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From The Oil Drum [Donate to them: because they are a respected source of information]

Fluid (oil and gas) will only move from one place to another if something is pushing it. For the fluid in the reservoir under the Gulf, this force pushing the oil out is the difference in pressure between the oil in the rock, and the pressure in the well. The pressure of the oil in the rock is 12,000 psi. When the well was drilled the pressure of the mud that filled the well was over 13,000 psi and no oil moved into the well. Just before the disaster the fluid in the well was changed from mud to seawater. This lowered the pressure of the fluid in the well below that of the fluid in the rock, a differential pressure now existed, and where there was a passage through which the oil and gas could flow, and they did.

The question has always been – how much? Gas flows more easily through cracks than oil, and the disaster was first evident when leaking gas reached the drilling rig, and then ignited. The BOP then, at least partially, functioned. After the rig sank, the riser also sank, bending the pipe just above the BOP. A couple of days later the flow was suggested at about 1,000 bd, and this then escalated to 5,000 bd. As cameras began to publicly monitor the outlet of the riser the estimates started to grow the flow out of the riser was estimated to be around 8,000 bd, with allowance for leaks, the overall flow was estimated to be perhaps 12,000 bd. Once the broken part of the riser was removed and a cap placed over the well, a significant portion of the escaping oil was captured and could then be measured as it flowed into the surface vessel recovering it. Those values are currently at around 15,500 bd. BP is currently planning on additional capture this week of up to another 10,000 bd, and preparing for a worst case scenario with a flow rate of 80,000 bd.
"The Oil Drum" is published by the Institute for the Study of Energy and Our Future (ISEOF), a 501(c)(3) non-profit corporation.