Coke Deposition During Air Injection Assisted Cyclic Steam Stimulation Process: Mechanism Study and Field Impact Analysis

Abstract

Abstract Air injection assisted cyclic steam stimulation (CSS) is an innovative and effective process for enhancing ultra heavy oil recovery and increasing the efficiency of steam injection, which has been successfully applied in over 200 wells in Liaohe oilfield (Northeast China) in recent years. During this process, low temperature oxidation (LTO) and pyrolysis of crude oils may take place spontaneously under elevated reservoir temperatures (around 300°C) due to steam and air coinjection. There have been concerns over the formation and deposition of coke-like substance in the reservoir and their impacts on reservoir permeability and oil production. In this study, the mechanisms of coke formation during this process were investigated based on experimental results, and the effects of LTO reactions on coking and oil pyrolysis were analyzed. A mechanistic model of coke formation in the presence of air was constructed, in which the coking process was modeled based on the partial equilibrium reaction. The model was then incorporated into a reservoir simulator, and a field scale reservoir simulation study of this process was conducted in order to reveal the impacts of coke deposition on formation properties and oil recovery performance. The simulation results demonstrate that coke deposition can reduce the fluid porosity and permeability in the near well region, which will induce a detrimental effect on heavy oil production. Therefore, the possible reduction of formation permeability due to precipitation of coke-like substance should be considered. The field performance of this technique conducted in a typical ultra heavy oilfield was simulated and presented along with the analysis of the possible coking effect. Injection of air with reduced oxygen content is proposed for reducing the coking effect, and the simulation results show that injection of air with oxygen content of 10 vol% will be beneficial to retarding coke formation and deposition, and moreover the safety hazard of explosion will also be eliminated in field operation. This study can provide new insights into the numerical modeling of the coking process and deepen the understanding of the air injection assisted CSS process for extracting oil from ultra heavy oil reservoirs.