ORIGIN OF RADIOACTIVITY IN MARCELLUS SHALE PRODUCED WATER

The origin of salinity in water produced during extraction of gas from Marcellus Shale is still the subject of scientific controversy. One hypothesis is that the elevated salinity in the produced water is a result of mixing between the frack fluid and formation brine in Marcellus or adjacent layers (e.g., Onondaga). Another hypothesis is that the elevated salinity originates from the dissolution of the rocks in the formation. Both hypotheses will be addressed in the presentation.

It is well known that Ra226 and Ra28 are progenies of U238 and Th232, respectively, but there were no previous studies addressing the source of Ra in the produced water (e.g., mixing with the formation brine or shale dissolution). Because radionuclides in the produced water are present in trace quantities, the origin of produced water radioactivity may not be the same as the produced water salinity. This study offers new insights to address the source of radionuclides, mainly Radium isotopes, in the produced water from Marcellus Shale.

Secular radioactive decay equilibrium is achieved when the ratio of activities of the successive members of the decay series remains constant. In particular, if the half-life of the parent compound is much longer than that of its progeny, the activity of the progeny would keep increasing until its activity equals the activity of the parent compound. In the case of Ra288 (half life = 5.8 years) and PB212 (half life = 10.6 hours), the activity ratio of Pb212:Ra228 would be 1 (secular equilibrium) only if Ra228 has been present in the liquid phase for about 35 years. Analysis of various brines collected in the Marcellus Shale region show that the activity ratio of Pb212:Ra228 is consistently close to 1, which implies that Ra228 has been present in the liquid phase for at least 35 years. Such finding supports the hypothesis about mixing between the frack fluid and formation brine as a sources of radionuclides.