PROCESS FOR SEQUENTIALLY APPLYING SAGD TO ADJACENT SECTIONS OF A PETROLEUM RESERVOIR

PROCESSUS PERMETTANT D'APPLIQUER SEQUENTIELLEMENT LE SAGD AUX SECTIONS ADJACENTES D'UN GISEMENT DE PETROLE

English Abstract

Steam assisted gravity drainage ("SAGD") is practised in a first section of a reservoir containing heavy oil. When production becomes uneconomic, steam injection into the first section is terminated. Non-condensible gas is then injected into the section to pressurize it and production of residual oil and steam condensate is continued. Concurrently with pressurization, SAGD is practised in an adjacent reservoir section. As a result, some of the residual oil in the first section is recovered and steam loss from the second section to the first section is minimized.

French Abstract

Un drainage par gravité au moyen de vapeur (« SAGD ») est pratiqué dans une première section d'un réservoir contenant du pétrole lourd. Lorsque la production n'est plus rentable, l'injection de vapeur dans la première section est arrêtée. Un gaz non condensable est ensuite injecté dans la section pour la pressuriser et la production de pétrole résiduel et de condensat de vapeur se poursuit. Simultanément à la pressurisation, un SAGD est pratiqué dans une section adjacente du réservoir. En conséquence, une partie du pétrole résiduel de la première section est récupéré et la perte de vapeur de la deuxième section à la première section est réduite au minimum.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS
FOLLOWS:
1. A method for recovering heavy oil from an underground reservoir,
comprising:
(a) injecting steam and producing heated oil and steam condensate
using steam assisted gravity drainage in a first section of the reservoir;
(b) preparing an adjoining section of the reservoir;
(c) terminating or reducing steam injection into the reservoir first
section;
(d) injecting steam and producing heated oil and steam condensate
using steam assisted gravity drainage in an adjacent second section of the
reservoir; and
(e) concurrently with step (d), injecting a non-condensible gas into
the first section to pressurize it and producing residual oil and steam
condensate from said first section.
2. The method as set forth in claim 1 wherein:
the first section is pressurized in step (e) to a pressure about equal with
the steam injection pressure in step (d).

3. In a method for recovering heavy oil from an underground reservoir
wherein a first section of the reservoir is at least partially depleted and an
adjoining
second section is less depleted, and wherein injected fluid can move from one
section
to the other, and wherein a non condensible gas is injected into the first
section while
steam is injected into the second section so that the pressure in the two
sections of
reservoir is about equal, the improvement comprising:
the first section has been depleted by practising steam assisted gravity
drainage using one or more horizontal pairs of injection and production wells;
practising steam assisted gravity drainage in the second section using one or
more horizontal pairs or injection and production wells by injecting steam
through the
injection wells and producing oil and steam condensate through the production
walls;
and concurrently injecting the non-condensible gas through the injection wells
of the first section while maintaining the pressure in the two sections of
reservoir
about equal.

Description

CA 02251157 1998-10-26
1 FIELD OF THE INVENTION
2 This invention relates to recovering heavy oil from an underground
3 reservoir using a staged process involving, in the first stage, steam
assisted
4 gravity drainage, and in the second stage, non-condensible gas injection and
reservoir pressurization.
6
7 BACKGROUND OF THE INVENTION
8 Steam assisted gravity drainage ("SAGD") is a process first proposed
9 by R. M. Butler and later developed and tested at the Underground Test
Facility ("UTF") of the Alberta Oil Sands Technology and Research Authority
11 ("ROSTRA"). The SAGD process was originally developed for use in heavy
12 oil or bitumen containing reservoirs, (hereinafter collectively referred to
as
13 'heavy oil reservoirs'), such as the Athabasca oil sands. The process, as
14 practised at the UTF, involved:
~ Drilling a pair of horizontal wells close to the base of the reservoir
16 containing the heavy oil. One well was directly above the other in
17 relatively close, co-extensive, spaced apart, parallel relationship.
18 The wells were spaced apart 5 - 7 meters and extended in parallel
19 horizontal relationship through several hundred meters of the oil
pay or reservoir;
21 ~ Then establishing fluid communication between the wells so that
22 fluid could move through the span of formation between them. This
23 was done by circulating steam through each of the wells to produce
24 a pair of "hot fingers". The span between the wells warmed by
conduction until the contained oil was sufficiently heated so that it
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CA 02251157 1998-10-26
1 could be driven by steam pressure from one well to the other. The
2 viscous oil in the span was replaced with steam and the wells were
3 then ready for production;
4 ~ Then converting to SAGD production. More particularly, the upper
well was used to inject steam and the lower well was used to
6 produce a product mixture of heated oil and condensed water. The
7 production well was operated under steam trap control. That is, the
8 production well was throttled to maintain the production temperature
9 below the saturated steam temperature corresponding to the
production pressure. Otherwise stated, the fluids being produced at
11 the production interval should be at undersaturated or "subcooled"
12 condition. (Subcool = steam temperature corresponding to the
13 measured producing production pressure - measured temperature.)
14 This was done to ensure a column of liquid over the production well,
to minimize "short-circuiting" by injected steam into the production
16 well. The injected steam began to form an upwardly enlarging
17 steam chamber in the reservoir. The chamber extended along the
18 length of the horizontal portions of the well pair. Oil that had
19 originally filled the chamber sand was heated, to mobilize it, and
drained, along with condensed water, down to the production well,
21 through which they were removed. The chamber was thus filled
22 with steam and was permeable to liquid flow. Newly injected steam
23 moved through the chamber and supplied heat to its peripheral
24 surface, thereby enlarging the chamber upwardly and outwardly as
3

CA 02251157 1998-10-26
1 the oil was mobilized and drained together with the condensed
2 water down to the production well.
3 This process is described in greater detail in Canadian patent 1,304,287
4 (Edmunds, Haston and Cordell).
The process was shown to be commercially viable and is now being
6 tested by several oil companies in a significant number of pilot projects.
7 Now, the operation of a single pair of wells practising SAGD has a finite
8 life. When the upwardly enlarging steam chamber reaches the overlying, cold
9 overburden, it can no longer expand upwardly and heat begins to be lost to
the overburden. If two well pairs are being operated side by side, their
11 laterally expanding chambers will eventually contact along their side edges
12 and further oil-producing lateral expansion comes to a halt as well. As a
13 result, oil production rate begins to drop off. As a consequence of these
two
14 occurrences, the steam/oil ratio ("SOR") begins to rise and continued SAGD
operation with the pair eventually becomes uneconomic.
16 If one considers two side-by-side SAGD well pairs which have been
17 produced to "maturity", as just described, it will be found that a ridge of
18 unheated oil is left between the well pairs. It is of course desirable to
19 minimize this loss of unrecovered oil.
In Canadian patent 2,015,460 (Kisman), assigned to the present
21 assignee, there is described a technique for limiting the escape of steam
into
22 a thief zone. For example, if steam is being injected into a relatively
23 undepleted reservoir section and there is a nearby more depleted reservoir
24 section, forming a low pressure sink, there is a likelihood that
pressurized
steam will migrate from the undepleted section into the more depleted section
4

CA 02251157 1998-10-26
1 - which is an undesired result. One wants to confine the steam to the
2 relatively undepleted section where there is lots of oil to be heated,
mobilized
3 and produced. The Kisman patent teaches injecting a non-condensible gas,
4 such as natural gas, into the more depleted section to raise its pressure
and
equalize it with the pressure in the relatively undepleted section. By this
6 means, the loss of steam from the one section to the other can be curtailed
or
7 minimized.
8 The Kisman patent further teaches that pressurizing the more depleted
9 section with natural gas has been characterized by an increase in production
rate from that section, if the production well penetrating the section is
11 produced during pressurization.
12
13 SUMMARY OF THE INVENTION
14 In accordance with the present invention, a novel process is provided
for producing adjacent sections of an underground reservoir containing heavy
16 oil. Each section is penetrated by one or more wells completed for SAGD
17 operation, preferably one or more pairs of horizontal injection and
production
18 wells. The process comprises:
19 (a) injecting steam into the first section of the reservoir to practice
SAGD and produce contained oil, until the steamloil ratio rises
21 sufficiently so that further production by SAGD from the section
22 is substantially uneconomic;
23 (b) then reducing or terminating steam injection into the first section
24 and injecting non-condensible gas into the section to maintain it
pressurized;
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CA 02251157 2002-05-23
1 (c) continuing to produce oil from the first section while it is
2 pressurized; and
3 (d) concurrently with step (c), injecting steam into the adjacent
4 second section to practice SAGD therein and produce contained
oil;
6 (e) while preferably maintaining the first section pressurized to
7 substantially the same pressure as exists in the second section
8 during step (d).
9 Steps (b) and (c) constitute a post-steam wind-down of oil production
from the first section. Over time, oil production rate will drop off during
wind-
11 down and eventually it will again become uneconomic to justify continuing
to
12 produce the first section. However it may still be desirable to continue
13 maintaining pressurization in the first section to limit steam loss from
the
14 second section.
The process provides a strategy for sequentially producing adjacent
16 sections across the reservoir. It takes advantage of gas pressurization to
17 prevent steam leakage from a less depleted section undergoing SAGD to a
18 mature, more depleted section. It also maximizes production from each
19 section by subjecting it to sequential SAGD and pressurization production
stages.
{ ET049043. DOC;1 }6

CA 02251157 1998-10-26
1 DESCRIPTION OF THE PREFERRED EMBODIMENT
2 In accordance with the best mode of the process known
to the
3 applicants,
it comprises:
4 (a) directionally drilling one or more pairs of wells
from ground
surface into a reservoir first section, to provide
generally parallel,
6 horizontal, co-extensive, spaced apart, upper and
lower well
7 portions extending through the section, and completing
the wells
8 for SAGD production;
9 (b) establishing fluid communication between the injection
and
production wells of each pair by circulating steam
through both
11 wells, to heat the span between the wells by heat
conduction,
12 and then displacing and draining the oil in the
span by injecting
13 steam through the upper injection well and opening
the lower
14 production well for production;
(c) practising SAGD in the reservoir first section by
injecting steam
16 through the injection wells and producing the produced
heated
17 oil and condensed water through the production wells
while
18 operating said production wells under steam trap
control;
19 (d) preparing a second adjoining section of the reservoir
for SAGD
production by carrying out the provision of wells
and establishing
21 fluid communication between the wells of each pair
as in steps
22 (a) and (b);
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CA 02251157 1998-10-26
1 (e) terminating or reducing steam injection into the reservoir first
2 section injection wells and initiating natural gas injection through
3 said injection wells to increase the pressure in the reservoir first
4 section to about the anticipated steam injection pressure in the
reservoir second section and maintaining the pressure at about
6 this level while simultaneously producing residual heated oil and
7 steam condensate through the production wells under steam
8 trap control; and
9 (f) concurrently with step (e), practising SAGD in the reservoir
second section.
11 In connection with practising steam trap control with wells extending
12 down from ground surface and having riser and horizontal production
13 sections, it is preferred to operate as follows:
14 ~ measuring the downhole temperature at the injection and
production wells of an operating pair, using thermocouples;
16 ~ establishing the temperature differential between the two wells and
17 throttling the production well to maintain the differential at a
18 generally constant value (say 7°);
19 ~ monitoring for significant surges in vapour production rate at the
ground surface production separator and for surges in steam
21 injection rate; and
22 ~ adjusting throttling to minimize the surges.
23 Otherwise stated, a generally constant liquid rate at the wellhead is
24 maintained and the bottomhole production temperature is allowed to vary
within a limited range.
8

CA 02251157 1998-10-26
1 The invention is characterized by the following advantages:
2 ~ additional oil is recovered from the mature wells during the gas
3 pressurization stage, while simultaneously reducing steam leakage
4 from the second reservoir section;
~ use is made of the residual heat left in the mature reservoir section;
6 and
7 ~ a finite steam-producing plant can be applied in sequence to a
8 plurality of adjacent sections of the reservoir, without severe steam
9 loss from a section undergoing SAGD to an adjacent depleted
section.
9