Run the Waterflood

Run It

The model is complete. Water goes into the down-dip flank and oil comes out up-dip, on the far side of the crest. Water is denser than oil, so injecting at the deepest point and producing up-dip puts the producer in the oil column rather than in the water leg. It still catches the transition zone at its base, which is why it makes some water from the first barrel.

The velocity field is solved from the permeability you built in stage 5, so the water cut and oil recovery that build over pore volumes injected belong to this field rather than to a uniform box. The cut faces carry the facies model, darkened below the oil-water contact.

Breakthrough and Sweep

Watch the water cut before you watch the front. It does not start at zero, because the producer is completed through the transition zone above the contact and makes some water from the first barrel. The cut then falls as the oil bank arrives ahead of the flood, and the turn at the bottom of that dip is breakthrough, the moment injected water reaches the well. After it the cut climbs steeply and recovery slows to a crawl.

The front follows the permeability, running ahead through the best sand and lagging where the rock is tight, so parts of the field are swept early and parts are never swept at all. That bypassed oil is most of why recovery stops well short of the movable volume.

Raise the oil viscosity and the mobility ratio turns unfavorable. Breakthrough comes earlier, the water cut climbs faster, and more oil stays in the ground.

Sweep and Support

Sweeping the oil is half of what the flood does. The other half is pressure support. As the producer withdraws fluid the reservoir pressure would fall, and injecting water to replace that volume holds it up. The voidage replacement ratio measures how much you put back: at one, the pressure holds at its initial value however much you produce.

What the pressure follows is the net volume withdrawn, so the ratio buys you production. Replace half the voidage and you can produce twice as much before the pressure has fallen as far, which is why the bend in the curve slides to the right as you raise the ratio. The dotted tie measures exactly that: it runs from where you are across to the point where no injection at all would have reached the same pressure, and its length is the production the injection bought you. That bend is the bubble point. Past it, gas comes out of solution and the drive becomes an inefficient solution-gas mechanism that leaves most of the oil behind.

Replacement above one repressurizes, and that has a ceiling of its own: the fracture pressure, past which the injectors part the rock instead of filling it. Injecting is how a waterflood pushes the oil and keeps the reservoir energized at the same time, and the ratio is the dial between doing too little and doing damage.

The Number That Decides

The recovery at the economic limit is the prize, oil-in-place times recovery factor, the reserves a development hangs on. From an empty structural envelope to this single number is the whole of reservoir modeling, and the whole of this course.