Nano- and Micro-Scale Contact Angle Measurement Under Pressure by Vertical Shift Interferometry

In petroleum industry, it is of essential importance to better understand the wettability phenomenon between the crude oil, underground water and the reservoir pore surface. Dependent on the geology condition, the reservoir pores may be distributed from a few hundred micrometers to a few micrometers. The pore pressure and temperature under subsurface may also change site by site and covers a broad range. To improve our understanding on the wettability of mineral surfaces by crude oil and fluid, an in situ observation is required down to micro or even nano scale.

In this study, the contact angle was measured for nano- and micro-droplet in high pressure and temperature cell using vertical shift interferometry (VSI). The high-pressure and -temperature cell that we built allows us to measure the contact angle up to 200 ^oC and 20 MPa. VSI can determine the shape without contacting the sample with a very high accuracy ~1 nm in the determination of the height and ~1 micro meter in the determination of the width of the droplet. By knowing the shape of the droplet, the contact angle was determined by theta/2 method.

By using these techniques above, we have succeeded in measuring the contact angle of water droplet on silica immersed in heptane or toluene fluid at 0, 2, 4, 6, 8 and 10 MPa. It was found that the contact angle of the droplet is ranged 35 to 60^o, while the contact angle of water in heptane is ranged 15 to 35^o. As anticipated, the pressure dependence is negligible. The detectable difference between heptane and toluene fluid was further discussed based on the different interactions between silica, water and the oil fluids, which further ensure the significance of the definition for the percentage contents of paraffinic (alkane), naphthenic (cycloalkane) and aromatic components in reservoir fluids (often referred to as the PNA distribution) when describing the crude oil.