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In-situ Determination of Gas Threshold Pressure Helps Forecasting... |
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In-situ Determination of Gas Threshold Pressure Helps Forecasting Gas Migration through Caprock in Underground Gas Storageand CO2 Sequestration Authors: L. Ostrowski, B. Ülker
Underground gas storage operations and CO2 disposal in aquifers relay on a sealing function of the
caprock. The leakage through the seal can occur by diffusion, capillary transport and two-phase migration.
Modeling of two-phase fl ow in porous media requires the specifi cation of the functional relationship
between capillary pressure, relative permeability and saturation. Both capillary pressure or gas threshold
pressure and relative permeability control the way the liquid, as wetting phase and gas, as non-wetting
phase interact. The injected gas moves to the top of the formation below the caprock due to gravity and
density differences. Therefore, the ability of a cap rock to seal fl uids is one of the key parameter for the
successful gas storage or long term disposal of CO2. Gas mobility is controlled by sealing properties of
a low permeability caprock. Capillary pressure data, which are critical for exact prediction of gas leakage
through the caprock are seldom available and yet necessary. An in-situ method of gas entry pressure
determination was developed and successfully implemented to help reducing uncertainties gas leakage
predictions. Zonal isolation of caprock is performed followed by exchange of wellbore liquid by gas.
Constant rate injection of gas is then conducted to determine the gas entry pressure into a fully water
saturated caprock. The gas threshold entry pressure is used in the reservoir model to predict the leakage
rates. Simulations runs accounting for relative permeability hysteresis were performed to investigate the
gas leakage through the caprock for a CO2 sequestration model. It was shown that the uncertainty of predictions
could be signifi cantly reduced by using data obtained from in-situ gas threshold determination. |
2011