Efficient Simulation of Thermal Enhanced Oil Recovery Processes

Book excerpt: Simulating thermal processes is usually computationally expensive because of the complexity of the problem and strong nonlinearities encountered. In this work, we explore novel and efficient simulation techniques to solve thermal enhanced oil recovery problems. We focus on two major topics: the extension of streamline simulation for thermal enhanced oil recovery and the efficient simulation of chemical reaction kinetics as applied to the in-situ combustion process. For thermal streamline simulation, we first study the extension to hot water flood processes, in which we have temperature induced viscosity changes and thermal volume changes. We first compute the pressure field on an Eulerian grid. We then solve for the advective parts of the mass balance and energy equations along the individual streamlines, accounting for the compressibility effects. At the end of each global time step, we account for the nonadvective terms on the Eulerian grid along with gravity using operator splitting. We test our streamline simulator and compare the results with a commercial thermal simulator. Sensitivity studies for compressibility, gravity and thermal conduction effects are presented. We further extended our thermal streamline simulation to steam flooding. Steam flooding exhibits large volume changes and compressibility associated with the phase behavior of steam, strong gravity segregation and override, and highly coupled energy and mass transport. To overcome these challenges we implement a novel pressure update along the streamlines, a Glowinski scheme operator splitting and a preliminary streamline/finite volume hybrid approach. We tested our streamline simulator on a series of test cases. We compared our thermal streamline results with those computed by a commercial thermal simulator for both accuracy and efficiency. For the cases investigated, we are able to retain solution accuracy, while reducing computational cost and gaining connectivity information from the streamlines. These aspects are useful for reservoir engineering purposes. In traditional thermal reactive reservoir simulation, mass and energy balance equations are solved numerically on discretized reservoir grid blocks. The reaction terms are calculated through Arrhenius kinetics using cell-averaged properties, such as averaged temperature and reactant concentrations. For the in-situ combustion process, the chemical reaction front is physically very narrow, typically a few inches thick. To capture accurately this front, centimeter-sized grids are required that are orders of magnitude smaller than the affordable grid block sizes for full field reservoir models. To solve this grid size effect problem, we propose a new method based on a non-Arrhenius reaction upscaling approach. We do not resolve the combustion front on the grid, but instead use a subgrid-scale model that captures the overall effects of the combustion reactions on flow and transport, i.e. the amount of heat released, the amount of oil burned and the reaction products generated. The subgrid-scale model is calibrated using fine-scale highly accurate numerical simulation and laboratory experiments. This approach significantly improves the computational speed of in-situ combustion simulation as compared to traditional methods. We propose the detailed procedures to implement this methodology in a field-scale simulator. Test cases illustrate the solution consistency when scaling up the grid sizes in multidimensional heterogeneous problems. The methodology is also applicable to other subsurface reactive flow modeling problems with fast chemical reactions and sharp fronts. Displacement front stability is a major concern in the design of all the enhanced oil recovery processes. Historically, premature combustion front break through has been an issue for field operations of in-situ combustion. In this work, we perform detailed analysis based on both analytical methods and numerical simulation. We identify the different flow regimes and several driving fronts in a typical 1D ISC process. For the ISC process in a conventional mobile heavy oil reservoir, we identify the most critical front as the front of steam plateau driving the cold oil bank. We discuss the five main contributors for this front stability/instability: viscous force, condensation, heat conduction, coke plugging and gravity. Detailed numerical tests are performed to test and rank the relative importance of all these different effects.