Page 30 - Enhanced Oil Recovery in Shale and Tight Reservoirs

20 Enhanced Oil Recovery in Shale and Tight Reservoirs

1
c t is the total compressibility in psi ; L is the characteristic length in ft; p is
D
the dimensionless pressure which is defined as
R R
t huff t puff
0 p avg dt 0 p avg dt
p ¼ p huff p puff ¼ (2.18)
D
S huff S puff
The subscripts huff and puff mean during the huff time and puff time,
p avg means the matrix average pressure. Refer to the areas marked in
Fig. 2.12,p huff and p puff are defined as
R R
t huff t huff
S1 0 p avg dt 0 p avg dt
p huff ¼ ¼ ¼ (2.19)
S2 S huff p max t huff
R R
t puff t puff
S3 0 p avg dt 0 p avg dt
p puff ¼ ¼ ¼ (2.20)
S4 p
S puff
max t puff
where S 1 , the blue area shown in the figure, represents the integral of
average matrix pressure over the huff time in a cycle (t huff ), S 2 , part of it
being the yellow area shown in the figure, represents the area defined by the
maximum average matrix pressure during the huff time, p max , times t huff ;
similarly, S 3 , the green area shown in the figure, represents the integral of the
average matrix pressure over the puff time in a cycle (t puff ), S4, part of it
being the pink area shown in the figure, represents the area defined by the
maximum average matrix pressure, p max , times t puff .
Using the above definitions, the curve of cumulative oil recovery factors
versus the dimensionless time for the simulation models of different matrix
sizes falls on almost the same curve, as shown in Fig. 2.13. In this figure, the
oil mobilities (permeability divided by oil viscosity) of the different scales are
the same. When the mobility is increased, the curve shifts to the right,
although variation of well operation constraints (e.g., injection and produc-
tion pressures) does not shift the curve. When the huff time and/or puff time
are changed, p D is changed. As p D is increased, t D is decreased, the curve
shifts to the left. Simulation model results seem to indicate that the optimum
p D for oil recovery is 0.8 (Li and Sheng, 2017b).

2.5 Effects of pressure and pressure depletion rate

In the beginning of the type of research in laboratory, the injection
pressure was in a few thousands of psi, and the pressure was suddenly released
to the atmospheric. It was observed that as the injection pressure was