By the butterfly effect I assume that you mean the multiplicative effects of changing history. This was misinterpreted the first time which is why the thread was deleted.
I think it is fair to say that while the door hasn't been closed entirely on travel to the past, it may be that the energy requirements would far exceed anything possible for the foreseeable future - maybe beyond anything that will ever be possible. There is also the [physics based] argument that one could only travel back to the point in time that the time machine is first turned on.
The Future of Spacetime  has a nice series of essays on this subject.