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Salt precipitation in permeable rocks is a risk faced by some energy sectors, particularly for gas storage in geological formations during operational phases (injection and extraction), when there is contact with saline aquifers. This is because the circulation of a gas phase in a partially brine-saturated porous medium induces salt crystallization and precipitation within the porous space. This precipitation reduces the space where fluids can circulate, altering rock permeability, or even leading to plugging under certain conditions.

In order to understand the underlying mechanisms behind this damaging phenomenon, experiments examining gas flow in a brine-saturated porous medium were conducted on IFPEN’s CAL-X flow test bench[1]. These experiments made it possible to observe the dynamics of salt migration in 2D, through X-ray radiography. However, the use of X-rays alone is not sufficient to both measure local water saturation and differentiate the states in which the salt is found. 

To overcome this limitation, these experiments were replicated on the D50T neutron line at the Laue-Langevin Institute (UGA-77 proposal)[2], within the framework of a partnership with the NeXT-Grenoble laboratory[3] and Equinor. This line also has an X-ray source, thereby making it possible to simultaneously produce X-ray imaging and neutron imaging observations. The complementary nature of these two methods thus makes it possible to monitor local water saturation and salt quantity, both dissolved and precipitated.

Figure 1 shows a series of neutron and X-ray radiographies during the injection of a dry gas (N2) into a rock, in contact with brine at 100 g/l of KBr in its lower part. The neutron radiography clearly shows the presence of water at the base of the sample, while the X-ray radiography reveals salt migration. 

Figure 2 illustrates the evolution over time (a) and space (b) of different magnitudes of interest (quantity of water, dissolved salt and precipitated salt). These observations have shown that salt precipitation results from the interaction of different parameters, i.e., the pressure gradient, brine concentration, capillary forces and water vapor partial pressure. Moreover, observations reveal that the injection of dry gas systematically cause salt to precipitate but permeability alteration is only observed if capillary contact is maintained with the brine.

This experimental work led to the construction of a 2D flow model incorporating the various physical and physicochemical mechanisms observed. Once calibrated, the model demonstrated good experiment predictability on a laboratory scale. Researchers will be able to use this model as part of a parametric study, with the possibility of scaling up through well simulation.

Click on pictures to enlarge

Science@ifpen n° 46
Figure 1: 2D radiographies during salt precipitation, taken at 100 min, 120 min and 140 min from left to right. The gas is injected from top to bottom, capillary contact is maintained at the bottom of the sample
Science@ifpen n° 46
Figure 2: Nitrogen injection test with 100 g/l KBr brine capillary contact maintained at the exit. a) gas saturation and flow, b) evolution over time of salt quantity and c) distribution of salt in the different phases (t = 150 min). 

 

Publications:

[1] O. Lopez, S. Youssef, A. Estublier, J. Alvestad et C. Weierholt Strandli. 3S Web Conf., 146 (2020) 03001. https://doi.org/10.1051/e3sconf/202014603001. 
   
[2] M. Mascle, O. Lopez, H. Deschamps, L. Rennan, N. Lenoir, A.Tengattini et S. Youssef. Int. Sym. of the Society of Core virtual, SCA 2021-06.
  
[3] A. Tengattini, N. Lenoir, E. Andò, B. Giroud, D. Atkins, J. Beaucour et G. Viggiani. Nuclear Instruments and Methods in Physics Research Section. Volume 968, 2020. https://doi.org/10.1016/j.nima.2020.163939.
 

Scientific contact: souhail.youssef@ifpen.fr

>> ISSUE 46 OF SCIENCE@IFPEN

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