How to merge values as given index in a single array

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;