The mixed injection of N

2

and CO

2

shows great potential for enhancing the productivity of low-yield wells and for CO

2

sequestration. This process involves complex interactions

including multicomponent gas competitive adsorption

multiphase porous flow

mass transfer

and heat exchange. However

the differential responses of coal reservoirs with different lithotypes to N

2

-CO

2

mixed gas injection for enhanced coalbed methane (ECBM) recovery and their optimal injection ratios remain unclear. This study focuses on high-rank coals in southern Qinshui Basin. Using a multi-scale pore structure characterization approach

the pore and fracture characteristics of four coal samples were systematically analyzed

as well as their differences in N

2

CO

2

and CH

4

adsorption behaviors. The evolution patterns of gas-water and CO

2

-CH

4

systems during the displacement process were investigated. Furthermore

thermo-hydro-mechanical (THM) coupled numerical simulations were conducted to reveal the differential response processes and underlying mechanisms of various coal reservoirs during N

2

-CO

2

injection for ECBM recovery. The results show as follows . (1) The pore systems of all coal types are dominated by micropores

with the specific surface area of micropores accounting for up

to 99%

indicating significant CO

2

sequestration potential. Mesopores and macropores are relatively well developed in semi-dull and dull coals. The overall adsorption capacity for multiple gases increases with increasing vitrinite content. Bright and semi-bright coals exhibit stronger adsorption capacities for N

2

CO

2

and CH

4

as well as more significant variances in Langmuir volume

implying more remarkable competitive adsorption effects. (2) During the displacement process

fracture water and seepage pore water are readily expelled

whereas adsorbed water requires higher pressure for adsorption-driven displacement. The volume of displaceable water increases nonlinearly with rising gas pressure and gradually approaches saturation. Due to its stronger adsorption affinity

CO

2

can significantly enhance CH

4

production efficiency and increase CO

2

sequestration capacity under high-pressure conditions. However

its displacement effectiveness is controlled by the adsorption properties and seepage conditions of the coal matrix

reflecting the strong coupling among gas displacement

water migration

and mechanical response during the ECBM process by CO

2

injection. (3) Numerical simulation results indicate that the mixed injection of N

2

and CO

2

promotes the migration of high-concentration CH

4

toward production wells under the effect of "far flooding and near displacement". Compared with pure CO

2

injection

the mixed injection increases CH

4

production by an average of 65.08%and enhances CO

2

sequestration capacity by 17.71%. Among different coal lithotypes

semi-dull and dull coals exhibit a more intense CH

4

concentration response and greater carbon storage capacity; however

their lower diffusion coefficients result in relatively reduced CH

4

production rates. (4) During the mixed gas injection process

the relatively rapid migration of N

2

tends to cause early breakthrough

thereby shortening the production cycle. Overall

as the proportion of CO

2

in the mixture increases

the N

2

breakthrough time exhibits a trend of first decreasing and then increasing

with the shortest production cycle occurring at a CO

2

fraction of 50%. For optimal CO

2

sequestration in reservoirs with different coal lithotypes

the recommended CO

2

concentration in gas mixture ranges from 55%to 90%

with the main differences reflected in the extent of CH

4

production enhancement. Comprehensive evaluation indicates that the optimal CO

2

injection ratios for different coal lithotypes are as follows: 55%-65%for bright and semi-bright coals

60%-70%for semi-dull coal

and 65%-75%for dull coal

balancing the enhancement of CH

4

production and the maximization of CO

2

sequestration. These results provide a theoretical basis and technical support for the implementation of ECBM recovery by N

2

-CO

2

mixed gas injection in reservoirs with different coal lithotypes.