Integration of microscale chemical debonding into underground reservoir modelling

Abstract:

To face the climate change, underground reservoirs are promising candidates to store greenhouse gas such as CO2. This injection into underground rocks alters the chemical composition of their natural pore fluid. By affecting the fluid reactivity, these reactions lead to mineral dissolution/precipitation that can modify different properties of the rock and threaten the mechanical integrity of the system.
We are here interested in sandstone reservoirs, which we consider to be a granular medium of silicate cemented by carbonate bonds. Considering the dissolution kinetics of these minerals, the alteration mode of this geomaterial can be simplified as a debonding phenomenon: the cement dissolves, while the granular skeleton remains intact.

Even if this debonding phenomenon has been previously investigated through both experiments and numerical simulations in a range of configurations, only a limited number of studies have focused on examining the evolution of the mechanical elastic properties. Microstructure-scale investigations on debonding are even more scarce. This scale is particularly relevant, as it allows the direct observation of the consequences of the chemical reactions through the microstructure evolution.

Consequently, the goal of this contribution is to develop a numerical framework where the microstructure of a reservoir rock and its evolution are explicitly described. Then, the variation of the elastic properties of this avatar is determined with a numerical homogenization scheme considering intermediate states between the bonded and unbonded configurations. Several parameters such as the initial degree of cementation or the intrinsic properties of the minerals that compose the microstructure are explored.

From this application of the framework, two pivotal mechanisms for the weakening of the elastic properties are revealed: a) the presence of inactive cement material and b) the decrease of the influence of the cement. Indeed, it appears that the softening remains small close to the initial bonded state as it dissolves inactive cement material, which does not participate in the stress transmission. In the same vein, the effect of the chemical destabilization reduces close to the final unbonded state, as the impact of the cement material on the sample behavior decreases.

Once the weakening laws have been obtained in this numerical homogenization framework, they are applied at the scale of the fractured material from the underground reservoir. In particular, the behavior of a preexisting fracture is investigated under the injection of a reactive fluid. We study the effect of several parameters, such as the amplitude of the degree of cementation, the fluid pressure, and the mechanical loading. In particular, an unexpected effect of chemical debonding is revealed: in the simulated scenarios, the integrity of the material is reinforced by the amplitude of the chemical solicitation. We find that the chemical debonding participates in a softening of the mechanical properties, and thus in a stress redistribution at the material scale, reducing the mechanical solicitation at the crack tip.
The numerical framework presented herein represents a flexible tool to consider chemo-mechanical couplings across the scales. It can be applied to explore the mechanisms that dictate the complex behavior of the geomaterials.