ABSTRACT:

Thermal recovery of heavy oil by injection of steam into the reservoir has been attempted in the Bohai oilfield of China during the past several years. However, field data showed that wellbore instability and sand production has become a major problem. Injection of high temperature steam into the reservoir increases the pore pressure and the stresses around the wellbore and meanwhile decreases the strength of the formation rock, resulting in failure of the formation. It is highly desirable to mitigate the wellbore instability and sand production by optimizing the injection and production parameters. In this work, we report results from detailed numerical modeling of the cyclic steam stimulation of a heavy oil reservoir. A fully coupled thermo-hydro-mechanical finite element model has been employed for characterizing the complicated process involving multiphase fluid flow, heat transfer and elastoplastic deformation of the formation rock. Both conduction and convection of heat are considered in the model to honor the fact that heavy oil reservoirs are usually of high permeability. Effects of formation permeability, injection rate, injection temperature as well as wellbore pressure drop on the stress and pore pressure distributions around the wellbore and the plastic deformation of the formation rock were presented. Results show that for relatively low permeability reservoir, high injection rate would lead to high injection pressure and thus result in wellbore expansion due to excessive plastic deformation of the near wellbore region. Some implications for maintaining wellbore stability and reducing sand production have been provided based on the numerical modeling results.

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