Chemical-structural Properties Of The Coke And Reactive Transport Of The Combustion Front During Crude Oil In-Situ Combustion

In-situ combustion?ISC?is an enhanced oil recovery technique for unconventional heavy crude oil recovery.The main criterion for a successful ISC process is the sustainable propagation of the combustion front through the reservoir.The complexity of the ISC process is increased because of the multi-physics and reactive transport in reservoir porous media,especially at the combustion front.Fundamental analyses are highly required to understand the complex phenomena at the combustion front.In the ISC Process,coke,an important solid product from low temperature oxidation?LTO?and thermal cracking of the crude oil,acts as a fuel for the self-sustaining combustion front.The study presents a novel methodological insight into the coke formation with the coke chemical-structural properties characterized using a combination of EDS,ATR-FTIR,Raman,XRD and HRTEM techniques.Such coke formation and characterization technique was demonstrated as an easy,quick and controlled method for fundamental studies.Effects of reaction temperature and gaseous atmosphere on the chemical-structural properties of the Xinjiang crude oil coke were then investigated.Element compositions,surface functional groups and carbon structures of the LTO coke and the pyrolytic coke were compared to further understand the coke formation during the crude oil ISC.The study shows that the LTO coke and the pyrolytic coke mainly consist of amorphous carbonaceous structures with trace amount of‘onion-like'structures in poor order.The characteristic ISC combustion front,less than 550^oC,cannot initiate the polycondenstion and planar stacking of the aromatic ring systems.Compared to the pyrolytic coke,the oxygen/carbon ratio and oxygen functionalities of the LTO coke is one order of magnitude higher than the pyrolytic coke.The products of the oxidative cross-linking reactions are unstable,leading to much higher apparent oxidation rate of the LTO coke than the pyrolytic coke.Coke deposition at the combustion front is an important phenomenon that significantly impacts the pore topology and permeability.X-ray computed microtomography and a specific image processing procedure were used to reconstruct the micro-tomographic images of coked packed beds with the pixel resolution of 500 nm.From the reconstructed images,the microstructural parameters related to the transport were analyzed,such as the pore size distribution,the constrictivity and the geodesic tortuosity.The Lattice Boltzmann method was used to simulate the species diffusion and fluid flow through the microstructures to quantify the mass diffusivity and permeability.The pore-scale characteristic of digital microstructures and the simulated permeability were calibrated against the experimentally determined results.The effects of the coke deposition on the pore topology and the permeability were analyzed.The coke deposition pattern shows significant deposition in the pore throat.Analyses of the pore size distribution lead to a more reasonable geometric approach to measure the effective hydraulic radius for the better permeability prediction.Based on the effective transport properties and the microstructural parameters,a developed permeability relation was introduced by factorizing the permeability into two distinct contributions from the wall friction effect and the microstructural effect.A main effect analysis reveals the decreasing porosity is the most important parameter on the permeability reduction,followed by the reduced pore throat size.Additionally,the study provides an empirical relation linking the permeability reduction rate and the coke concentration for the engineering application of ISC or THAI processes in the well-sorted sandstone reservoirs.At the combustion front of the crude oil ISC process,the multiple physicochemical and thermal processes simultaneously takes place in porous media,including the Naiver-Stokes fluid flow,species transport,conjugate heat transfer and heterogeneous reactions.This study presents a pore-scale numerical model based on lattice Boltzmann method to investigate the coupled reactive transport.A thermal counter-slip algorithm was developed to solve the conjugate heat transfer and heterogeneous reactions on the gas-solid interface,while volume of pixels method was used to track the structure evolution with the coke burned out.The accuracy of the numerical model was calibrated against analytical solutions,benchmark cases and COMSOL software.The pore-scale simulation was then applied to explore the dynamics of the in-situ combustion front for a wide range of Péclet and Damk?hler number in a homogenous porous medium with the combustible coke coated on the rock particles.Non-local thermal equilibrium and the reaction-trailing structure were observed at the combustion front.The temporal evolutions of the microscopic temperature and O₂ concentration field and the residual coke distribution were analyzed to yield the knowledge of the oxygen utilization rate,temperature increase rate and front velocity as a function of the Péclet and Damk?hler number.Four control mechanisms of the reactive transport were identified at different Péclet and Damk?hler number regimes,including diffusion-controlled,convection-controlled,kinetic-controlled and control transition regimes.In the diffusion-controlled regime,the temperature of the combustion front can maintain quasi-steady-state due to the energy balance,while the front velocity is determined by the O₂ diffusion flux.By comparison,our work suggests the diffusion-controlled process is desirable in terms of the manageable temperature,moderate front velocity and complete fuel and oxygen conversion.The critical Péclet and Damk?hler numbers were derived for the diffusion-controlled regime in the given porous medium.Finally,some insights were discussed to help engineers concerned with the combustion front control.