The drilling induced tensile fractures can be observed from borehole image logs but they must be differentiated from natural fractures. This paper focuses on the understanding of hydraulically or artificially induced tensile fractures while drilling. We present an inversion method using rigorous principles of mechanics, to determine the in-situ formation stress state from observed induced tensile fractures. Contrary to common practices where only vertical or near vertical wells can be analysed, the present method is applicable to wellbores of all orientations. For a geological rock formation and area where the in-situ stress regime can be assumed to be similar, all the relevant borehole image logs can be included to provide information to yield the most probable subsurface in-situ stress state. The proposed inversion method directly solves for the in-situ stress states given any single set of observed tensile fracture location and orientation. It provides not only an estimate for the minimum horizontal stress magnitude and direction, but also the maximum horizontal stress magnitude which is usually very difficult to pin down. The resultant equations are non-linear and a simple numerical scheme is adopted for the solution. Although published data on borehole images of fractures with corresponding in-situ stress information are scarce, two observed field data from published papers are chosen for comparison.
In-situ stresses in deep rock formations reached equilibrium over geologic time; but drilling, production and injection processes cause deformation in the rock mass and change these stresses, and often negatively impact the operator's development plan. If they are not well anticipated, the challenges and costs of managing a prospect can far exceed the initial field development expectation. Therefore, it is vital to characterize the undisturbed in-situ stress and evaluate potential impact of field development induced rock stress changes. Two examples that come to mind are 1) enhanced oil recovery field development where injection scheme takes place under hydraulic fracturing condition and 2), the development of unconventional resources such as shale gas, where the effectiveness of hydraulic fracturing to access hydrocarbon is impacted by the in-situ stress regime. The latter, for instance, is encountered in China's Sichuan basin where operators are striving to determine whether hydraulic fractures in certain areas of the basin are developed under thrust-faulting or reverse-faulting stress regime (Yuan et al, 2013). A reverse-faulting stress regime implies that the hydraulic fractures would grow in a horizontal 'pancake' fashion, diminishing its effectiveness in draining the reservoir.
