Flowing wells can be very difficult. Suffice to say if you are going into an area you suspect to have flowing wells ie: recharge zones or valley train in elevated intermontane valleys always use double casing with a grout seal. More expensive, sure but take it from me saves a pile of worry and "egg on face" in the end. For sealing an already flowing well with no outer conductor casing it completely depends on shut in pressure. 2-10 psi probally packers and injection of heavy grout. Greater than 10 psi and you may want to consider directional drilling with PK set up and set a radial grout plug. In any case dont shut off flow if >5 psi, regardless of how well you think casing is sealed into native materials (skin friction) as it may start leaking around annulus and then you are hooped.