Determination method of total pore structure of caprock

Overview of methods

This method provides a method to determine the total pore structure of rocks by double gas chromatography and mercury intrusion method. The pore size measured by double gas chromatography is 0.75 ~ 6.3 nm, and the pore size measured by mercury intrusion method is 6.3 ~ 75000nm. This method is suitable for determining the pore structure of various massive rock samples.

Double gas chromatography. According to the principle of multi-layer adsorption and capillary condensation of gas on the pore wall of porous materials, rock samples adsorb nitrogen in nitrogen-helium mixed gas environment at liquid nitrogen temperature, and the smaller the pore radius, the sooner they are filled with nitrogen condensate. When liquid nitrogen is removed from the adsorption balance, the sample tube is raised from low temperature to room temperature, and the nitrogen adsorbed and condensed in the rock sample is desorbed by heating, and the larger the pore radius, the faster the desorption. With the carrier gas passing through the sample tube and the measuring chamber of the thermal conductivity detector, the pore size distribution, capillary pressure curve and specific surface area of the rock sample can be calculated according to the unbalanced signal generated by the bridge.

Mercury injection method. According to the principle of capillary action, using the non-wettability of mercury to rocks, mercury is pressed into the corresponding pores in rocks under different external forces, and the corresponding amount of mercury is measured. The pore radius distribution map and capillary pressure curve of rock are drawn by calculation.

Instruments and equipment

The pore radius measured by the specific surface area aperture meter ranges from 0.75 ~ 15 nm, and the device is shown in Figure 72.22.

The maximum working pressure of the pore structure meter is 120MPa, and the device is shown in Figure 72.23.

Oven room temperature ~ 200℃.

The sensitivity of analytical balance is 1mg.

Rock sample drilling and cutting machine.

The capacity of liquid nitrogen tank is 10kg.

The sample bowl is broken.

Standard sieve 2 ~ 3mm

Reagents and materials

Helium bottle, the purity is not less than 99.99%.

Nitrogen steel cylinder, the purity is not less than 99.99%.

The purity of liquid nitrogen is 99.9%.

Mercury.

No.358 light oil.

Anhydrous ethanol.

sample preparation

1) dual gas chromatography. Oil-bearing rock samples should be extracted and cleaned first. Crushing and sieving the sample, taking a particle sample with a particle size of 2 ~ 3 mm, placing it in a constant temperature box, drying it at 105℃ for at least 8 hours, taking it out and storing it in a dryer for testing.

2) Mercury injection method. Oily rock samples should be extracted and degreased first. In general, rock samples can be drilled with 25mm sampling drill, while loose argillaceous rock samples can be prepared by hand, but the samples should not be hammered to avoid artificial microcracks. The surface of a cylinder with a sample size of $25mm and a length of 15 ~ 30mm or a block sample equivalent to this size should be as flat as possible to reduce the surface effect and improve the measurement accuracy.

Put the prepared rock samples into a thermostat, bake at 105℃ for at least 8 hours, take them out, put them into a dryer, cool them, weigh them, and make records. Put the weighed sample in a dryer for analysis.

Send the remaining samples to measure porosity and apparent specific gravity.

Impurities should be removed before using mercury, and then mercury should be poured into the mercury storage bottle.

Determination step

1) double gas chromatography (determination of pore size r≤6.3nm), determination procedure of desorption branch of adsorption isotherm.

As shown in fig. 72.22, open the gas path first, and then turn on the power supply of the instrument to stabilize the instrument1h. Set the relevant parameters on the computer, adjust the carrier gas flow to 50mL/min, and measure the current to 75mA. Install a clean sample tube in the gas path of the six-way valve, and test the blank value of the sample tube first. Unload the empty sample tube and put the dried sample into the sample tube to fill the "belly" of the sample tube and weigh it. After inserting fine glass rods at both ends of the sample tube, connect them to the gas path position of the six-way valve, switch the six-way valve to the adsorption position, put on a heating cup, ventilate and heat at 100℃ for 30 minutes, and then remove the heating cup. After the sample tube is cooled, both six-way valves are switched to the adsorption position, and the sample tube is covered with a liquid nitrogen cup. After N2 adsorption for 5 ~ 6 minutes, push the helium valve to desorb the mixed gas for 6 minutes, and record the flow of r N2 and RHe. When the mixed gas is desorbed and balanced, click the desorption button in the program to switch the six-way valve of the calibration tube to the desorption position. After the calibration tube peaks, click the desorption button in the program to switch the six-way valve of the sample tube to the desorption position, then remove the liquid nitrogen cup, put the cooling water cup on it, click the finish button after the peak of the sample tube is completed, and save the measurement data. Repeat the above steps to measure five points, and their relative pressures are 0.828, 0.722, 0.538, 0.340, 0.11(specifically, it can be calculated by adjusting the relative flow rates of RN2 and Rt). After the measurement, cut off the power supply first, and then close the gas path.

Fig. 72.22 Device diagram of specific surface and aperture tester

2) Mercury injection method (measuring pore radius r≥6.3nm) is shown in Figure 72.23.

Figure 72.23 Flowchart of Pore Structure Instrument

Determination of instrument blank value. Turn on the instrument circuit, and adjust the pressure transmitter and capacitor amplifier after 1h is stabilized; Putting a solid sample made of stainless steel into a rock chamber; Start the vacuum pump, open the vacuum valve of the rock cavity, and vacuum the rock cavity; When the vacuum degree in the rock chamber reaches 6.67× 10-6MPa, open the mercury bottle vacuum valve; After 3 minutes, open the mercury injection valve first, and then open the stop valves 5 and 4; When the indicator light of the probe at the upper end of the ventricle is on, the mercury injection valve automatically closes; Close stop valves 5, 6 and 1 in turn according to the program, then stop the vacuum pump and close the solenoid valve of the vacuum system; Adjust the initial value of capacitance measurement, and then control the booster pump through the computer; Gradually increase the pressure from 0MPa to 1 19MPa, and record the pressure point and the capacitance change value corresponding to each pressure point, measuring 2 1 point in total; After pressurization, the booster pump automatically depressurizes to 0MPa, and the stop valve 1 is opened; First, shut off the cut-off valve 4, then open the cut-off valve 6 and cut-off valve 5, pump in the air valve and the mercury unloading valve, and after the mercury in the stone chamber is discharged, close the mercury unloading valve and the air inlet valve to clean the stone chamber; Repeat the above steps, measure the blank value of the instrument at least twice, and the repeatability relative error of the second measurement should be less than 5%.

Then measure the sample. Put the weighed and preheated (100℃) rock sample into the rock chamber. The determination steps are the same as the operation procedure for determining the blank value; After the measurement, open the mercury suction valve, stop valve 5 and mercury unloading valve, put the mercury in the pipeline into the mercury storage bottle, then close the mercury unloading valve, install the rock chamber, vacuum for a while, and finally turn off the power supply.

3) Determine the specific surface area of the sample (see Figure 72.22). Ventilate the circuit first, and then turn on the power supply to stabilize the instrument1h; Connect 2-3 in the figure with a cold trap tube, and connect the sample tube and the standard sample between 1-4; Put the dried sample into the sample tube, the sample amount is estimated according to the size of specific surface area, and it is appropriate to weigh it at 1/3 of the "belly" of the sample tube, and then plug a little glass wool at both ends of the sample tube; Connect the sampling tube to the gas path of the six-way valve, put on a heating cup, and heat at 100 ~ 120℃ for 30 minutes. At this time, the six-way valve should be in the adsorption position. Set the relevant parameters on the computer and input the mass and specific surface area of the standard sample into the computer. At the same time, adjust the nitrogen flow rate to 20mL/min, the helium flow rate to 80mL/min, and the measuring current to1000 ma. Take off the heating cup after heating, and switch the two-way valve to the desorption position after the sample tube is cooled; Outside the standard sample tube and the sample tube to be tested, a Dewar cup filled with liquid nitrogen is respectively sleeved, the immersion height should be equal, and the adsorption time should be 12 ~ 15 min at the temperature of liquid nitrogen (depending on the specific surface area of the sample to be tested, the larger the specific surface area, the longer the adsorption time, and vice versa). After the adsorption is balanced, click the computer desorption button first. Desorption should be carried out in the order of first desorbing the standard sample and then desorbing the tested sample (remember that the liquid nitrogen cup must be placed on the cold water cup immediately after it is removed), and the adsorption and desorption of the sample depends on the upper and lower parts of the liquid nitrogen cup. After all desorption, the computer automatically calculates the specific surface area of the tested sample, and directly prints out the corresponding data and atlas; After the measurement, turn off the power supply first, and then turn off the air supply.

calculate

1) dual gas chromatography. Calculation of adsorption capacity: