Integrated geochemical and geophysical monitoring of CO2-rich fluids in carbonate samples.

Abstract:

Percolation of CO₂-rich fluids in limestones causes the dissolution (and eventual reprecipitation) of calcium carbonate minerals, which affects the rock microstructure and changes the rock petrophysical properties (i.e. hydraulic, electrical and elastic properties). In addition, microstructural changes further feedback to affect flow paths as well as the location and magnitude of fluid-rock interactions. To better understand this complex coupled problem and to assess the possibility of geophysical monitoring in chemically reactive geosystems, we performed percolation laboratory experiments on two well-characterized carbonate samples (99% calcite), from Estaillades and St Maximin (France), 10 cm in diameter and 35 and 16 cm in length, respectively. We monitored aqueous chemistry parameters (pH, calcium concentration and total alkalinity) and petrophysical properties (permeability, electrical formation factor and acoustic velocities). X-ray tomography imaging of the rock samples were also performed before and after the flow experiments. The measured chemical and electrical parameters allowed rapid detection of the dissolution of calcite in the downstream fluid. After circulating fluids of various salinities at 5mL·min^-1 for 32 days (about 290 pore sample volumes) at a pCO₂ of 1 atm (pH = 4) in the Estaillades sample, porosity increased by 7%, permeability increased by one order of magnitude, electrical formation factor decreased by 15% and P- and S-wave velocities, measured every cm along the sample main axis, decreased non-uniformly by less than 1% to up to 14%. X-ray microtomography revealed the creation of a ramified wormhole; these, along with the convex curvature of the permeability-porosity relationship, are consistent with a transport-controlled dissolution regime for which advection processes are greater than diffusion processes. Similar results were obtained for the St Maximin sample, except that the wormhole is more compact, which is most likely due to a more homogeneous initial microstructure. This study shows that both seismic and non-seismic geophysical techniques (i.e. electrical measurements) are promising for monitoring geochemical changes within the subsurface due to fluid-rock interactions.