Abstract
While exploring deeper water depths, large hydrocarbon deposits have been found below salt formations. These reservoirs are located in formations called "pre salts". Salt formations create favorable conditions for trapping hydrocarbons but also introduce challenges in well drilling and completion because of their mobile nature. The rock salts behave in a visco-elastic manner and deform under pressure. This deformation is called "creep" and occurs over time once the salt is disturbed. Creep begins the instant the drilling penetrates the salt formation and occurs because of instability and formation stresses. Completion of the well does not stop the creep process. This continual creep causes an increase in load on the casing and causes excess stress on the well completions during the lifetime of the well. Creep can eventually cause the casing to collapse.
Poorly designed casing, centralizer placement against the salt formation, and the cement job can be at risk for casing collapse during production. Previous studies have demonstrated eccentricity can increase the stress load on casing dramatically in salt formations. To avoid eccentricity, the placement of centralizers in the salt formation is very important. Presently, there is no method for optimizing centralizer placement; hence, it remains a challenging problem.
This study presents a geomechanical model, which treats the formation, cement, and casing as a whole system. The eccentricities of casing in wellbores having different borehole inclination and centralizer placement are derived from analytic solutions. The creep behavior of the salt formation and the stress load on the casing are calculated using the finite element method. The results of this model can be used as input parameters for optimum wellbore design, centralizer placement, and wellbore integrity analysis to predict the in-situ casing stress throughout the lifetime of a well. This paper also presents derivations of the generalized methodology for optimum placement of centralizers and the application of the model in a field case. Simulations examples have been performed and are reviewed in this paper. In addition, this paper documents the mathematical simulation results under different well profile and well geometry scenarios.
