Use logical indexing:
porindex1 = por<=0.47; %% por<=0.5 porindex2 = por>0.47&por<=0.63; %% por>0.47& por<=0.63 porindex3 = por>0.63;
exponent = zeros(size(por)); exponent(porindex1) = 1.2; exponent(porindex2) = 1.6; exponent(porindex3) = 2;
k_fl = (k_water./Sw - k_co2).*(1 - Sg).^exponent + k_co2;