Page 76 - Primer on Enhanced Oil Recovery

Thermal EOR 67

Figure 8.1 Steam injection into a formation.

60 C, one kilogram of steam will realize more than 5000 kJ. This is considerable
amount of thermal energy which will increase temperature of rock with oil. Oil vis-
cosity will be reduced and oil will move more easily towards the production well.
Moreover, the steam is very effective in removing oil from small cavities.
It is convenient and useful for process understanding to divide processes in the
formation under steam influence into few zones. In oil containing strata three dis-
tinctive zones can be identified. Those zones are classified by the significant tem-
perature differences and by the prevailing mobile phases.
On the Fig. 8.1 it is assumed that we inject steam at the temperature at around

400 C (supercritical fluid) and the reservoir temperature is at around 45 80 C.
Zone 1. Temperature is almost the same as temperature of injected fluid. Partial

steam condensation starts at temperatures below 374 C (critical temperature). In
this zone we have oil distillation, partial extraction of oil light fractions and
removal of those fractions into the next zone.
Zone 2. In this zone we have temperatures at which most of steam condenses
into the hot water. Hot water proceeds through the formation and displaces the hot
oil towards production well.
Zone 3. No water steam left. Temperature slowly drops over distance to the nor-
mal oil formation temperature. Oil mostly displaced by the water, which now has
properties (temperature and salinity) close to connate water.
After the beginning of steam injection zones one and two grow and zone three
shrinks. The injection process continues to the point when breakthrough occurs.
Injection stops when water cut approaches approximately 80% and the process
becomes not economically feasible anymore.
Many other processes develop during oil formation heating by the steam. We
have already mentioned oil viscosity reduction and partial oil distillation (upgrad-
ing). We also have materials volume increase, phase penetration (mixing) changes,
change of rock wettability by the reservoir fluids, modification of connate water