Jean-Baptiste Clavaud

Percolation of CO2-rich fluids in a limestone sample: Evolution of hydraulic, electrical, chemical, and structural properties

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.

Formation Evaluation in Thin Sand/Shale Laminations

Formation evaluation in thin sand-shale lamination seeks first to determine sand resistivity, volume fraction, and porosity. Afterwards, saturation and volume are simple Archie applications. Resistivity anisotropy techniques can provide estimates of sand resistivity and volume fraction, but good results depend on the choice of the anisotropic shale point. The same shale point should be used in the determination of sand porosity. Difficulties will arise when anisotropy is not caused by sand-shale laminations, when no sand-shale point exists, or when the nearby thick sand-shale is not representative of the sand-shale in the laminations. In producing fields that have undergone several waterfloods, water resistivity is often unknown in the swept thick sands and might not be representative of the water in the unswept thin sands.

On the Interest of Bulk Conductivity Measurements for Hydraulic Dispersivity Estimation from Miscible Displacement Experiments in Rock Samples

The determination of the hydraulic dispersivity and effective fraction of porous medium contributing to transport on soil and rock sample in the laboratory is important to understand and model the evolution of miscible contaminant plumes in groundwater. Classical methods are based on the interpretation of the breakthrough curve, i.e., the evolution of the concentration in contaminant at the downstream end-face of a sample into which a front of contaminant is advected. Here we present an experimental device aimed at performing such measurements, but also allowing the bulk electrical conductivity of the sample to be measured. We show that the dispersivity and effective fraction can be inferred from this electrical measurement, and that the combined use of both out-flowing fluid conductivity and bulk conductivity allows the incertitude on the dispersivity and effective fraction to be significantly enhanced.