The ability to obtain detailed physical measurements from subsurface samples in a controlled laboratory setting crucially depends on the quality of the retrieved core. The key to maximize core quality is to limit avoidable damage during drilling, retrieval, handling, transport and analysis. This paper focusses on the retrieval process, where core integrity can be compromised when the core is retrieved too quickly without allowing sufficient time for pore pressure equalization. Core integrity, in particular avoiding decompression damage, is a distinctive concern for low-permeability shales. A common notion is to retrieve slowly to allow for pore pressure equilibration. Operational considerations, on the other hand, demand the shortest possible retrieval time. This paper discusses a resolution to these two conflicting requirements by identifying the optimal core retrieval time using finite-element modeling and optimization techniques.
In this paper, a core retrieval effort is modeled using an axisymmetric, finite-element representation of a core with pore fluid that is subjected to successively decreasing external pore-pressure boundary conditions and pressure loads. The modeling results are analyzed for the maximum pore-pressure difference and compared with rock strength criteria to predict potential core damage.
The optimal retrieval time for given geometric and material parameters is determined with an optimization algorithm. The combined use of finite element modeling and optimization techniques in a reliability study enables the determination of core damage probability. This technique provides an analytical-based guideline for the shortest core-retrieval times without compromising core quality due to decompression.