Overview

In the previous chapters, we have discussed the mechanical behaviour of rocks in terms of continuum mechanics. That is, we have treated the rocks as homogeneous materials. However, sedimentary rocks are made up of small particles, and are largely heterogeneous materials on a length scale comparable to the particle size (Fig. 1.18). Furthermore, inho-mogeneous distribution of particle types and particle sizes in the form of layers or clusters, as well as fractures, produce heterogeneities on larger length scales. The mechanical properties of the rock will differ largely from one part to another if measured on a length scale which is small or comparable to the size of the heterogeneities. The continuum approach can be applied with confidence only as an average, and on a length scale which is large compared to the size of the heterogeneities.

For many practical applications the continuum approach is valid. However, it is intuitively clear that the mechanical properties of the rock even on a large scale must depend in some way on the microscopical nature of the rock. We have already touched this problem when we introduced the poroelastic formalism (Section 1.6). The theory of poroelasticity is based on the assumption that the rock consists of both a solid part and a fluid part, which are separated on a microscopic scale, but coexist on a macroscopic scale. One of the statements of this theory is that the rock s framework has its own moduli, separate from those of the solid material and the pore...