Can we use nanoindentation to derive the Poroelastic Parameters of Microporous Rocks? - Experimental Evidence

Abstract

The knowledge of creep is crucial in predicting different phenomena such as reservoir subsidence associated with production, wellbore stability and proppant embedment. In this report, we explored the ability of nanoindentation technique to evaluate the pore scale interactions between the microporous solid and pore fluid. The specimens included a natural carbonate and an analog synthetic micro-porous alumina ceramic. The homogeneity and uniform pore size (mean=8.1 nm) of the ceramic was very helpful to isolate the fluid effects. We measured the creep deformation of the porous specimens over a period of 3 minutes under a constant maximum force using a nano-DMA transducer. The experiments were performed on dry samples as well as saturated with water (1 cp and buffered with 30 ppm calcite powder) and silicone oil (100 cp). Thus, the fluids presented a wide variation in viscosity and chemical reactivity. Saturation with water and oil had contrasting effects on the indentations in both carbonate and ceramic samples. The indentations in the water-saturated carbonate showed a drastically reduced Young’s modulus (from 38 to 6 GPa in carbonate and from 8 to 2 GPa in alumina) and increased creep compared with the dry indentations. We attribute these large differences to the possible chemical weakening of the porous solid in the presence of water. This is further confirmed by comparing the hardness values, which showed that water softened the matrix (from 0.87 to 0.20 GPa in carbonate and from 0.19 to 0.01 GPa in alumina). The oil-saturated sample, on the other hand, showed a higher modulus (47 GPa in carbonate and 17 GPa in alumina) and greater hardness (1.39 GPa in carbonate and 0.47 GPa in alumina), while the creep magnitude was less than that observed in the dry sample. The viscous displacement of oil during consolidation of the poroelastic matrix can explain the higher modulus and lower creep. The loading rate-dependency and size (maximum load) sensitivity of the observed creep also corroborate the poroelastic nature of deformation. We used Agbezuge and Deresiewicz’s (1974) solution to derive poroelastic constants based on the recorded amount of creep. The analysis yields estimates of the diffusivity constant of the rock and the equilibrium creep depth. In summary, the oil was displaced during the creep time, while water weakened the matrix in addition to its displacement during the creep time. The drastic water weakening effect has important implications for the mentioned reservoir scale time-dependent phenomena. Instrumented nanoindentations could be complementary measurements for understanding the matrix permeability of tight rocks.