Does the in situ stress field control the orientation of open natural fractures in sub-surface reservoirs?

Abstract

In order to evaluate naturally fractured reservoirs, it is critical to assess whether natural fracture sets believed to exist at depth (eg, from surface mapping and/or seismic data) are likely to be open and productive or closed and non-productive. The semi-log relation between stress and the closure of natural fractures is combined with the effective normal stress acting on fractures to yield: [see full text for equation] which relates fracture closure (d), the constants in the semi-log fracture closure/stress relation (k and s), and maximum effective horizontal stress (sH¢) magnitude with the effective horizontal stress ratio (n), and the angle between the normal to the fracture and the sH direction (q). This relation shows that: (i) for a given fracture, the sensitivity of fracture closure to the anisotropy of the in situ stress field can be constrained by the effective horizontal stress ratio (n); (ii) natural fracture closure is sensitive to fracture orientation with respect to the in situ stress field where n is high; (iii) the sensitivity of natural fracture closure to its orientation with respect to the in situ stress field decreases markedly as n drops; and (iv) the rate of change of closure with changing orientation is relatively low at very low and very high misalignment angles, and much greater at intermediate angles.