Effect of anisotropic confining stresses on hydraulically-induced fracture propagation from perforated cased-hole in unconsolidated sands

摘要

In order to clarify the mechanisms of hydraulic fracturing in unconsolidated formation, we carried out laboratory tests in this study. For a simulated borehole, a casing pipe was buried vertically in a cubical specimen of unconsolidated sands with moderate permeability of 5 mD. After the specimen was subjected to anisotropic triaxial compressive stresses of a few MPa, a fracturing fluid of a viscous machine oil injected into the casing pipe, and it came out through an axial slit of the casing pipe into sands. After the test, the specimens were cut off bit by bit in order to check fracture formation and invasion of the fracturing fluid. Then we observed that fractures were induced actually and the fracture pattern obviously changed with azimuthal orientation of the slit relative to azimuth of the maximum horizontal stress. A single, double or multiple fractures were induced by combination of the slit orientation and a manner of pressurization. It is inferred from the elasto-plastic FEM analysis that such fracture patterns were caused by variation of stress state around the casing pipe due to sliding of pre-induced fractures and yielding. Those result suggest the possibility to control not only length and volume but also patterns of the fractures hydraulically induced in unconsolidated formation.