Low-temperature Oxidation Models And Thermal Effect Study For Light Oil Reservoir Air Injection

Air injection into light oil reservoiris is an effective technique for improved oil recovery,and this technique mainly depends on a spontaneous low temperature oxidation (LTO) whichoccurs at original reservoir temperatures. During LTO process of air injection, oxygen in theinjected air is consumed and carbon dioxide can be produced in the reservoir, leading to fluegas flooding and producing a significant amount of heat.In this paper, the mechanisms of air injection IOR and LTO reaction are analyzed, andan improved LTO reaction model is established to facilitate reaction mechanism and reservoirsimulation studies. A series of high-pressure LTO experiments using typical light oils werecarried out in a temperature range from70℃to170℃to measure the reaction rates, verifythe reaction model, and establish the kinetic model of LTO reaction.Heat transfer analysis in the experimental process was conducted to examine theexothermic characteristics of the reaction using different reactors and at various experimentalconditions. Typical light oils LTO experiments results showed that a significant temperaturerise occurred only at high temperatures (170℃) due to the thermal effect of LTO reaction.This phenomenon confirmed that no temperature rise could be observed with the availabletemperature detector because of the low reaction rate and heat loss during the reactions at lowtemperatures (<160℃). Model calculation results showed that there might be a temperaturerise of over100℃in the reactor if there was no heat loss. An accelerating stage exists inthe LTO process, and when temperature enters this stage, the reaction rate will increaserapidly, leading to a faster heating rate and an enhanced thermal effect.A conceptual simulation model of typical low-permeability reservoir was built based onexperimental study using CMG software. Another two models, N2injection model andisothermal controlled air injection model, were also built to serve as comparison, in order toreveal the IOR effect contributed by LTO thermal effect. The simulation results showed theIOR mechanisms of LTO thermal effect are: a) To cause a significant temperature rise, whichreduces the viscosity of crude oil, even cracking of the oil molecules, which make it easier toflow underground. b) To boost LTO reactions, leading to more CO2to be produced, and this will enhance the effect of flue gas flooding. c) More CO2will dissolve in the oil, reducing oilviscosity and causing the oil to swell, adding to the elastic energy of the reservoir. d) Gas inthe formation will swell because of the temperature rise, which will contribute to themaintenance of reservoir pressure.