In　shale　oil　production　using　in-situ　thermal　recovery　method,　the　temperature　in　the　borehole　is　high　or　even　ultra-high,　and　it　is　easy　to　result　in　failure　of　the　sealing　integrity　of　the　cement　sheath.　It　is　thus　important　to　study　the　changes　of　the　mechanical　characteristics　and　the　porosity　and　permeability　of　a　set　cement　under　high　or　ultra-high　temperatures.　Using　digital　image　technology,　changes　of　the　failure　mode　of　a　set　cement　after　high　temperature　treatment　is　analyzed.　Using　low　field　NMR,　the　changes　of　the　porosity　and　permeability　of　a　set　cement　after　high　temperature　treatment　are　studied,　and　the　correlation　of　porosity　and　permeability　is　analyzed.　The　study　shows　that　for　a　high　temperature　treated　set　cement　sample,　as　the　temperature　increases,　the　monoaxial　compressive　strength　of　the　set　cement　first　increases　and　then　decreases.　The　porosity　and　the　permeability　of　the　set　cement　increase　continuously;　at　first,　the　increase　goes　slowly　and　then　goes　fast.　This　is　because　at　lower　temperatures,　there　are　only　micro　and　small　pores　produced　inside　the　cement　and　these　pores　are　not　inter-connected　with　each　other.　As　the　temperature　increases,　some　micro　and　small　pores　combine　into　medium　and　large　pores,　and　pores　of　different　sizes　begin　to　connect　with　each　other,　thereby　increasing　the　permeability　of　the　set　cement.　The　achievements　of　the　study　have　important　reference　significance　for　protecting　the　sealing　integrity　of　cement　sheath　and　optimizing　cement　slurry　formulation　in　in-situ　thermal　recovery　of　shale　oil.　©　2022　North　China　Petroleum　Administration　Drilling　Technology　Research　Institute.　All　rights　reserved.