Experimental Study on Rock Dissolution and Scale Formation by Strong/Weak Alkali During the Alkali–Surfactant–Polymer Flooding in an Oilfield in China

This study utilizes the temperature–pressure reactor to simulate the real conditions of the reservoir to study rock dissolution and scale formation caused by strong and weak alkali during the ASP flooding in an oilfield in China. Mercury injection experiments showed that the porosity and permeability of rock increased by 10.3% and 15.3% under the action of strong alkali, while they increased by 7.2% and 10.1% under the action of weak alkali, indicating that both strong and weak alkali can cause rock dissolution. The structural morphology of the rock demonstrated that the clay content between the grains decreased significantly. The semi-quantitative analysis of XRD indicated that the content of kaolinite decreased from the initial 7% to 0%. The recrystallized carbonate was found, and the carbonate content increased from the initial 0% to 12%. According to the SEM, EDS, and Raman analyses of the scale, the scale formation was complex in the strong alkaline system, including silicate scale, carbonate scale, and hydroxide scale. In contrast, only carbonate scale was found in the weak alkaline system. The ICP-AES test for the liquid system revealed that the rock dissolution releases substantial Ca2+, Mg2+, Fe2+, SiO32− and AlO2− ions, among which Si concentration can reach around 560 ppm. The chemical mechanism of rock dissolution and scale formation by strong and weak alkali includes the exchange of mineral cations by Na+ and the destruction of Si-O and Al-O bonds by OH−. These released ions migrate with the composite fluid, then recrystallize under the saturation state to form the scale. The dissolution of rock by strong alkali is more intense, while the dissolution of weak alkali is relatively mild. Moreover, the scale type in the weak alkaline system is simpler, which would be convenient to develop inhibitors.