Control of Generalized Capillary Number on Immiscible Displacement Path: NMR Online and Network Simulation of Fluid Displacement Mechanism

Abstract

Tight sandstone has a complex pore-throat structure and usually has multiple immiscible displacement paths. Capillary number may control and affect this process, but its control mechanism is still unclear. Therefore, this work aims to study the control process of generalized capillary number on the immiscible displacement process of tight sandstone. Through nuclear magnetic resonance (NMR) online experiments, real-time monitoring of changes in remaining oil in different-radius pores was performed under the control of different generalized capillary numbers. Combined with the microscopic experimental results of a scanning electron microscope and thin-section and constant-speed mercury intrusion, the pore-throat distribution law was obtained, and a pore network model based on a real core was established. The results show that under the control of low capillary numbers, the immiscible displacement path shifts to large and medium pores, while under the control of medium and high capillary numbers, the displacement path shifts to medium and small pores. Moreover, the simulation results also showed interesting phenomena that were not realized in the experiment. When the number of capillaries increases to a certain value (Ca ≈ 6.734 × 10–2 in this study area), unfavorable viscous displacement will occur in tight sandstones, that is, the displacement efficiency will decrease rapidly.