oa Fluid-rock interaction in carbonates - the impact of flow rate and grain size distribution on limestone dissolution at the laboratory column scale

As part of the project studying the fundamentals of carbonate reservoir pore/fracture scale physics and chemistry within the Qatar Carbonates and Carbon Storage Research Centre at Imperial College London, sponsored by Qatar Petroleum, Shell and Qatar Science and Technology Park, we present experimental data on the dynamics of fluid-rock interaction during acid injection in carbonate rock. This has implications for CO2 sequestration in geological sinks as well as in well acidization that has been used in carbonate reservoirs to enhance oil recovery.

The effect of grain size distribution and flow rate on dissolution kinetics was studied in laboratory columns packed with calcite grains at ambient conditions. For each set of different experiments the columns were packed with 150-250µm (fine), 300-500µm (medium) and 600-850µm (coarse) calcite grains. The evolution of fluid-rock interaction was investigated by using inductively coupled plasma-atomic emission spectroscopy (ICP-AES) to study the time dependent profiles in Ca²⁺ cation concentrations inside the column and in the effluent stream. A scanning electron microscope (SEM) imaging technique was performed prior to, and after, acid injection to illuminate the nature of calcite dissolution at the rock surface.

ICP-AES and SEM analysis highlighted the complex nature of the dissolution, characterized by the creation of additional surface roughness and wormholes in single grains that resulted in the formation of a more heterogeneous porous medium. The in situ Ca²⁺ concentration, measured by slicing the column at the outlet, is greater than the effluent concentration, confirming that Ca²⁺ residesin stagnant regions of the pore space.

After starting acid injection, the chemical reaction occurs in the column, resulting in a gradual increase in Ca²⁺ concentration in the effluent that eventually reaches a steady-state value. Thus, the time needed to reach this steady state defines an important time-dependent reaction dynamics regime. The duration of this regime is longer as the grain size distribution becomes finer. As the finer media has a more complex structural heterogeneity, the corresponding surface area takes a longer time to be reached by the injected acid and the transport of the created products takes a longer time to breakthrough. This implies a transport-limited reaction.