ABSTRACT: Laboratory hydraulic fracturing tests were performed in cylindrical specimens of highly porous Berea sandstones (17% and 25%). We were able to raise the borehole pressure so as to induce hydraulic fractures by increasing the viscosity of the borehole injection-fluid as well as the controlled flow rate. A series of tests in unjacketed specimens yielded apparent breakdown pressures that increased linearly with the pore (=confining) pressure, and were well approximated by the two classical criteria. Tests in jacketed specimens, in which the far-field stresses were kept constant, revealed that the apparent breakdown pressure decreased near-linearly with the increase in the initial pore pressure, as expected theoretically. The two hydrofrac criteria served as the upper and lower limit of the experimental data points. The shut-in pressures recorded yielded excellent approximations of the horizontal stress, regardless of porosity, fluid viscosity, or pore pressure. Our results suggest that hydraulic fracturing stress measurements in highly porous sandstone are feasible. However, a more appropriate relationship between in situ stress and breakdown pressure requires better understanding of the poroelastic behavior due to injection-fluid infiltration into the surrounding permeable medium..

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