def time_finite_acting(perm, poro, mu, ct, rw, re):
  """Calculate time at flow starts behaving infinite-acting"""
  r_D = re / rw
  t_Dw = 0.25 * r_D**2
  return (poro * mu * ct * (rw**2) * t_Dw) / (.0002637 * perm)