CA1208539A - Solvent stimulation of heavy oil reservoirs - Google Patents

Abstract

SOLVENT STIMULATION OF HEAVY OIL RESERVOIRS

ABSTRACT OF THE DISCLOSURE

This invention provides a method for cyclic solvent stimulation of production of heavy oil from an underground reservoir penetrated by a well which comprises:
(a) injecting into the reservoir a solvent in an amount between .79 and 4.0 m3 per meter of oil-bearing formation (5 barrels and 25 barrels per foot);
(b) letting the reservoir stand for a soak time of between less than one hour and 48 hours; and (c) producing a solvent-oil mixture.

Description

12t)8539 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 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.

- lZ(~8539 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 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.

Claims (9)

CLAIMS:

1. A method for solvent stimulation of production of heavy crude oil from an underground reservoir penetrated by a well, which comprises:
(a) injecting into the reservoir a solvent in an amount between .79 and 4.0 m3 per meter of oil-bearing formation (5 barrels and 25 barrels per foot);
(b) letting the reservoir stand for a soak time of between less than one hour and 48 hours; and (c) producing a solvent-oil mixture.

2. The method of Claim 1, wherein the ratio of crude viscosity to solvent viscosity is at least 10.

3. The method of Claim 2, wherein the ratio is at least 100.

4. The method of Claim 1, wherein the amount of solvent injected in step (a) is between 1.6 and 3.2 m3 per meter of oil-bearing formation (10 barrels and 20 barrels per foot).

5. The method of Claim 1, wherein the soak time in step (b) is between one hour and 24 hours.

6. The method of Claim 1, wherein the production of step (c) is carried out until the amount of solvent in the produced solvent-oil mixture drops below 12 percent.

7. The method of Claim 1 wherein step (a), (b), and (c) are repeated.

8. The method of Claim 7, wherein at least a portion of the injected solvent is solvent-rich production from a previous cycle.

9. The method of Claim 1, 2 or 3 wherein the solvent is a light crude oil.