The dynamics of hydraulic fracture water confined in nano-pores in shale reservoirs

Hydraulic fracturing treatments and horizontal well technology are central to the success of unconventional oil and gas development. In spite of this success, replicated over several thousand wells over diverse shale plays, hydraulic fracturing for shale wells remains poorly understood. This includes the poor recovery of hydraulic fracture water, the inability to explain the progressive increases in produced water salinity and an incomplete understanding of the potential trapping mechanisms for hydraulic fracture water.

In this work, we focus on describing the distribution of saline water in organic and inorganic pores as a function of pore size and pore morphology with the purpose of providing fundamental insights into above questions. A kerogen model is constructed by mimicking the maturation process in a molecular dynamics simulator and it incorporates structural features observed in SEM images including the surface roughness, tortuous paths, material disorder and imperfect pore openings of kerogen pores. This work also extends this kerogen model through the use of oxygenated functional groups to study fluid behavior in partially mature shales associated with non-zero oxygen to carbon ratios.

Our results demonstrate that water entrapment mechanism and the distribution of water and ions in organic and inorganic pores are strongly related to the pore-surface mineralogy and pore width. The work in this paper also underscores the importance of kerogen thermal maturity and pore roughness on the accessibility of the kerogen material to water.