Coupled Mechanical and Permeability Model for Grain Crushing and Pore Collapse

ABSTRACT The coupled hydromechanical behavior of porous rocks at high compressive stresses is studied. At such stresses, grain crushing, or pore collapse are the primary failure modes. The need to study and model such behaviors is necessary as exploitation of reservoirs reaches depths where stress regimes for such failures are becoming relevant. Experimental data from triaxial compression and Ko tests at different stress ratios are analyzed to model the effects of grain crushing or pore collapse on the stress-strain behavior but also on the changes in the permeability. A stress-dependent permeability model is developed to account for the large drops in permeability associated with these compaction failures. The permeability model modifies the Kozeny-Carman model for permeability as a function porosity to include a stress dependent term linked to grain crushing or pore collapse. Comparison with the experimental results show the validity of the model in capturing permeability changes during compaction failure. INTRODUCTION Reservoir rocks are sometimes loaded at high compressive mean stress at deep reservoirs and high depletions. Moreover, proppant or other completions that support the face of boreholes or perforations may result in both high deviatoric and high mean stresses. Such loadings can lead to grain crushing and/or pore collapse of reservoir rocks. Sandstones often experience grain crushing at high mean stresses. A combination of deviatoric and hydrostatic loading facilitates this phenomenon (Papamichos et al. 1993). High porosity chalks on the other hand experience pore collapse where the open skeleton structure of the material breaks, and destructures giving a sharp decline in porosity (e.g., Papamichos et al. 1997). Both phenomena cause fundamental changes in the structure of the rock, the pore space and thus significantly affect the rock permeability. This work proposes a permeability model that is linked to the mechanical failure of the rock due to pore collapse and grain crushing and attempts to simulate the sharp permeability decline observed during grain crushing and pore collapse and which cannot be captured with only porosity dependent permeability models løike the Kozeny-Carman. Experiments are presented for two sandstones and a chalk where these phenomena had been investigated to a certain extent. Based on the experimental results a model was advanced that appears to provide reasonable simulations of the experimental results.

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