CA1239088A - Method for controlling water coning in oil wells - Google Patents

Abstract

"METHOD FOR CONTROLLING WATER CONING IN OIL WELLS" In a reservoir having an oil zone underlain by an aquifer, injection and production wells are completed by perforating them in the oil zone, well above the oil/water interface. A non-condensable gas is then injected at the injection well while the production well is simultaneously produced. In due course, the gas establishes communication with the production well along the oil/water interface. A layer having a relatively high gas saturation is produced along said interface. Injection of gas is continued at a rate higher than gas is being produced at the production well. Primary production of the producing well is continued in conjunction with the gas injection. It is found that water coning at the production well is suppressed by the gas "blanket".

Description

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2 This invention relates to a method for reducing water

3 coning in connection with a pattern of injection and production wells

4 producing oil from a reservoir comprising an oil zone underlain by a contiguous aquifers zone, said oil zone being producible by natural 6 primary production and said reservoir being susceptible to water coning 7 at the production wells.

g The present invention has to do with a method for suppressing lo water coning in a particular type of reservoir. Such a reservoir contains 11 mobile oil which can be initially produced by natural primary production 12 (i.e. flowing or pumping).
It is common for such an oil zone to be underlain by and contiguous with a zone containing water under pressure. When a production well is completed in the oil zone and produced, there is a tendency 16 in many cases for the water to move toward the low pressure sink created 17 by the well. As the water has a lower viscosity than the oil, its capacity 18 to move through the formation is greater than that of the oil. Depending 19 to some extent on factors such as the positioning of the producing well casing perforations, the permeability and thickness of the oil reservoir, 21 and the pressure regimes involved, there is always the possibility that 22 the water may "cone" up to the perforations and compete with the oil to 23 be the produced fluid.
24 This problem is particularly acute in reservoirs where the oil is heavy and viscous and thus moves with greater difficulty through the 26 formation to the producing well. In such reservoirs, the disparity in 27 mobility between the oil and water is great, and coning can quickly occur 28 and choke off oil production.

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1 The present invention was developed in connection with a 2 particular reservoir plagued to a serious extent by this coning problem.
3 The reservoir was the South Penner Upper Manville "J" pool, located in the4 Subfield Military Range in Southeastern Alberta. While the invention will find application in other fields, it is discussed herein in connection with 6 the South Penner field and the problems and conditions associated therewith.
7 The reservoir of interest in the South Penner field is 8 a sandstone bed having a thickness of about 45 meters. The upper 20 9 meters contain the producible oil. The bottom 25 meters contain the active aquifer. The reservoir is characterized by high porosity (27%) 11 and good horizontal and vertical permeability (500 and 200 my. respectively).
12 The viscosity of the oil in place is about 170 maps Its gravity is about 13 APE. The initial reservoir pressure is about 10,500 spa.
14 Heretofore, when a typical South Penner well was placed on primary pumping production it would initially produce at a rate of 16 about 15 - 60 BOND, without any water. But the water would soon commence 17 to be produced and often within 3 - 6 months the water cut would be so 18 high (typically 90 to 95%), that the well could no longer economically 19 be produced.
Cumulative primary production for a well in the field will 21 vary quite widely - but in order of magnitude, the primary production 22 might amount to about 0.5% of the original oil in place (OOIP).
23 Heretofore, the solution practiced with respect to the 24 South Penner wells to ameliorate this watering out problem was simply to perforate as high as possible in the Gil zone.
26 Turning now to the prior art literature, a patent of 27 interest is US. 3,997,004, issued to We. This patent teaches:

C3~ `3~3 (a) providing adjacent cased injection and production wells 2 penetrating a viscous oil zone and an underlying water 3 zone, said oil zone not being producible by primary 4 pumping or flowing production and further being resistant to air injection;
6 (b) perforating the injection well at the base of the oil 7 zone and at the top of the water zone;
8 (c) perforating the production well across the full 9 thickness of the oil zone and at the top of the water zone;
11 (d) injecting heated air at the oil-water interface 12 and initiating forward combustion - the combustion 13 front moves along the oil-water interface toward the 14 production well, with concurrent heating of oil above the thin, radially extending combustion zone by upward 16 conduction and convection by combustion gases;
7 (e) the air injection and forward combustion is maintained 18 until the viscosity in the upper region of the oil 19 zone is in the order of 10 - 100 centipoise, or until the formation temperature is 200 - 300F, or for a 21 period of 180 - 360 days following the noting of 22 increasing temperature at the production well 23 (f) at this stage, We teaches switching to a second phase, 24 comprising: terminating air injection and forward combustion, filling the burned out zone with water, 26 cementing off the perforations into the water zone in 27 each of the wells, perforating the injection well 28 across the entire thickness of the oil zone, and 29 initiating a steam drive at the injection well and propagating it to the production well.

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1 This reference is mentioned because We does teach that 2 injected air tends to move along the oil-water interface. over it 3 will be noted that mu is working with tar sand, perforates into the 4 water zone at the production well, and never mentions water coning.
When applicant was considering a combustion process for its S South Penner wells, certain possibilities were of concern:
7 (1) that the injected air, due to the effect of gravity, 8 would move from the injection well perforations g through the oil zone on an ascending path, to override the bulk of the oil, so that the combustion 11 reaction, taking place at the leading edge of the air 12 flow, would do little to heat the oil; or 13 (2) that the injected air would instead follow the path 14 of least resistance and move down into and through the water zone, so that combustion would be snuffed out 16 or much heat would be lost to the water; or 17 (3) that the injected air and its associated combustion lo front would follow the path of least resistance 19 (fractures or the water zone) and arrive prematurely at the production well (so that heating of the oil 21 would be minimal), in which case the well would have to 22 be closed in or thermal damage to it would occur.
23 In spite of these concerns, an experimental pilot project, 24 involving a pattern comprising a central injection well, four equidistantlyoutwardly spaced producers, and three observation wells positioned at 26 different distances between the injection and production well, was under-27 taken in the South Penner field in Alberta, Canada. It was known that the 28 oil zone was amenable to air injection.

3L~3~3~ 3 2 At this pilot project, the injection and production wells3 were perforated only in the oil interval, well above the oil-water 4 interface. The injection and producing wells were then produced by pumping for a period of time, specifically 13 months in this particular case. The 6 water cut in the production increased until on termination of primary 7 production, it was in the order of 40%. On termination of primary pumping, 8 the producing wells were left open and air injection and forward wet 9 combustion was initiated at the injection well. The observation wells, positioned between the injection well and the producing wells, soon 11 indicated that the observable gas movement and temperature increase was 12 taking place substantially exclusively along the oil-water interface.
13 Over an extended period, oil was produced, along with combustion gases -14 this oil was being produced at approximately the same rate as oil from that well was produced initially during primary, unassisted production.
16 The temperature increase arising from combustion was noted to be very 17 slow in advancing horizontally through the formation. It appeared that 18 the combustion front was being fed from above by downwardly migrating 19 heated oil and was therefore advancing horizontally only very slowly.
In addition, the rate of water production from the producing wells 21 declined sharply, relative to the rate during the original primary 22 production, once gaseous communication was established and maintained 23 between the injection well and the producers along the oil-water interface.
24 In summary then, the combustion process was virtually stalled, with the result that the production wells were basically 26 still on primary production. The oil production rate during this 27 phase was comparable to the original production rate prior to air 28 injection. And the water cut was substantially diminished and increased 29 only marginally with time. It appeared that a "gas blanket", evidenced by high gas saturation concentrated at the oil-water interface, was 31 suppressing water coning.

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There was thus tested and provided a long term production 2 strategy for this oil reservoir, Which is amenable to gas injection.
3 The production wells would be perforated high in the oil zone, remote 4 from the oil-water interface. Gus would be infected into the perforated oil zone at a rate sufficient to create and maintain a high gas 6 saturation between the wells at the oil-water interface, with concomitant 7 production of gases at the production wells. Simultaneously the oil 8 zone would be produced. The "gas blanket" would function to suppress 9 water coning at the producing wells where it occurs.
There is no requirement for combustion, in order to gain Al the benefit of the invention. As an alternative to air another gas, 12 non-condensable at reservoir conditions, such as nitrogen, could be 3 injected to achieve the same end.
4 Broadly stated, the invention is a method for reducing

5 water coning in connection with the production of oil from a subterranean 16 oil reservoir which comprises an oil zone and an underlying aquifers 17 zone, said reservoir being penetrated by adjacent cased injection and 18 production wells, said oil zone being producible by natural primary 19 production and said reservoir being susceptible to water coning at the 20 production well when primary production is practiced. The method comprises:
21 (a) completing the wells by perforating them only in the oil zone along an 22 interval spaced above and remote from the oil-water interface; (b) in-23 jetting gas through the injection well while simultaneously producing the 24 production well, said gas being injected at a rate in excess of that at 25 which gas is produced through the production well, to cause the bulk 26 of the gas to move along the oil-water interface and create a zone, 27 extending between the wells along said interface, which zone is relatively 28 high in gas saturation and (c) continuing to inject gas as aforesaid 29 while producing oil through the production well, to deplete the reservoir 30 while maintaining the zone of high gas saturation at the oil-water inter-31 face, to reduce water coning at the production well.

2 Figure 1 is a plan view of the well pattern employed at the 3 pilot project, conducted at the South Penner field, which gave rise to the 4 present invention;
Figure 2 is a plot showing water and air injection rates

6 into the central injection well If of the well pattern;

7 Figure 3 is a plot showing the water/oil ratio (WOW), gas/oil

8 ration (GO) and oil production rate for the pattern, from the initiation

9 of primary production in January, 1981~ through the initiation of forwardcombustion in March, 1982, through the initiation of water injection in Al about February, 1983, up to about March, 1984;
12 Figure 4 is a fanciful representation indicating the 13 cumulative oil recovery, as a percentage of the oil in place, for the 14 production wells of the pattern, after practice of the process for a period of 3~1/2 years (including the initial primary production period);
16 Figure 5 is a plot of the data obtained by taking temperature 17 surveys in Aug./84 - Join, showing the variation of temperature with 18 depth in the observation wells;
19 Figure 6 shows the compensated neutron logs for the observation wells taken at two points in time specifically before 21 initiation of air injection and 6 months following thereafter;
22 Figure 7 is a plot of the WOW for the pattern over the life23 of the test; and 24 Figure 8 is a plot of the nitrogen content in the gas production of the pattern over a period of time when air injection rate 26 was less than gas production rate.

2 The invention is illustrated by the following example.
3 An inverted five-spot pattern of wells was drilled, as 4 shown in Figure 1, in the South Penner field. The wells were completed in the oil interval. More particularly, the wells were drilled into 6 the oil zone, cased to total depth, and cemented. They were then 7 perforated in the upper portion of the oil zone. Table I sets out the 8 pertinent data for the injection well, one of the production wells 9 which is typical of the otherwise and one of the observation wells (which is typical of the others).

12 Injection Well I-l 3 Ground Elevation 749.6 m 4 KB to ground 5.1 m Top of Glauconitic or 16 oil zone 911.51 mob 17 Perforation interval 915 - 917 mob 18 Clean out depth 925 m 19 Casing landing depth 930 m Total depth 930.5 m 21 Oil-water interface 932 m 22 Production Well P-l 23 Ground elevation 750.03 m 24 KB to ground 5.2 m Top of Glauconitic 909.7 m 26 Perforation intervals 909.4 - 912.6 m 27 914.5 - 919 m 28 Clean out depth 929.5 m 29 Total depth 931 m Oil-water interface 934 m ~.3908~

1 Observation Well Of 2 Ground elevation 749.8 m 3 KB to ground 5.35 m 4 Top of glauconitic 910.9 m Oil-water interface 932.5 m 6 Total depth 963 m 7 It will be noted that the injection well was perforated 8 at the upper end of the oil zone and the producing wells were per-9 forayed at the top of the oil zone. The observation wells were cased through the oil zone into the water zone, but were not perforated.
11 The production wells were all placed on primary production 12 for about 16 months before combustion began in about March, 1982. The 13 injection well was also placed on primary production for about 6 months 14 and then shut in for approximately 5 months before air injection began.
The injection rates for air and water at the injection well 16 If are set forth in Figure 2 . Typically, air was injected for 10 days, 17 then water for 10 days, and then the cycle would be repeated.
18 It is necessary to infect the gas at a rate higher than 19 that at which it is produced, in order to control water coning. During a recent interval of the operation of the pilot protect, air injection was 21 reduced, due to injection well problems. The gas production rate dropped, 22 but the injection rate dropped more. As a result, the rate of gas pro-23 diction exceeded the rate of air infection. The results are indicated 24 in Figures 7 and 8. This figure compares a plot of the produced WOW with a plot of the rolling four month average nitrogen recovery percentage 26 (i.e. the instantaneous rather than cumulative gas recovery rate) from 27 the production wells. It will be noted that the percentage of nitrogen 28 recovered rose sharply from September, 1984, to February, 1985~ from 29 about 70% (four month rolling average) to about 110%. Over this same period, the WOW of the produced fluids rose from about 0.3 to 0.7.

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l It appears that the "over-production" of gas during this 2 period may have depleted the gas saturated zone along the oil-water 3 interface and altered the water coning characteristics of the wells.
4 Commencing in February 1983, water was injected to modify the flow path of the air in the pattern to stop oxygen production 6 which very quickly appeared at the production wells. The injection of 7 water was successful in this respect and was continued.
8 Figure 3 sets forth the total production rate of oil from 9 the 4 producers over the life of the pilot test, together with the accompanying gasloil ratio and water/oil ratio.
if It will be noted that the oil production rate from the 12 pattern remained fairly constant. The long term oil production showed 3 marked improvement in terms of the percentage of the original oil in 4 place. The % OOIP recovery per quadrant of the pattern is set forth in Figure 4 and ranges from 2.7 to 12.2%. This % OOIP recovery is 16 significantly higher than the typical primary production value for the17 reservoir which, as previously mentioned, is in the order of 0.5%.
18 In summary then, the injection of air and attendant combustion process 19 did not result in an increase in oil production rate from the pattern, but the rate did remain constant relative to primary oil production rate 21 and the overall recovery was much improved. And water production was 22 controlled to economic volumes.
23 It will further be noted that the water/oil ratio (WOW) 24 for the pattern increased rapidly during initial primary production.
When gas production first commenced at the production wells following 26 initiation of air injection, the WOW dropped virtually to zero. The 27 WOW remained low until water injection commenced in February, 1983. The 28 WOW then increased over the first half of 1983 to a value of about 0.2 29 In mid-1983 the air injection rate was increased, in an attempt to speed up the combustion process, and the WOW stabilized at about 0.2, with some 31 fluctuations.

The temperature profiles in the three observation wells, 2 taken over the interval August, 1984 - January 1985, is shown in Figure 3 5. It will be noted that the maximum temperature was located just below 4 the oil-water interface. This indicates that combustion was taking place 5 along the oil-water interface.
6 To confirm the lQcatiQn of the injection air and combustion 7 gases, compensated neutron 1Q95 were run in October, 1980 (before 8 injection) and August, 1982, (after injection) in the observation wells.
9 The relevant sections of the logs are shown in Figure 6. The logs on 0 observation well 01 indicated an overall increase in gas saturation in

11 addition to large peaks just below the oil/water interface. The logs

12 on wells 02 and 03 indicated an increase in gas saturation only adjacent 3 the oil-water interface.
4 The temperature survey and neutron log results indicate 15 that the air, combustion front, and combustion gases were under-running 16 the oil zone along the oil-water interface.
7 There was no significant increase in temperature at any I of the production wells. Temperature surveys at those wells were run 19 about every month. These results indicate that the combustion front was moving approximately radially. but very slowly.
21 From the foregoing, certain observations can be made:
22 (1) water coning at a production well can be controlled by 23 injecting gas into the oil zone of a permeable reservoir 24 and circulating it along the oil-water interface, to produce part of it at surrounding production wells;

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1 (2) the oil can be produced over the long term in 2 accordance with primary production methods at a 3 rate comparable to primary production, while the water 4 is controlled in the manner stated in (1);
(3) in the event that the gas injected is air, and wet 6 forward combustion is practiced, (which is not 7 necessary to practice the invention), this combustion 8 process remains localized along the oil-water inter-9 face and moves toward the production wells only very slowly. Thus it heats the oil above it in a manner 11 conducive to orderly production of the oil.

Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for reducing water coning in connection with the production of oil from a subterranean reservoir which comprises an oil zone and an underlying aquifers zone said reservoir being penetrated by adjacent cased injection and production wells, said oil zone being producible by natural primary production and said reservoir being susceptible to water coning at the production well when primary production is practiced, said method comprising:

(a) completing the wells by perforating them only in the oil zone along an interval spaced above and remote from the oil-water interface;
(b) injecting gas through the injection well while simultane-ously producing the production well, said gas being injected at a rate in excess of that at which gas is produced through the production well, to cause the bulk of the gas to move along the oil-water interface and create a zone, extending between the wells along said interface, which zone is relatively high in gas saturation; and (c) continuing to inject gas as aforesaid while producing oil through the production well, to deplete the reservoir while maintaining the zone of high gas saturation at the oil-water interface, to reduce water coning.

2. The process as set forth in claim 1 wherein:
the gas is air and forward combustion is initiated and maintained in connection with the air injection.

3. The process as set forth in claim 2 wherein:
water is intermittently injected with the air whereby wet forward combustion is initiated and maintained.