A numerical scheme to couple pore fluid flow in a 2-dimensional discrete particle model is developed based on the commercial code PFC (Particle Flow Code). The fluid-coupled particle model is used to study the permeability change of a simulated granular material while subjected to stress change. The particle model simulates granular material like sandstones with an aggregate of cemented particles (balls or disks). The pore fluid flow network is simulated with pores connected by pipes. It is solved in an explicit scheme. The pores and pipes are associated with the particle model. They are updated with the deformation and damage development of the solid part. The fluid pressure around a particle is integrated and applied to the particle. The fundaments of the coupling scheme and some modeling results are presented in this paper.
Hydrocarbons are often produced with pore pressure depletion. This changes the in-situ effective stresses in the reservoir. The stress changes may induce damage and shear-enhanced compaction, which may affect the permeability of the reservoir materials. Reliable estimates of permeability and its stress de-pendence are crucial to successful planning and management of petroleum reservoirs. Many researchers have investigated the relatioaship between permeability changes and stress changes. When porous materials deform elastically the permeability reduction is quite small and can often he predicted by, for example, the semi-empirical Kozeny-Carman equation (Holt, 2000). Network models can also make good evaluation of the permeability (Bryant, 1993). When the materials yield, however, significant permeability alteration may occur (Holt, 1990; Bruno, 1994; Ruistuen, 1997; Ferfera et al., 1997; Bout?ca et al., 2000). The permeability change of the reservoir materials depends on the constitutive mechanical behavior of the rock (dilatancy or non-dilatancy) and on the stress path. A rigid relationship is not yet established. Particle models have been applied to study mechanical properties of weakly cemented sandstones with success (Ruistuen, 1997; Holt et al., 2000). Compared to laboratory experiments, numerical modeling is economic and efficient. The parameters in numerical models are easy to change, and hence can he thoroughly studied. Numerical modeling is a good tool to improve the design of experiments and the data interpretation, even though it is not a substitution.
Combining it with a flow network model, Bruno (1994) applied a discrete particle model to study stress-induced permeability anisotropy. Considering the mutual effects between the fluid and solid part, fluid coupled particle models have been developed (Cundall, 1999) and applied to study stress dependent permeability (Li et al., 2000).
This paper presents a numerical scheme of the fluid coupled particle model (2D) and results by applying the model to study stress dependent permeability. A commercial code, PFC2D is used as the basis of the developments.