## The than 30 days, the oil recovery

The injection times ranges
from several days to several years. So how to choose a reasonable injection
time? Tablexx1shows the relationship among injection time, average pressure
(during huff or puff period) and the oil recovery in the three cycles for the
field model . In this study, the production time is constant. The only variable
is injection time. Fig.xx 1shown
the influence of injection time on oil recovery factor. Figxx2
gives the matrix average pressure distribution with each
injection time. Fig.xx3 summarized
the influences of injection time on oil recovery and matrix average pressure
together. When injection time is 10 and 30 the matrix average pressures were
21368.82 and 29832.78 psi respectively. And their oil recoveries are 0.2 and
0.4, respectively. When the injection time is longer than 30 days, the oil
recovery and the matrix pressure grows slowly. From 30 to 60 days, the average
matrix pressure increases from 29832.78 to 29847.36 psi. The oil recovery factor
increases by 0.2% from 0.4 to 0.5% The longer injection time yields the higher
average pressure during the huff process, thus more oil recovered from the
matrix in the huff-n-puff cycle figurexx3. Before reaching the set injection
pressure 30000 psi, the oil recovery in one huff-n-puff cycle increases as the
injection times increases. After reaching the set injection pressure,
increasing the huff time will not lead to a significantly more oil recovery as
shown in figurxx1.1. In this case, it can be considered that the injection time
less than 30 days is not longer enough. But longer injection time, such as more
than 55- 60 days in this case, the further increased injection time contributes
little to the oil recovery. This extra longer time can be considered as  an invalid time.

Actually,
the increased injection time boosted the matrix average pressure both in huff
and puff period. In this paper, we define a term called pressure gradient (_P/_d),
which is described as the difference of matrix average pressure during
huff-n-puff process divided by the model scale (here we use radius, it’s the
same when discussing the effect of total volume later). Table
xx 1 show the

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value
for this models. We can see that the oil recovery factor rises synchronously as
of the pressure gradient increases. The method to get a higher oil recovery is
to maximize the pressure gradient by increasing matrix average pressure in huff
process (enlarging injection time) and decreasing that in puff process
(elongating production time or increasing production rate). But the enlarged
time will increase project time. Therefore, we need to do the optimization to get
the optimal value of injection and production time. In this model, the optimal
injection time should be 55 to 60 days, as the oil recovery does not increase
significantly as the injection time is longer than 60 days.

Tablexx1
and Figs.
xxi–xxn show the
relationship among injection time, matrix average pressure and the oil recovery
in the four cycle for the field model .The results illustrate that
oil recovery and matrix average pressure increases with increases
injection time . After the injection time exceeds 60 days further increase of
injection time contributes little to the matrix average pressure as well as the
oil recovery
as shown in tablexx1.
we can find that with the same injection time 10 days, the oil recovery  is 0.2 .The reason is that10 days is too short
for the larger scale model., while after 10days injection the average reservoir
pressure is only 21368.82 psi. The lower pressure gradient gives little driving
force to produce oil, thus it yields a lower oil recovery. However, if the
injection time is longer enough (when injection time is longer than 60 days, see
Table xx1). Thus
it is important to choose the appropriate injection time for enhancing oil
recovery.

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