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Intermittent gas lift for liquid loaded horizontal wells in tight shale gas reservoirs
Croce, Daniel
Croce, Daniel
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2020
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2021-06-04
Abstract
As the near-wellbore pressure of tight gas shale formations, drops sharply within the first two years of production, the gas entering the completion slows down, reducing its capacity to drag liquids to the surface. This causes the accumulation of liquids in the horizontal section (i.e. liquid loading). The stagnant liquids must be produced to allow the continuous flow of new hydrocarbons into the wellbore. Current artificial lift (AL) systems have proven to be inefficient and unsustainable for this purpose, due to the large gas volume fraction; the horizontal and tortuous nature of the wellbores; the presence of solids (formation sands and proppant) and the tight space available within the completion. The objective of this work is to evaluate the design of a new artificial lift method to unload horizontal wells, relying on the study of intermittent injection of compressed gas at low rates, together with the use of ionic, polysaccharidic and sucrose solutions. The device consists of a check valve set in a concentric coiled tubing, which is installed inside the horizontal section of the wellbore. The check valve allows the flow of fluids into the coil while open, then closing when internal pressure is applied with gas at the surface. The accumulated liquids are swept back to the surface by the gas, which is injected into the annular space of the concentric tubing. The sweeping effect of the gas is improved by the use of polymers and ionic solutions, which affect the shape and size of the nose of the gas slug, by altering the Eotvos number (Eo), the Viscosity number (Nvis) and the Froude number (Fr). To model the operation of this device, an analytical model is presented, reflecting the flow of gas slugs across stagnant liquids in pipes of small ID’s (from 0.375 to 0.75 inches).An experimental flow loop including a vertical and a horizontal section was used to test the novel device using water and compressed air. The results show a liquid removal efficiency (ratio of volume of liquid produced to volume of liquid available in the device) of up to 70 percent, while showing a volumetric efficiency (volume of liquid recovered to volume of gas injected per cycle) of up to 33%. The removal efficiency increased in 15% when dissolving xanthan gum in the stagnant liquids at a concentration of 0.05% w/w. High speed video recordings, indicate a flattening of the nose of the Taylor bubble, as same as an increase in the liquid flowline conversion angle measured between the body of the bubble and the wall of the pipe, going from the nose of the bubble to its tail. This restricts liquid fall back, causing a better liquid sweep by the bubble. The analytical model successfully predicts the surface pressure, exit velocity of the produced liquid slugs, and the produced volume of liquids per cycle.
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