Mixing characteristics in perforation due to the delivery of combustion-supporting agent by jet for in-situ combustion of methane fracturing

CAI Chengzheng; GAO Ya'nan; GAO Mingzhong; TAO Zhixiang; SUN Weifu; WANG BO; ZHU Zhengchao; ZOU Zengxin

doi:10.13225/j.cnki.jccs.2024.1138

Abstract：As a disruptive reservoir stimulation technology in unconventional natural gas development,in-situ methane combustion fracturing generates complex fracture networks through high-temperature and high-pressure shockwaves pro-duced by the combustion of reservoir-desorbed methane mixed with surface-injected combustion-supporting agent at the wellbore bottom.The core mechanism lies in creating methane and combustion-supporting agent mixtures within specific concentration ranges to induce sufficient combustion pressure for reservoir fracturing in confined spaces.However,pre-cise combustion-supporting agent injection and mixing control under the complex flow field conditions of the wellbore-reservoir system remain critical bottlenecks hindering its engineering application.To address the limitations of traditional fixed injection methods,such as low mixing efficiency and elevated wellbore safety risks,this study proposes a mobile jet-based combustion-supporting agent injection approach.A three-dimensional computational fluid dynamics(CFD)model was established to simulate the methane-combustion-supporting agent mixing flow field in the wellbore and formation per-forations.The evolution of the mixing flow field under mobile jet action was investigated,with comparative analyses of four nozzle orientations:axial forward,oblique forward,radial sideward,and oblique backward.The results indicate that when the jet moves along the wellbore,intense shear interactions between the jet boundary layer and perforation inlet gen-erate turbulent flow fields dominated by vortex structures near the perforation entrance and within the perforation channel.After mass exchange at the perforation entrance,the combustion-supporting agent and methane undergo further diffusion and mixing driven by tangential velocity within the perforation.By optimizing nozzle orientation and injection parameters,the methane volume fraction inside the perforation can be controlled within the combustion limit,while diluting the meth-ane concentration in the wellbore below the lower explosive limit,ensuring localized combustion exclusively within the perforation.Significant differences in mixing efficiency were observed among nozzle orientations:radial sideward nozzles generate strong swirling flow fields(tangential velocity:1.05 m/s)through orthogonal jet impingement,promoting circum-ferential uniform diffusion of the combustion-supporting agent along the perforation channel,with mixing inhomogeneity notably lower than other orientations.Although axial forward nozzles exhibit weaker perforation mixing uniformity com-pared to radial sideward nozzles,they achieve optimal wellbore dilution(methane volume fraction<4%).Comprehensive evaluation of methane concentration and flow field heterogeneity ranks the mixing performance of perforation combustion-supporting agent injection in descending order:radial sideward jets,oblique forward jets,oblique backward jets,and axial forward jets.Axial forward jets excel in wellbore methane dilution,whereas radial sideward jets demonstrate superior per-foration-channel mixing uniformity.To further enhance injection efficacy,a hybrid configuration combining radial side-ward and axial forward nozzles is proposed.This strategy improves both combustion-supporting agent injection and mix-ing homogeneity within the perforation while maintaining wellbore safety.The findings provide theoretical guidance for optimizing combustion-supporting agent injection in in-situ methane combustion fracturing operations.