Page 49 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Huff-n-puff gas injection in oil reservoirs 37

2.9 Minimum miscible pressure
For gas EOR, one of the important mechanisms is the miscibility of
gas and oil. Then a miscibility pressure needs to be measured. One of the
conventional methods is to use slimtube tests. One example of such an
experimental setup is shown in Fig. 2.27. The part in the figure marked
“Coil/Column” is a slimtube which is packed with sand and the sand is satu-
rated with a dead oil initially. About 1.2 pore volumes (PV) of gas (CO 2 in
this figure) are injected to the slimtube. Some of the gas is dissolved in the oil
to swell the oil and to reduce oil viscosity; some of the gas bypasses the oil;
and some of the gas displaces out the oil in the slimtube. The produced oil is
collected at the downstream where a back-pressure regulator (BPR) is
installed. It can be understood that as the injection pressure is higher,
more oil can be displaced out. When the pressures are low, an increase in
pressure will result in a significant increase in oil production. But when
the pressures are high, the increase in pressure may not lead to as much in-
crease as in the low pressures. One example of the oil recovery factors at
different pressures is shown in Fig. 2.28. From this figure, it can be seen
that the increase in oil recovery slows down when the pressure is higher

Figure 2.27 A schematic to measure the miscible pressure using slimtube tests.