Page 178 - Fundamentals of Reservoir Engineering

DARCY'S LAW AND APPLICATIONS 116

applied to effect this viscosity reduction is steam soaking. Steam is injected into the
reservoir and, in the simple model shown in fig. 4.6(b), extends to a radius r h, the
magnitude of which is primarily a function of the amount of steam injected, usually
several thousand tons, over a period of several days. During injection heat is lost in the
wellbore and to the cap and base rock, but since steam is used, these losses are
reflected as a reduction in latent heat and therefore take place without a significant
change of temperature.

150

hot zone

µ (cp) p
µ o

r w r h r e
µ w 2 cp (b)
0
150 T (°F) 400

(a)

Fig. 4.6 (a) Typical oil and water viscosities as functions of temperature, and
(b) pressure profile within the drainage radius of a steam soaked well

Following injection, the well is opened on production and the cold oil crossing into the
heated annular region has its viscosity greatly reduced and consequently the PI is
increased. A typical steam soak production rate, in comparison to the unstimulated
rate, is shown in fig. 4.7. There is an initial surge in production followed by a steady
decline as the temperature in the hot zone is reduced, due to the continual loss of heat
to the cap and base rock, as a function of time, and the removal of heat with the
produced fluids. When the production rate declines towards the unstimulated rate, the
cycle is repeated.

100 steam soak
production
oil
rate
(stb/d)

10
unstimulated production

1 2 3 time (yrs)

Fig. 4.7 Oil production rate as a function of time during a multi-cycle steam soak