Temperature and pressure effects on the mechanical behavior of porous carbonates saturated by viscous fluids

Pressure, temperature, and infilling fluids influence the petrophysical properties and the associated damaging processes of rocks at all scales and at all depths. Moreover, each fluid-rock system possesses peculiar mechanical behavior, being this particularly complex in carbonate rocks. We focus on an outcropping carbonate-bearing reservoir (Majella, Central Italy), that represents a very good analogue for buried reservoirs. We performed hydrostatic and triaxial deformation tests up to a temperature of 100 °C and a confining pressure up to 100 MPa on both clean and naturally hydrocarbon-filled samples. Results show increasing seismic velocity and Young's modulus with increasing confining pressures for both clean and saturated samples. However, different results are observed when the temperature is increased. At low temperatures, saturated samples show larger seismic velocity and rigidity with respect to clean samples due to the solid state of the hydrocarbon at ambient conditions, whilst at higher temperatures the opposite occurs. When temperature is raised up to 100 °C the Young's modulus of the saturated samples decreases by ∼25 %, being coupled by a volume reduction of ∼1 cm³ even during hydrostatic tests (no differential stress applied). Accordingly, microstructural observations highlight crackle breccia and grain crushing microstructures with a large number (more than 30/100 μm) of randomly distributed cracks in saturated samples after both hydrostatic and triaxial tests. On the contrary, tested clean samples are characterized by few microfractures (less than 1/μm), pointing out the primary role played by melting hydrocarbons. Thus, the presence of melted hydrocarbons weakens the rock promoting fracturing, whilst at lower temperature the presence of solid hydrocarbons increases the mechanical properties of hydrocarbon-bearing rock. These observations have a large impact on the risk related to mining or porous carbonate reservoirs depletion and for understanding microscale to mesoscale mechanisms of deformation and viscous fluids movement along rock volumes.