CO2 — From Fizz to Fracture
By Keith Hall
June, 2009 |
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Where readily available and economically feasible, carbon dioxide is increasingly being used in the petroleum industry to fracture deep oil wells and for enhanced oil recovery (EOR), where the CO2 may also be sequestered (essentially taking the place of the oil). In well fracture, CO2 is pumped under high pressure to open up fractures deep within the well. Sand or another medium is pumped down-hole and used to hold the fractures open. In enhanced oil recovery efforts, CO2 is continuously injected under high pressure, i.e. 5000 to 15,000 psig, directly into production wells, or into nearby wells. The CO2 dissolves into the crude (similar to the way it goes into solution in your soda pop), reducing its viscosity and enabling it to more readily flow from the surrounding substrate. Additionally, the high-pressure CO2 displaces and forces the freed oil into the well.
Current primary and secondary oil recovery technologies, including traditional CO2 EOR technologies (used where economically feasible), only recover about one-tenth to one-third of an oilfield’s reserves. The US Department of Energy estimates that “next generation” CO2 EOR technologies will be able to recover more than 60 percent of in-place oil, and expand the use of CO2 EOR to a broader range of domestic oil reserves. Here is how CO2 in EOR and well fracturing processes work.
Vessels that contain CO2 are not perfectly insulated and allow heat to leak into the liquid CO2 during transport and storage. This causes the CO2 to boil and creates a corresponding increase in tank pressure. The additional pressure presses down upon the surface of the liquid, slowing the boiling action. With heat leak, this boiling and counter-acting pressure increase process continues, with the product becoming more saturated with heat over time. This process of saturation is similar to the thermodynamics of your car’s cooling system (see CGI, December 2007, pp. 40-42).
When CO2 is being used to “frac” a well, the liquid contained in the transport will boil at an increasingly higher rate as product is withdrawn due to the corresponding pressure decrease (the vapor space above the liquid is increasing in volume). With the reduction of pressure, the heat saturated within the liquid causes it to boil, resulting in vapor being entrained in the liquid. This is referred to as two-phase product.
The introduction of a two-phase product into the high-pressure fracturing (piston) pump used in well fracturing processes causes vapor lock, very unstable pumping conditions, and damage to the pump. Detrimental boiling is also created within the high-pressure pump itself. To avoid introducing a two-phase product into the high-pressure pump, a phase separator and a vane-type boost pump are used upstream of the high-pressure pump. A phase separator partially acts as an accumulator, in that it provides a temporary “reservoir” for the CO2 to settle in on its way to the pump. When operated at the correct liquid/vapor level, entrained vapor can rise to the top of the phase separator, where it is vented. The denser liquid remains near the bottom where it is introduced into the suction inlet of the boost pump.
The increased pressure created by the boost pump stops the liquid from boiling as it enters the suction manifold on the high-pressure pump. Entrained vapor is turned back into liquid as the pressure is increased, and the tendency for vapor locking, and damage to the high-pressure pump, are greatly reduced. Vane-type pumps can pump some two-phase product (liquid with entrained vapor) as cavitation does not occur as it would with a centrifugal pump. They are self-priming and can even run dry for short durations (although this condition should be avoided).
Vane pumps employ relief by-pass valves to protect the pump from excessive pressure. Their seals can be replaced without removing the pump from the piping system, making them easy to maintain. If the pump is driven by a hydraulic pumping system, versus a direct drive electrical motor, pump speed/pressure can easily be controlled, avoiding the unnecessary recirculation of product through the by-pass valve, which adds heat to the liquid. Once supplied with “good” liquid from the boost pump, the tendency for boiling on the suction stroke of a high-pressure piston pump is greatly diminished and the life expectancy of the high-pressure pump enhanced. The high-pressure CO2 pump will dependably do its job of increasing down-hole pressure, resulting in enhanced oil recovery. CO2 doesn’t just put fizz in your soda pop — it puts pizzazz in your gas tank, too.
Keith Hall is the Engineering Manager at Cryogenic Vessel Alternatives, located in Mont Belvieu, TX. He can be reached at khall@cvatanks.com.
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