As presented in the discussion of laboratory (core)-based geomechanics versus log-based geomechanics, laboratory-derived values for mechanical properties, from properly conducted tests, are often the most accurate values for mechanical properties. In addition, unlike log-based evaluations, core-based tests can, for example, determine if inelastic behavior is important or evaluate the mechanical properties of the rock fabric as opposed to just the matrix of the rock. As such, core-based geomechanical testing – though expense, time-consuming and limited in coverage – must play a central role in any complete geomechanical analysis or evaluation. Nonetheless, careful core handling and core testing is required in order to obtain reliable and accurate results. Not surprising, then, a quality laboratory-based core testing program starts well before the well is even drilled.

A proper laboratory geomechanics program begins with the evaluation of the geomechanical challenges and their associated lithologies – whether that be in the overburden, the reservoir or, in the case of solution mining, the salt layer itself. These key lithologies then become the target of coring operations. This is an important – but overlooked – critical first step. Because most coring operations are limited in scope, coring must be targeted at critical formations and depths, and these formations and depths need to be carefully selected for the geomechanics challenge being evaluated. Many geomechanics evaluations have been short-circuited because core points and sample selection were driven by other core customers and geomechanical testing was left to pick from the scraps.

Core handling and preservation – from the wellsite to the laboratory is also a critical step and cannot be overlooked. However, once the core has been successfully delivered to the testing laboratory (and accounting, again, for the necessary coring, core handling, and preservation efforts), sample selection is then made (based upon key lithologies, available core material, test type and, ultimately, budget). The critical issue with plugging of the core, like the selection of the core points themselves, is that geomechanics testing cannot be an afterthought on core scraps if the goal is to develop solid geomechanics data for a geomechanics evaluation. 

OFG has a history with core handling and testing from the early 1980’s and can design and supervise your coring and rock mechanics laboratory program for the specific field application. We can:

  • – Assist in the evaluation and determination of key lithologies;
  • – Evaluate and support coring operations and core handling and preservation procedures;
  • – Evaluate core for sample selection and test type and number of tests;
  • – Assure the use of ISRM/ASTM guidelines for proper rock testing;
  • – Design the testing program including confining pressures and stress trajectories; and
  • – Supervise and QC testing and results
Typical geomechanics load frame (left) and corresponding failed samples post-testing in compression (right). 
Common depiction of axial and radial strain versus axial stress for an unconfined compression test. The slopes of these lines are used to determine the elastic parameters Young’s modulus and Poisson’s ratio.
As Young’s modulus is properly derived from the tangent line to an axial strain vs. axial stress curve, Young’s modulus could be continuously derived during a given test as shown above. Note that rock samples, even companion samples, can reflect heterogeneous behavior. Equally important, elastic parameters, like Young’s modulus, are often a function of the stress level. Consequently, it is very important that the reported values (shown in the inset table) are taken at the appropriate reservoir stress conditions or the reported values, as here, may be off by a factor of two or more from expected reservoir behavior.