I find this very interesting and thanks for confirming about the disposal well which is the question I asked about in my first post here back in April. Knowing nothing about all of this I have to wonder how the two wells would interact or has Devon given up the idea of drilling the second Harstine well at all or are the disposal well plans on hold or both. I know there was a slow down on disposal wells with the state I think but do not know where that stands now. Of course the results of the Sensibaugh well will be critical. Perhaps they would not interact with each other but with no knowledge of how these things work I wonder and as I write this I see Fang's post.
Without having access to Devon's development plan and drilling schedule, which I don't, it would not be inconceivable to me for Devon to slot in a disposal well into their 2012 well planning or to use serendipity to convert a well that intersected more favorable disposal zone(s) above the Marcellus shale. Most operator's annual development plans are very fluid, and it has been my experience to have wells deferred or accelerated into a current year's schedule, depending on rig, equipment, crew and material availability, of course.
The disposal zone would be different than the productive zone and also would be separated by hundreds or thousands of feet of rock and kept separated by steel casing and cement, if all is done properly, competently, and to industry standards.
A disposal zone above the Marcellus doesn't technically exist that far west as the formation is essentially non-existent out there. The best formation for an injection well is likely the Mt. Simon sandstone, which is a Cambrian age formation that is a quartzose sandstone with interbedded shale and dolomite. Above that is the Conasauga group, which lays over the Rome formation and is considered a safe place to keep fluid migration to a minimum. Underlying the Mt. Simon is a Pre-Cambrian layer of what is essentially granite, aka the Grenville Province. This formation is basically impermeable. That's where the majority of Ohio's injection wells are drilled to.