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Water Saturation Relations and Their Diffusion‐Limited Equilibration in Gas Shale: Implications for Gas Flow in Unconventional Reservoirs
Author(s) -
Tokunaga Tetsu K.,
Shen Weijun,
Wan Jiamin,
Kim Yongman,
Cihan Abdullah,
Zhang Yingqi,
Finsterle Stefan
Publication year - 2017
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1002/2017wr021153
Subject(s) - oil shale , saturation (graph theory) , imbibition , capillary pressure , permeability (electromagnetism) , hydraulic fracturing , desorption , geology , relative permeability , drainage , petroleum engineering , unconventional oil , water retention , adsorption , soil science , porosity , chemistry , porous medium , geotechnical engineering , soil water , mathematics , membrane , ecology , biology , paleontology , biochemistry , germination , botany , organic chemistry , combinatorics
Large volumes of water are used for hydraulic fracturing of low permeability shale reservoirs to stimulate gas production, with most of the water remaining unrecovered and distributed in a poorly understood manner within stimulated regions. Because water partitioning into shale pores controls gas release, we measured the water saturation dependence on relative humidity ( rh ) and capillary pressure ( P c ) for imbibition (adsorption) as well as drainage (desorption) on samples of Woodford Shale. Experiments and modeling of water vapor adsorption into shale laminae at rh = 0.31 demonstrated that long times are needed to characterize equilibrium in larger (5 mm thick) pieces of shales, and yielded effective diffusion coefficients from 9 × 10 −9 to 3 × 10 −8 m 2 s −1 , similar in magnitude to the literature values for typical low porosity and low permeability rocks. Most of the experiments, conducted at 50°C on crushed shale grains in order to facilitate rapid equilibration, showed significant saturation hysteresis, and that very large P c (∼1 MPa) are required to drain the shales. These results quantify the severity of the water blocking problem, and suggest that gas production from unconventional reservoirs is largely associated with stimulated regions that have had little or no exposure to injected water. Gravity drainage of water from fractures residing above horizontal wells reconciles gas production in the presence of largely unrecovered injected water, and is discussed in the broader context of unsaturated flow in fractures.