Dynamic capillarity during displacement process in fractured tight reservoirs with multiple fluid viscosities

Abstract

Dynamic capillarity commonly exists for multiphase flow in porous media, during which fluid viscosity varies and has strong influence. Displacement experiments are conducted on water-wet, fractured tight rock at in situ pressure and temperature of an oil reservoir via a specially designed apparatus to investigate the effects of fluid viscosity on the dynamic capillarity. The dynamic effect in the matrix is examined through the measurement and calculation of capillary pressure, the dynamic coefficient, and the fluid flow behavior. The results show that with a higher oil viscosity:(a) both the steady and the dynamic capillary pressures reverse their directions more quickly and behave as larger resistances in the matrix; (b) the difference between the steady and the dynamic capillary pressures becomes around 5%-19% more significant; (c) water saturation changes more slowly corresponding to the lower water relative permeability, while oil relative permeability quickly becomes lower than that during the basic displacement process; and (d) the dynamic coefficient becomes 2-3 times higher, and the dynamic contact angle becomes 10%-25% larger, showing a more variable interface. A contact angle advancement coefficient is proposed to identify the significance of contact angle advancement and the competition between capillary pressure and viscous force. The findings of this study can help for better understanding of multiphase flow in tight reservoirs and enhancing oil recovery.

Transient Two-Phase Flow in Porous Media: A Literature Review and Engineering Application in Geotechnics

“…Later, Tang, Lu, Zhan, Wenqjie and Ma [56] revisited the same research objective using numerical simulation, concluding the dynamic coefficient of fractured porous media is higher than that of unfractured porous media. Similar findings were repeatedly confirmed in later studies for fractured tight reservoirs [50,163,173].…”

“…The primary purpose of their work is to validate the conclusion that the dynamic coefficient can be enlarged with increasing the effective viscosity (µ eff = µ n S n + µ w S w ) proposed by Barenblatt,et al [172]. Li, et al [173] also drew a similar conclusion for the fractured tight reservoir that the dynamic coefficient is proportional to the effective relative viscosity (µ ew = µ eff /µ w ) defined by them. Other studies did not consider those newly defined terms.…”

Mathematical Model of Imbibition Replacement and Optimization of Soaking Time for Massively Fractured Tight Oil Reservoirs

“…In addition, we calculate the water saturation and its variation at each time step to simulate the dynamic capillary force effect, by the following equation p c d = p n w − p w = p c s − τ false( ∂ S w / ∂ t false) …”

Mechanisms of Stress Sensitivity on Artificial Fracture Conductivity in the Flowback Stage of Shale Gas Wells

“…In this situation, the prop effect weakens, so the fractures close, leading to a reduction in fracture conductivity. In addition, the seepage feature is extremely complex due to the two-phase flow of gas fracturing fluid [8][9][10][11]. With the continuous production of fracturing fluid and gas, the pressure in fractures drops gradually, and under the action of overlying pressure, proppant embedment and fragmentation are inevitable.…”