Why is playing near well sites dangerous?

Water or OIl? Both have heavy equipment, usually in dubious condition, set up by people who are busy and not particularly concerned about your safety. Both use various chemicals, hydraulics and electricity in ways an uninformed person would not expect. Oil wells are capable of venting non breathable and poisonous gases, as well as flammable ones. Under some circumstances both can undermine the ground you are standing on and drop you into a hole that you never even saw. Find someplace else to play.