Wettability alteration appears to be an important physiochemical process during low salinity waterflooding in carbonate reservoirs. However, the leading factor(s) behind wettability alteration remains unclear. In particular, the relative contribution of calcite dissolution induced pH and Ca²⁺ increase has not been explicitly quantified. We thus hypothesized that the pH increase plays a significant role in the wettability alteration, whereas the increase of Ca²⁺ plays a minor role. To test this hypothesis, we performed a geochemical study and examined the effect of salinity level on calcite dissolution and pH increase against the recovery factor profile obtained from the literature. The calcite dissolution was evaluated by equilibrating different brines with calcite. The equilibrated pH and brine compositions were then to be employed to quantify the Ca²⁺ increase and calculate surface species and electrostatic force between oil-brine-calcite interfaces.

Our results show that low salinity water indeed triggers calcite dissolution, which induces the pH increase from 1 to 2 units, whereas only causes less than 10% increase of Ca²⁺ concentration in the low salinity water. Surface complexation modelling implies that Ca²⁺ concentration variation may not lead to significant chemical surface species variation on calcite surfaces, but the pH increase would play an important role on oil-brine interfaces especially for the oils with high base component (-NH⁺). Moreover, geochemical modelling shows that at least 5 times dilution of the original brine is required to obtain a pH increase from 6.5–7.0 to 8.0–8.5, which seems to be essential for triggering the wettability alteration. We argue that high salinity formation brines with the low initial pH (5.5 to 6.5), high basic component oils, and at least five-time dilution could be the three key favourable elements to trigger the low salinity effect in carbonate reservoirs. This work provides a practical framework to screen potential carbonate reservoirs and predict low salinity waterflooding (LSWF) performance.

碳酸盐岩储层低盐度注水过程中，润湿性变化似乎是重要的理化过程。然而，润湿性改变背后的主要因素仍然不清楚。特别是，方解石溶解引起的pH值和Ca ²⁺增加的相对贡献尚未明确量化。因此，我们假设pH的升高在润湿性改变中起着重要的作用，而Ca ²⁺的增加扮演次要角色。为了验证这一假设，我们进行了一项地球化学研究，并根据从文献中获得的恢复因子曲线，研究了盐度水平对方解石溶解和pH升高的影响。通过用方解石平衡不同的盐水来评估方解石的溶解。然后将平衡的pH和盐水成分用于量化Ca ^2+的增加，并计算油-盐水-方解石界面之间的表面物种和静电力。

我们的结果表明，低盐度水确实会触发方解石溶解，从而导致pH从1增至2个单位，而仅导致低盐度水中Ca ²⁺浓度增加不到10％。表面络合模型表明，Ca ²⁺浓度变化可能不会导致方解石表面化学表面物种发生明显变化，但pH值的升高将在油-盐水界面中起重要作用，尤其是对于具有高碱成分（-NH ⁺）。此外，地球化学模型显示，要使pH从6.5-7.0增加到8.0-8.5，需要至少稀释5倍的原始盐水，这似乎是引发润湿性改变的必要条件。我们认为，初始pH值低（5.5至6.5）的高盐度地层盐水，高碱性组分油以及至少五倍的稀释度可能是触发碳酸盐岩储层低盐度效应的三个关键有利因素。这项工作为筛选潜在的碳酸盐岩储层和预测低盐度注水（LSWF）性能提供了实用的框架。