Geomechanical testing with large rotary side-wall cores to reduce uncertainty of mechanical properties in the overburden

Summary

Understanding the strength of overburden rocks, and how they respond to changes in applied stress is essential for developing 3D mechanical earth models. Reliable prediction of the in-situ stress changes over time can only be undertaken with detailed knowledge of the overburden rock mechanical properties. Similarly, monitoring of surface deformation to interpret subsurface changes from fluids injection or production requires knowledge of overburden rock properties. Finally, rock mechanics tests of the overburden formation provide data to reduce the risk of wellbore instability problems for future drilling campaigns.

Acquiring conventional core through the overburden formations can be extremely costly. The XL-Rock tool allows the collection of side wall cores of sufficient size (1.5in x 2.5 in) to extract vertical sub-plugs for rock mechanics testing.

These samples can be used in cases where acquiring full core is not an option. Test results from conventional core have less uncertainty but XL-Rock samples have the following advantages:

1. Significantly lower acquisition cost than conventional coring.

2. Ability to take samples from widely separated formations, rather than just a relatively short continuous interval as is the case in conventional coring.

3. Sample selection can be closely targeted based on prior logging runs, to obtain samples from specific zones of interest.

This paper describes a case history, where, thirty-five XL-Rock samples were successfully acquired targeting overburden formations ranging in depth from 300-1600m. The samples were taken from different lithologies with a wide range of rock mechanical properties.