Modeling of vertical oil well drilled on salt rocks using equivalent subdomains

Resumo

This work proposes an alternative approach based on equivalent subdomains for the numerical modeling
of vertical oil well drilling in Brazilian pre-salt. These regions are composed of large salt rock layers, which, under

high stress and temperature, present creep behavior when they are drilled. Creep causes progressive and time-
dependent strains in the direction of wellbore closure, which it can generate stuck pipe and/or well drilling delay.

Thus, it is common to use computational simulations to predict the salt rock behavior over its drilling. In the con-
ventional approach, the full salt layer is numerically mapped into a single finite element mesh and one can evaluate

the wellbore closure over time using a full plane axisymmetric analysis (Full2D-model) around the oil well vertical
axis. In this work, the proposed methodology subdivides the salt layer (domain) into equivalent and independent
horizontal stripes (subdomains) to distribute their analysis into several computational threads (multiprocessing),
which drastically reduces the computational cost of the wellbore closure evaluation. For each subdomain, a plane

axisymmetric analysis (2D-model) or an even faster one-dimensional axisymmetric analysis (1D-model) for sev-
eral depth values can be used. The 1D-models are simpler and more efficient since they represent the mechanical

behavior of a single independent lithology on a given depth. However, on lithology interfaces, the associated me-
chanical discontinuity is properly managed only by 2D-models because they aggregate a multi-lithology behavior

along a given depth interval. Since 1D-models are faster, first a full1D-model is used to estimate the error in the
displacement field on the wellbore. The error is measured based on a reference Full2D-model. Then, given a
target admissible error, an adaptive scheme that gradually increases 2D-model subdomains over critical regions
is employed. This scheme ensures that the maximum error at the wellbore displacement field is the considered
target error and defines an optimized subdomain partition. This proposal aims to use different case study scenarios
and observe how optimized subdomain partitions behave, in order to identify partitioning patterns according to the
characteristics of the scenario. It is expected that the proposed strategy produces good approximations, allowing
faster and more practical simulations aiming to assist oil well design and operational demands.