Model And Calculation of In-Situ Stresses In Anisotropic Formations

Abstract In-situ stress has great influence on wellbore stability, well trace controlling and hydraulic fracturing design, therefore its estimation is considerably important. Previous work has been based upon simple isotropic, so these results were observed difference between practical stresses and calculated stresses. In this paper, according to sedimentary rocks having a laminated structures, the formations are considered as transversely isotropic material. The calculating equations of three horizontal principal in-situ stresses and a new formation fracture pressure model are to be presented. It is also introduced that how to determine elastic parameters of anisotropic rocks in lab. Meanwhile, the above methods are applied in Tuha oilfield of China to detect formation fracture pressure. The results show that errors between the theoretic calculating pressures and practical formation fracture pressures are smaller than obtained by isotropic method. This has important significance for increasing the effectiveness of drilling. Introduction In-situ stress is a widely existing natural force. The orientations and magnitudes of in-situ stresses strongly influence several reservoir characteristics related to geology, drilling, stimulation, and production. During drilling operations, for example, the building profiles of formation pressure and formation fracture pressure, mud weight limits required to avoid wellbore collapse, and calculation of creep rate in creep formation such as shale, all these have relations to the magnitudes and orientations of in-situ stresses. During production, determinations of fracturing parameters, forecast of sanding problems and optimization of sand control, work out development plan, rational pattern arrangement, all these have close relations to them. In the past, formation, rocks are considered as isotropic material in nearly all investigations of in-situ stresses, many calculating equations of in-situ stresses and their applied equations are derived[1]. Although these have guiding significance in practical work, calculating results have some errors as compared to practical values, one of the reasons is that rock anisotropy is not considered. Chenevert's work[2] showed that sandstones can be classified as very anisotropic, shale's as moderately anisotropic, and limestone's as isotropic. Rahn's work[3] also showed that the module of elasticity of a foliated rock can vary by about 50% if measured normally and parallel to the bedding plane. In China, most formations of drilling sections are anisotropic sandstones and shale's. Because of their depositional features, they have distinct beddings. This can be best described as transversely isotropic material. Theories of in-situ stresses in anisotropic rock formations are well documented. Leknitskii[4], in his pioneering work, provided a comprehensive analytical solution for anisotropic elastic bodies. Amadei further studied wellbore stability in anisotropic formations. Andnoy[5] presented a model of anisotropic stress to study the fracture and collapse behavior of boreholes. Recently, See Hong Ong[6] on the Andonoy's work developed a wellbore-stability "design code" that can be used to drill, complete inclined wells optimally so as to keep wellbore-stability. In addition, Inversion[7] presented some mechanical conceptions of in-situ stresses in anisotropic formations. The deficiency of above studies lies in that there is not an easily mastered method in field like isotropic formations, most are qualitative equations.