## 2 Materials and Methods
### Core Flooding Experiments  
Limestone cores were saturated with 3.5 wt% NaCl brine. Brine saturation was performed in an acrylic vacuum chamber. The cores were then saturated with crude oil using the SRP 350 core flooding apparatus by Vinci Technologies displayed in Figure 2. The oil saturation process consisted of injecting the oil at a progressively increasing rate for 20 h. After oil saturation, the cores were aged in oil at 60 \({}^{\circ}\)C for at least 14 days. The oil's API gravity was 35.0. At 22 \({}^{\circ}\)C, the oil had a density of 0.86 g/cm\({}^{3}\) and a viscosity of 7.70 cp.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Core ID**} & **Diameter** & **Length** & **Gas** & **Pore Volume** & **Porosity** \\  & **(mm)** & **(mm)** & **Permeability** & **(cm)** & **(cc)** & **(\%)** \\ \hline IDLS\_1 & 25.52 & 50.78 & 229.444 & 4.515 & 17.38 \\ IDLS\_2 & 25.33 & 50.78 & 212.025 & 4.405 & 17.21 \\ IDLS\_3 & 25.27 & 50.82 & 208.178 & 4.315 & 16.93 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Dimensions and measured properties of limestone cores used for core flooding.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Material** & **Concentration (wt\%)** \\ \hline \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) nanoparticles & 0.1 \\ NaCl salt & 0.1 \\ Concentrated HCl acid & 0.007 \\ CTAB surfactant & 0.03 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Concentrations of each material added to distilled water to prepare the \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) nanofluid with a pH value of 4.5.  
Core flooding experiments were performed using the same core flooding apparatus displayed in Figure 2. Three core flooding experiments were conducted, and each of the experiments had the following three steps: the first step was low-salinity water (LSW) injection. The second step was either nanofluid alternating CO\({}_{2}\) injection or only nanofluid injection. The final step was the injection of five pore volumes (PVs) of the sweeping fluid, which was LSW with CTAB surfactant and had a pH value of 4.5. The sweeping fluid was prepared to maintain the same surface charge for the pore surface and the nanoparticles remaining in the pores during the sweeping step. Each of the first two steps lasted until no oil was produced for two consecutive PVs.  
The first core flooding experiment (IDLS_1) was LSW injection followed by nanofluid alternating CO\({}_{2}\); the ratio was 1 PV of nanofluid to 6 PVs of the CO\({}_{2}\), which was selected after some experimentation that was effective in achieving and maintaining a low pH value over a long period, as illustrated in Figure 3. In the second core flooding experiment (IDLS_2), after LSW injection, only the \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) nanofluid was injected. The final core flooding experiment (IDLS_3) was the same as the second experiment, except the amount of HCl in the nanofluid was doubled to investigate the effect of a high HCl concentration on pH alteration and permeability damage results. The nanofluid in the third core flooding experiment had a pH of 2.8 instead of 4.5.  
Figure 3: pH of the effluent during nanofluid alternating CO\({}_{2}\) injection after each injection cycle.  
Figure 2: Schematic of the SRP 350 core flooding apparatus by Vinci Technologies.