Page 27 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

Concept of Gas Wettability and Research Status CHAPTER 1 11

3. Intermediate wettability is conducive to reducing damage to grains
migration in hydrocarbon reservoirs. During oil-gas field development,
migration of particles is an important reason for the formation damage.
Water-wet formation particles usually flow with water, and oil-wet for-
mation particles normally flow with oil. Only when the formation parti-
cles are intermediate-wet can they remain on the interface. It is difficult
to move with the fluid.
4. Intermediate wettability can enhance oil recovery: No capillary pressure
exists on the intermediate-wet surface strata, and the seepage of oil and
water phases in the reservoir pores are free from the influence of capillary
forces. Therefore, the water block effect and Jamin effect do not exist for
oil and gas wells when oil and gas accumulates toward the wellbore. In
water injection wells, injected water will move forward and evenly along
big and small pores of the strata, improving the sweep efficiency of water
and enhancing recovery rate. However, in the strong water-wet strata,
when oil and gas accumulate into the wellbore, the strong water blockage
effect must be overcome, and the oil is likely to be trapped in big pores
during water displacement. In strong oil-wet strata, oil occupies small
pores of the strata; it is difficult for water to drive the oil in small pores.
In the water displacement experiment, the saturation of water-
displacement residual oil is lowest in the intermediate-wet strata—i.e.,
intermediate-wet is beneficial to improving recovery.
5. Electroneutral strata surface is beneficial for increasing oil recovery: If
the electrical property of the strata surface is the same as that of the oil
surface, then it is difficult for oil to get through the pore media. If the
electrical properties of strata surface are different from that of the oil
surface, then crude oil can spread on to the strata surface, causing an
electrical wetting effect and reducing oil displacement efficiency. Hence
electroneutral strata surface does not only allow oil to easily enter into a
porous medium, but also does not produce an electrical wetting phe-
nomenon, thus facilitating oil recovery.
In 2005, Ren Xiaojuan et al. [25], studied the impact of changing the water-
wet surface of low permeability sandstone on the effective permeability of the
gas-water-phase through wettability alternation treatment of the low perme-
ability core of the Upper Paleozoic sandstone, in the Changqin gas field, and
that of low permeability sandstone in certain reservoirs in Xinjiang. A self-
developed wettability alternation agent LW-1 was used, which changed the
property of the surface of water-wet rocks (water-wet is changed into neutral
weak oil-wet). Experimental results show that LW-1 can alter the properties of
rock surface to neutral and oil-wet wettability. The effective permeability of
gas can be increased by 1.06 times on average when the wettability of the rock
surface is changed. The effective permeability of water phase can be improved
by 2.6 times on average when the core of water-wet reservoirs with low perme-
ability is processed and the wettability of the rock surface is changed. The rea-
son for this is that LW-1 changes low permeability of the rock surface, and the