Note: Descriptions are shown in the official language in which they were submitted.
12t)8539
F-1269 -1-
SOLVENT STIMULATION OF HEAVY OIE RESERVOIRS
This invention is concerned with the stimulation of production
of heavy oil.
In the past, solvents have been injected into heavy oil wells
in order to improve production from those wells. Generally, the
solvents used were light hydrocarbons, such as a rich gas, or light
liquid hydrocarbons containing additives for such purposes as
dissolving deposited solid hydrocarbons or for breaking oil-water
emulsions. However, such mixtures were much more expensive than equal
volumes of the crude they were designed to help produce. Reported
results have indicated that well clean-up and emulsion reduction have
been the primary mechanisms responsible for any increased production
resulting from solvent injection. Production improvement as a result
of the viscosity reduction due to solvent dilution has been discounted
as a practical procedure.
It is the unexpected discovery of this invention that the
production of heavy oil can be greatly increased by solvent dilution of
heavy crude alone, the dilution resulting in a reduced viscosity of the
solvent/crude mixture. The production increase is a result of proper
design criteria of the solvent stimulat~on procedure, involving the
volume of solvent used and the soak period prior to resumption of
production. The continuity of production, once begun, is also
important to the procedure. It has also been found that the same well
can be successfully solvent stimulated many times, depending on the
~5 choice of proper intervals between solvent in~ections. In addition, it
has been determined that relatively inexpensive hydrocarbons with~ut
additlves, such as light crude oils, can be used as the solvent.
As used in specification and claims, "heavy" crude oil is a
viscous crude oil that has poor flow characteristics in the reservoir.
In general, it is a crude oil that has a API gravity of about 20
degrees or lower.
The solvent used should be substantially, but not necessarily
completely, miscible with the crude oil. It must, however, have
viscosity lower than that of the crude. In general, the ratio of crude
`` 12V~539
F-1269 -2-
viscosity to solvent viscosity at reservoir conditions should be at
least 10, preferably 100 or more. Suitable solvents are light crude
oil, syncrude, diesel fuel, condensate, cutter stock, or other light
hydrocarbons. It is within the contemplation of this invention that
about a third of the injected solvent can be solvent-rich production,
i.e., the initial production from a solvent stimulated production that
is rich in solvent content. The amount of solvent that is injected is
between .79 and 4.0 m3 per meter of oil bearing formation (5 barrels
and 25 barrels per foot), prefe~ably between 1.6 and 3.2 m3 (10 and
20 barrels).
jAfter solvent injection has been completed, there should be
little or no soak time, i.e., the time between the end of solvent
injection and the start of production. Generally, the soak time will
be between less than an hour and 48 hours, preferably less than 24
hours. In accordance with this invention, there is little advantage,
if any,"in an appreciable soak time to effect diffusive mixing of
solvent and heavy oil. It appears that prolonged soak times of several
days or more isolates solvent and interrupts flow paths, so that there
is little increase in production over that obtained in unstimulated
production.
Production, when commenced after solvent injection is
completed, should be maintained continuously. Any shut-down should be
kept under 48 hours, preferably under 24 hours. Production should be
continued until the fraction of solvent in the produced oil has dropped
to 12% or less, regardless of production rate. At this point,
additlonal solvent or solvent and solvent-rich production can be
injected into the reservoir followed by resumed production.
These cycles of solvent injection and production can be
continued until the reservoir is exhausted. Essentially all (about
97%) the solvent injected into a formation in a multi-cycle solvent
stimulation is returned with produced oil. Thus, it can be separated
from the produced oil on site, if desirable or practical, by some
separation method, such as in a topping plant, and used in subsequent
injections. Alternatively, the mixture of solvent and heavy oil can be
used directly as a refinery charge stock and it has the advantage of
being easily pipelined.
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F-1269 ~3~
Example
The well used was a Cox Penn Sand Unit well located in Carter
County, Oklahoma. It is completed in 4th Deese sand which contains oil
with a gravity of 15~ API. The thickness of $he oil bearing formation
is about 13 meters (43 feet). The unstimulated rate of production from
this well was 1.35 m3 per day (8.5 barrels per day (BOPD)). The
solvent used was Graham-Deese light crude oil (about 34 API), unless
otherwise noted.
The well was subjected to five injection~ production cycles.
The first three cycles each consisted of solvent injection of about
63.6 m3 (400 barrels). Dycle 1 used as,solvent fresh Graham-Deese
crude for the full volume. The first 21.5 m3 (135 barrels) of
solvent-rich production (~28 API, containing about 70~ solvent) was
saved and used as the middle portion of the following injection, the
remainder being the fresh light crude. This procedure was followed
through the subsequent cycles~ Each cycle involved injection one day
with production starting the next morning for convenience in rnonitoring
the returning fluids.
For the fourth cycle, the injected volume was increased to
about 191. m3 (1200 barrels) to determine whether average production
rate or the incremental barrel/injected barrel ratio was changed using
a larger injected volume. ~oth were dramatically lower, showing that
the in~ected volume can be too large and that multiple small injections
produce more incremental oil.
In the fifth cycle, in order to make a direct comparison with
the first three, about 63.6 m3 (400 barrels) were injected. The
initial production rate was too high and average rate and incremental
volumes suffered in comparison, but incremental oil was produced and
increased average rate achieved more nearly like the first three 63.6
m3 (400 barrel) cycles than the 191. m3 (1200 barrel) cycle.
These runs and the results are set forth in the following
Table:
12~539
F-1269 -4-
o
~ a~
C C h
~1 > O a~ a~ ~ o
u~ a
O U~
~1) ~ 1~ ~ ~ O~
Q _,
o
O~ ~ O ~ ~O
O)* O ~
E N N N _/ N
o
Q~ ~ ~
-
N ~ --
a~ ~ E¦ ~ 0 a~
~
V~ O
~ N 1~ ~t u~
385;39
F-1269 ~5~
From the data in the Table, it will be noted that the four
cycles of 63.6 m3 (400 barrels) of injected solvent produced an
incremental 29.4 to 44.5 m3 (185 to 280 barrels) of oil (0.48 to 0.71
m3/injected m3) at an average rate of .84 to 1.1 m3/day (5.3 to
6.8 BOPD) greater than the unstimulated rate of 1.35 m3/day (8.5
BOPD). This amounts to a 6Z% to 80~ increase in production. The 187.8
m3 (1181 barrel) injection produced 55.7 m3 (350 barrels) of
incremental oil (0.3 m3/injected m3) at a rate of .6û m3/day (3.8
BOP0) greatér than the unsti~ulated rate, a 45% increase in production.
Additional experiments in seven other wells in five other
heavy oil reservoirs have shown results similar to or better than those
cited above, in terms of either increased average production rate or
incremental oil barrels/barrel of solvent injected.
