Mechanical behavior of porous carbonates as a function of pressure, temperature, and fluid content from laboratory experiments and correlation with larger scale structures

Pressure, temperature, and infilling fluids influence the petrophysical properties and the associated damaging processes of rocks at all scales. Moreover, each fluid-rock system possesses peculiar mechanical behaviours being these particularly complex in carbonate rocks hosting fluids. In this work, we analyze the laboratory results of deformed clean and hydrocarbon-filled limestones under varying pressure and temperature, providing links between recorded physical properties (seismic velocity), fluid behavior, and damaging. We focus on carbonate-bearing reservoir (Bolognano Formation) rocks, sampled in the Majella massif (Central Italy) that represents a very good analogue for buried carbonate reservoirs. This reservoir is composed by calcarenites with connected porosity of about 20% saturated by hydrocarbon in the solid state at the outcrop conditions. We performed hydrostatic, triaxial and true-triaxial deformation tests up to a temperature of 100º C and a confining pressure up to 100 MPa on both clean and naturally hydrocarbon-filled limestone samples. Results show increasing seismic velocity and Young’s modulus with increasing confining pressures for both clean and saturated samples as expected. 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 whilst at higher temperatures the opposite occurs. In particular, when temperature is rised up to 100º C the Young’s modulus of the saturated samples dramatically decreases, being this coupled by a clear volume reduction even during hydrostatic tests (no differential stress applied). Accordingly, microstructural observations highlight grain crushing related to a large amount of randomly distributed cracks within saturated samples. On the contrary, clean samples are characterized by few microfractures, pointing out the primary role played by liquid hydrocarbons. These observations are in good agreement with meso and microstructural features observed on outcropping hydrocarbon-filled carbonate-bearing faults. The presence of fluid hydrocarbons (high temperature) severely weakens the rock promoting fracturing whilst at lower temperature the presence of solid hydrocarbons increases the mechanical properties of hydrocarbon-bearing rocks. These observations have a large impact for the petrophysical characterization of reservoirs and for the understanding microscale to mesoscale mechanisms of deformation and fluids movement along deformed rock volumes.