Water Distribution in Overmature Organic-Rich Shales: Implications from Water Adsorption Experiments

Exploratory and developmental data show that connate water has a significant effect on the gas content of a shale. On the basis of its mobility in shales, connate water could be classified as free water and irreducible water, and the latter is generally dominant in highly mature or overmature shales. However, it is still not fully understood how the irreducible water is distributed in different nanopores of shales at the overmature stage. In this study, two groups of overmature Lower Paleozoic shale samples from southern China were selected for water adsorption experiments based on the ASTM standard (D1412-07, 2010), and the distribution of irreducible water in different nanopores was investigated. The results show that the equilibrium irreducible water contents for the two sample sets range from 5.4 mg/g rock to 15.58 mg/g rock. Although irreducible water is actually mainly stored in inorganic pores for the most of our shale samples, the equilibrium irreducible water contents of these shales are found to be positively correlated with TOC contents, indicating that irreducible water in shales could be hosted in organic pores as well as in inorganic pores. For a given irreducible water content, the percentages of nonmicropore surface area and pore volume occupied by irreducible water are larger than those of the microspore surface area and pore volume, implying that irreducible water is more easily stored in nonmicropores than in micropores for our overmature shale samples, and that the irreducible water may be mainly stored in an absorbed state in nonmicropores but in a condensed state in micropores. Furthermore, there is still approximately 50% of micropore volume or surface area available for gas adsorption even when the shale sample is equilibrated with absorbed water at a relative humidity of 96-97%. This study provides new data for understanding the distribution of irreducible water in the nanopores of highly mature or overmature shales and is helpful for accurately evaluating the shale gas content under geological conditions.