Page 182 - Enhanced Oil Recovery in Shale and Tight Reservoirs

166 Enhanced Oil Recovery in Shale and Tight Reservoirs

7.6 Water-alternating-gas (WAG)
To overcome the gravity override of gas and the gravity underride of
water or to combine the benefits of water and gas, water-alternating-gas is
widely used in conventional reservoirs. In shale and tight reservoirs, the
gravity-related problem should be less severe. But Yang et al. (2015) evalu-
ated the performance of water-alternating-CO 2 injection in laboratory with
core permeability less than 0.5 mD, and the oil viscosity is 2.17 cP at 20 C

and at atmospheric pressure. They observed that when the ratio of water to
CO 2 slug sizes was decreased, the fluid injectivity was improved, but the
recovery efficiency was decreased because sweep efficiency was decreased.
In terms of the effect of the ratio on oil recovery efficiency, the result is
consistent with the simulation result from Ghaderi et al. (2012). In their
model, the horizontal reservoir permeability is 0.61 mD and the vertical
permeability is 0.061 mD. The oil viscosity at the bubble point is 0.63 cP.
Three horizontal wells (two edge producers and one middle injector)
have transverse fractures in a staggered configuration. Such configuration
helps to maximize the contact area with the formation and to maximize
the distance between fractures to delay breakthrough and improve sweep ef-
ficiency. Their result shows that WAG performs better than continuous
CO 2 injection, and higher water-to-gas ratio results in higher oil recovery;
for example, after one pore volume of injection, the oil recovery factors for
water-to-gas ratios of 0.5, 1.0, and 2.0 are 16.7%, 19.8%, and 21.7%, respec-
tively. But tertiary recovery for CO 2 injection should be better as CO 2 con-
tacts with oil, reducing residual oil saturation through miscible displacement.
In a specific reservoir, there should exist an optimal water-to-gas ratio. Their
results also show that when the water-to-gas ratio is identical, as the WAG
cycle length becomes shorter, higher oil recovery is obtained. This is because
more cycles can be performed within a fixed time interval. This result is
consistent with that for a huff-n-puff gas injection.

7.7 Huff-n-puff water and surfactant injection
Water injection can build up reservoir pressure to a depleted reservoir.
Zhang et al. (2019) used field-scale simulation models to demonstrate the
potentials of water injection and surfactant solution injection. In the simu-
lation models, the matrix permeability is 150 nD and the natural fracture
spacing is 0.5 ft. The surfactants have the functions to change rock wetta-
bility to more water-wet and reduce IFT. The surfactants of two gpt