Evaluation of in situ stress conditions is important for determining wellbore stability in the oil and gas industry, or assessing excavation stability for mining and civil engineering projects. As a result, accurate and reliable methods of in situ stress determination are necessary to adequately define stress conditions in order to optimize engineering design of a given structure. Since it is challenging to collect reliable stress information at significant depth from surface, observational methods based primarily on borehole reconstruction tools have increasingly been used to constrain stress conditions. This paper provides an overview of some of the traditional methods used for determining in situ stress conditions in deep boreholes, including a summary of the methodology and a critical assessment of the limitations associated with each technique. Some of the issues associated with properly assigning material properties are examined, both in terms of scale dependency and the variation of material properties from the borehole wall to the rock mass. Particular emphasis is placed on the use of these methods in a variety of difficult ground conditions including high stress environments and horizontally layered sedimentary rock.
In situ stress is a critical engineering design input for oil and gas, mining, and civil projects, but yet it is one of the most difficult parameters to infer with some degree of accuracy and reliability. It has been estimated that an error of approximately ± 10-20% for both stress magnitude and orientation should be expected in rock masses with few fractures, while in poorer quality rock masses, accurate measurement is very difficult [2]. In order to constrain the stress data to an acceptable level, it is generally recommended that stress measurements be collected using several different methods and at multiple locations [3]
