According to tweets from BP the top kill procedure has begun.
Oil_Spill_2010
RT @BP_America: BP has started"top kill" operations to stop the flow of oil from the MC252 well in the Gulf of Mexico. #oilspill
[Update #1] BP has changed the source of their video feed. It now shows a close up of the blow out preventer.
[Update #2] CNN has a multi camera view available here.
I have been watching the video feeds and the video feed from bp showed the 4" pipe that was siphoning 2000 bpd accoring to bp yesterday now it shows an open pipe with oil flowing out of it and broken pieces of the siphon line. i fist saw it at 11:30 eastern but there hasn't been any reports of the change in condition on the news.
Here is the source page of all BP videos.
Here is my guesswork scenario based on a fluid flow courses I took in "pseudo* college".
They need to get the mud under enough pressure such that is creates a complete layer and overcomes the pressure from the oil and stops the flow so the cement can set.
They need to control keep the pressure and flow controlled so the mud stays together and fills the pipe.
The mud would prefer to flow out of both leaks so the mud added must be at enough of a flow rate such that there is more mud than the leaks can hold.
If they put too much mud in the flow will reverse until the mud exits the bottom of the pipe in the reservoir and the leak will begin again.
There are a few points where flow will stop and the cementing can begin.
The back pressure at the wellhead will decrease as the mud fills the pipe since the weight of the mud will counter the pressure from below.
At some point the BOP's shut off valves might be enough to hold that lower pressure.
If back flow into the pipe begins from too much mud the BOP's valves might seal and create a vacuum lock.
If back flow continues a balance point might be reached where the mud balances the reservoir's pressure and flow stops.
All those scenarios are temporary and the cementing needs to be complete before any pressure imbalance causes the still liquefied cement to be pushed out before it sets.
There is a chance the mud wont go down the riser pipe to the reservoir but would be enough to flow at pressure out the leak holes while holding the oil at the wellhead.
If they can sustain the mud flow until the relief well takes the pressure off the pipe this could be enough to stop the leak.
The question is can they get enough mud and keep it flowing into the BOP until the LMRP cap or the second BOP in installed.
*My "pseudo" college was considered, by those who taught it, to be the third must demanding engineering school in the country (and I may have already said too much).