Comparative analysis of wellbore closure in salt rock formations considering primary creep

Resumo

This study presents a comparative analysis of vertical wells wellbore closure behaviour considering constitutive models with and without the incorporation of primary creep in salt rocks. In the context of drilling wells in the pre-salt region, one aspect that can jeopardise the operation is the phenomenon of creep, as time-dependent deformations accumulate and can lead to irreversible drilling column entrapment due to wellbore wall closure. Because of the complexity of drilling in salt formations, vertical trajectories are usually adopted in this region. During the design stage, well closure simulations are necessary to predict undesirable scenarios and thus assist in the appropriate selection of drilling fluids and the planning of reaming operations. Therefore, it is crucial to employ constitutive models capable of adequately representing the behaviour of salt rocks in these simulations. The constitutive model traditionally adopted for Brazilian rocks is the so-called Double-Mechanism of deformation. This model describes only the secondary creep stage, which is dominant over longer periods of time. This study focuses on comparing the differences between the results of well closure simulations with the Double-Mechanism and with the EDMT model (Enhanced Double-Mechanism using a Transient Function), which incorporates primary creep into the Double-Mechanism of deformation, thus being able to better represent short-term deformations. To achieve this objective, a case study is employed with the modeling of a synthetic well, using data consistent with real pre-salt wells. Elastic and viscoelastic parameters of saline rocks are obtained from the literature. The numerical simulation is performed using the finite element method. The well closure profiles of a well section obtained over time are compared. In the comparative analysis, the importance of primary creep in the well wall closure is further discussed by comparing critical time windows for restricting the passage of drilling equipment.