In addition to the microscopic remaining oil distribution, the oil-water-rock three-phase contact angle is also a paramount indicator of oil-water flow. The three-phase contact angle under the condition of only capillary force can reflect the wettability of rocks. It is of great significance to recognize the dynamic changes of the three- phase contact angle in water flooding to understand the law of oil-water flow
[13]. Initially, the three-phase contact angle was mainly measured by using the contact angle measuring instrument to measure the contact angle of a single droplet in the open space
[14⇓-16]. This method considers a single droplet and focuses on the characterization of the fluid-solid contact interface from an image perspective, but it neglects the distribution state of the droplet in the pore-throat environment. Micro-CT scanning image technology has also been applied to measure the contact angle in the pore-throat system. Andrew et al.
[17] and Klise et al.
[18] established a pixel-level in-situ contact angle automatic measurement method based on the micro-CT three-dimensional scanning images and studied the distribution of contact angle under different rock structures. Scanziani et al.
[19] separated rock, water, and oil in the 3D core reconstruction data, carried out linear fitting on the rock edge, and made circular fitting and merging on the oil-water interface to take the tangent line. Then, they completed the automatic measurement of the three-phase contact angle by measuring the included angle of the two lines. This method realizes a more accurate measurement from microscopic images, but it reflects the characteristics of a three-phase contact angle under static conditions. Mirzaei et al.
[20] determined the exact position of the contact point through the Harris corner detection function according to the edge curve and realized the measurement of the droplet contact angle. AlRatrout et al.
[21] used the constant fluid- fluid interface curvature to find the contact line and normal vector, and calculated the contact angle, so that the in-situ contact angle of immiscible fluids could be measured. Ibekwe et al.
[22] realized the accurate measurement of microscopic three-phase contact angles in images by means of contact point identification, rough estimate of contact angles, and angle precision. It is shown that the above efforts take less account of the abstract representation of the contact angle, and cannot make statistical analysis of a large number of contact angles in the image. However, these achievements have reference significance for the image processing and analysis strategy of three-phase contact angle. Considering the visual characteristics of microfluidic experimental images, such as clear threshold, uneven exposure, and few phase states, as well as relevant requirements for the measurement of the contact interface between oil, water, and rock, this paper proposes a method to automatically measure three-phase contact angles based on digital images from microfluidic experiments, according to the approach of Scanziani et al.
[19] and AlRatrout et al.
[21] to measure the micro three-phase contact angle by imitating CT reconstruction data.