Comprehensive Model for Oil Transport Behavior in Nanopores: Interactions between Oil and Pore Surface

Abstract

A clear knowledge of fluid flow at the nanoscale will greatly contribute to recovery of unconventional oil/gas reservoirs. The distinction of nanoconfined fluid flow behavior with that of the bulk phase stems from strong fluid–surface interactions, which favor the emergence of slip phenomenon as well as spatial variation of viscosity and further affects transport capacity. However, elaboration of the above essential relationship remains challenging nowadays. Oil possesses a complex molecular structure and therefore leads to fruitful technical content regarding oil–surface interplay, posing a huge resistance to gaining a good understanding. Furthermore, the physical properties of tight oil reservoirs exhibit a wide variation range, including rock wettability, pore dimensions, and temperature, and their influences on oil transport are fascinating for in-depth investigation. The aim of this research is to establish a physics fully coupled model for nanoconfined oil transport behavior, which is capable of quantifying the contribution of each influential factor. The excellent agreement against the collected data from published literature claims the reliability of the proposed model. Results indicate the following: (i) A wide pore size can effectively suppress the nanoscale-induced influence on the oil flow behavior, including slip boundary and spatial viscosity variation features. (ii) The velocity distribution characteristic exhibits a plug-like shape as a result of a relatively large slip length by tuning surface wettability. (iii) Slip length shows a negative correlation with increasing temperature, and the feasibility of thermal recovery for tight oil reservoirs is not beneficial compared with that applied in conventional heavy oil reservoirs.

New Model of Oil Migration in Shale Nanopores Considering Microscopic Deformation Induced by Stress and Pore Pressure

“…Therefore, the calculation of the slip length of oil migration in nanopores cannot use the theory of water migration in nanopores. 9,18 In this study, the slip length is calculated using the following empirical equations…”

“…However, studies have shown that the bulk oil viscosity is almost identical to that of unconfined oil, the near wall oil viscosity is greatly controlled by the oil-wall interaction, and the critical thickness of near wall oil/adsorbed oil can be reasonably determined as two layers of liquid molecules. ,,, Therefore, it is assumed that the viscosity of both bulk and near wall oil/adsorbed oil is fixed for simplicity . Meanwhile, considering that the oil migration in nanopores belongs to the laminar flow, the oil migration is dominated by the viscous resistance . Therefore, the oil flow in the two regions can be described by the basic continuity equation μ normalb r ∂ ∂ r ( r ∂ v b ∂ r ) = ∂ p ∂ x , .25em r ∈ [ 0 , r b ] μ normalw r ∂ ∂ r ( r ∂ v w ∂ r ) = ∂ p ∂ x , .25em r ∈ [ r b , r 0 ] where p is the fluid pressure, MPa; μ b and μ w are the viscosity of bulk oil and near wall oil/adsorbed oil, respectively, mPa·s; r represents the distance from oil molecule to centerline of nanopores, nm; v b and v w are the velocity of bulk oil and near wall oil/adsorbed oil, m/s; and x denotes the oil flow direction, m. r b and r 0 represent the radi...…”

“…17−19 Therefore, the Hagen−Poiseuille law without considering viscosity change and slip effect cannot describe the fluid migration in nanopores. 9,18 To this end, the researchers investigated the migration behavior of fluid in shale nanopores.…”

“…26,37 However, due to the significant difference in the structure of oil and water molecules (oil molecules are generally non-polar or weakly polar, while water molecules are strongly polar), the water slip length formula in nanopores is not suitable for oil migration. 9,18 In addition, numerous studies have shown that the viscosity and density of near wall oil are greater than those of bulk oil, regardless of whether it is in IM or OM nanopores. 7,9,18,30,31 As for water migration, the viscosity and density of water near wall in IM nanopores are higher than those of bulk water, which is opposite in OM nanopores.…”

“…in CNTs is 1 to 3 orders of magnitude higher than that predicted by non-slip Hagen–Poiseuille law through molecular dynamics simulation (MDS). The experimental and simulation results show that the slip effect during liquid migration in nanopores is obvious, which cannot be ignored. − For simplicity, many studies have represented IM nanopore in shale as quartz nanotube and OM nanopore as CNT or graphene. ,,, Researchers have conducted a large number of studies on water migration in IM and OM nanopores, − and a slip length calculation formula considering wettability is established to describe the slip behavior of water in nanopores. , However, due to the significant difference in the structure of oil and water molecules (oil molecules are generally non-polar or weakly polar, while water molecules are strongly polar), the water slip length formula in nanopores is not suitable for oil migration. , In addition, numerous studies have shown that the viscosity and density of near wall oil are greater than those of bulk oil, regardless of whether it is in IM or OM nanopores. ,,,, As for water migration, the viscosity and density of water near wall in IM nanopores are higher than those of bulk water, which is opposite in OM nanopores. ,, Therefore, the relevant theories describing water migration in nanopores cannot be directly applied to oil migration, and further research on oil migration in shale nanopores is required.…”