Salt Water Disposal Wells

Are salt water disposal wells normally installed every so many miles if there's salt water production in the area? Can the injection of salt water push more oil out of nearby wells? Does the presence of a nearby salt water well encourage or discourage the drilling of an oil production well nearby? If the salt water well is 6500 feet deep, would anyone expect it to ever seep into potable water at the 100' to 200' depths? I already have salty water at about 200' in Sec17, T19N R3E.