Limestone pore structure strongly influences dissolution and associated reactive transport. These effects are critical in limestone diagenesis and but also in engineering operations such as carbon capture and storage (CCS). However, detailed studies on how CO₂‐enriched (acidic) brine changes this pore structure at relevant reservoir storage conditions are very limited. Thus, to provide further quantitative information and more fundamental understanding about these key processes, we studied the dissolution patterns of a homogeneous, a fractured, and a vuggy limestone when flooded with CO₂‐saturated brine at representative storage conditions. The pore structured of these limestones showed drastically different responses to the acidic brine flood. As such, preferential channels surrounded by branched channels were formed in the homogeneous sample, while fractures became the main flow path in the fractured sample. In contrast, only one dominant channel formed in the vuggy sample, which resulted in a sharp permeability increase. These dissolution patterns reflect the associated Damköhler number, which significantly lower in the homogeneous, representing uniform dissolution. However, after injecting sufficient reactive fluid (1,000 PV), this uniform dissolution pattern transformed into a single preferential channel growth. Moreover, we conclude that increasing complexity of the pore geometry leads to more nonuniform dissolution. These dissolution patterns indicate the effect of initial pore structure on preferential channel growth and reaction transport. Our work provides key fundamental data for further quantifying limestone dissolution patterns in CCS, indicating that the CO₂ injection may cause the reactivation of geological faults and damage around wellbore, thus aids in the implementation of industrial‐scale CCS.

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).

Fundamental understanding of pore‐scale methane hydrate dissociation in porous media is important to evaluate submarine slope stability and potential utilization of methane resources. In this paper, a general pore‐scale framework based on the lattice Boltzmann (LB) method is established for reactive transport coupled with nonisothermal multiple physicochemical processes in porous media. The framework combines the gas hydrate dissociation kinetic model, the single‐phase flow LB model, the mass transport LB model, and the conjugate heat transfer LB model. The pore‐scale framework is validated by several benchmark problems and then employed to investigate the endothermic dissociation process of methane hydrate with pore‐filling and grain‐coating habits in porous media. The methane hydrate endothermic dissociation behavior coupled with nonlinear nonisothermal multiple physicochemical processes involving intrinsic dissociation dynamics, gas flow, mass transport, phase change heat transfer, and conjugate heat transfer is well captured by the framework. The phase change of methane hydrate dissociation and pore structure evolution for different pore habits of hydrate are well depicted, and some insights about the dissociation front advancement and temperature distributions are also obtained. In addition, the effects of temperature field, inlet temperature, and inlet pressure on methane hydrate dissociation are investigated. The pore‐scale methane hydrate dissociation helps to advance our understanding of permeability‐saturation variation relation for continuum models.