Abstract

This paper presents a methodology to analyze the casing integrity in salt layers considering the effects of creep and dissolution on the wellbore closure. The finite element software ABAQUS was adopted to perform the simulations. The dissolution was considered homogeneous and inhomogeneous through a function that turns off the elements located on the wellbore. The progress of material removal obeys to the characteristic dissolution curve of rock salt considered, and the creep was analyzed with a power law function, available in the ABAQUS library. Four cases have been studied: a) wellbore creep without dissolution, b) homogeneous dissolution of the entire wellbore, c) partial dissolution of the wellbore and, d) homogeneous dissolution considering casing decentralization. The results show that the dissolution may slow the contact of the salt with the casing whenever it is homogeneous. For the condition of inhomogeneity point loads can be generated by increasing the stresses on the casing, causing the potential pipe collapse, in addition to modeling time around 5 years, numerical problems are presented due to the high distortions of the elements finite mesh being necessary to use adaptive remeshing techniques to treat such problems.

1. INTRODUCTION

Large oil fields were discovered on the Brazil coast (Santos Basin) in 2007, in a geological formation known as pre-salt. In the pre-salt region, the depth of water varies between 1000-2000 meters thickness and the seabed varies between 4.000-6.000 meters. Thus, the total depth of the reservoirs below the salt layer can reach 8.000 meters. The Figure 1 shows one example of an oil reservoir in the pre-salt region.

One of the main challenges to cross the salt layers is to ensure the casing integrity because these wells are drilled in complex geological formations, both in the geometric aspect as the mechanical behavior. There are two main effects that can be emphasized: 1) the dissolution of the salt rock due to drilling fluid and 2) the creep after drilling. These effects have opposite results, while the dissolution increases the diameter of the wellbore, the creep decreases it.

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