CA2358555C - Steam injection pressure in a steam-assisted gravity drainage process - Google Patents

Abstract

The pressure at which cold water is initially injectable into an oil sand formation is determined. In the practice of steam-assisted gravity drainage in that formation, steam is injected at a pressure greater than the cold water initial injectivity pressure but no more than 1500 kPa grater than that pressure.

Description

1 "STEAM INJECTION PRESSURE IN A STEAM-ASSISTED

2 GRAVITY DRAINAGE PROCESS"

3

4 FIELD OF THE INVENTION
The present invention relates to a method for practicing steam-assisted gravity 6 drainage.

9 Steam-assisted gravity drainage ("SAGD") is the accepted label for an in situ thermal oil recovery process now being used commercially in the subterranean 11 McMurray oil sand deposits in Northern Alberta and Saskatchewan.
12 The oil (or bitumen) in these deposits is heavy and very viscous. It will not 13 flow readily and therefore must be heated by steam injection to reduce its viscosity, so 14 that it becomes mobile and can be moved through the formation to a production well.
The SAGD process which is used involves the following steps:
16 ~ A pair of parallel co-extensive horizontal wells are established in close 17 proximity (for example 7 meters apart), one above the other, in the oil zone;
18 ~ Fluid communication through the span of oil sand between the wells is 19 established - this may be done by circulating steam through the two wells simultaneously to create "hot fingers" which heat the span by conductance;
21 ~ When the span is hot, steam is injected through the upper well to displace 22 oil in the span into the lower well, through which it is produced to ground 23 surface;
{ EM 177270.DOC;1 }

1 ~ The upper well is then dedicated to steam injection and the lower well to 2 fluid production. Steam, injected through the upper injection well, rises and 3 heats cold oil in the reservoir. The heated oil and steam condensate drains 4 by gravity to the production well and is produced. An upwardly ascending chamber, depleted of oil and filled with steam, is gradually developed. This 6 chamber is capable of readily allowing fluid to move therethrough. Newly 7 injected steam can rise through the chamber to heat cold oil along the 8 chamber boundary and the heated oil and steam condensate can drain down 9 under the impetus of gravity to the production well.
The conventional wisdom in the industry is that steam injection needs to be 11 carried out at less than formation break through pressure. The McMurray oil sand is at 12 relatively shallow depth (for example 300 m). If steam is injected at a pressure greater 13 than break through pressure, there is a strong possibility that vertical channeling will 14 occur into an adjacent or close porous permeable formation (referred to as a "thief zone") and injected steam will preferentially move into the thief zone and be wasted.
16 However, the process proceeds slowly when steam is injected at a pressure less 17 than formation break through pressure. It is desirable to find a workable technique 18 whereby the rate of growth of the steam chamber may be substantially accelerated.

SUMMARY OF THE INVENTION
21 The present invention is based on the discovery that steam injection can be 22 advantageously conducted at an elevated pressure that is:
23 ~ greater than the pressure at which cold water begins to be readily injectable 24 into the oil zone; but {EM177270.DOC;1 ) 1 ~ less than the pressure at which steam begins to preferentially escape into a 2 thief zone.
3 By way of example, in the Fort McMurray region of Alberta, we have 4 determined:
~ that steam injection rate into the McMurray oil formation is low (less than 6 10 m3/day, cold water equivalent volume) and steam chamber growth is 7 negligible at a steam injection pressure of 2000-4300 kPa;
8 ~ that cold water can be effectively injected into the oil formation 9 commencing at a pressure above 4300 kPa;
~ that steam injection rate is higher (over 200 m3/day, cold water equivalent 11 volume) and steam chamber growth is rapid at a steam injection pressure in 12 the range 4500 - 5200 kPa; and 13 ~ that steam begins to be lost to the overlying thief zone if the steam is 14 injected at a pressure above 6500 kPa.
In summary then, we have discovered that steam chamber development can be 16 surprisingly enhanced, without breakthrough to the overlying thief zone, if steam 17 injection in connection with the SAGD process is conducted at a pressure narrowly 18 greater than the pressure at which cold water first becomes injectable into the heavy oil 19 zone. Preferably, the steam injection pressure should be less than 1500 kPa greater than the pressure at which cold water first is injectable. In numbers, if the initial cold 21 water injectivity pressure is 4500 kPa, then steam injection should be conducted in the 22 range 4500 - 6000 kPa.
~ EM 177270.DOC;1 }

2 By way of explanation, we believe that the McMurray oil formation can be 3 mildly fractured by injecting steam at a pressure greater than but close to the cold 4 water initial injectivity pressure, without having the fractures penetrate to the thief zone. We find that relatively good steam injectivity and steam chamber growth can 6 be achieved, without steam loss to the overlying thief zone, provided that the steam 7 injection pressure is preferably within 1500 kPa of the cold water initial injectivity 8 pressure.
9 In one embodiment, the invention is concerned with an improvement in an in-situ steam-assisted gravity drainage process for the recover of heavy oil from a 11 subterranean oil formation, comprising: determining the pressure at which cold water 12 is initially injectable into the formation; and establishing steam injection and steam 13 chamber growth by injecting steam at a pressure greater than the cold water initial 14 injectivity pressure and less than 1500 kPa greater than the cold water initial injectivity pressure.

18 Figure 1 is a plot of pressure versus injection rate for a typical mini-frac test, 19 showing the pressure at which cold water is initially injectable into the McMurray oil zone in the Fort McMurray region; and 21 Figure 2 is a plot of the height of the steam chamber associated with a SAGD
22 steam injection well over time, as observed by a nearby injection well.

(E4241862.DOC;1 f 1 4a 3 The steps of the process are exemplified by the following examples.
4 Example I
S A "mini-frac" test was carried out to establish the pressure at which cold water 6 is initially injectable into the McMurray formation.
7 More particularly a vertical steam injection well was completed in the 8 McMurray oil formation. The well was filled with cold water and the water was 9 pressured up. 32 liters of water at a rate of 8 liters/minute were fed into the formation, commencing when the pressure reached 4800 kPa, as shown in Figure 1.

{E424I862.DOC;1 f I Example II
2 Figure 2 is a plot showing the growth rate of the steam chamber associated with 3 a steam injection well being used in a SAGD operation being conducted in the Lower 4 McMurray oil sand formation.

5 As shown, steam was initially injected for a period of about 7 months at a

6 pressure of 5100 kPa and injection rate of 300 m3/day. This was an injection pressure

7 greater than the pressure (4500 kPa) at which cold water could initially be injected.

8 The steam chamber grew at a rate of 3.0 to 5.0 cm/day. It was then necessary for a

9 period of about 9 months to reduce the injection pressure to 4400 kPa, due to a shortage of available steam. As shown, the growth of the steam chamber became negligible. At 11 this point, more steam became available and the injection pressure was increased to 12 5200 kPa with an accompanying growth of the steam chamber at a rate of about 3.0 to 13 6.0 cm/day.

Examine III
16 At another nearby SAGD injection well, steam was injected into the McMurray 17 formation at a pressure of 6500 kPa. Significant losses of steam to the overlying thief I 8 zone were experienced.
( EM 177270.DOC;1 }

Claims (2)

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

1. In an in-situ steam-assisted gravity drainage process for the recover of heavy oil from a subterranean oil formation, the improvement comprising:
determining the pressure at which cold water is initially injectable into the formation; and establishing steam injection and steam chamber growth by injecting steam at a pressure greater than but within 1 500kPa of the cold water initial injectivity pressure.

2. The improvement as set forth in claim 1 wherein:
the oil formation is the McMurray oil sand.