In thermal recovery, reservoir and wellbore temperature distributions are important parameters that must be calculated precisely. Most drilling and completion simulators use an overall constant heat transfer rate for the reservoir-wellbore heat balance. This approach is not accurate for thermal applications, as temperature profiles and thermal properties of the fluids change both in space and time. This article describes the development of a combined wellbore-near well region simulator capable of fast and accurate modeling of temperature and flow profiles in the stated region without the difficulties of building a three-dimensional (3D) reservoir model.
This work discusses the development of a coupled combined wellbore-near well region thermal simulator for completion design in thermal recovery fields. It consists of two parts: a two-dimensional (2D) wellbore simulator and a thermal hydraulic near-wellbore reservoir model assuming elliptical symmetry of the temperature and flow profiles around the wellbore. The simulation domain is fully discretized along the longitudinal and radial directions. Heat and mass transfer profiles in the wellbore and formation are explicitly coupled at each time step.
Reservoir models usually use rectangular grids, while the wellbore proximity (3 to 10 m) suggests radial topology is more optimal for thermal recovery modeling. The developed thermal simulator accounts for the dependence of fluid properties and thermal conductivities on both pressure and temperature. Wellbore heat transfer and production varies along the well length and with time and should consider the completion type. For testing and validation of the developed thermal solver, a 3D finite volume computational fluid dynamics (CFD) simulator was used, and good agreement was demonstrated in tested cases. For one of the validation examples, an injection of hot water into the formation was modeled. The simulator accurately predicted both the propagation of the water front and temperature field variation with the time. In contrast to many traditional wellbore simulators that account only for thermal conductivity in the reservoir, the developed simulator also considers convective heat transfer (by the movement of the fluids), making it applicable for lower completion zones.
In completion design simulations, the local near-wellbore scale of thermal recovery methods often does not justify the application of a full-field 3D reservoir simulator. Because of the 2D topology used, the proposed simulator exhibits excellent stability and speed, which are usually the weak points of thermal reservoir simulators, and provides engineers with an easy tool for modeling fluid and heat exchange between the reservoir and a wellbore over time.
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