting a 2-stage loading history. When the SHMax or Shmin orientations from different wells are plotted on a map, they give a picture of the regional state-of-stress. This is exactly the information that is displayed on the World Stress Map Project website. The orientation of SHMax and Shmin is commonly uniform across broad regions. In many other instances, the orientation of these features is seen to rotate, sometimes across even very short distances. Stress rotation is an indication of tectonic influence.
