Hydrate-based　CO2　sequestration　(HBCS)　in　post-exploitation　natural　gas　hydrate　(NGH)　reservoirs　via　forming　CO2-CH4　mixed　hydrate　has　been　proposed　to　achieve　CO2　sequestration　and　geological　restoration　simultaneously.　However,　existing　numerical　models　struggle　to　accurately　simulate　the　mixed　hydrate　formation　and　geomechanical　responses.　Thus,　we　proposed　a　numerical　method　to　solve　the　limitations　of　the　existing　model　that　can　only　consider　pure　CH4　and　CO2　hydrates,　which　was　verified　by　laboratory　experiments　with　an　error　of　≤3.54　%.　Then,　we　evaluated　the　HBCS　in　the　post-exploitation　NGH　reservoir　under　medium-temperature　and　high-pressure　conditions　(10–15　°C,　11–16　MPa)　based　on　geological　data　at　Site　W17.　Results　show:　(i)　Depressurization-induced　NGH　dissociation　enlarges　pore　spaces　and　causes　subsidence,　while　CO2　injection　restores　pressure　and　forms　mixed　hydrates　with　residual　CH4　for　structural　support.　(ii)　The　injection　process　may　cause　hydrate　accumulation　near　the　wellbore,　blocking　the　seepage　channel　and　inhibiting　the　sequestration　effect.　(iii)　Some　NGH　stimulation　measures　have　a　synergistic　effect　with　HBCS.　For　example,　complex　structural　wells　allow　CO2　to　be　injected　more　dispersed,　forming　a　wider　area　of　mixed　hydrates　and　avoiding　local　blockage.　(iv)　The　high　permeability　case　(20　mD)　exhibits　superior　synergy　effects　with　an　energy　efficiency　of　448　and　a　sequestration　capacity　of　5786　tons,　but　the　higher　the　exploitation　degree,　the　worse　the　geological　restoration　effect.　These　findings　provide　a　reference　for　future　HBCS　in　post-exploitation　NGH　reservoirs,　including　blockage　risk　mitigation　and　well　design　optimization,　obtaining　additional　benefits　from　carbon　trading　to　reduce　costs.　©　2025　Elsevier　Ltd