Page 24 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

8 Gas Wettability of Reservoir Rock Surfaces with Porous Media

oil-based system in 2004. Results indicate that an anionic surfactant is better
than a cationic surfactant in changing the surface of carbonate rock into an
intermediate gas-wet or water-wet status. The main reason is that carbonate
rocks are positively charged under reservoir conditions and favor the adsorp-
tion of negatively charged anionic surfactant. If sodium carbonate is added,
it makes the surface of carbonate rock negatively charged and reduces the
adsorption of the anionic surfactant, thereby preventing the rock surface
from becoming water-wet.
In the same year, George et al. [15], proposed that for a fractured dual media
reservoir, the sweep efficiency is an equally important factor affecting oil
recovery. Although polymers can be used to control the fluidity of the surfac-
tant, it will retard the penetration of the surfactant solution into the matrix. If
the wettability changes to preferential water-wetting and/or capillary pressure
reduces to ultralow interfacial tension, oil can be driven upwards by buoyancy
so that crude oil flows out of the bedrock and into the fractures.
Correspondingly, the injected oil-displacement fluid (such as water) will dis-
place crude oil in the bedrock.
In 2006, Panga et al. [16], with the method of injecting gas into cores satu-
rated with saline water under differential pressure, investigated the relation-
ship between pressure gradient, displacement liquid quantity, and residual
fluid quantity with core wettability. Panga also evaluated the effects of five
treatment agents, A1-A5 (A1, A2, and A3 are surfactants; A4 and A5 are fluoro-
polymers), in preventing “water block” damage in high temperature reservoirs.
The CA, spontaneous imbibition, and core flow experiments all show that A5
is stable under the condition of a simulated well and has good adsorption on
core. It also has the minimum damage to core permeability. The surface of
rocks can be transformed into preferential gas-wet from preferential liquid-wet
to effectively relieve water blockage. Subsequently, they screened [17] 41 dif-
ferent types of chemical treatments with the same method in 2007. It turned
out that in all these treatments, the properties of A5 are still the most superior
and the wettability alternation ability is the strongest with the least damage to
the reservoirs. For the core processed by the formula 5%A5 1 95%KCl, the
capillary pressure is reduced significantly and the cleaning efficiency of resid-
ual water is remarkably improved.
In 2007, Fahes and Firoozabadi [18] evaluated the gas-wet alternation capabil-
ities of ten different chemical treatments from the company 3M and compared
them with the gas-wet alternation effects of fluorocarbon polymers FC759 and
FC722 used by Li Kewen. Results indicate that the ten chemical treatments can
be adsorbed on the surface of the core by an acid-base effect to reduce the sur-
face free energy and lead to increasing lyophobic gas-wet characteristics of the
core surface. However, FC759, FC722, and products from 3M lose efficacy at
140 C. Finally, two fluoride treatments, 11-12P and L-18941, were screened

out. They were made into solutions with diethyl ether, hexyl-methylketone,
water, and acetic acid in different proportions that can transform the