In our work, counter-propagating THz pulses were used to accelerate electrons from a Krypton gas jet. Using numerical simulations, we investigated three different ionization arrangements: (i) the ionizing laser pulse propagates in the same direction as the accelerated electrons, (ii) the ionizing laser pulse propagates in the opposite direction to the accelerated electrons, (iii) two counterpropagating ionizing laser pulses generate the electrons. We showed that the ionization process has a significant effect on the acceleration efficiency and the energy spectra. We investigated the acceleration with fC-pC initial charge using the three different ionization techniques.