Methods and Equipment for the Determination of Two-Phase-Flow Properties of Low Permeability Natural Gas Reservoir Rocks available at the Institute of Petroleum Engineering (ITE)

1. Capillary Pressure Measurements after the Restored-State-Method

The physical principle of the measurement of the capillary pressure functions employing the semi-permeable membrane or ceramic plate techniques is often called in literature "restored state method". This method is the most reliable direct method for the determination of the capillary pressure functions of low permeability gas reservoir rocks (permeability class below 1 mD).
The essential part of the apparatus for Restored-State-Method measurements at ambient stress conditions is the pressure cell shown in Fig. 1. This multifunctional cell designed and built by ErgoTech company, (UK) enables to measure entrance pressure and capillary pressure of the porous materials at ambient stress conditions. A pre-saturated test sample is positioned on the semi-permeable ceramic plate. For the drainage capillary pressure measurement the displacing nitrogen gas is injected at incremental pressure steps into the cell from the top section. After establishing the capillary equilibrium at each pressure step the displaced water volume is measured in the graduated buerette connected to the outlet of the cell. The volume measurement has a resolution of 0.02 ml. It enables to detect about 0.5 % change in saturation of the plugs. The accuracy depends upon the pore volume of the plugs.

The imbibition capillary pressure function is measured by reversing the above process decreasing the gas pressure step-by-step. The "Restored-State" capillary pressure measurements on the low permeability core samples are time consumable. A complete measurement of drainage capillary function (approx. 10 points) requires from 2-3 weeks time, the drainage and the imbibition functions correspondingly 4-6 weeks. Five ErgoTech cells are available at the ITE enabling parallel measurements on five cores.

The experimental set-up for the "Restored-State" capillary pressure measurement at in-situ stress conditions shown in Fig. 2 was designed and built at the ITE. The measurement technique is basically the same as the described above. The differences are in the core preparation. The saturated core will be covered by a the led mantle before the measurement and put under the confining stress. If there are significant heterogeneities in the core (control by CT scanning) non-uniform deformation may occur, which can lead to a ceramic disk failure. Therefore careful sample selection and preparation is needed. Only one such cell is available at ITE.

2. Irreducible Pore Fluid Saturation Determination

The irreducible water saturation is a very important parameter for correlation of the relative gas permeabilities using standard correlation methods (Corey-Brooks etc.). The irreducible water saturation of low permeability gas reservoir rocks often cannot be approached byt restored state experiments due the gas threshold pressure limit of the ceramic disk. The irreducible water saturation of the test cores can then be measured by centrifuge method at rotation speeds corresponding to the existing capillary pressure level. A prerequisite of the applicability of this procedure is a sufficient mechanical stability of the test core.
A high performance rock centrifuge Beckman Avanty J25 is available at the ITE (Fig. 3). Up to 6 test cores can be centrifuged at the same time.

3. End-point Permeability Measurements and Correlation of the Gas/Water Relative Permeability Functions

The gas/water drainage capillary pressure functions are commonly used for correlation of the gas/water relative permeability functions. For the widely used Corey-Brooks correlation method the relative gas permeabilitiy at the irreducible water saturation ("end-point" relative gas permeabilitiy) and the specific water permeability are required. The measurements of these reference permeabilities at ambient as well as at in-situ stress conditions can be performed by using the two-phase-flow experimental apparatus available at the ITE (see Fig. 4). The specific water permeability is determined by steady-state one-phase water flow experiments on fully water saturated test cores. The "end-point" gas permeability is determined by steady-state gas flow experiments on test cores desaturated by "Restored-State" or centrifuge measurement.

4. NMR-Quality Check the Desaturation State of the Test Cores and Determination of the Immobile Water Saturation of the Low Permeability Gas Reservoir Rock Cores

The capillary fixed and clay bound immobile water saturation of the test rock cores can be estimated by means of the NMR-T2 relaxometry. The technical data of the NMR-Tomograph Maran 7 available at the ITE and principles of the NMR-T2 relaxometry are shown in the Fig. 5. Before the start of capillary pressure measurements the fully water saturated test cores are measured by means of the NMR-T2 relaxometry and the immobile water saturation is determined from the T2-distribution. If this value is not approached at the restored state experiment, the T2-distribution of the partially desaturated core is measured again. If a significant part of the hydraulically mobile water is identified, the additional desaturation of the core by centrifuging at the relevant capillary pressure range can be carried out in case of sufficient mechanical stability of the core.
If a sufficient number of capillary pressure measurements and corresponding NMR-T2 measurements on lithologically identical core material were performed, it is possible to determine a correlation between the NMR-T2 distribution and the capillary pressure function. The capillary pressure function of rock samples belonging to this rock type can then be evaluated by non-destructive NMR-T2-measurements in then lab or even in the well by NMR-Logging.


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