Tailored-Pulse Loading Conditions for Multiple Fracturing of Boreholes

摘要

Tailored-pulse loading conditions to produce multiple fracturing of boreholes are examined for sandstone samples having a dry density of 2.42 Mg/m exp 3 and 8.9% porosity. Experiments are performed with borehole pressures ranging from 10 to 100 MPa, loading rates from 10 exp -4 to 10 exp 2 MPa/ms, and pulse durations from 1 to 50 ms. Samples are tested for initial states of dry, quasi-dry, and total saturation, with confinement pressure varying from 0.1 to 50 MPa. Data show sensitivity of multiple fracture initiation on loading rate, water content, and confining pressure. The threshold-breakdown pressure increases proportionately with the loading rate. For quasi-dry and wet conditions, the loading rate for the onset of multiple fracturing decreases with increasing water content and with increasing confining pressure. When fluid intrusion around the borehole is prevented, only two fractures occur in dry samples, regardless of the loading rate or confinement conditions. These effects indicate that pore-fluid mechanisms, which result in a transient increase in pore pressure in the vicinity of the borehole, are responsible for multiple fracture initiation at intermediate loading rates. This influence of pore fluid suggests that higher permeable source rock may be more susceptible to multiple fracturing than low permeable rock. 6 figures. (ERA citation 06:028167)