NMR imaging application to the study of adsorption/precipitation of chemicals inside porous media.

Chemical inhibitors are widely used to prevent undesired precipitation of salts (e.g., CaCO3 or CaSO4) inside formations. The phase behaviour of solutions of phosphine-polyacrylic acids, (PPAA) which belong to a widely used class of threshold inhibitor, investigated in presence of Ca2+ ions and placed in a column packed with quartz sand. Solutions were prepared using deuterated water (99.9%) as solvent. T1-weighted spin-echo images gave profiles of the distribution of phosphine-polyacrylic acids during flow of the solutions inside the sandpack, while spatially resolved T2 estimation (by nonlinear least-square fitting of several images acquired by varying the echo time) allowed precipitation phenomenon to be probed. In fact, spin-spin relaxation, which is particularly sensitive to slow motions, was affected by a precipitation process that caused a relevant T2 decrease in the area of interest. No adsorption occurred under the conditions adopted, whereas precipitation was visualized at the interface area between the different solutions. The phenomenon was very slow and was mainly governed by diffusion of the chemicals.