Numerical modeling of wellbore stability in layered rock masses
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ORIGINAL PAPER
Numerical modeling of wellbore stability in layered rock masses H. Parsamehr 1 & S. D. Mohammadi 2 & P. Moarefvand 1
Received: 17 April 2014 / Accepted: 21 May 2015 # Saudi Society for Geosciences 2015
Abstract Borehole instabilities during drilling are more common in bedding plane rocks than in most other rock formations. Bedding plane rocks make up more than 80 % of rocks in siliciclastic environments, and about three quarters of borehole problems are caused by bedding plane rocks instability. The assessment of in situ stress and analysis of borehole failure due to instability and weak bedding plane represents one of the most critical factors when evaluating borehole stability that causes borehole failure. This paper is based on elastoplastic and isotropic model for stresses around the wellbore, with the aim of trying to understand the general behavior of inclined boreholes due to anisotropy. It was found that borehole collapse was caused predominantly mainly not only by shear but also by tensile failure. It is seen that bedding exposed depends not only on inclination but also on dip of the formation, attack angle, and azimuth. The numerical analyses presented in this paper were carried out using a threedimensional numerical program. The effects of several dips and dip directions of rock mass layering and angles of well and three different field stress conditions have been investigated. It is known that the differential stress has an important influence on wellbore instability. Also, the effect of a high differential stress is exacerbated by the layer geometry and well angles. In other words, some dip and dip direction of bedding plane causes maximum displacement toward wellbore and significantly affect wellbore stability during drilling. * S. D. Mohammadi [email protected]; [email protected] 1
Department of Mining and Metallurgy, Amirkabir University of Technology, Tehran, Iran
2
Department of Geology, Bu-Ali Sina University, Hamedan, Iran
Keywords Instability . Well direction . Bedding plane . Distinct element
Introduction Wellbore instability remains an important issue in wellbore drilling, with major drilling problems being encountered in many gas and oil fields. Recent field observations have shown that despite their relatively small diameters, wellbores can also be affected by the presence of geological structures such as layering, fracturing, and folding in rock masses. The increasing demand for wellbore stability analyses during the planning stage of a field arises from economic considerations and the escalating use of deviated, extended reach and horizontal wells. Wellbore instability can result in lost circulation where tensile failure has occurred and spalling and/or hole closure in the case of compressive failure (Bandis et al. 1987; Barton 2007). Tensile failure occurs when the effective stress across some plane in a rock exceeds a critical limit (Fjær et al. 2008). For rocks, this limit is the tensile strength T0. The tensile strength is very sensitive to the presen
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