Fluid flow in porous systems driven by capillary pressure is one of the most ubiquitous phenomena in nature and industry, including petroleum and hydraulic engineering as well as material and life sciences. The classical Lucas–Washburn (LW) equation and its modified forms were developed and have been applied extensively to elucidate the fundamental mechanisms underlying the basic statics and dynamics of the capillary-driven flow in porous systems. The LW equation assumes that fluids are incompressible Newton ones and that capillary channels all have the same radii. This kind of hypothesis is not true for many natural situations, however, where porous systems comprise complicated pore and capillary channel structures at microscales. The LW equation therefore often leads to inaccurate capillary imbibition predictions in such situations. Numerous studies have been conducted in recent years to develop and assess the modifications and extensions of the LW equation in various porous systems. Significant progresses in computational techniques have also been attained to further improve our understanding of imbibition dynamics. A state-of-the-art review is therefore needed to summarize the recent significant models and numerical simulation techniques as well as to discuss key ongoing research topics arising from various new engineering practices. The theoretical basis of the LW equation is first introduced in this review and recent progress in mathematical models is then summarized to demonstrate the modifications and extensions of this equation to various microchannels and porous media. These include capillary tubes with nonuniform and noncircular cross sections, discrete fractures, and capillary tubes that are not straight as well as heterogeneous porous media. Numerical studies on the LW equation are also reviewed, and comments on future works and research directions for LW-based capillary-driven flows in porous systems are listed.

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Spontaneous imbibition (SI) is one of the primary mechanisms of oil production from matrix system in fractured reservoirs. The main driving force for SI is capillary pressure. Researches relating to SI are moving fast. In the past few years, amount of literature on the development of SI with respect to many variables, such as mechanism of imbibition, scaling of imbibition data and wettability of matrix blocks. In this review, we first introduced the fundamental physics mechanism of SI through capillary tube models and micromodels. Then both conventional and more novel experimental methods of measuring oil production are discussed thoughtfully. This is followed by reviewing the oil production performance under various boundary conditions and the characteristic length in scaling equations that have been used to account for different cores shape and boundary conditions. The effect of fluid viscosity on the rate of oil production and final oil recovery as well as the development of viscosity term in the scaling equation are reported. The commonly used methods to quantitatively evaluate the wettability of cores and the SI under mix-and oil-wet conditions are introduced. And last but not least, the methods and mechanism of wettability alteration for enhanced oil recovery in mix-or oil-wet fractured reservoirs are presented.Keywords: Spontaneous imbibition, fractured reservoirs, boundary condition, viscosity ratio, wettability.Citation: Meng, Q., Liu, H., Wang, J. A critical review on fundamental mechanisms of spontaneous imbibition and the impact of boundary condition, fluid viscosity and wettability.

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Oil displacement from water-wet fractured reservoirs by spontaneous imbibition leads to the entrapment of a considerable fraction of oil. Understanding the entrapment phenomenon and mechanism is significant for designing recovery processes. In this study, experiments of pure co-current spontaneous imbibition are conducted with packed columns. The packed columns are filled with either glass beads or quartz sands and saturated completely with oil or gas. The geometry of the glass bead is regular with a narrow size distribution. In contrast, the geometry of the quartz sand is irregular with a wide size distribution. In experiments, one end of the packed column is in contact with brine and the other end is in contact with oil or gas. Both the oil/ gas production and the advancing distance of the imbibition front versus imbibition time are measured. The relative permeability to brine behind the imbibition front and the capillary pressure at the imbibition front are estimated as well. For glass-bead-packed columns, the magnitude of entrapment and the relative permeability to brine behind the imbibition front have no significant variation with the increase in oil/gas viscosity. However, for quartz-sand-packed columns, the magnitude of entrapment increases rapidly and the relative permeability decreases rapidly with the increase in oil/gas viscosity. The reason for this behavior is the difference in the pore size distribution of the packed columns. The capillary pressure at the imbibition front estimated by the piston-like model is always smaller than that measured under restricted counter-current imbibition. This work will help us better understand the process of the spontaneous imbibition, which is significant for the oil/gas recovery in fractured reservoirs.

High-pressure methane sorption isotherms at 40–101 °C and pressures up to 25 MPa were measured on Jurassic lacustrine and Silurian marine shales from China. Shale samples span a thermal maturity range from low mature (oil window) to overmature (dry gas window). Low-pressure CO2 and N2 adsorption techniques were used to quantify specific surface area, pore volume, and pore size distributions. The thermodynamic characteristic of methane sorption on shales was assessed based on the experimental multitemperature isotherms. The effects of physical and chemical properties of shales on thermodynamic properties were analyzed and discussed. Finally, standard enthalpy of sorption was first introduced to evaluate the sorption affinity of methane on shales, and a general pattern describing the evolution of methane sorption as a function of thermal maturity was proposed. The Langmuir sorption capacity of these shales varies from 0.09 to 0.16 mmol/g. The low total organic matter carbon, clay-rich lacustrine shales have comparable methane sorption capacities as those of organic-rich, high thermal maturity marine shales, though high thermal maturity shale samples tend to have larger micropore volume than low mature shales. Clay minerals, especially I/S mixed layer minerals, contribute a lot to methane sorption of lacustrine shales in oil window. The isosteric heat of methane sorption on these shales decreases with increasing absorbed amount. The commonly used Clausius–Clapeyron equation, which neglects the real gas behavior and adsorbed volume, tends to overvalue the isosteric heat. The standard enthalpy of sorption reflects the comprehensive effect of physical and chemical properties of shales on gas sorption and shows a parabolic-like pattern with thermal maturity. The standard enthalpy of sorption first decreases with increasing thermal maturity up to the condensate window shales (late mature, equilibrated vitrinite reflectance 1.0–1.1%) and subsequently increases toward the overmature shales.