Update: early this evening, per NYT, BP reports it resumed the effort about 3:45 Central. Also, OilDrum now has a plausible explanation of what BP may be doing with each pause/resumption]

The New York Times is reporting that BP suspended its "top kill" efforts late last night (2:30 a.m.) after it became apparent the effort to clog up the blowout preventer (BOP) with a "junk shot" was failing to keep mud from gushing out the BOP and riser pipe. If you can’t keep enough mud in, it can’t work. That’s not good.

From the Times:

BP’s renewed efforts at plugging the flow of oil from its runaway well in the Gulf of Mexico stalled again on Friday, as the company suspended pumping operations for the second time in two days, according to a technician involved with the response effort.

In an operation known as a “junk shot,” BP engineers poured pieces of rubber, golf balls and other materials into the crippled blowout preventer, trying to clog the device that sits atop the wellhead. The maneuver was designed to work in conjunction with the continuing “top kill” operation, in which heavy drilling liquids are pumped into the well to counteract the pressure of the gushing oil.

If the efforts succeeded, officials intended to pump cement into the well to seal it. But the company suspended pumping operations at 2:30 a.m. Friday after two junk shot attempts, said the technician, who spoke on the condition of anonymity because he was not authorized to speak publicly about the efforts. . . .

On ABC’s “Good Morning America” on Friday, Admiral Allen said the top kill effort was continuing, and that BP engineers had been able “to push the hydrocarbons and the oil down with the mud.”

But the technician working on the effort said later Friday that despite the injections at various pressure levels, engineers had been able to keep less than 10 percent of the injection fluids inside the stack of pipes above the well. He said that was barely an improvement on Wednesday’s results when the operation began and was suspended after 11 hours. BP resumed the pumping effort Thursday evening for about 10 more hours.

“I won’t say progress was zero, but I don’t know if we can round up enough mud to make it work,” the technician said. “Everyone is disappointed at this time.”

So, just as we described in this post last evening, they’re losing too much mud being blown out of the BOP and out the riser pipe by the pressure from oil/gas gushing up the well. That means they can’t yet get enough mud down the column to overcome the massive pressure of gas/oil gushing up the well and force that column of fluids back down the well. And the "junk shot" has so far not clogged up the BOP enough to reduce the loss of mud out the BOP top.

Tough problem. What next?

And note that Admiral Allen’s media appearances this morning, which used the standard BP line, "it’s going according to plan" and that they’d stopped the oil gushing out (which may be true only for the moment) once again did not reflect what BP knew. Was he misled? And what did he tell the President during the briefing?

More:
The folks over at OilDrum assess the result earlier today:

UPDATE 5: 11:09 AM The feed has gone back to the riser, and we are back to the oil and gas flows that we were saw at the beginning. Not quite the same shapes as earlier, so perhaps the block in the BOP was partially effective, but BP have now apparently filled the well twice and failed to get enough weight into the mud to hold the driving pressure from the rock. They could try again with a higher density mud, I am presuming that the second shot had a higher weight than the first, and that while the first left a small pressure imbalance, that the second was closer, but as yet no banana. (Though the Admiral did say that they had stabilized the flow). My presumption is that they will mix up another batch and try again – though whether they will try another junk shot is not clear.

The way in which you try to stop leaks is that you put the big stuff in first. If you can get enough of that to stick, it still leaves large flow channels, and so the second shot uses smaller pieces that fit into the gaps. Then you try smaller shots etc until you get as good a seal as you can. Doing this to plug water flows into tunnels can take several shots to get a total seal, working with sequentially smaller sizes of particles.