Percolation of a CO2-enriched fluid in a limestone’s core: Evolutions of the hydraulic, electrical, chemical and structural properties

Abstract

Percolation of CO2-rich fluids in limestones causes the dissolution (and eventual reprecipitation) of calcium carbonate minerals, which affect the rock microstructure and change the rock petrophysical properties (i.e., hydraulic, electrical, and elastic properties). In addition, microstructural changes further feed back to affect the chemical reactions. To better understand this coupled problem and to assess the possibility of geophysical monitoring, we performed reactive percolation laboratory experiments on a well-characterized carbonate sample 35 cm in length and 10 cm in diameter. In a comprehensive study, we present integrated measurements of aqueous chemistry (pH, calcium concentration, and total alkalinity), petrophysical properties (permeability, electrical formation factor, and acoustic velocities), and X-ray tomography imaging. 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), porosity increased by 7% (from 0.29 to 0.31), permeability increased by 1 order of magnitude (from 0.12 D to 0.97 D), and the electrical formation factor decreased by 15% (from 15.7 to 13.3). X-ray microtomography revealed the creation of wormholes; 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, confirming results from previous numerical studies. This study shows that nonseismic geophysical techniques (i.e., electrical measurements) are promising for monitoring geochemical changes within the subsurface due to fluid-rock interactions.