Page 78 - Primer on Enhanced Oil Recovery

Thermal EOR 69

If the oil containing strata is relatively thin then this leads to the situation when
ratio of strata volume to the surface becomes relatively low and this ultimately
leads to high thermal energy loss into the surrounding oil-less formation. This ulti-
mately means that the maximum temperature in the reservoir will be lower and
temperature uniformity will be compromised. The same can happens if the density
of production/injection well ratio is below one per hectare.
It can be reasonably expected that for the oil formation depth at around 900 m
and initial oil viscosity 1000 mPa-s it is possible to extract around 55% OIP with
steam injection against 15 18% OIP by water flooding.
In the favorable conditions for one additional oil barrel extracted with the steam
injection one will need at around 2.5 3.5 barrels of water converted into the steam
and injected into the formation.
Oil is usually burned in the steam generators. It is realistic to expect to spend
one barrel of oil to generate 2.5 3.5 barrels of water converted into the steam. The
final balance means that for each barrel of oil burned in the steam generator one
expects to extract 4 5 barrels of oil. This is sensible economy expectation for
steam injection project.
In the case of loose sand oil containing formation steam injection process acti-
vates sand mobility and a lot of sand is pumped to the surface. This makes overall
process more difficult and lowers profit margin.
One needs to bear in mind that the stem and hot water after steam condensation
have much lover viscosity than the connate or injected water at the formation tem-
perature. This leads to much more unstable water/oil displacement front and earlier
breakthrough.
For deep formations, at or above 900 m, heat loss from the steam in the injection
well becomes very significant, as despite efforts to insulate the injection well from
the surrounding rock it is expected that the heat losses from the steam will be at
around 4% per every 100 m of depth. This loss can be reduced by good insulation
but still remains very substantial at high depth. One way to deal with this problem
is to increase injection speed as this reduces heat losses.
Instead of continuous steam injection it is possible to do cyclic injection. This
reduces cost of injection process but reduces oil extraction to 10 30%. Porosity for
cyclic steam injection should be above 25% and thickness of formation above 6 m
at the depth 900 m. It was shown that variable cyclic periods can gain additional
10% on the numbers mentioned above.
Huff and puff method requires the lowest initial investment outlay but only pro-
duces 3 4% of OIP.
For successful SAGD implementation the formation should be formed by not
fully densified sand-type with high vertical permeability.
It is shown on the Fig. 8.2 major criteria for steam injection implementation.

8.1.3 Implementation methods

There are two major implementation methods. In the traditional one (see Fig. 8.3)
80% saturated stem is produced in a steam generator (Fig. 8.4). This method