Underground » Strata Control and Windblasts
This project brings together two techniques as a relatively cheap and rapid means of assessing underground horizontal stress conditions. These techniques are measurement of drill core relaxation using a laser micrometer and core orientation using palaeomagnetism. Drill core able to be monitored using a laser micrometer within a few hours of drilling has exhibited real-time relaxation. This is interpreted to the be the response to relaxation of stress. For vertical drill holes and horizontal strata, and in the absence of any obvious anisotropy, the axis of greatest strain presumably corresponds to the major principle horizontal stress axis (sh1).
Measurement of the remanent magnetisation of the core permits orientation since the remanent magnetisation direction of the Sydney-Bowen Basin sediments has been established through palaeomagnetic studies (ACARP C1615). This method is appealing in several respects:
- it is simple and non-destructive,
- it can be applied to old core and it is not very expensive.
The method may break down when any of the following conditions apply:
- the sediment is too weakly magnetised for a consistent palaeomagnetic direction to be determined;
- the stress anisotropy is too low to produce significant oval-shaped drill core cross-sections;
- the rheological properties of the lithology are such that the core shape varies during transport and storage, or with humidity and temperature; or
- the drill core is taken too close to the surface and stress differences that may exist at depth are compensated for by surface accommodation.
Case studies from the Sydney Basin where the method has given consistent results are described in this report. A comparison is made between Hydrafrac results and (sh1) determined for Pacific Power's PHKE1 hole at Eveleigh. The (sh1) observed in most drill holes at Ulan appear to be N-S. At Oakdale, there is a consistent relationship between (sh1) above and below the coal seam to the west of an inferred fault. Below the seam, axes are more E-W, while below they swing around the SE-NW.
These case studies demonstrate the soundness of the underlying concepts of the method. With carefully selected samples, and when all the assumptions inherent in the method are met, including the magnetic requirements, systematic relationships have been found, suggesting that the underlying concepts are basically sound.
There is scope for further research to determine why some lithologies encountered in other areas are unsuitable, and moreover, to determine means of establishing absolute stress magnitudes. Both of these might be addressed through gaining a thorough understanding of rheological properties of different lithologies through studying the behaviour of synthetic materials during and after loading.
Results
During the course of the project several opportunities have arisen to attempt to observe the real-time relaxation of a drill core sample. Several drilling campaigns have been conducted close enough to the CSIRO Rock Magnetism Laboratory at North Ryde for samples to be transported and monitored within a few hours of their extraction from the ground.
These have included Pacific Power's Eveleigh deep hole and programs at Tahmoor and Ulan.
The example shown in Figure 2 is from Ulan's 1994 drilling program. This is a compilation of real-time relaxation data for sample C448, which was collected on site at Ulan on 14 January 1994 and immediately taken to North Ryde.
That evening and throughout the following day the sample was continuously monitored. The plot shows how the sample slowly becomes elliptical over an 18 hour period. It is noteworthy that even by 7.25pm (0 hr) the sample had a measurable oval shape, whose long axis remains fairly constant throughout observation.
The diameter difference increased from 19µ m to 29µ m, 47µ m, 50µ m and finally 51µ m. Other results from the Sydney Basin suggest that final values of 40µ m to 80µ m are normal.
Conclusions
Drill core has been monitored using a laser micrometer within a few hours of drilling and real-time relaxation has been observed. This imparts a good deal of confidence that the oval shaped cross-sections do indeed reflect stress relaxation.
There are promising similarities between the laser micrometer results and the Hydrafrac results from the Eveleigh deep hole. The predominant stress orientation at Eveleigh appears to be NE/SW. Gross differences between the micrometer and Hydrafrac results can be explained in terms of either orientation error from steep remanent inclinations or siltstones and mudstones which do not appear to be suitable lithologies.
Careful attention to sample selection should greatly reduce inconsistencies between the methods.The determination of stress orientation from drill core relaxation and palaeomagnetism has been apparently quite successful at Ulan, although independent stress orientation data are not available at the time of reporting for comparison.
Fine even-grained sandstones yield the most consistent and easily interpretable data. Shales and other clay-rich lithologies often give inconsistent and, in some cases, unrepeatable results.
This corroborates the results from Eveleigh. For these reasons fine even-grained sandstones are the preferred lithology for these tests.