Page 173 - Fundamentals of Enhanced Oil and Gas Recovery

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Thermal Recovery Processes

and Jiang [56] experiments, a packed bed model of glass bead was saturated with
870 mPa s viscose oil. Results revealed that choosing propane as solvent increased the

production rate by 21.5% as temperature increased from 21 to 27 C. Enhancement in

oil production rate was 18% as temperature raised in the range of 10 19 C [58]. The
method of hybrid and warm VAPEX was compared to normal VAPEX by Frauenfeld
et al. [48]. He declared that application of heat lead to faster communication from
injection to production wells by more reduction in viscosity in the area near the well.
Hybrid VAPEX process was also studied, in which steam and solvent were alterna-
tively or simultaneously injected. Farouq Ali [6] revealed this idea in 1976 and Butler [59]
compared SAGD with solvent-added SAGD which lead to decrease in steam requirement
up to 30% and the recovery of propane up to 99%. As dew point of water is higher in
comparison to light hydrocarbons, Mokrys and Butler [59] reported steam trapping
which results in reduction of steam-to-oil ratio and energy consumption. A group of
researchers at Texas A&M University studied on the effects of propane injection into a
limited region of the reservoir contained with 160 170 C temperature steam. Results

indicated that the steam injectivity increases by adding the solvent into steam. The starting
time and energy consumption decrease and overall recovery increases. This process is sim-
ulated using STARS module by Deng [60] and Mamora et al. [61] to consider heat effects
in VAPEX process. The results agreed well with experimental values and confirm it as a
hybrid process. Moreover, Zhao [62] developed a combined SAGD VAPEX process
with alternatively injection of solvent and steam. The properties of the system were as
SAGD. This was similar to Allen [50] in which “huff-and-puff” process was studied with
injection of solvents in definite cycles. Zhao [62] studied on steam alternating solvent
(SAS) process and compared it to SAGD process. The results of his studies revealed that
for the same production rate, the required energy decreased 18% for SAS. The results of
his studies were also simulated using STARS and CMG for a typical Cold Lake reservoir
conditions and compared it to SAGD process. He showed that SAS process lead to higher
production rates in comparison to SAGD.

5.2.6.1 Comparison of Steam/Solvent-Based Hybrid Processes
There are a number of advantages of solvent-aided processes in comparison to SAGD,
namely, high energy efficiency, low operating and capital costs, and more oil recovery.
In comparison of VAPEX to SAGD, it should be noted that VAPEX needs less energy
due to lower latent heat value of hydrocarbon in comparison to water. The tempera-
ture in this process is also lower. Singhal et al. [63] found that energy consumption for
VAPEX is 3% of the same project done by SAGD process. One deficiency of such
processes is transfer of the generated heat to the solid structure, overburden and
underburden, and connate water of the well in addition to the heavy oil in place. The
temperature of the reservoir in steam applications rises significantly. However in

VAPEX, the temperature increase is 5 10 C [63]. As Das [64] reported for 1 kg of