Page 117 - Enhanced Oil Recovery in Shale and Tight Reservoirs

104 Enhanced Oil Recovery in Shale and Tight Reservoirs

For the purpose of comparison, methane huff-n-puff injection was
performed. During a huff period, gas (methane) was injected into the
core through the valve A (with B and C) closed for 30 min at 2000 psi
which was higher than the dew point pressure. During a puff period,
the core was depleted through A as well to the BPR for 30 min. No
soaking time was applied. The CT number was recorded at the end of
puff period. The liquid saturation in the core was calculated accordingly.
The cycle was repeated 5 times. Within these 5 cycles, the condensate
saturation was reduced from 0.2745 to 0.0957, resulting in 65.14% liquid
recovery factor.
During a huff period of surfactant solution injection, the surfactant solu-
tion was injected into a core in 2.5 h at 2000 psi through the valve A with B
and C closed. After that, the surfactant solution could not be further injected
because of the low compressibility. And it was difficult to flow back because
of the higher liquid viscosity and low shale permeability. As a result,
only one cycle of surfactant huff-n-puff injection was performed in the
experiment. The condensate saturation was reduced from 0.225 to 0.2218
with 1.42% liquid recovery factor. Note that any condensate whose
saturation was lower than a critical condensate saturation could not be
recovered by huff-n-puff surfactant injection. Injected surfactant could
only change rock wettability to preferentially gas welling so that gas and
liquid condensate relative permeabilities might be improved. One problem
is that the radius or penetration of the surfactant treatment during huff-n-
puff injection was small. Compared with methane huff-n-puff injection
(65.14% RF), the RF from surfactant huff-n-puff (1.42% 5 ¼ 7.1%)
was about 10 times lower, even assuming 5 cycles had been able to be
performed. Methane injection could have the revaporization mechanism
which could recover liquid condensate both at higher and lower the critical
condensate saturation. Note that the two cores in the two experiments were
from the same Eagle Ford outcrop.
To further evaluate the feasibility of surfactant huff-n-puff injection, sur-
factant durability and economics were studied. The surfactant adsorption
increased almost linearly with pore volumes injected. It reached 6.1 mg/
g rock after 14 pore volume injection. The data indicates that the adsorption
could be very high as a high pore volume of surfactant solution is injected
because shale and tight rocks have a high surface area. Our laboratory
experience revealed that it was difficult to change wettability using this
surfactant after more than 15 pore volumes of flooding treatment (such
treatment is not feasible in real reservoirs).