## 3 Results and Discussion  
### Experimental and ANOVA Results  
Among all the acid types used for acidizing treatment, the most commonly used acids are inorganic ones, including Hydrochloric acid (HCl). This acid is widely used to enhance the permeability of carbonate formations [30]. In the current work, a combined acid formulation was used based on 12% HCl: Oxalic acid. According to our best knowledge, Oxalic acid has never been used in an acidizing treatment; therefore, the acid combination formulation is considered a new one. The reason behind the selection of Oxalic acid is its weak ionization and slow reaction. It causes less corrosion to the well equipment and allows for a longer reaction period. The concentration of Oxalic acid in the formulation ranges from 3.8-8.8%. Mixtures of equal volumes of the diluted acids were used based on the adopted experimental design. The mineralogical composition of the rock formation has a significant effect on the rate of the acid-carbonate rock reaction. Carbonate reservoirs composed of limestones (CaCO\({}_{3}\)) and/or dolomites [CaMg (CO\({}_{3}\))\({}_{2}\)] are able to dissolve in acids. The chemical reaction between carbonate rocks and the injected combined acid could be clarified by the following chemical equations:  
\[\text{Calcite: CaCO}_{3}+2\text{HCl}\rightarrow\text{CaCl}_{2}+\text{CO}_{2}+ \text{H}_{2}\text{O} \tag{2}\]  
\[\text{CaCO}_{3}+\text{H}_{2}\text{C}_{2}\text{O}_{4}=\text{Ca}_{2}\text{O}_{4 }+\text{CO}_{2}+\text{H}_{2}\text{O} \tag{3}\] \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Experiment No.** & **1 \({}^{\star}\)** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **a** & **b** & **c** & **d** \\ \hline Oxalic acid \% & 6 & 8 & 3.2 & 6 & 8.8 & 8 & 6 & 4 & 4 & 12 & 4 & 4 \\ \hline Temp.\({}^{\circ}\)C & 50 & 70 & 50 & 21.7 & 50 & 30 & 78.3 & 70 & 30 & 50 & 50 & 40 & Room Temp. \\ \hline \hline \end{tabular} \({}^{\star}\)Repetition.  
\end{table} 
Table 1: The actual levels of the operating variable for acidizing experiments. Dolomite: CaMg (CO\({}_{3}\))\({}_{2}\) + 4 HCl \(\rightarrow\) CaCl\({}_{2}\) + MgCl\({}_{2}\) + 2 CO\({}_{2}\) + H\({}_{2}\)O  
\[\text{CaMg (CO${}_{3}$)}_{2}+\text{H${}_{2}$C${}_{2}$O$}_{4}\rightarrow\text{CaC${}_ {2}$O$}_{4}+\text{MgC${}_{2}$O$}_{4}+2\text{CO${}_{2}$}+\text{H${}_{2}$O} \tag{5}\]  
The reaction of calcite with HCl (Ka value 1.3 \(\times\) 10\({}^{6}\)) occurs with a rapid release of carbon dioxide gas, with bubbles of effervescence appearing when a single drop of the acid contacts the calcite surface. However, the reaction of HCl on dolomite generates only a subtle effervescence. This is because of the strong ionic bonds between calcium and magnesium cations and carbonate anions owing to the closer atomic packing within dolomite's orthorhombic crystal lattice. On another hand, Oxalic acid is a relatively strong organic acid compared to other carboxylic acids (Ka value 5.4 \(\times\) 10\({}^{-2}\)). It may be more environmentally acceptable relative to other acids such as HCl. It has been reported that Oxalic acid acts both as a depressant and pH modifier in treating the surface chemistry of calcite and dolomite, and other minerals of a similar nature [31]. The oxalate anion acts to fix Mg and Ca ions in solution through chelation throughout forming bridging chelates in two or more directions, owing to the abundant hydrogen bonds accepting sites [32]. The laboratory acidizing experimental results using constant concentrations of the combined acid (12% HCl: 4% Oxalic acid) and different acidizing temperatures are illustrated in Figures 2 and 3. The results indicate that the solubility (%) increases with an increasing temperature up to 40 \({}^{\circ}\)C, then decreases steadily (Figure 2). This solubility increase may be attributed to an increase in the acid ionization rate with temperature increase, and so its dissolving power will increase leading to high reaction and easier dissolution of the lithology.