Wells – Processes – Distinct – separate injection and producing wells
Reexamination Certificate
1998-10-21
2001-02-13
Neuder, William P. (Department: 3672)
Wells
Processes
Distinct, separate injection and producing wells
C166S272700
Reexamination Certificate
active
06186232
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to improving a fluid drive or steam assisted gravity drainage (“SAGD”) process for recovering oil from a 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 the production well to an oil-wet condition, to thereby obtain enhanced oil recovery.
BACKGROUND OF THE INVENTION
In a SAGD process, steam is injected into a reservoir through a horizontal injection well to develop a vertically enlarging steam chamber. Heated oil and water are produced from the chamber through a horizontal production well which extends in closely spaced and parallel relation to the injection well. The wells are positioned with the injection well directly over the production well or they may be side by side.
SAGD was originally field tested with respect to recovering bitumen from the Athabasca oil sands in the Fort McMurray region of Alberta. This test was conducted at the Underground Test Facility (“UTF”) of the present assignee. The process, as practiced, involved:
completing a pair of horizontal wells in vertically spaced apart, parallel, co-extensive relationship near the bottom of the reservoir;
starting up by circulating steam through both wells at the same time to create hot elements which functioned to slowly heat the span of formation between the wells by heat conductance, until the viscous bitumen in the span was heated and mobilized and could be displaced by steam injection to the production well, thereby establishing fluid communication from the developing chamber down to the production well; and
then injecting steam through the upper well and producing heated bitumen and condensate water through the lower well. The steam rose in the developing bitumen-depleted steam chamber, heated cold bitumen at the peripheral surface of the chamber and condensed, with the result that heated bitumen and condensate water drained, moved through the interwell span and were produced through the production well.
This process, as practised at the UTF, is described in greater detail in Canadian patent 2,096,999.
Successful recovery of bitumen during the SAGD process depends upon the efficient drainage of the mobilized bitumen from the produced zone to the production well.
One object of the present invention is to achieve improved drainage, as evidenced by increased oil recovery.
SUMMARY OF THE INVENTION
The present invention had its beginnings in a research program investigating the effect of wetting characteristics of oil reservoir sand on oil recovery. Athabasca oil sand from the Fort McMurray region is water-wet in its natural state. The following experiments were performed using water-wet sand saturated with oil to mimic the naturally occurring oil sand.
Three pressure driven flood experimental runs from the program were of interest. In each of these runs, oil-saturated, water-wet sand was packed into a horizontal, cylindrical column and several pore volumes of brine were 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 “production end”). The oil and brine were separated and the amount of oil quantified. In the first run, the column was packed entirely with oil-saturated water-wet sand and the brine was pumped continuously. In the second run, a 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 injection of the brine was continuous. There was no appreciable difference in oil recovery between runs
1
and
2
. In the third run, the column was packed as in run
2
and a thin, oil-wet membrane added to the production end. However, in this run the injection of brine was intermittent. There were significant pauses or shut-downs (having a length anywhere from several hours to several days) in pumping of the brine. The oil recovery from the third run was significantly greater than had been the case for runs
1
and
2
.
From these experiments and additional work, it was concluded and hypothesized:
that provision of an oil-wet oil membrane at the production end of a column of oil-saturated, water-wet sand was beneficial to recovery;
that the pumping shut-downs or cyclic injection provided quiescent periods during which we postulated that oil was drawn by capillary effects or imbibed into the oil-wet membrane with corresponding displacement of resident water; and
that this combination of features enabled oil to flow more easily through the production end, leading to improved oil production rate and recovery.
From this beginning it was further postulated that adding oil-wet sand to surround the production well and then practising the SAGD process might provide an opportunity for imbibing to materialize (the SAGD process typically does not involve large pressure differentials and might therefore provide a quiescent condition similar to that occurring during the cyclic injection used in the third pressure driven flood run).
At this point, a bench scale cell was used in a laboratory circuit, to simulate an SAGD process. More specifically, an upper horizontal steam injection well was mounted to extend into the cell, together with a lower horizontal oil/water production well. Two runs of interest were conducted. In 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 produced through the production well. In the second run, oil-wet sand was provided to form a lower layer in the cell and the production well was located in this layer; oil-saturated, water-wet oil sand formed the upper layer and 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 production well. In the first run, about 27% of the oil in place was recovered after 200 minutes of steam injection. In the second run, about 40% of the oil was recovered over the same period. The oil production rate in the second run was also higher than that for the first run.
In summary then, the invention has two broad aspects.
In one aspect, the invention provides an improvement to a conventional pressure driven fluid flood or drive process conducted in an oil-containing reservoir formed of water-wet sand using injection and production wells. The improvement comprises: providing a body of oil-wet sand in the near-bore region of the production well and injecting the drive fluid intermittently.
In another aspect, the invention provides an improvement to a conventional steam-assisted gravity drainage process conducted in an oil-containing reservoir formed of water-wet sand using injection and production 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.
The body of oil-wet sand may be emplaced in the near-bore region by any conventional method such as: completing the well with a gravel pack-type liner carrying the sand; or circulating the sand down the well to position it 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 region extending radially outward from the center line of the production string to a depth of about 3 feet into the reservoir and extending longitudinally along that portion of the production well in the reservoir.
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 combined with a low pressure differential regime, causes oil to imbibe into the region and has the effect of easing oil flow into the well, which leads to enhanced recovery.
REFERENCES:
patent: 3102589 (1963-09-01), Graham et al.
patent: 3338306 (1967-08-01), Cook
patent: 3498378 (1970-03-01), Stone et al.
patent: 3580338 (1971-05-01), Sparlin
patent: 3910350 (1975-10-01), Mastroi
Babchin Alex
Isaacs Eddy
Nasr Tawfik
Alberta Oil Sands Technology and Research Authority
Neuder William P.
Tassel Kurt D. Van
Van Tassel & Associates
VandenHoff Deborah G.
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