This study presents new workflows to estimate in-situ horizontal stresses using routinely acquired caliper logs while drilling. A physics-based, analytical solution is used to relate borehole deformations to the in-situ stresses, rock mechanical properties, and drilling mud pressure. A machine learning model is used as a mapping function to predict the minimum and maximum horizontal stresses given other known and unknown parameters in the analytical solution. Through incorporating probability analysis using Monte Carlo simulation, estimates of the two horizontal stresses are provided. When a field measurement of the minimum horizontal stress is available, a modified workflow can be used to estimate the maximum horizontal stress only. The workflows have been demonstrated in this paper through an application to a field case study in the Appalachian Basin.
Estimating the magnitude and orientation of in-situ stresses is of practical importance in many petroleum applications such as hydraulic fracturing, wellbore stability, and reactivation of pre-existing fractures or faults. In terms of magnitude, the minimum horizontal stress is often adequately measured using micro-fracs, mini-fracs, leakoff tests, or extended leakoff tests. The maximum horizontal stress, on the other hand, is typically measured indirectly and generally more challenging to estimate. Constraining the magnitude of the maximum horizontal stress requires detailed observations of tensile borehole failures (drilling-induced fractures) and/or compressive borehole failures (breakouts) (Zoback 2010).
Since specialized well logs are needed, such detailed observations of the borehole may not always be available. What is almost always available in vertical or slightly deviated wells is four-arm caliper log measurements, which provide useful information about the shape of the borehole after drilling.
If we assume that borehole deformations remain within the elastic limit (minimal yielding/failure), one can relate the deformed borehole shape, geomechanical properties and in-situ stresses through Equations 1 and 2 for a vertical borehole (Han and Yin 2018):
