# Investigation of Properties Alternation during Super-Critical CO\({}_{2}\) Injection in Shale
## 3 Results and Discussion
### Pore Structure Alteration  
The low-temperature N\({}_{2}\) adsorption isotherms of the samples with varied Sc-CO\({}_{2}\) injection time (0 day, 10 days, 30 days) are shown in Figure 5. According to the isotherm classification from the International Union of Pure and Applied Chemistry (IUPAC) [24; 25], all the curves obtained are following the Type III Adsorption Isotherm curve. At the beginning under the extremely low P/Po applied, the pores of the both samples present micro-pore filling behavior and micro pore volume is related to the adsorption amount. Then, with the relative pressure(P/Po) building up, the isotherm adsorption curve illustrates that the multi-molecular layer adsorption occurred. At higher P/Po, the fluid is at the liquid phase and thus the capillary condensation exists. Comparing the isotherm curves under different Sc-CO\({}_{2}\) injection times for the two samples separately, we proposed several interpretations of the effect on the pore structure. Firstly, with the Sc-CO\({}_{2}\) flooding time increasing, the adsorption amount trend to increase for the first depth sample while the reverse tendency occurred for the sample at the depth of 9850 ft. Also, the similar isotherm curve for all time slots means that the pore shape did not vary significantly during the CO\({}_{2}\) injection, and the slit pore and cylindrical pore dominate the main pore shapes. While the hysteresis loop which illustrated the meso-pore existence, was divided into two scenarios dramatically. For the samples from the depth at 9848 ft (Figure 5a),  
Figure 4: [PERSON]’s modulus with variable Sc-CO\({}_{2}\) injection time. The upper curve is for the sample with depth at 9848 ft, the lower curve is for the sample with depth at 9850 ft.  
the width of the hysteresis loop gets wider with the Sc-CO\({}_{2}\) injection time extension, which means that after the Sc-CO\({}_{2}\) treatment, the presentation of the meso-pore becomes abundant. Meanwhile, the wideness tendency represents that the other pore type shapes intent to the silt pore conversion. For the samples from the depth at 9850 ft (Figure 5b), the hysteresis loop starts to narrow with the Sc-CO\({}_{2}\) treatment time increasing, which reservedly indicated that the percentage of the meso-pore tends to reduce. It is also indicated that the pore shape tends to be converted to the plate core from the other types of the pore shape. The plate-shape pores in the Middle Bakken and the slit-shape pores in the Upper and Lower Bakken is advantageous for the flow of the hydrocarbon due to their excellent openness. Both the plate pore and slit pore are advantageous for the hydrocarbon flow in the porous media due to their excellent openness. The Brunauer-Emmett-Teller (BET) method was applied to characterize the surface area change during the varied Sc-CO\({}_{2}\) treatment period. The BET equation is:  
\[\frac{1}{W\!\left(\left(\frac{P_{\rho}}{P}\right)-1\right)}=\frac{1}{W_{m}C}+ \frac{C-1}{W_{m}C}(\frac{P}{P_{o}}) \tag{4}\]  
In order to acquire the surface area, two main parameters were required. The slope, which is,  
\[s=\frac{C-1}{W_{m}C} \tag{5}\]  
and the intercept, which is,  
\[i=\frac{1}{W_{m}C} \tag{6}\]  
Combining the Equations (5) and (6), we have  
\[W_{m}=\frac{1}{s+i} \tag{7}\]  
Then the BET surface area can be calculated as:  
\[S_{t}=\frac{W_{m}\,N\,A_{cs}}{M} \tag{8}\]  
where:  
N: Avogadro's number (6.0221415 \(\times\) 1023 molecules/mol);  
Figure 5: Isotherm curves with different Sc-CO\({}_{2}\) injection time. (**a**) sample at depth of 9848 ft, (**b**) sample at depth of 9850 ft.  
Wm: weight of adsorbate constituting a monolayer of surface coverage;  
Acs: molecular cross section of the adsorbate molecule, (16.2 A2 for N2 at 77 K);  
M: molar mass (molecular weight) of the adsorbate N2 gas (28.0123). Figure 6 clearly demonstrates the parameters acquired for the surface area calculation. It is obvious that the slope gets increase with the Sc-CO2 treatment time increasing.