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Use of TransAT to study air-lift system for oil extraction

cnpc china
cnpc china
cnpc china

Overview: In the air-lift oil-extraction technique (upper image), gas is injected at the bottom of a hydrocarbon production pipe reducing the gravitational pressure drop in the well. With this, the produced oil flow rate in the pipe is increased. Gas lift is a widely employed Enhanced Oil Recovery (EOR) technique to boost production from depleted wells, and its efficiency could have enormous financial impacts. Gas is injected from valves of variable complexity attached to the pipe wall, which generates large bubbles.

The challenge: The challenge is to optimize the nature of the bubbly flow in the pipe, since it seems that gas-lift efficiency can be improved by injecting rather small bubbles. Gravitational pressure drop is then reduced because: (i) the rise velocity of small bubbles is lower, and hence the residence time and void fraction in the pipe are higher, (ii) small bubbles are more evenly distributed in the pipe, which increases the gas void fraction, and (iii) small bubbles postpone the transition from bubbly flow to slug flow, which is an undesirable operating condition for gas-lift.

TransAT Solution: Interface tracking techniques (ITM) were first tested to track bubble dynamics in a prototype idealized riser, as shown in the 2nd image left, even if phase-average models would have been cheaper. Immersed surface Technology (IST) combined with Block-Mesh Refinement (BMR) are used to simplify the meshing of complex gas-injection mechanisms as shown next in the real case from CNPC. Unsteady RANS (or V-LES) turbulence modeling provide a more faithful picture of the flow, in particular when use is made of ITM’s, which is a more adequate model combination. To get an efficient solution in terms of quality-response time, the phase-average N-Phase model was preferred at the end for the CNPC case to calculate the void fraction of gas in gas lift valve as shown in left. Here the density is decreased by gas injection and the mixture of oil and gas will be lifted by bottom pressure. Different diameters of the valve were considered to improve the flow rate of oil.

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