Stress Sensitivity of Fractured and Vuggy Carbonate: An X-Ray Computed Tomography Analysis

Fractures, vugs, and pores constitute the main pore space in carbonate reservoirs. The associated pore structure-which is of key importance in terms of hydrocarbon production and fluid flow-varies with effective stress. However, there is a serious lack of data with regard to how precisely the pore morphology changes as a function of effective stress; we thus carried out in situ loading-unloading experiments (up to 20 MPa effective stress) where two carbonate samples (fractured and vuggy) were examined with X-ray computed tomography at high resolution in 3-D. The results showed that after loading, porosity decreased exponentially, followed by an increase during unloading where it did not recover to its initial value. This was mainly because of the stress effect on large pore space (i.e., fractures and vugs). For fractured carbonate, the unrecovered porosity can be linked to similar reduction of the surface area and the average aperture of the fracture. Clearly, the changes in pore morphology were more significant in the fractured carbonate (including extension, connection, and disconnection of fractures) during both loading and unloading, while the vuggy carbonate experienced irreversible structural damage. The vuggy carbonate has bigger porosity variation than fractured carbonate to the mechanical loading-unloading cycle. This work thus demonstrates how carbonate pore morphology changes with depth (higher effective stresses are encountered deeper in the reservoir) or during production (with hydrocarbon depletion the effective stress increases).