Could Atomic-Force Microscopy Force Mapping Be a Fast Alternative to Core-Plug Tests for Optimizing Injection-Water Salinity for Enhanced Oil Recovery in Sandstone?

Core tests demonstrated that decreasing the salinity of injection water can increase oil recovery. Optimizing injection-water salinity, however, would offer a clear economic advantage for several reasons. Too-low salinity risks swelling of the clays that would lead to permanent reservoir damage, but evidence of effectiveness with moderate-salinity solutions would make it less difficult to dispose of produced water. The goal is to define boundary conditions so injection-water salinity is high enough to prevent reservoir damage and low enough to induce the low-salinity (LS) effect, while keeping costs and operational requirements at a minimum. Traditional core-plug testing for optimizing conditions has some limitations. Each test requires a fresh sample; core-testing requires sophisticated and expensive equipment; and reliable core-test data require several months because cores must be cleaned, restored, and aged before the tests can begin. It is also difficult to compare data from one core with results from another because no two cores are identical, making it difficult to distinguish between effects resulting from different conditions and effects resulting from different cores. Gathering statistics is limited by the time required for each test and the fact that core material is in short supply. Thus, our aim was to explore the possibility of a less-expensive, faster alternative by probing the fundamental chemical mechanisms behind the LS effect. We developed a method that uses atomic-force microscopy (AFM) to investigate the relationship between the wettability of pore surfaces and water salinity. We functionalize AFM tips with organic molecules and use them to represent tiny oil droplets of nonpolar molecules, and we use sand grains removed from core plugs to represent the pore walls in sandstone. We bring our oil-wet tip close to the sand-grain surface and measure the work of adhesion between the tip and the surface. Repeated probing of the surface with the tip produces data that one can convert to maps of adhesion, and we can estimate contact angle. Adhesion work is proportional to wettability and is directly correlated with the salinity of the fluid in contact with the tip and the particle surface. From our measurements, the threshold values for the onset of the LS response are 5,000 to 8,000 ppm,which benchmark remarkably well with observations from core-plug tests. From a mechanistic perspective, the correlation between salinity and adhesion provides evidence for the role of electrical-double-layer (EDL) expansion in the LS response; expansion of the double layer decreases oil wettability. Because AFM experiments can be performed relatively quickly on very little material, they give the possibility of testing salinity response on many samples through-out a reservoir and for gathering statistics. Our approach provides a range of data that one can use to screen conditions to maximize the value of the core-plug testing and to provide extra data that would be too time consuming or too expensive to gather with traditional methods alone. Thus, AFM force mapping is an excellent complement to traditional core-plug testing.