Improved Streamline-based Simulation for CO2 Storage
Creators: name: Lazaro Vallejo, Lorena
Publisher: name: Imperial College London
Year: 2012

description: CO2 Storage is one of the key technologies to mitigate climate change at a large scale and ensuring that the injected CO2 stays trapped underground is one of the main challenges. It is critical to develop fast and more physically accurate methods for CO2 storage simulations, otherwise computation times become prohibitive, especially when geological uncertainty is large, as in deep saline aquifers. Injection strategies and geological uncertainty have an impact on how much CO2 can be trapped as residual saturation. Fast and accurate simulators such as the one in this work are necessary to run the large number of simulations used in optimising CO2 sequestration. Our existing research streamline simulator has been extended with two improved thermodynamic models to maintain thermodynamic equilibrium along the streamlines. This minimises time-step size dependence and convergence errors. 1D simulation along the streamline was compared against analytical solution. Models were validated on 2D and 3D sections of the SPE 10th Comparative Model using water alternating gas (WAG) injection followed by chase brine. Results show that both new thermodynamic algorithms are faster (lower CPU cost) and have a faster convergence of results than the previous algorithm. Based on the validated model, we run 3D simulations for a single well strategy for the stage 2 CO2CRC Otway Project to test residual trapping. Simulation results were compared to TOUGH2 (finite-difference simulator) simulation results to study numerical dispersion, convergence of results and CPU times. Streamline simulations decreased computational time by a factor of five but results were not in agreement. Streamline simulations simulate advection processes accurately. However, there are other non-advective processes, such as diffusion, dispersion and buoyancy effects, which streamlines cannot simulate properly. This could cause the differences between streamline and TOUGH2 simulation results. Incompressibility was one of the main assumptions of the streamline-based simulator and this could pose some challenges when trying to simulate CO2 sequestration projects where injection strategy is complex. The CO2 streamline code was extended to add compressibility. Supercritical CO2 is slightly compressible so including compressibility in the streamline code is important to be able to model the physics more accurately. Streamlines can now end anywhere in the reservoir. Expansion or contraction of fluids can create source or sink cells which act as injection/production cells. Initially the pressure profile obtained numerically was compared to the analytical solution for radial single-phase flow and 1D simulations were run to study the effect of compressibility on the saturation profile. 2D simulations of a slightly compressible case on the SPE10 geological model were compared to ECLIPSE simulations, resulting in good matching. Then, the 3D Otway field case was re-simulated using the compressible code and results were compared to those obtained by TOUGH2 without obtaining a good agreement due to the complexity of the case. With most of the storage potential being in geological formations which are poorly characterised, monitoring will be a central part of any CO2 storage project. We have adapted a new approach for streamline-based history matching which exploits the analogy between the propagation of a wave front and the pressure front in the reservoir. This approach uses diffusive time-of-flight which determines the velocity at which pressure propagates as a function of static and fluid properties. This tool enables us to reconcile response data with static geological data at an earlier time, improving the management of the project. This approach has been applied to drawndown-buildup well test for a 2D synthetic case and a 3D real field case. Results for both cases were satisfactory, showing a clear improvement in the pressure matching after the 10th iteration in most cases.