Hydraulic Fracturing in Porus and Nonporus Rock and Its Potential for Determining in-Situ Stresses at Great Depth

摘要

The process of Hydraulic Fracturing as a method of determining in-situ stresses in brittle elastic formations at great depth is analyzed both theoretically and experimentally. Theoretically, it is found that in attempting to relate the recorded hydraulic fracturing pressures to tectonic stresses, it is essential to investigate whether the fluid used to induce and extend fracture penetrates and flows into the rock strata. This flow raises the pore fluid pressure and creates additional stresses and displacements which lower the critical pressure required to initiate fracture and reduce the width of the newly-formed fracture. Based on this additional stress field, a realistic evaluation of in-situ stresses in permeable strata is obtained. Experimentally, hollow cylindrical and cubical specimens of porous and nonporus rock were tested under constant triaxial external loading and increasing internal (borehole) fluid pressure. In the cubical samples, three uniform unequal principal stresses were induced to simulate general conditions at great depth. The experimental results show that hydraulic fracturing occurred when the internal pressure achieved a critical value that could usually be predicted by the theoretical criteria for fracture initiation. (Author)