Experimental Studies On The Characteristics Of Immiscible Displacements In Microscale Quartz Capillaries

Fractured reservoirs with low permeability are widely distributed in China, spontaneous imbibition is generally recognized as the dominant mechanism underlying the oil recovery from such kind of reservoirs. Since the spontaneous imbibition is driven by capillary pressure, it is natural that capillary pressure plays paramount part in terms of the oil recovery from fractured reservoirs with low permeability, consequently, it is of great importance to investigate the variation of capillary pressure during imbibition displacement in porous matrix within fractured reservoirs, both from the theoretical and practical points of view. On the other hand, the problem concerning the presence of threshold pressure gradient caused by the microscale effect in the course of fluid flow in porous matrix is still in controversy and further experimental verification is needed. In this dissertation, the microscale quartz capillaries with radii close to the pore size of porous matrix have been used to conduct imbibition displacements and a comprehensive investigation focusing on the variations of capillary pressure in the process of displacements is presented. Furthermore, a preliminary probe into the pendent puzzle regarding the existence of threshold pressure gradient during fluid flow in porous matrix within fractured reservoirs is included in this dissertation as well.The main contents of this study can be described as following.1. An experimental apparatus which can be used to perform immiscible fluid-fluid displacements in horizontally oriented microscale capillaries has been fabricated, and an approach dealing with the experimental investigation of immiscible displacements in microscale capillaries has been developed.2. Based on the experimental set-up and approach, the radii of the four kinds of capillaries used in our experiments have been measured by using pure water as working liquid. Through comparison, it shows that our method of measuring capillary radius in micrometer order is significantly superior to the conventionally adopted SEM scanning method in terms of the convenience in operation as well as in the analysis of the homogeneity of radius distribution.3. The effect of wettability on the variations of capillary pressure in the process of immiscible displacements has been studied by carrying out displacement experiments under different wettability conditions. The experimental results show that the variations of capillary pressure are significantly affected by wettability. Specifically, the capillary pressure during displacements stays unchanged in strong wetting case, while it decreases gradually as the dispal cement proceeds in both weak wetting and non-wetting cases, however, the values of capillary pressure in these two different cases are opposite, being positive in the weak wetting case throughtout the displacements. The reason behind the difference in variations of capillary pressure may be ascribed to the wettability hysteresis during displacements.4. The velocity dependence of dynamic capillary pressure and its correlation with water/oil viscosity ratio have been elucidated by performing imbibition displacements in hydrophilic capillaries under constant external pressure drops. The radius of the capillaries used ranges from0.92μm to6.53μm and the water/oil viscosity ratio varies between0.11and4.2. Experimental results show that the velocity dependence of dynamic capillary pressure is closely correlated with the relative viscosities of the liquids involved. When the liquids are equal in viscosity, the displacement proceeds with constant rate and the dynamic capillary pressure stays almost unchanged, while it increases gradually with a increase in velocity in the case of water displacing more viscous oil, in contrast, it decreases gradually with a reduction in velocity when water displaces less viscous oil. The root cause underlying the experimentally observed velocity dependence of capillary pressure maybe due to the dynamic effect that is present in the course of imbibition displacements.5. A preliminary probe into the controversial problem regarding the presence of threshold pressure gradient during fluid flow in porous matrix within fractured reservoirs has been conducted through re-analysis of the experimental data obtained from imbibition displacements. The results show that the threshold pressure gradient is absent in all the imbibition displacements presented in this dissertation. In addition, it has been found that the change rate of displacement velocity with pressure gradient is markedly influenced by the wettability of capillary inner surface, the magnitude of capillary radius and the liquid viscosity. Specifically, the change rate of displacement velocity with pressure gradient corresponding to the displacements in hydrophobic capillaries is higher than its counterpart in hydrophilic capillaries, while it is linearly proportional to the capillary radii and inversely proportional to the total viscosities of the liquids involved in displacements.