The vast majority of pores in oil (gas) producing rocks (coal samples) are interconnected, it not only has the ability to store oil and gas, but also has the ability for fluids to seep through them. Under a certain pressure difference, the nature of the rock that allows fluid to pass through is called the permeability of the rock, and the quantitative measurement of the permeability parameters of rocks is called the permeability of rocks

The gas permeability tester has the advantages of simple structure, convenient operation, fast determination speed, high precision, etc. It is an indispensable experimental device for routine core analysis.

II. Working Principle

This instrument was designed based on Darcy's Law.

A rock with cross-sectional area A and length L is provided and clamped it in a core holding unit as shown in Figure (1), a fluid with viscosity µ passes through the core under a pressure difference ∆P and the flow rate Q is measured. Experiments show that the volume flow rate Q per unit time passes through the core is proportional to the cross-sectional area of the core A of the pressure difference △P, and inversely proportional to the length L of the core and the viscosity of the fluid

III. Instrument Structure and the Description of the Components

This instrument can be mounted on any test bench (table), it is best that the table surface is shockproof, and away from the door to avoid sudden changes in temperature, then the stable results can be measured, see Figure 2:

2: Flow Chart of Gas Permeability Tester

 

Instrument Components:

 (1). A high-pressure cylinder with a regulator, the pressure inside the cylinder is about 15MPa, which is used as the gas source of the instrument. (User-provided)

 (2). Core holding unit: This is used to hold cores for permeability determination. This holding unit can also be used to connect to a porosimeter to determine core pore volume.

 (3). Ring pressure gauge: Use this to indicate the value of the pressure applied to the outside of the rubber cylinder.

 (4). Upflow pressure gauge: It is used to indicate the upflow pressure value P₁ of the core when determining the permeability; it is divided into high pressure gauge and low pressure gauge;

 (5). Pressure regulator: This valve is used to regulate the gas coming in from the gas source and depressurize to control the value of operating pressure required at the upflow end of the core; it is divided into Q261 and valuator.

 (6). Air release valve: Open this valve to release the ring pressure, so that the pressure of the rubber cylinder reaches atmospheric pressure.

 (7). Core valve: Open this valve, allow high pressure gas to enter the ring space between the core holding unit and the rubber cylinder, allowing the rubber cylinder to tightly cling to the rock sample and the upper and lower end plugs of the holding unit.

 (8). Air source valve: supply helium and nitrogen gas below 1 MPa to the permeability meter regulator, and then generate the appropriate upflow pressure through the regulation of the regulator.

 (9). Dryer: to dry the gas before it enters the rock sample ad then enters the core.

 (10). Rotor flow meter: it is used to measure the flow of gas at the outlet end of the rock sample.

 IV. Test Preparation

1. Sample Preparation

 (1). Sample processing: The sample to be measured is drilled with a drilling machine to a diameter of 25mm and a length of 25-76mm, with both end surfaces ground flat and perpendicular to the sample axis.

 (2). Sample treatment: Oil-containing samples are firstly washed with oil, then dried in an oven at 105℃ (the length of time can be determined on a case-by-case basis), and then put into a desiccator to cool down to room temperature and wait for measurement.

2. Instrument Preparation

Turn on the power switch, after connecting according to the process, after leakage detection, close all the valves and regulators on the panel, and close the needle valve of the flow meter.

 (1). Upflow leakage detection: apply soap to each joint, if there is a leak, you can see the soap bubbles, check each joint with the valve and pipeline.

 (2). Downflow leakage detection: The lower end of the rock sample should never have leakage, if leakage is found, it should be repaired immediately.

 (3). Core holding unit leakage detection: leakage detection of the core holding unit system may be performed as follows:

① Put a piece of sample with large pore space and permeability to the core holding unit, holding unit rubber cylinder pressurized 1MPa, the length of the sample must be the maximum sample length that the holding unit can test;

②. Turn off the ring pressure valve and the ring gauge pressure will show when the pressure in the rubber cylinder drops. This indicates a leakage. If there is no leakage when checking the relevant pipeline and joints, it is judged that there is a leakage in the holding unit, and the location of the leakage can be checked as follows:

Hold the upflow plastic tube to the core holding unit with a clamp and open the needle valve of the flow meter, if the meter shows, it means that the core holding unit rubber cylinder has a small hole leakage and must be replaced with a new rubber cylinder.

If it's not the rubber cylinder of the core holding unit that's leaking, tighten the upper and lower holding unit nuts, and if it's still leaking after tightening, use a soapy solution to locate the leakage and make a reasonable repair.

V. Test Procedure

 (1). Remove a rock sample from the dryer that fits the diameter of the holding unit and calculate its cross-sectional area A by measuring the length and diameter of the rock sample with a vernier caliper; the geometric sizes must be measured before making the determination. If it is necessary to cut off a portion of the core to keep the core ends clean, its length should be retaken.

 (2). First check that whether the valves on the panel and the handwheel on the holding unit are closed.

 (3). After loosening the handwheels that hold the brackets on each side of the core holding unit, lower the brackets and remove the pressurized steel plunger from the holding unit.

 (4). Load the rock sample with the measured geometry size into the rubber cylinder of the core holding unit, lift the rock sample upward with the pressurized steel plunger until both ends of the rock sample are tightened by the upper end of the holding unit against the pressurized steel plunger, and tighten the handwheel.

 (5). Open the air release valve and close the air release valve.

 (6). Open the high-pressure cylinder valve, set the pressure gauge on the cylinder to 2 MPa, open the core valve so that the ring pressure gauge shows 2 MPa, and close the core valve.

 (7). Adjust the pressure regulator (general pressure adjustment from small to large), adjusted to the required upflow pressure to 0.2MPa or so, if the required upflow pressure is low, adjusted through the valuator.

 (8). Select a flow meter to read flow at different upflow pressures

 (9). Adjust the pressure regulator to reduce the upflow pressure to zero, open the air release valve to reduce the ring pressure to zero, repeat the steps and remove the rock sample.

 (10). If the test is to be continued, repeat steps 4 to 10; if the experiment is completed, push the pressurized plunger into the holding unit, tighten the handwheel, close all valves, and the test is complete.

 (11). Record the data:

①. Record the atmospheric pressure for the day;

②. Record all gas categories;

③. Record the upflow pressure;

④. Record the flow rate;

⑤. Record rock sample number, length, and diameter.

 (12). Parameters measured above are filled in the raw data recording form (see attached table)

 (13). Notes:

① Both end faces of the rock sample must be perpendicular to the axis of the sample and the ends should be parallel to each other; irregularities in the end faces of the rock sample may crease or scratch the rubber cylinder.

②. Ring pressure should never be applied when the sample is not placed in the core holding unit or the rubber cylinder will be damaged.

③. When the diameter of the rock sample is 1 to 1.5mm smaller than the diameter of the core holding unit, it will not damage the rubber cylinder when it is put into the holding unit, and if it is smaller, it should be handled in an appropriate way so that it will not damage the rubber cylinder.

④． When determining the gas flow rate, a flow meter is used.

VI. Data Treatment

1. Because the monophasic fluid passes through the rock samples, its seepage law does not always obey Darcy's law, only when the pressure gradient is small, the flow rate is low, the monophasic fluid flow in porous media obey Darcy's linear seepage. When the pressure gradient exceeds a certain limit value, it no longer obeys Darcy's law, but obeys a nonlinear seepage law.

 Main Technical Parameters of the Instrument

1. Measuring range: 0.01～10000×10-3um2;

2. Flow meter: digital display type, display accuracy ≤ 5%, 0-5L/min;

3. Digital display type, measuring range 0～1MPa, display accuracy: 0.5%F.S;

4. Core holding unit A: φ25×25～76mm; (optional)

5. Core holding unit B: φ38×38～76mm; (optional)

6. Measurement accuracy: middle and high permeability (≥10×10-3um2) ≤5%, low permeability ≤10%;

7. External dimensions: length * height * width   610 * 690 * 400mm;

8. Range of confining pressure adjustment: 0~3MPa;

9. Working medium: nitrogen

10. Instrument supporting: one high-pressure cylinder (user-provided)

One air pressure gauge (user-provided)

One vernier caliper (user-provided)

One dryer (for storing rock samples) (user-provided)

5. Test range: 0.01×10^-3 µm² ～6µm²

6. Measurement error: K 〈 10 × 10^-3 µm²   Relative error ≤ 5%

^-K〉10×10^-3 µm²   Relative error ≤ 5%