Why does it need to be filtered? It is a disposal well. If there is any contamination, then there must be either a poor cement job or a hole in the casing.
All wells, oil or gas, produce a mix of oil and water. The amount of the water is called “the cut”. The water is crudely separated, but is slick, emulsified and oily.
In PA, the Governor has put a moratorium on using disposal wells, as the oily slop is definitely having adverse effects of groundwater quality. Drillers and operators are paying to haul the produced water to OH and WV for disposal, putting a serious dent in the bottom line.