Page 212 - Gas Wettability of Reservoir Rock Surfaces with Porous Media

196 Gas Wettability of Reservoir Rock Surfaces with Porous Media

methods don’t take into account reservoir damage due to transient filtrate.
Also, in some cases, the plugging effect caused by these methods needs to be
improved further. Damage to low- and extra-low-permeability reservoirs can
be caused by water-blocks, hence Zhang Hongxia proposed a method of
adding surfactant to the drilling fluid to prevent damage from water-blocks.
The method has also been applied to low-permeability gas reservoirs in
Qiudong. This concept maybe considered for protecting such reservoirs while
using drilling completion technology. However, further improvements and
advancements in both, choosing surfactants and establishing protection
technology is required [15].

6.1.2 Preparation and Properties of Fluorinated Acrylate
Water-Block Resistance Agent FCS-08
While exploring reservoirs, if external fluids, including filtrates of drilling fluid
and fracturing fluid, invade a reservoir, the invaded fluid cannot be completely
discharged due to a capillary effect, which leads to increased water saturation
in reservoirs and decreased permeability of oil and gas phases. These phenom-
ena are generally called water-block effects [16,17]. Research shows that water
blocking is the main and the most severe damage to low-permeability, low-
pore reservoirs.
Drilling fluid is the first external fluid that comes in contact with the
reservoirs. It is crucial to prevent filtrate from invading reservoirs, leading to
water-block damage [18]. However, when drilling filtrates invade reservoirs,
they cannot be eliminated completely. The measure used frequently is to add
a water-block resistance agent in the drilling fluid. The water-block damage
decreases due to reduced surface tension and interfacial tension of filtrates
[19,20]. More recently, two types of frequently used drilling fluid water-block
resistance agents are being used: polyhydric alcohols and surfactants [21,22].
However, for irregular reservoirs with low porosity and low permeability, and
under complex geological conditions and with severe water-block resistance
damage such as in the case of shale gas reservoirs, tight sands tone gas reser-
voirs, condensate gas reservoirs, etc., these traditional water-block resistance
agents could not meet the need. Moreover, changes in reservoir wettability
caused by filtrates that have invaded cannot be ignored, as they can’tbe
completely eliminated.
The water-block resistance agents used in low-pore and low-permeability reser-
voirs have lower surface tension and interfacial tension, lesser amount of liq-
uid block, lower core permeability damage rate, and better compatibility. This
section deals with a new kind of water-block resistance agent—FCS-08 fluori-
nated acrylate polymer, based on the gas wettability theory. Hard monomers
with higher glass transition temperature, soft monomers with stronger adhe-
sive ability, and functional monomers with perfluoroalkyl side chain are
contained in its molecular structure. FCS-08 reduces surface tension and inter-
facial tension of drilling fluid filtrates by a large degree. When the transient