What do I mean by conformance control, well it's just a fancy engineering term for producing, or injecting, more of what you want and less of what you don't want. In the oilfield we use the word to indicate that we have done something to the downhole configuration to direct fluids where we want them to go.
We have done it forever. Way back in the mists of time when I was working offshore on Forties, if a well's water cut rose too high we would run a bridge plug into the well to shut off the water producing zone and let the oil flow freely again. In a trice a well that was dead could be resurrected and flow say 15,000 bbls per day again – I'm looking at you FB15 (egg box 5 well 6) in the spring of '87.
That was one of the great advantages of having vertical or near-vertical wells in high quality homogenous reservoirs. The oil came in at the bottom of the wells and a simple bridge plug astutely placed could make a dramatic difference. In heterogenous reservoirs we would have had to use casing patches or something much more complex, or just have to put up with processing the water and gas we didn't really want.
Horizontal wells have changed all that. Now, in a very homogenous reservoir we would expect water or gas to start to cone into the heel of the well, as that is where the pressure sink is deepest; a bridge plug would be of no use to us there, it would block off production from the whole well. So interventions can be difficult and really something that needs to be planned in to the well design upfront.
For steam floods, the key to success is stopping steam when it breaks through, so that the whole reservoir, not just a small part of it, can be swept by the steam. For steam floods using horizontal wells, stopping steam breakthrough is even more important as each horizontal well is doing the work of maybe ten to fifteen vertical wells. When steam hits the producer we want to shut off that part of the well and force the steam front to sweep the rest of the reservoir. For Pilot, our initial plan was to run a distributed temperature monitoring system in the well and place a series of sliding sleeves in the completion so that we could shut off production from the hottest parts of the well.
Then we found this technology – an autonomous inflow control valve, the AICV®. They are made by an innovative young company from Norway called Inflow Control. Halliburton and Baker Hughes make similar devices and Schlumberger have something akin to it in the works, but as I have studied the AICV® I'll try to explain how that particular device works.
In each and every joint of sand screen installed in the well you install one of these valves. The sand screens need to be fitted with an internal blank pipe so that all the fluids that come through the sand screens are diverted through this device before entering the production tubing.
So how does the valve work?
Well, there are two flow paths through the valve, one which is always open, let's call it the pilot flow path (just like the pilot flame on your gas boiler) and another which is either closed or open. The pilot flow pathway is designed with two flow restrictions built in; one which operates in the laminar flow regime and one which triggers turbulent flow, something like an orifice plate. High viscosity fluids struggle through the laminar restrictor so midway through the pilot flow path the pressure is low. Low viscosity fluids fly through the laminar restrictor, but suffer a big pressure drop as they pass through the turbulent restrictor, so, if the fluids have a low viscosity, midway through the pilot flow pathway the pressure is high.
Those differences in pressure are harnessed to keep the main flow pathway open when the fluid has a high viscosity. The thickest blue arrow shows the inlet of the main flow to the valve, and the two horizontal arrows show the outlet of the main flow into the base pipe. The thin blue vertical arrow represents the outlet of the pilot flow. The yellow piston is actuated by the pressure midway through the the pilot flow path. When the fluid is viscous, the pressure below the yellow piston is low and the valve stays open.
When the fluid is less viscous the pressure beneath the yellow piston is relatively high, the yellow piston is forced upwards and, in doing so, shuts off the main flow pathway. When the valve is fully open only 1% to 5% of the fluid passes through the pilot flow path. So when the valve closes, it can autonomously shut off over 95% of the flow from that joint of pipe.
Just one moving part, no electronics and no attempt to send signals back up the wellbore; so simple, so elegant, so clever, and potentially a game changing technology with lots of applications.
Inflow Control is testing a specially designed version of the AICV® which can tolerate temperatures as high as 310ºC in a SAGD production well in Canada this summer. We will be waiting to hear how the trials go and we hope the product lives up to their (and our) expectations and that it works as well in practice as it does in this video.
© and courtesy of Inflow Control AS