Note: Descriptions are shown in the official language in which they were submitted.
CA 02250648 1998-10-19
1 FIELD OF THE INVENTION
2 The present invention relates to improving a fluid drive or steam
3 assisted gravity drainage ("SAGD") process for recovering oil from a
4 subterranean, oil-containing, water-wet sand reservoir. More particularly
the
invention relates to altering the nature of the sand in the near bore region
of
6 the production well to an oil-wet condition, to thereby obtain enhanced oil
7 recovery.
8
9 BACKGROUND OF THE INVENTION
In a SAGD process, steam is injected into a reservoir through a
11 horizontal injection well to develop a vertically enlarging steam chamber.
12 Heated oil and water are produced from the chamber through a horizontal
13 production well which extends in closely spaced and parallel relation to
the
14 injection well. The wells are positioned with the injection well directly
over the
production well or they may be side by side.
16 SAGD was originally field tested with respect to recovering bitumen
17 from the Athabasca oil sands in the Fort McMurray region of Alberta. This
18 test was conducted at the Underground Test Facility ("UTF") of the present
19 assignee. The process, as practiced, involved:
~ completing a pair of horizontal wells in vertically spaced apart,
21 parallel, co-extensive relationship near the bottom of the reservoir;
22 ~ starting up by circulating steam through both wells at the same time
23 to create hot elements which functioned to slowly heat the span of
24 formation between the wells by heat conductance, until the viscous
bitumen in the span was heated and mobilized and could be
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1 displaced by steam injection to the production well, thereby
2 establishing fluid communication from the developing chamber
3 down to the production well; and
4 ~ then injecting steam through the upper well and producing heated
bitumen and condensate water through the lower well. The steam
6 rose in the developing bitumen-depleted steam chamber, heated
7 cold bitumen at the peripheral surface of the chamber and
8 condensed, with the result that heated bitumen and condensate
9 water drained, moved through the interwell span and were
produced through the production well.
11 This process, as practised at the UTF, is described in greater detail in
12 Canadian patent 2,096,999.
13 Successful recovery of bitumen during the SAGD process depends
14 upon the efficient drainage of the mobilized bitumen from the produced zone
to the production well.
16 One object of the present invention is to achieve improved drainage, as
17 evidenced by increased oil recovery.
18
19 SUMMARY OF THE INVENTION
The present invention had its beginnings in a research program
21 investigating the effect of wetting characteristics of oil reservoir sand
on oil
22 recovery. Athabasca oil sand from the Fort McMurray region is water-wet in
23 its natural state. The following experiments were performed using water-wet
24 sand saturated with oil to mimic the naturally occurring oil sand.
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1 Three pressure driven flood experimental runs from the program were
2 of interest. In each of these runs, oil-saturated, water-wet sand was packed
3 into a horizontal, cylindrical column and several pore volumes of brine were
4 injected under pressure through one end of the column (the "injection end").
Oil and brine were produced at the opposite end of the column (the
6 "production end"). The oil and brine were separated and the amount of oil
7 quantified. In the first run, the column was packed entirely with oil-
saturated,
8 water-wet sand and the brine was pumped continuously. In the second run, a
9 thin, oil-wet membrane was added to the production end of a column that had
been packed with water-wet sand and oil-saturated as in run 1. Again, the
11 injection of the brine was continuous. There was no appreciable difference
in
12 oil recovery between runs 1 and 2. In the third run, the column was packed
13 as in run 2 and a thin, oil-wet membrane added to the production end.
14 However, in this run the injection of brine was intermittent. There were
significant pauses or shut-downs (having a length anywhere from several
16 hours to several days) in pumping of the brine. The oil recovery from the
third
17 run was significantly greater than had been the case for runs 1 and 2.
18 From these experiments and additional work, it was concluded and
19 hypothesized:
~ that provision of an oil-wet oil membrane at the production end of a
21 column of oil-saturated, water-wet sand was beneficial to recovery;
22 ~ that the pumping shut-downs or cyclic injection provided quiescent
23 periods during which we postulated that oil was drawn by capillary
24 effects or imbibed into the oil-wet membrane with corresponding
displacement of resident water; and
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1 ~ that this combination of features enabled oil to flow more easily
2 through the production end, leading to improved oil production rate
3 and recovery.
4 From this beginning it was further postulated that adding oil-wet sand
to surround the production well and then practising the SAGD process might
6 provide an opportunity for imbibing to materialize (the SAGD process
typically
7 does not involve large pressure differentials and might therefore provide a
8 quiescent condition similar to that occurring during the cyclic injection
used in
9 the third pressure driven flood run).
At this point, a bench scale cell was used in a laboratory circuit, to
11 simulate an SAGD process. More specifically, an upper horizontal steam
12 injection well was mounted to extend into the cell, together with a lower
13 horizontal oil/water production well. Two runs of interest were conducted.
In
14 the first run, the cell was packed entirely with oil-saturated, water-wet
sand.
Steam was injected through the upper well and oil and condensed water were
16 produced through the production well. In the second run, oil-wet sand was
17 provided to form a lower layer in the cell and the production well was
located
18 in this layer; oil-saturated, water-wet oil sand formed the upper layer and
19 contained the injection well. As in the first run, steam was injected
through
the upper well and oil and condensed water were produced through the
21 production well. In the first run, about 27% of the oil in place was
recovered
22 after 200 minutes of steam injection. In the second run, about 40% of the
oil
23 was recovered over the same period. The oil production rate in the second
24 run was also higher than that for the first run.
In summary then, the invention has two broad aspects.
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1 In one aspect, the invention provides an improvement to a conventional
2 pressure driven fluid flood or drive process conducted in an oil-containing
3 reservoir formed of water-wet sand using injection and production wells. The
4 improvement comprises: providing a body of oil-wet sand in the near-bore
region of the production well and injecting the drive fluid intermittently.
6 In another aspect, the invention provides an improvement to a
7 conventional steam-assisted gravity drainage process conducted in an oil-
8 containing reservoir formed of water-wet sand using injection and production
9 wells. The improvement comprises: providing a body of oil-wet sand in the
near-bore region of the production well and then applying the SAGD process.
11 The body of oil-wet sand may be emplaced in the near-bore region by
12 any conventional method such as: completing the well with a gravel pack-
13 type liner carrying the sand; or circulating the sand down the well to
position it
14 in the annular space between the wellbore surface and the production
string.
The "near well-bore region" is intended to mean any portion of that
16 region extending radially outward from the center line of the production
string
17 to a depth of about 3 feet into the reservoir and extending longitudinally
along
18 that portion of the production well in the reservoir.
19 By way of explanation, we believe that placement of oil-wet sand in the
near well-bore region serves to maintain a continuous oil flow. This, when
21 combined with a low pressure differential regime, causes oil to imbibe into
the
22 region and has the effect of easing oil flow into the well, which leads to
23 enhanced recovery.
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1 DESCRIPTION OF THE DRAWINGS
2 Figure 1 is a simplified schematic vertical cross-section of a well
3 configuration for practicing the invention in the field;
4 Figure 2 is a schematic end view in section of the well configuration of
Figure 1;
6 Figure 3 is a schematic of the laboratory column circuit used to carry
7 out the pressure drive runs;
8 Figure 4 is a schematic of the laboratory visualization cell circuit used
9 to carry out the SAGD runs;
Figure 5 is an expanded view of the cell of Figure 4 showing the sand
11 packing for the 2"d SAGD run;
12 Figure 6 is a plot of oil displacement versus pore volume injected
13 showing the effect of cyclic imbibition on oil recovery;
14 Figure 7 is a plot of the percent oil recovery versus time;
Figure 8 is a bar graph showing the percent recovery of oil after 200
16 minutes; and
17 Figure 9 is a plot of the cumulative oil production versus time in days.
18
19 DESCRIPTION OF THE PREFERRED EMBODIMENT
The invention is concerned with modifying a conventional SAGD
21 system. Having reference to Figures 1 and 2, an SAGD system comprises
22 steam injection and oil/water production wells 1,2. The wells have
horizontal
23 sections 1 a, 2a completed in an oil sand reservoir 3 so that the injection
well
24 section 1 a overlies the production well section 2a. The reservoir 3 is
formed
of water-wet sand or other solids. The injection well 1 is equipped with a
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1 tubular steam injection string 4 having a slotted liner 5 positioned in the
2 horizontal section 1 a. The production well 2 is equipped with a tubular
3 production string 6 having a slotted liner 7 positioned in the horizontal
section
4 2a. Fluid communication is established between the wells 1,2, for example by
circulating steam through each of the wells to heat the span 8 by conduction,
6 so that the oil in the span is mobilized and drains into the production
well.
7 ~ Steam injection is then commenced at the injection well. The steam rises
and
8 heats oil which drains, along with condensed water, down to the production
9 well and is produced. An expanding steam chest 9 is gradually developed as
injection proceeds.
11 In accordance with the invention, a layer 10 of oil-wet sand is emplaced
12 along at least part of the horizontal section 2a of the production well.
This
13 may be accomplished by circulating the sand into place or packing it at
14 ground surtace into a gravel-pack type liner before running it into the
well as
part of the production string. Alternatively, one could treat the sand in-
place
16 with a suitable solution to render the sand oil-wet. For example, one could
17 apply an acid wash to the formation in the near well-bore region.
18 The experimental work underlying the invention is now described.
19 Water-wet sand was used in the following experiments unless
otherwise stated. The water-wet sand was packed in either a column or a test
21 cell and saturated with oil. About eighty-five percent (85%) of the pore
22 volume of the packed sand was oil, saturated.
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1 Examale I
2 This example describes the treatment used to convert water-wet sand
3 to an oil-wet condition. This treatment involved coating the sand with
4 asphaltene to render it oil-wet.
It further describes a test used to assess the wetted nature of the
6 treated sand.
7 More particularly, water-wet sand was first dried by heating it at
500°C
8 for several hours. Asphaltenes were extracted from Athabasca bitumen and
9 diluted in toluene to give a 10 weight % asphaltene/toluene solution. The
asphaltene/toluene solution was added to the dry sand in an amount sufficient
11 to totally coat the sand particles with asphaltene without having the sand
12 particles sticking together. Typically the amount of the asphaltenes added
per
13 volume of sand was about 0.1 %. The asphaltene/toluene/sand mixture was
14 put in a rotary evaporator to evaporate the toluene. As the toluene
evaporated, the asphaltene stuck to the sand particles in a thin film. The
16 treated sand was then heated in an oven at 150°C for several hours.
17 Wetting tests were conducted on the treated sand to determine
18 whether it was oil-wet. More particularly, treated sand saturated with oil
was
19 placed in a glass tube and water was poured into the tube. Observation that
no oil was displaced from the sand by the water was accepted as an
21 indication that the grains were oil-wet. In the case of non-treated water-
wet
22 sand, the oil was easily displaced by water and flowed to the top by
gravity.
23 This was accepted as an indication that the sand grains were water-wet.
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r
1 The effect of steam on the oil-wet properties on the treated sand was
2 also tested. It was observed that when the treated sand was subjected to
3 steam at 115°C for 20 hours, it maintained its oil-wet properties in
accordance
4 with the test described above.
Example II
6 This example describes 3 runs that showed that the provision of an oil-
? wet membrane at the production end of a column would increase oil recovery
8 when coupled with intermittent flooding with brine.
9 More particularly, a laboratory circuit shown in Figure 3 was used. The
entire volume of a 30 cm x 10 cm diameter column was packed with water-
11 wet sand and then saturated with oil so that about 85% of the pore volume
12 was oil. The column was run in the horizontal position.
13 In run 1, brine was pumped through one end of the column (the
14 "injection end") at a constant rate of 25 cc/hr until it had been washed
with 6
pore volumes of brine. Fractions of eluate were collected from the opposite
16 end of the column (the "production end"). The oil and brine were separated
17 and the amount of oil in each fraction quantified.
18 In runs 2 and 3, the column was packed with water-wet sand and
19 saturated with oil as in run 1. However, an oil-wet membrane (a 5mm
metallic
porous membrane that had been treated with organosaline) was placed at the
21 production end in both runs.
22 In run 2, the column was washed at a constant rate of 25 cc/hr with
23 three pore volumes of brine, fractions of eluate collected and the oil
content in
24 each fraction quantified.
CA 02250648 1998-10-19
1 In run 3, the column was washed intermittently with brine. Brine was
2 pumped through the column at a rate of 25 cc/hr. However, after one pore
3 volume of brine had been pumped, the pump was shut off and the column
4 allowed to "rest" for several hours. Pumping of brine was resumed at a rate
of
25 cc/hr for a short period of time and then pumping was stopped again. The
6 pumping of brine was resumed after several hours. The pumping was
7 stopped and restarted at least 15 times in total until 3 pore volumes of
brine
8 had been added to the column. The stop periods would vary anywhere from
9 several hours to several days. Throughout the stop-start procedure,
fractions
of eluate were collected and oil content measured.
11 Figure 6 is a plot of oil displacement versus pore volume injected for
12 each of runs 1, 2 and 3. After injection of 2.7 pore volumes of brine, run
1
13 displaced 47.5% of the oil, run 2 displaced 49.2% of the oil and run 3
14 displaced 62.5% of the oil. The results indicate that the addition of the
oil-wet
membrane in run 2 did not markedly affect oil recovery. However, when the
16 oil-wet membrane was coupled with intermittent washes as in run 3, oil
17 recovery increased by about 50% relative to run 1.
18 Example III
19 This example describes 2 SAGD runs conducted in a test cell. The
runs show that provision of oil-wet oil sand in the near-bore region of the
21 production well, when coupled with SAGD, increases recovery when
22 compared to the case where only water-wet oil sand is used.
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1 More particularly, a 0.6 m x 0.21 m x 0.03 m thickness scaled
2 visualization cell 1 was used. The sides of the cell were transparent. An
3 upper injection well 2 and a lower production well 3 were provided. The
wells
4 were horizontal and spaced one above the other in parallel relationship.
Both
wells were constructed from 0.64 cm diameter stainless steel tube that was
6 slotted with 0.11 cm wide by 5.1 cm long slots. A schematic illustration of
the
7 experimental set-up is shown in Figure 4. Steam flow rate was measured
8 using an orifice meter 4. A control valve 5 was used to deliver steam to the
9 injection well at about 20 kPa (~ 3 psig). An in-line ARI resistance heater
6
and a heat trace were used to maintain a maximum of 10°C superheating
at
11 the point of injection. To achieve "enthalpy control" (steam trap) control
over
12 the production of fluids, a valve 7 was thermostatically controlled to
throttle
13 the production well and ensure that only oil and condensate were produced.
14 In the baseline first run, the cell was entirely filled with oil-saturated,
water-wet sand. In the second run, as shown in Figure 5, the bottom section
16 8 of the cell was packed with a layer of oil-wet sand treated in accordance
17 with Example I and the upper section 9 was packed with non-treated oil-
18 . saturated, water-wet sand. In the second run, the steam injection well 2
was
19 located in the upper water-wet section 9 and the production well 3 was
located in the lower oil-wet section 8.
21 The initialization of gravity drainage was achieved by injecting steam
22 for 30 minutes into both wells at once for about 30 minutes while producing
23 from both wells at the same time. Following the initialization period,
steam
24 was injected into the top well only and production fluids were obtained
from
the bottom well. The experiment lasted for a total of 700 minutes. The
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1 production fluids were collected every 15 minutes, the oil and water
2 separated, and the amount of oil recovered measured.
3 Both runs were done in duplicate and Figure 7 is a plot of the percent
4 oil recovery versus time in minutes for all four runs. It can be clearly
seen
from this plot that the addition of oil-wet sand around the production well
6 increased both the rate of oil recovery and the percent of oil recovery.
Having
7 reference to Figure 7, is can be seen that in the runs without the addition
of
8 oil-wet sand, it took an average of 425 minutes to achieve 40% oil recovery.
9 However, in the runs where an oil-wet sand layer surrounded the production
well, it took less than half the time (175 minutes) to achieve 40% oil
recovery.
11 Figure 8 is a bar graph showing the percent recovery of oil for all runs
after
12 200 minutes. The average recovery of oil for the runs without the oil-wet
sand
13 layer was 27.5%. However, the average recovery of oil for the runs with the
14 oil-wet sand layer was 43%. This represents a 64% increase in the percent
of
oil recovered.
16 Example IV
17 The improvement in oil production observed during laboratory
18 experiments when an oil-wet region surrounded the production well was
19 further investigated using a numerical simulator to examine if the above
phenomenon would prevail on a field scale. A 500 m deep reservoir was
21 assumed in a numerical model, which had a pay-zone thickness of 21 m.
22 Two superimposed horizontal wells, each 500 m long, were placed near the
23 bottom of the pay-zone 4 m apart from one another. A SAGD process was
24 simulated whereby steam was injected into the top well (the "injection
well") at
a pressure of 3.1 MPa and oil was collected in the bottom well (the
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1 "production well"). In one instance, the reservoir surrounding the
production
2 well remained water-wet. In another instance, an oil-wet zone was placed
3 around the production well. This was achieved by using capillary pressure
4 and relative permeability functions for water-wet and oil-wet sands.
The field scale numerical results are shown in Figure 9, a plot of the
6 cumulative oil production versus time in days. It was clear that oil
production
7 rates increased when an oil-wet region was added to the production zone.
8 Further, the results show that the starting of oil production can be
advanced
9 when an oil-wet zone is placed around the production well. The effect of the
oil-wet region was most significant during the first two years of operation.
11 Example V
12 Bottom water drive experiments were done in order to test the
13 effectiveness of various anti-coning agents in preventing penetration of
the
14 production well by reservoir water. It was observed that when the porous
region around the production well was rendered oil-wet, the coning of the
16 water was significantly reduced. The oil recovery in the oil-wet case was
17 higher by as much as 20% over that of the water-wet case.
18 Bottom-water drive experiments were done using visualization cells as
19 described in Paper 96-13 of the Petroleum Society of the CIM 47th Annual
Technical Meeting, June 10 - 12, 1996. It was observed that when only
21 water-wet sand was used, coning around the production well occurred due to
22 imbibition and early breakthrough of water. By contrast, when oil-wet sand
23 was packed around the production well, water breakthrough to the producer
24 was delayed and therefore coning was also delayed.
14
