Micro-mechanisms of pore collapse in limestone

Many poorly-consolidated reservoir rocks undergo irreversible deformation (compaction) as a result of an increase in effective confining pressure during production of hydrocarbon. Such compaction is usually the result of pore collapse caused by changes at the microscopic level within the rock mass. Pore collapse is considered to be a potential problem in many producing reservoirs. The main goal of this study was to gain an in-depth understanding of the micro-mechanisms operative in a calcite-cemented, porous, clastic limestone during compaction. To achieve this goal, an experimental program was designed and pursued including both macroscopic and microscopic aspects. Hydrostatic compression tests were performed on cubical samples of Cordoba Cream limestone (24.5% porosity) in High Capacity Cubical Device (HCCD). The samples were loaded to three different levels of confining pressures representing three stages of pore collapse, namely: pre-pore collapse, pore collapse, and post-pore collapse. The samples were unloaded after the desired stress levels were reached. Load deformation response of limestone was observed from these test results. Standard thin sections were prepared from both tested and untested samples. Microscopic changes that accompany different stages of pore collapse were examined by an optical microscope. Microscopic changes are correlated with macroscopic responses (stress-strain behavior), and are illustrated to identify the micro-mechanisms and their sequences during the progression of pore collapse. Although cataclastic flow and crystal plasticity are found to be the dominant micro-mechanical processes that are operative throughout the pore collapse, the contribution of a single mechanism to the total deformation mechanism causing pore collapse varies during specific collapse stages. The findings of this study can be effectively used in understanding the macro-micro correlations of deformation and in developing realistic constitutive models and accurate prediction methods for calculating reservoir subsidence because these require an in-depth understanding of the actual phenomena.