Page 104 - Fundamentals of Enhanced Oil and Gas Recovery

92 Ramin Moghadasi et al.

flow lines as asphaltene deposition occurs through the transporting lines. Finally, a
CO 2 flood should be conducted in a way that gas mobility is controlled. Injection of
water alternative CO 2 or injection of CO 2 foam using surfactants are the common
options for a mobility control process. All of these mentioned situations need careful
evaluation to avoid loss of cost and time.

3.6 CO 2 INJECTION IN TIGHT RESERVOIR

Tight reservoirs are categorized as unconventional resources due to their poor
reservoir characteristics. At the time of discovery, these type of reservoirs were
believed to be uneconomical because of their low production rate. However, in recent
years, as the energy demand of the world is increasing, the interest toward production
from these reservoirs is also increasing. New technologies like multilateral drilling
have been implemented on these reservoirs for the purpose of oil recovery. On the
other hand, the use of the appropriate EOR method would further increase the oil
recovery from tight reservoirs [74,75].
Commonly, water flooding would be the first potential EOR method to all kinds
of reservoirs. However, for the case of tight reservoirs, water flooding would be very
challenging and in some cases impossible to be conducted. This is because of very
low permeability of these reservoirs, which leads to large growth of unattainable pres-
sure during injection process. In other words, injectivity of water into tight reservoirs
is very low. Such a problem would be assisted by using an injection fluid, which is less
viscous than water. Among all types of fluids, gases have much lower viscosity than
any other fluid. As a result, gas injection can serve a high injectivity into tight reser-
voirs compared with common water flooding [76].
Among all types of gases suitable for gas injection, CO 2 has received much more
attention as it serves lower MMP, which means higher potential for miscibility devel-
opment. In tight reservoirs, viscous force and gravity drainage are less important while
molecular diffusion will be the dominating mechanism. It has also been reported that
in tight reservoirs, implementation of Huff-n-Puff injection is much more efficient
compared with a WAG or continuous CO 2 injection process. In the following, two
field case studies of CO 2 injection into tight reservoirs are discussed [77 81].
• Yu-Shu-Lin Field, Daqing [81]
The reservoir of this field, Fuyang, has an absolute permeability of 0.96 mD.
The oil viscosity was 3.6 cP. The reservoir was water flooded before CO 2 injec-
tion. There were seven injection wells in a five-spot pattern along with 17 produc-
ing wells. The injection of CO 2 started in December 2007 with two injection
wells. The five injector wells were also added to the process in July 2008. Totally,