Hilner E, Andersson M P, Hassenkam T, Matthiesen J, Salino P A, Stipp S L S
Nano-Science Center, Department of Chemistry, University of Copenhagen, Denmark.
BP Exploration, Sunbury, U.K.
Sci Rep. 2015 Apr 22;5:9933. doi: 10.1038/srep09933.
Core flood and field tests have demonstrated that decreasing injection water salinity increases oil recovery from sandstone reservoirs. However, the microscopic mechanism behind the effect is still under debate. One hypothesis is that as salinity decreases, expansion of the electrical double layer decreases attraction between organic molecules and pore surfaces. We have developed a method that uses atomic force microscopy (AFM) in chemical force mapping (CFM) mode to explore the relationship between wettability and salinity. We functionalised AFM tips with alkanes and used them to represent tiny nonpolar oil droplets. In repeated measurements, we brought our "oil" close to the surface of sand grains taken from core plugs and we measured the adhesion between the tip and sample. Adhesion was constant in high salinity solutions but below a threshold of 5,000 to 8,000 ppm, adhesion decreased as salinity decreased, rendering the surface less oil wet. The effect was consistent, reproducible and reversible. The threshold for the onset of low salinity response fits remarkably well with observations from core plug experiments and field tests. The results demonstrate that the electric double layer force always contributes at least in part to the low salinity effect, decreasing oil wettability when salinity is low.
岩心驱替实验和现场测试表明，降低注入水的盐度可提高砂岩油藏的采收率。然而，这一效果背后的微观机制仍存在争议。一种假设是，随着盐度降低，双电层的扩张会减弱有机分子与孔隙表面之间的吸引力。我们开发了一种方法，利用原子力显微镜（AFM）的化学力映射（CFM）模式来探究润湿性与盐度之间的关系。我们用烷烃对AFM探针进行功能化处理，并用它们来代表微小的非极性油滴。在重复测量中，我们将“油滴”靠近取自岩心塞的砂粒表面，并测量探针与样品之间的粘附力。在高盐度溶液中，粘附力保持恒定，但当盐度低于5000至8000 ppm的阈值时，随着盐度降低，粘附力减小，使表面的油湿性降低。这种效应具有一致性、可重复性和可逆性。低盐度响应开始的阈值与岩心塞实验和现场测试的观察结果非常吻合。结果表明，双电层力总是至少部分地促成低盐度效应，在低盐度时降低油湿性。
