What Does Borehole Slotting Mean?
Borehole slotting is a 2-D stress measurement method for in-situ stress in a borehole based on the principle of local stress relief. The method involves sawing relief slots parallel to the borehole axis. The method is cost-effective and is designed for quick operation in boreholes for high-density measurements. The equipment can be fully recovered.
The borehole slotting method has been developed to overcome problems with overcoring and hydraulic fracturing. Some common problems include high cost, limitations of a 2-D stress measurement, and propagation of hydrofracs and borehole wall fractures where the borehole axis and the principal in-situ stress direction do not coincide.
In the borehole slotting method, using a pneumatically driven diamond saw, relief slots about 1 mm wide and up to 20 mm deep are sawed parallel to the borehole axis. A contact strain sensor is pressed next to the slot on the borehole wall with a specific force while slotting is in progress. At least three slots, 120° apart are sawed in different sections in the test location which enable the 2-D stress state to be determined.
At another borehole location, at least 10 cm above or below the first location, three more slotting tests are made. The tested area is considered to be linearly elastic, homogenous, and isotropic. Using the perforated disc model, the primary stress state can be re-calculated using Kirsch's equation.
Poisson's ratio and modulus of elasticity must be determined from uniaxial compression tests on cores taken from the borehole for the input values for stress state evaluation.
The borehole slotting method can also be used to determine the 3-D stress state in the rock by conducting the procedure in three boreholes with different azimuth and dip angle.
