Stability Studies for Experimental Horizontal Well at Fazenda Belem (CE), Brazil
Authors: da Fontoura S.A.B. , dos Santos H.M.R. 
Published: December 2007

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Abstract
An analytical procedure to evaluate safe drilling mud weight in inclined wellbores is discussed herein. This procedure permits to consider a general stress held and the relative orientation between the wellbore and the in situ stresses. An attempt to validate this procedure is presented, firstly the fracturing procedure is presented, firstly the fracturing gradient calculated by the present method for an idealized situation is compared with the result predicted by an existing method and, secondly, the breakdown pressure of small diameter horizontal holes obtained in pressure of small diameter horizontal holes obtained in laboratory tests is compared with the results predicted by the present method. In both cases the comparison seemed very satisfactory. At the end, a real case of an experimental 500m long horizontal wellbore drilled onshore in the Northeast of Brazil is analyzed and the drilling mud weight is compared with the predicted value.

Introduction
The use of horizontal and extended reach wells increased very fast in the last few years, and nowadays there are more than 100 horizontal wells in operation throughout the world. During the 90's the oil industry expectation is that around 3 out of ten wells will be horizontals. There are several advantages associated with the use of horizontal wells: productivity increase in fractured reservoirs and with high-viscosity oil, reduction in gas and water conning, increase in the oil recovery factor among others. On the other hand, there is a need to improve technology to complete horizontal wells as well as a need to investigate in details the stability of the well, defining the fracturing and collapse gradients, very much influenced by the direction and magnitudes of the in-situ stress. Following the world tendency, the Brazilian State Oil Company, Petrobras, started a qualification program in horizontal drilling operations and today three program in horizontal drilling operations and today three horizontal wells have already been drilled. The goal is to drill horizontal wells to develop off-shore fields and these three wells drilled on shore, were steps to gain experience. As wellbore stability is responsible for the economic success of the well, a detailed stability analysis should be conducted during the design stage of the well, especially in inclined and horizontal wells, to avoid future problems. The aim of this paper is to describe the methodology currently in implementation for stability studies of inclined wellbores and its application to the analysis of an experimental horizontal well. The paper initially presents the methodology sued to analyse wellbore presents the methodology sued to analyse wellbore stability being given the material properties, the in -situ stress and fluid formation pressure and the direction of the wellbore.