Page 383 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Fracturing fluid flow back 355

back for fracturing fluid. However, as the above discussion about Fakchar-
oenphol et al.’s (2014) work implies, the osmotic mechanism would be
possible only if the shale matrix has higher affinity for aqueous fluid over hy-
drocarbon, because for an oil-wet shale matrix, they had to use a capillary
pressure close to zero which is not the reality. In other words, the shale needs
to be water-wet or mixed-wet.

12.3.5 Evaporation
When a gas phase flows through a porous medium that is partially occupied
by a volatile liquid phase, evaporation occurs, even if the gas is saturated, due
to its volume expansion. This process is referred to as flow-through drying.
Such process is important in a wide variety of natural and industrial applica-
tions, such as natural gas production, convective drying of paper, catalysts,
and membranes. The papers in the petroleum literature mainly discuss the
water vaporization near the gas wells.
Another type of drying that is related to flow-through drying is referred
to as pass-over drying. In this type of drying, the rate of mass transfer is
controlled by the diffusion of volatile species within the pore space. The
pressure drop is negligible and the gas flow rate is constant. However, in
flow-through drying, the gas flows through a porous medium owing to a
pressure drop. Its mass-transfer is controlled by the convection of gas
(Mahadevan, 2005).
It is understandable that when gas is unsaturated, water will be vaporized
until the water solubility in the gas at the system pressure, temperature and
salinity is reached. Fig. 12.13 shows the water solubility in the nonaqueous

phase of a methane/NaCl brine system at 121 C as a function of pressure
and brine salinity. The symbols represent measured data and the solid lines
represent that calculated by the PR EOS (Peng and Robinson, 1976).
From this figure, we can see that the mole fraction of water in the hydro-
carbon phase is increased, as the pressure is decreased. Then during hydro-
carbon gas production, when the reservoir pressure is decreased, more water
will be vaporized into the hydrocarbon phase. Therefore, the water blocking
near the wellbore will be mitigated as gas is produced. Here we use the PR
EOS data to explain water evaporation phenomenon, as the gas is expanded
(the pressure is decreased). We can combine the Raoult’s law with Dalton’s
law to explain this phenomenon.