Page 28 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

12 Gas Wettability of Reservoir Rock Surfaces with Porous Media

water-wet surface becomes a weak oil-wet one, which reduces the irreducible
water saturation of gas reservoirs, thereby significantly improving the effective
permeability of gas and reducing water invasion damage on permeable gas
reservoirs. Changes in wettability of water-wet reservoirs with low permeability
can notably improve the effective permeability of the water phase, enhancing
the injection ability of reservoirs with low permeability and water-wet surfaces,
to finally achieve the goal of augmenting production and injection.
In 2008, Song Xinwang et al. [26], processed three groups of water-measured
2
permeability, artificial cores of 0.030, 0.197, and 0.508 µm respectively, into
three kinds of surfaces, namely: water-wet, intermediate-wet, and oil-wet. The
core with water-wet surface was processed with water-wet outcrop sands; the
intermediate-wet core was processed with methylsilicone oil; and the oil-wet
core was processed with a mixture of oil and kerosene at the Shengli Oil Field.
The core permeability was measured by simulated Shengli Oil Field strata
water with 19.4 g/L salinity and simulated oil. The recovery of cores with dif-
ferent permeability was investigated from three aspects: water seepage charac-
teristics, oil seepage characteristics, and water-displacement recovery. The
experimental results are as follows:
1. Influence of wettability on water seepage: There is highest seepage when
the core surface is water-wet, followed by intermediate-wet and oil-wet,
where seepage is the least. The variation decreases with increasing per-
meability and is up to a maximum of 600 times. This is an important
reason for high water injection pressure in reservoirs with low perme-
ability. When the wettability of the core is transformed into an oil-wet
surface, energy is expended to overcome capillary pressure when water
passes through the core pores. The lower the permeability is, the smaller
the average size of pores and the larger the capillary resistance, which
makes seepage resistance higher. When the water-wet surface of the core
is converted into intermediate-wet, the capillary resistance during water
seepage is almost zero, but cohesion from walls needs to be overcome.
Energy expended on cohesion is in proportion with the flow area. The
higher the permeability is, the larger the flow area of water and the
higher the energy expended if seepage needs to overcome cohesion.
Hence the seepage resistance is relatively higher. For intermediate-wet
core, seepage resistance of core with high permeability is the highest,
more than that of the core with intermediate permeability, and that of
the core with low permeability is the lowest.
2. Influence of wettability on oil seepage: When wettability of core is trans-
formed into intermediate-wet and oil-wet from water-wet, the value of
core permeability measured by oil increases initially and then decreases.
It is the highest on an intermediate-wet surface. Regardless of high or
low core permeability, the intermediate-wet condition is the best for oil
flow.
3. Influence of wettability on water-displacement recovery: The water-
displacement recovery of the core is the highest when the surface of the