Pore-scale study of coke formation and combustion in porous media using lattice Boltzmann method

Abstract

In-situ combustion (ISC) has long been recognized as a promising technique for heavy oil recovery. However, ISC includes multiple physicochemical processes, which are still poorly understood and difficult to predict and control. This study establishes a lattice Boltzmann (LB) model to simulate the two important aspects of ISC at the pore scale: coke formation and combustion. The LB model includes thermal expansion effects and solves the reactive air-coke interface without iterations. Moreover, this model improves upon previous models by considering both coke formation and two-step coke combustion, as well as the growth of solid geometry. Results show that the LB model correctly captures coke combustion properties. Meanwhile, the newly introduced coke formation and two-step combustion yield important findings. As heat released from combustion transfers downstream, oil cracking and coke formation ahead of the combustion front are successfully tracked. The generated coke fuels the upstream combustion, making the system self-sustained. During coke formation and combustion, four coke transition states are identified. In addition, a parametric study demonstrates that the large inlet oxygen content and driving force are desirable, while too high a driving force should be avoided as it causes high burning temperature. Furthermore, it suggests that the inlet air temperature should be set appropriately. On one hand, a high temperature may promote coke formation and retard the front propagation. On the other hand, a low temperature may slow down the combustion of coke 2, even though it is high enough to ensure the ignition of coke 1. The decelerated coke 2 combustion may further cause the insufficient heat release and the failed coke formation, thus inducing the early termination of combustion. Such effects of the inlet temperature indicate the necessity of considering coke formation and two-step coke combustion. These results help to improve the understanding and facilitate the development of ISC.