Darcy-Stokes Streamline Simulation for the Tahe-Fractured Reservoir With Cavities

Abstract

Summary The Tahe-fractured reservoir has very special cavities and fractures. There is no flow in the rock matrix; and cavities provide the main storage space and even have the scale of meters. Consequently, the reservoir cannot be considered as a traditional continuous porous medium. Instead of a dual-porosity flow model, a Darcy-Stokes compound single-porosity model is developed with the whole flow area divided into a Darcy-flow region, which obeys Darcy's Law, and a free-flow region, which satisfies a Navier-Stokes flow. Through a Tahe real reservoir case, it is found that a cavity-fracture dual-porosity model is unable to reflect the rapid bottom-water breakthrough, and that finite difference method has a hard time getting convergence in the strong heterogeneous reservoir. On the other hand, streamline simulation with a Darcy-Stokes model developed in this paper successfully demonstrates flow behavior of bottom- and edge-water flowing into the wellbore through the cavity: water advances fast, water breakthrough happens in a short time, and water cut rises rapidly. In application, a streamline numerical model for Darcy-Stokes flow is built by combining two-phase Navier-Stokes streamline modeling and streamline-based simulation of Darcy flow. Examples show that both a Darcy-Stokes model and conventional Darcy model give the similar simulation results of saturation and pressure distributions in the Darcy-flow region. However, fluid flow behaviors from these two models differ in the free-flow region. Such a difference lies in the different treatment of velocities: Our Darcy-Stokes model considers the fluid velocity difference caused by a shear stress effect; and the Darcy model only uses the average velocity of the fluid in the cavity, which means the fluid almost moves at the same speed in the free-flow zone and does not agree with the reality.