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

pressure gradient

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.