How can I calculate critical velocity of injected water and gas in horizontal well during the WAG process in black oil reservoir? should I consider critical velocity at all?

I have written data file of CMG, GEM simulator for WAG process through the horizontal well injector and producer. without attention to critical velocity, with increasing injection velocity, the max recovery factor achieved in earlier time. 

but water breakthrough occurs very faster than the case with critical velocity injection. What should I do ? can any one help me?