Sigra In-situ Stress Testing (IST) by Overcoring System
Sigra measures in-situ rock stress primarily by the use of its IST tool. This is an overcore device which operates in conjunction with the Boart Longyear HQ wireline coring system and permits stresses to be measured at up to 2000 m depth. The tool is a re-usable device which returns high quality two dimensional stress information quickly, where the rock stress is not high enough to cause rock breakage of the pilot hole. Most Australian coal mines have used the Sigra IST system as part of their exploration programmes.
The Sigra in-situ stress (IST) measurement technique is designed to provide the best possible combination of desirable features. In essence the tool is similar to the United States Bureau of Mines borehole deformation gauge (Merrill, 1967) in that it is a biaxial deformation device used to measure the change in diameter of a pilot hole. The advantages of the IST tool are that it is smaller, measures six diameters of the pilot hole, and does not use a cable for communication. This means that the use of tool is not restricted by depth. It has been used in measurements to 1 km in depth and can be used to 2 km. The overcore system is primarily set up to be used as part of a Boart Longyear HQ wireline coring system.
The process is outlined in Figure 1. It involves pulling the core, then in place of the inner barrel a stump grinding and countersinking bit is run and used to remove any upstanding core stump and centralise hole for the subsequent pilot hole. This is withdrawn and a pilot hole drill is used to create a hole 500 mm long and 25.5 – 26.5 mm in diameter. The pilot hole drill is withdrawn on wireline and the tool lowered into the hole where it locks into place. The rods are pulled back so that the on board orientation tools can detect the location of the tool free from magnetic interference. The core barrel is then pumped into the rods and coring commences.
During the overcoring operation a record of diameter change is obtained and stored electronically. Once coring has been completed, the core containing the tool is pulled. The diameter measurements and those taken from the accelerometers and magnetometers are downloaded. The core is tested for Young’s modulus and Poisson’s Ratio and the results are used with the deformation information to arrive at the biaxial stress field perpendicular to the borehole. Because the orientation of the tool is measured the direction of the principal stresses can be found.
The tool is a biaxial device and if testing is only conducted in a single borehole an assumption must be made as to the stress in the axial direction of the hole. As the tool is normally used in vertical drilling from surface the assumption is usually that the vertical stress is that of overburden weight. Where this has the most limitation is within zones where reverse faulting leads to areas of increased vertical stress and adjacent zones of reduced stress. Because of the biaxial nature of the measurement process, it is not possible to deduce any shear components of stress that are not perpendicular to the borehole.
Despite these limitations, the ability to perform a stress measurement at 800 m depth in about 3 hours, and to be able to examine the overcore trace directly on retrieval of the tool along with the core,provide very significant advantages compared to other systems.
The steps in the stress measurement process using the IST are:
- Drilling the hole
- Pulling core by wireline
- Pumping in a countersink tool
- Grinding the core stump
- Pulling the countersink tool by wireline
- Pumping in the pilot hole tool
- Drilling a pilot hole
- Pulling the pilot hole drill by wireline
- Running the IST tool into the hole on a setting tool on wireline
- Pulling the setting tool on wireline while leaving the stress tool locked in place
- Pulling the drill string back so that the IST tool magnetometers are not influenced by the magnetic effects of the core barrel
- Waiting for magnetometer and accelerometer data to be recorded
- Lowering the core barrel back over the IST tool
- Coring over the IST tool while it records changes in diameter
- Pulling the core containing the IST tool to surface by wireline
- Downloading the tool of data gathered during the process
- Checking the borehole deformation profile of the hole as measured by the six pin sets of the tool
- Testing the core for Young’s modulus and Poisson’s ratio
- Calculating the horizontal stresses and their orientation based upon the hole deformation and elastic parameters and orientating the results based upon the magnetic and gravitational field measured by the tool.
A pictorial view of the overcore is shown in Figure 2. The overcore traces can be viewed as soon as the tool is brought to surface and downloaded and an analysis can be conducted based upon estimated elastic parameters. Sometimes a reasonable solution can be obtained even if minor breakout affects no more than two pin sets. A typical stress measurement operation at 500 m depth interrupts coring by 2 ½ hours. Deeper tests take longer because of the time involved in running the tools up and down the borehole.
Overcoring process is shown in Fig. 2 while examples of the diameter traces during the overcore are shown in Fig. 3. Fig. 4shows the best fit of a theoretical pilot hole deformation to the measured deformations.
These tools have been in successful use in some 2000 stress measurements made at depths up to 800 m mostly over the coal and gas field areas of eastern Australia but also in the USA and South Africa. In these areas the diameter change to the 26 mm pilot hole lies typically in a range of 0.005 to 0.25 mm. The tool reads to about 0.0005 mm accuracy. Hard rocks have also been the subject of the use of the tool. Notable in these was the testing of ignimbrite at the site of the Burdekin dam in North Queensland. Here a major stress of 30 MPa was repeatably determined in rock of 350 MPa UCS with a Young’s modulus of 80 GPa.
The prime limitations of the technique are, from a practical viewpoint, the smoothness of the drilling and the evenness of the pump pressure used while drilling. Excessive vibration causes problems with measurement as does pulsating drilling fluid, as this loads the pilot hole. These issues are generally managed by proper drilling technique to provide successful measurements.
Theoretically, the most complex problem is dealing with non-linearly elastic rocks, as are moderately frequently encountered in sedimentary rock. Generally Sigra uses the unloading modulus from a uniaxial test. However where there is any reason to believe that significant anisotropy exists, the core can be tested with axial and radial stress. This is done in a stepwise manner using the unloading behaviour.
The analysis takes account of the effects of stress relief due to the overcore process and the effects of fluid pressure within the hole.