Page 374 - Enhanced Oil Recovery in Shale and Tight Reservoirs

346 Enhanced Oil Recovery in Shale and Tight Reservoirs

during injection and decrease during production, and water imbibition into
shale matrix by capillary pressure. Generally, formation brine in a shale basin
could be high (e.g.,150,000 ppm). And a typical fracturing fluid comprises
low-salinity water; in many cases it is in the range of 1000 ppm. The signif-
icant salinity contrast can lead to substantial chemical potential differences
that can create a large osmotic pressure and drive the filtrate from natural
fractures into shale matrix blocks. Current research work shows that the
following mechanisms are responsible for the phenomenon of low flow
back.
12.3.1 Subirreducible initial water saturation

Some gas reservoirs and many strongly oil-wet reservoirs exhibit abnormally
low initial water saturations which is lower than irreducible saturation. This
abnormally low initial water saturation is called subirreducible initial water
saturation. The corresponding formation is called a dehydrated or desiccated
formation. The initial water saturation in some gas reservoirs in Michigan
Reef is even close to zero (Katz ad Lundy, 1982). If an oil or gas reservoir
was initially 100% saturated with water, then later oil or gas influx could
not reduce the water saturation below an irreducible water saturation
because water could not move before reaching that low level. In such
situations, later fracturing water remains in the formation and cannot flow
back if the water saturation is below the irreducible water saturation. Several
hypotheses have been proposed to explain the subirreducible water
saturation.

12.3.1.1 Vaporization (gas reservoirs)
We know that as the temperature is increased, more water will be vaporized,
as shown in Fig. 12.6. If the initial pressure and temperature of a gas reservoir
were low, and later they increased due to tectonic and/or geothermal activ-
ities, then more water will be vaporized. When gas migrated, the vaporized
water would be carried away. Thus the water saturation would be lower
than the irreducible water saturation, if the initial water saturation was at
the irreducible level.

12.3.1.2 Geological compression and diagenesis
Owing to geological deposition, the increased overburden made the reser-
voir more compressed, resulting in the initial saturation lower than the orig-
inally irreducible water saturation. And reservoir diagenesis processes could
contribute to the formation of clays of high surface area and other authigenic