Gas Breakthrough

Petrophysics

by Amalia Aventurin (Author)

Internship Report 2013 16 Pages

Excerpt

I. Introduction: Principle Measurement

II. 1. Steady-State single-phase gas permeability on a dry sample
II. 2. Steady-State gas permeability on a saturated sample
II. 3. Non-steady state single-phase flow on a saturated sample

III. Error calculation

IV. Conclusion

V. References

I. Introduction: Principle Measurement

By inducing a gas (non-wetting fluid) into a water saturated rock, the gas will displace the water (wetting fluid), but this process just take place, when the capillary pressure is above the capillary entry pressure (gas pressure difference). First the largest pores near the sample surface are drained (drainage process). At higher capillary pressures even the smallest pores are filled with gasThe relation between capillary pressure and pore radius is given by the Washburn equation (1921), “The intrusion of a non-wetting fluid into a cylindrical capillary of radius r only occurs if the capillary pressure Pc […] within a pore is exceeded”:

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Where Pc is the capillary pressure, γ is the interfacial tension, r the pore radius and θ the wetting angle of the fluid in the capillary.

At gas breakthrough gas starts to flow through the sample and displace water from the pore system (drainage path). The effective permeability of the gas-phase after the gas breakthrough is a function of the gas/water saturation and is no longer a rock property like the absolute permeability (single-phase flow). The effective permeability can be determined using the following equation:

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A decrease of capillary pressure leads to an imbibition of water and gas saturation decreases until the last capillary pore is blockaded and gas flow is restricted (snap-off pressure). Complete pressure equilibrium is not established.

As the experiments were conducted with gas Darcy’s law for compressible media is used and the data are Klinkenberg corrected. This is done to determine the absolute gas permeability. At low mean fluid pressures, the slip flow effect causes a non-zero velocity at the pore walls. As a consequence the average flow velocity is higher. At infinite high fluid pressures the mean free path length of the gas molecules is strongly reduced and fluid flow is comparable to those of liquids.

The Klinkenberg correction is done by the following equation:

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A correction has not be done for liquid fluids, as here a normal velocity profile is given (zero at the pore walls).

Two different gas breakthrough experiments were done. The first was performed under steady-state conditions (constant pressure and constant volume flux) as shown in figure 1, using a dried and saturated sample. The experiment on the dried sample represents a single-phase flow. The experiment on the saturated sample represents a two-phase flow after the gas breakthrough occurred and gas displaced the water phase. The second experiment was conducted under non-steady-state conditions. Here a high pressure difference was created in the beginning of the experiment and the imbibition process was measured with final detection of the snap-off pressure (final pressure difference).

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Fig. 1: Apparatus for the single-phase gas and water permeability experiment.

In the first experiment, the sample is placed in a PE-cell. No confining pressure is applied. Bypass of the gas is prevented by an O-ring. One side held constant at atmospheric pressure, the other side with increasing p1-values due to induced helium. For the dried sample a Klinkenberg correction has to be done to calculate the absolute permeability, which is not necessary for the saturated sample. Here the result is a desaturation of the system and therefore we get the effective permeability. Gas flow rates are determined on the low pressure side using a bubble flow meter.

Before the gas breakthrough experiment could be started, a calibration was performed (figure 2) in order to receive the volumes of the upstream and downstream pressure cell. A schematic overview of the gas breakthrough is given in figure 3.

Calibration:

To calibrate the sample cup there has to be three values constant. These values are temperature (T), Railey-constant (R) and (n) as the amount of substance. The values which get measured are volume and pressure and applied in the ideal gas law:

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Out of the ideal gas law the formula to calculate the unknown volume (VX) in the cell:

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With VR as reference volume.

Four different calibration steps has to be done by closing four different valves in different ways.These four steps get repeated with the valves in the volume calibration set up five times in each step five repetitions. If this is done the sample in the sample cup get measured during a few hours. It’s a closed system without a constant flux, in this case the induced pressure (different in the upper and bottom part) comes in balance. Depending on how fast the flux is you can measure the permeability.

Imbibition experiment:

This experiment is under non-steady state condition. It’s a closed system without a constant flux, in this case the induced pressures (different in the upper and bottom part) start to equilibrate. From the flux (pressure drop, increase in closed volumes V1, V2) the permeability is calculated.

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http://www.lek.rwth-aachen.de/cms/index.php?id=142 (28.01.13)

Details

Pages
16
Year
2013
ISBN (eBook)
9783656644880
ISBN (Book)
9783656644866
File size
971 KB
Language
English
Catalog Number
v272606
Institution / College
RWTH Aachen University – Lehrstuhl für Geologie, Geochemie und Lagerstätten des Erdöls und der Kohle 