Overview

This presentation describes the processes used in the investigations for a deep compressed air storage cavern in a complex metamorphic environment. The exploration process involved drilling using HQ-3 coring techniques from surface and also from an adjacent underground mine. The latter were two 460 m boreholes drilled just sub horizontally across the planned cavern location.

The core from these holes was logged for structure and sampled for rock property testing. Because a significant portion of the rock was highly anisotropic the test methods used had to be adapted. This included alternative ways of working with point load testing, triaxial testing for anisotropy of moduli and some special shear tests.  Because of the importance of determining potential leakage of groundwater into the cavern, or of air out of the cavern, the correct measurement of the ground fluid pressure and the permeability of the rock mass were extremely important.

Testing had to be conducted in stages to cover the entire length of the holes. The method used was to fit the drill rods with a straddle packer assembly, downhole valve system and suitable precision pressure monitoring with operator read out. First the test zone was shut in to recover pressure. This was followed by a period of injection or production which was followed by another shut in period. These tests followed oilfield practice but with tools designed to work with HRQ drill rod and used horizontally.

The rock stress was determined by the use of three dimensional overcore tools used up to 441 m laterally. Other techniques could have also been advantageously used to fill in the sparse overcore data in the extremely complex geological environment. The vertical and the horizontal holes were logged using both acoustic televiewer and full wave sonic sondes.

The conclusions of the study were that the rock mass was heterogeneous but fit for purpose provided that the correct design decisions were made. The presentation also looks at what can be achieved from open hole drilling test techniques compared to coring and the comparative costs of these. This has particular relevance to tunnel investigations.

Program timeline

Online

6.00pm – Presentation commences

7.00pm – Presentation concludes

In-Person

5.30pm – Registration, networking & catering

6.00pm – Presentation commences, networking

7.00pm – Event concludes

Learning outcomes

• You should gain an appreciation of the importance of geology.
• You will gain an understanding of the methods used to determine the states of stress in rock and the measurement of anisotropic rock properties.
• The methods used and the importance of determining fluid pressures and permeability properly are extremely important and should totally replace conventional packer testing.

About the speaker

Ian Gray

Managing Director, Sigra Pty Ltd

Dr Ian Gray is the managing director of Sigra Pty Ltd, a role he has held for 32 years. In his career he has had the roles of mining, civil, geotechnical and reservoir engineers, engineering geologist and hydrogeologist. He has worked in 20 countries dealing with mines, tunnels, slopes, petroleum reservoirs and anything else in the ground. One of his passions is actually measuring things in the ground properly. To do this he has developed a suite of equipment and techniques. He also works with a variety of drilling systems.