In-Situ Combustion Frontal Stability Analysis

Abstract

Abstract Due to the complex chemical reactions and multi-phase flow physics, the displacement front stability for in-situ combustion (ISC) oil recovery processes is not well understood. In this work, we present the theory and numerical simulation for establishing analytical front stability criteria for ISC processes. We first analyze the four influencing factors for thermal displacement stability: viscous force, heat conduction, matrix permeability changes, and gravity. A thorough analysis of the different zones and displacement fronts in a typical ISC process is conducted, with the most unstable front identified. Second, we establish the analytical solutions for judging the frontal stability. Third, numerical reservoir simulation is performed to study the frontal stability/instability and also to validate the analytical theory. We have carefully selected differential schemes, spatial and temporal discretization to ensure the accuracy of these simulations. We have identified four major zones and three displacement fronts (reaction zone, leading edge of steam plateau, and oil bank leading edge) in a typical 1D ISC process. The most unstable front with the largest pressure gradient contrast is the leading edge of steam plateau. By establishing material and energy balance and solving the wavy perturbation of the steam front, we obtain the analytical equation for deciding the ISC flood frontal stability. In numerical simulations, we are able to obtain results with enough accuracy to capture unstable ISC displacements and show fingering behavior in different conditions. We have found matrix permeability reduction due to coke deposition has minimal impact on frontal stability. The simulation results are successfully validated with the analytical work for conditions where the ISC process is stable or unstable, which demonstrates its predictive capability for frontal stability. In conclusion, we have established a theoretical framework to analyze at certain conditions whether the displacement of an ISC process is stable or not. Numerical simulations confirm its predictive capability. It serves as a new reservoir engineering tool for the implementation and design of practical ISC projects.