Increasingly, operators around the world are using wells with multiple tubing strings to improve both injection and production of fluids in a reservoir. Many operators are beginning to use this technology for designing and optimizing thermal, heavy oil recovery. For example, steam chamber development in a Steam Assisted Gravity Drainage (SAGD) process along a horizontal well pair is often irregular and dual tubing strings may help to even out both injection and production. Problems that operators have with multiple tubing strings in wells include both their control and design. There is a need for a rigorous, accurate and robust method to model these wells as part of the reservoir simulation stage of field planning and development in order to design these wells to improve and optimize oil production and net present value (NPV).
This paper describes a unified thermal wellbore model that fulfills these requirements. The wellbore model allows flexible control of THP, gas lift rates, injection and production rates for each of the tubing strings. There are additional features for optimizing various aspects of the well operation using instantaneous parameters. Improved accuracy in modeling the multiphase flow of fluids and energy is obtained by allowing the tubing strings to consist of multiple segments and branches containing groups of segments. Each segment has its own set of physical parameters including pressure, temperature, phase holdups and frictional/gravitational/accelerational heads. Any number of segments are allowed, the segments are not confined to the grid and can even be specified outside the grid (up to surface) and a flexible conductive heat transfer facility allows multiple heat transfer paths to exist to and from each segment. Additional facilities are described which allow control and optimization of wells and/or individual tubing strings in addition to controllable devices (ICDs, FCVs) using instantaneous or longer term parameters. Two case studies are presented which illustrate some of the simulation workflows. These studies are based on fluid and reservoir properties and well designs typical of the Surmont reservoir which is part of the McMurray formation in Alberta Canada.