# A Novel Application of New High-Strength Plugging Agent in Baikouquan Oilfield
## 3 Performance Evaluation by Laboratory Tests
As a result, the plugging agent with the characteristics of micro-expansion is more advantageous than cement for plugging leakage formation and casing damage in wells, because the cement solidified body easily shrinks in field applications, resulting in unsuccessful plugging. The compactness of the solidified body is enhanced by the micro-expansion property of the plugging agent, which improves the bond strength between the solidified body and the formation, casing and cement sheath, reducing the risk of gas channeling [48]. Figure 5: Plugging strength variation at different temperatures and different densities. Figure 6: Plugging strength variation in terms of time at 80 \({}^{\circ}\)C and 100 \({}^{\circ}\)C.  
Expansion mechanism of plugging agent. The new type of micro-expansion plugging agent possesses a certain space network structure and its expansion rate changes with an increase in time at a certain temperature. Through the hydration reaction and water absorption expansion characteristics of the material, the solidified body of the plugging agent can achieve a specific expansion effect within a certain time. For example, the CaO, Al\({}_{2}\)O\({}_{3}\) and CaSO\({}_{4}\) will undergo a hydration reaction to form ettrinigite when gypsum is added into sulphate aluminum cement. The lattice of the product occupies a larger space than that of the original compound, resulting in an obvious expansion effect [49; 50]. The reaction of calcium oxide and magnesium oxide in the plugging material produces calcium hydroxide and magnesium hydroxide, which leads to crystal shape changes and volume increase, resulting in the phenomenon of micro-expansion [51]. The volume expansion of calcium oxide and magnesium oxide also occurs at high temperatures [52]. Besides, the sepiolite powder and wood fiber, etc., easily absorb water to expand [53], which makes the expansion system more stable, to some extent. Rheological properties of plugging agent. Table 1 presents the rheological property evaluation of plugging agent slurries with different densities at room temperature. The values for each evaluation parameter of the plugging agent slurries increase with the increase in plugging agent slurry density. When the density of the plugging agent slurry is 1.70 g/cm\({}^{3}\), the dynamic shear force, plastic viscosity and apparent viscosity of the plugging agent are 12 Pa, 43 mPa\(\cdot\)s and 55 mPa\(\cdot\)s, respectively. Based on the experimental data, it can be seen that the plugging agent has favorable thixotropy and particle suspension ability, so that the particles cannot settle down in the slurry. The plugging slurry has favorable fluidity, meeting the pumping requirements of the field, which ensures construction safety. Salt resistance of plugging agent. The salt resistance of the plugging agent is shown in Figure 8. It can be seen that when the salinity of water was 0 mg/L, 5.0 \(\times\) 10\({}^{4}\) mg/L, 1.0 \(\times\) 10\({}^{5}\) mg/L, 1.5 \(\times\) 10\({}^{5}\) mg/L and 2.0 \(\times\) 10\({}^{5}\) mg/L, the corresponding plugging strength of the solidified body was 48 MPa, 47 MPa, 46.3 MPa, 46.1 MPa and 45.8 MPa, respectively, under the condition that the temperature was 100 \({}^{\circ}\)C. The experimental results show that there is little difference in plugging strength at different salinities. The strength variation is less than 3 MPa, which shows that the formation water with different salinities has little influence on the plugging strength of the plugging agent, showing favorable salt resistance. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Density (g/cm\({}^{3}\))**} & **Initial Gel Strength (Pa)** & **Final Gel Strength (Pa)** & **Dynamic Shear Force (Pa)** & **Plastic Viscosity (mPa\(\cdot\)s)** & **Apparent Viscosity (mPa\(\cdot\)s)** \\ \hline
1.45 & 2.7 & 5 & 1.5 & 15 & 16.5 \\
1.60 & 3 & 6 & 3.5 & 25 & 28.8 \\
1.70 & 3.5 & 7 & 12 & 43 & 55 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of rheological properties of different density plugging agents.