Effect of flaw shape and penetrating fluids on hydraulic fracture initiation and propagation in porous material (gypsum)

Abstract

The need of new energy sources to supply the energy consumption requirements of modern societies, in conjunction with the decrease in the conventional natural reserves of oil and gas and the improvement of the technological capabilities, led to a considerable growth in the exploitation of unconventional oil and gas resources since the 1980s. Hydraulic fracturing is the process employed to extract hydrocarbons from these formations, which mostly characterize with a low permeability. The extension of this technique arose the necessity for the Rock Mechanics field of a better and complete understating of its fundaments in order to improve the efficiency of the methods applied. This thesis focus in the analysis of some of these fundamentals: the effect of the geometry of the pressurized flaw and the seepage of penetrating fluids into the matrix of the rock in the initiation and propagation of hydraulic fractures. For this purpose, prismatic gypsum specimens were tested under external biaxial loads and hydraulic pressurized flaws. Four type of flaw shapes were tested: 5 mm circular flaw, single vertical flaw, and 5 mm circular flaw with short notches (2.5 mm length), and with long notches (5 mm length). A special case of a circle with very long notches (15 mm) was also tested. Moreover, in some specimens, the circular flaw and single vertical flaw were internally sealed with wax in order to avoid the seepage into the matrix of the rock. Results have shown an influence of the flaw shape and penetrating fluids in the breakdown pressure, being lower for geometries with longer notches and with seepage into the rock, but no effect was observed on the fracturing propagation pattern.