Acoustic Emissions During Anelastic Strain Recovery of Cores from Deep Boreholes

摘要

Acoustic emissions and compressional wave velocity measurements were monitored during anelastic strain recovery measurements on sandstone, siltstone, and chalk cores, immediately after the cores were retrieved from 2.7 to 3.2 km deep boreholes. Strain recovery occurred in all of the cores and acoustic emissions were always detected during recovery. The acoustic emission rate decreased exponentially with time, similar to the strain relaxation behavior of the cores, and acoustic emission activity ceased when strain relaxation no longer occurred. The cumulative number of acoustic emission events was linear with volume expansion of the cores. No significant decrease in velocity occurred during strain relaxation, monitored 6--52 hours after the cores were cut, because the strains were small (less than 300 microstrains). However, laboratory measurements of compressional velocity on relaxed, air dried cores show a well-defined anisotropy in low porosity cores, which had a strong preferred orientation of the principal recovery strains. The slowest velocity was in the direction of maximum expansion. The field and laboratory results suggest that a statistically aligned microcrack population evolves during strain relaxation, with microcracks opening more in the direction of maximum strain recovery. It is proposed that the degree of anisotropy is dependent on the relative difference in the magnitude of the in situ principal stresses and corresponding strain relaxation, as well as the degree of heterogeneity of the rock. In high porosity rocks, the fractional increase in porosity due to relaxation microcracks is much smaller. Therefore, differential expansion of a core, due to opening of relaxation microcracks with preferred orientations, does not produce a measurable anisotropy. 16 refs., 3 figs.