Abstract
Abstract A laboratory experiment was developed to study the effects of pressurizing boreholes of rock samples with tailored pulses at intermediate loading rates. The experiment focused on ascertaining the loading conditions needed to initiate multiple-fracture growth from a borehole. Experiments were performed under controlled loading conditions with borehole pressures ranging from 10 to 100 MPa, loading rates from 1 to 100 MPa/ms, and pulse durations on the order of from 1 to 50 ms. The initial conditions of samples were varied to include dry, quasidry, and fully saturated states along with pressure confinement of the samples up to 50 MPa. Observations on samples of a medium-porosity sandstone showed that the degree of multiple fracturing is quite sensitive to the condition of the borehole as well as to the loading rate and appears strongly influenced by transient pore-pressure effects. The loading rate for the onset of multiple fracturing decreased with an increase in sample water content. The influence of water was much more marked at loading rates below 30 MPa/ms than at higher rates. For loading rates above the onset, the number of fractures initiated increased with an increase in loading rate, with about 10 to 12 at a loading rate of 100 MPa/ms. The fracture threshold-breakdown pressure increased proportionately with loading rate, a dependence that is attributed to transient water-intrusion effects around the borehole. The effect of sample confinement caused the threshold-breakdown pressure to increase and noticeably lowered the loading rate for multiple-fracture onset. Only two fractures (e.g., a typical hydrofracture) occurred when water was prevented from invading the borehole region. The tailored-pulse loading tests demonstrated their viability to determine the susceptibility of various rock types to multiple fracturing at intermediate loading rates.
Published Version
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