Note: Descriptions are shown in the official language in which they were submitted.
CA 02418195 2003-O1-31
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TITLE OF THE INVENTION:
Method of steam injec:t.ian through a horizontal well bore
to stimulate oil well production
FIELD OF THE INVENTION
The present invention relates to a method of steam
injection through a har~.zont.al. well bore to stimulate oii
well production
BACKGROUND OF THE INVENTION
It is recognized by persons skilled in the art that the
best manner of stimulating oil production by steam injection
is to have the steam injection as uniform as possible along a
horizontal well bore of an injection well.
a c
One strategy f.or providing uniform steam distribution is
to position additional well strings inside the well bore.
This approach adds to the well cost and creates higher
pressure gradients along the well than would be created by an
20 open well bore. Tn order to address the higher pressure
gradients and accommodate the additional well strings, the
well bore diameter is often increased, which further
increases the well r_.ast. This strategy has lead t:o Steam
Assisted Gravity Drainage (SAGD) designs with injector wells
having larger diameter:~s than the producing wells.
Another strategy is to provide for varying flow
resistance over the well interval by varying the density
distribution of inje~:tian openings along the well bore. This
30 strategy is illustrated and described in Canadian Patent
2,292,278.
Another strategy is to use " limited entry" injection in
which the flaw rate is governed by the sonic velocity of gas
CA 02418195 2003-O1-31
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passing through the injection openings. The total open area
of formation exposed tcs the well bore must be substantially
less than the cross-sectional area of the injection wei.l.
Well bore injection pressure must be substantially above
formation pressure in <>rder to generate sonic or near-sonic
flow velocities through t=he injectic>n openings. The
exceedingly small open area requires that the injection be at
widely spaced intervals with very small injection openings.
.r
~. J
SUN~iARY OF THE INVENTION
The present invention relates to an alternative method
of steam injection through a horizontal well bore to
stimulate oil well production.
According to the present invention there is provided a
method of steam injection, through a horizontal well bore
having a well liner, to stimulate oil well production. A
first step involves providing an injection opening density
~0 in the well liner with a plurality of small injection
openings that are distributed substantially constantly over
the length of the horizontal well bore. The injection
opening density must be greater than a cross-sectional area
of the well bore and less than 0.3° of the area of the well
liner. A second :step involves injectz.ng steam along the
horizontal well bore and into the formation through the
injection openings at pressures that are marginally greater
than reservoir pressure.
BO
With the method, as described above, the injection
outflow in controlled by the radial flow resistance provided
by the injection openings and the differential well bore
pressure, relative to the reservoir pressure.
CA 02418195 2003-O1-31
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BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the in~aention will become
more apparent from the following description in which
reference is made to the appended drawings, the drawings are
for the purpose o- i~lustratior~ only and are not intended to
in any way limit the scope of the invention to the particular
embodiment or embodiments shown, wherein:
THE FIGURE -Ls a perspective view, in section, of a
i0 horizontal well bore being used for reservoir stimulation in
accordance with the teachings of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred method of steam injection, through a
15 horizontal well bore having a well liner, to stimulate oil
well production wi:Ll now be described with reference to THE
FI GURE
This is illustrated by a formation 10 in which is
20 positioned a horizontal- well bore 12 by a well liner 14. A
first step in the method involves providing an injection
opening density in the well liner with a plurality of small
injection openings 16 that are distributed substantially
constantly over the length of horizontal well bore 12. The
?5 combined area of all openings must be greater than the
linear cross-sectional area, generally indicated by
reference numeral 18, c>f well liner 14 and less than 0.3°s of
the area of well liner 14. A second step involves injecting
steam along horizontal well bore 12 into formation 10
3(~ through injection openings 7.6 at pressures that are
marginally greater than reservoir pressure. The injection
is initiated through a steam injection unit 20 positioned at
surface. The path of steam being injected along well liner
14 and out injection openings 16 is indicated by arrows 22.
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The above described method uses a reduced density of
injection openings to generate an outflow resistance that is
relatively large in comparison with the flow resistance to
the steam along the wellbore. The ou'.flow resistance is
more reliably characte:ri,.ed in terms ct opening density,
rather than by operv area, However, in order to characterize
the design parameters relative to other inventions, the
opening density will be expressed in terms of open area for
!.0 a range of typical_ opening geometries.
Smaller openings are preferred because they can be more
closely spaced for more i.iniform flow distribution in the
near-wellbore zone. They also provide the sand retention
~5 requ.ired under backflow conditions, when injection
operations are interrupted. Slotted l.:iners provide the
preferred opening geometry because they can be produced in
opening widths that provide the required sand control and
can be machined ec:onorraically in a range of densities to
~0 provide the radial flow controlled required for specific
reservoir conditions and injector wellbore configurations.
An injector opening density is dependent on the
reservoir properties, wellborP length and diameter, and
tolerance for injection rate variation. When a uniform slot
density is provided in a homogeneous reservoir, there will
always be some variation in injection rate because of
pressure variation due to pipe flow loss along the wellbore.
If the outflow resistance is negligible, as is the case with
3) conventional screen designs, the injection variation in
high-permeability formations is too large because of the
pressure variation :due to the pipe flow loss. But as the
density of openings is reduced, the added radial flow loss
becomes significance relative to the pipe flow loss, and the
CA 02418195 2003-O1-31
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variation in injection rate is reduced. 'The system requires
an increase in injection pressure, but the viscosity of
injection steam is very low, so the added pressure required
is small relative to the formation pressure.
a
Significant reductions in open area relative to
industry practice are necessary to redistribute the
injection rates within acceptable tolerances, particularly
if the method is used to provide acceptable performance with
?.Ci smaller wellbore diameters than conventional designs require
for a given well length. The slot configurations use a
constant slot density over the well length that provide less
than 0.30 open area. ~t is preferred that the slot density
be approximately 0.1 0. The total open area must be larger
J5 than the wellbore cross section area to avoid limited entry.
The minimum open area is given by the expression
A.open.min - 0.25~/L, and ensures the local injection
velocity is well below the critical velocity. For example,
~0 in a typical 1000m long injection well 0.177m in diameter,
the minimum open area in the ranged claimed would be
0.00440.
The effectiveness of the design was revealed through
flow analysis that 1_eads to the design configurations in
this range. The analysis conclusions are non-obvious to
persons skilled in the art of horizonta..L well design. We,
therefore, lay claim tc:, this range of liner design
configurations for providing more efficient and cost
8:) effective injection of steam to reservoirs through
horizontal wellbores.
Example:
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The diagram below illustrates what we are trying to
resolve by adding flow resistance to the wellbore of steam
injectors to redistribt.ite the injection. It shows injection
pressure distributions for a 700m long well interval
injecting 20,OOOm'/day ;at downhole conditions) of steam into
a high-permeability reservoir. The solid line shows a flat
pressure distribution corresponding to the injection
pressure at the formation wellbore for uniform injection
(ignoring end-effects), with an injection pressure of
0.42kPa. The dashed lire shows the wellbore pressure inside
the liner corresponding to a uniformly distributed inflow.
Comparing the two pressure distributions, one observes the
large incompatibility between the pressure distribution
required to flow steam inside the liner and that required to
flow steam radially into the formation.
_;
The distribution labelled P, is representative of a
conventional liner with large open area and low radial flow
resistance. In this example, the differential pressure
varies from 28kPa at the heel to 20kPa at the toe, so the
20 injection variation is ~17> of the mediar: rate.
The distribution labelled PT. has an additional radial
flow resistance added by reducing the open area. It has the
same 8kPa variation over the length because the total
injection rate is the same, but the mean injection pressure
?'> differential is increased from 24kPa to 64kPa. The injection
variation about the mean for this configuration is ~60. If
the production plan requires the steam distribution to be
within ~20, additional flow resistance would need to be
added to increase the mean pressure to 200kPa. This is large
~o relative to the differential pressure, but small compared
with the reservoir pressure, which is t.ypi;_.ally about
4000kPa in a SAGD well. the open area has to be reduced
sufficiently to create this additional radial flow
resistance to equalize the injection distribution.
CA 02418195 2003-O1-31
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mm w ~w
16
14
lfVetlbore
12
Procure he ad
For 10
Dint
.
Unitbrm
Injection
(kPa) 6
Formation I i
4 head
2
0
700 500
600 400
3011
200
100
0
(heel) Dl~rnnca
from
Toe
(m)
'',
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Thus, unless some measure is taken to isolate the
formation from the large wellbore pressure variation, a
large variation in injection rate is created from the heel
to the toe. The following figure shows how the
incompatibility is resolved in the example configuration by
concentrating injection at the point where steam is
delivered to the liner. The injection is given as a ratio
between the local and average specific injection rates. It
shows most steam is injected in the first quarter of the
well, and the majority of the well does not delivered the
productivity benefits of thermal stimulation.
r err r~r err
7
1
6 1
1
5 1
iz~d l
Injection
4
tie
3
2
1
. .., _
...
0
T00 600
500 400
300 200
100 0
distance
loom Tos
(m)
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The method of reducing opening density provides a
controlled increase in the radial flow resistance from the
wellbore to the formation. Increasing the radial flow
resistance reduces the radial injection variation by forcing
more flow toward the heel of the well. The following figure
shows includes two additional pressure distributions in
designs where the slot density is reduced to provide the
additional flow resistance to redistribute the flow. The
highest density configuration would actually be below the
lower bound of conventional designs, providing about 0.5$
open area. Comparing the ratio of injection pressure between
the heel and, one notes the high-resistance, low-density
design has a substantially lower injection pressure ratio,
4.3 vs. 37.4. This is indicative of the ratio between the
injection rates at the heel and toe.
14
slot
density
12 (slotsJm)
~~5
10 20
DHtial _ _
_
130
(~,~p~~)
8 '
In je~fon
Prossune
(kPa)
f
4 I
v
2
1_
700 8 00 00
X00 0
400
300
200
1
dlst~wce from doe (m)
y , CA 02418195 2003-O1-31
The last figure (below) for the example shows the
improvement in injection distribution by increasing the
radial flow resistance with the slotting modifications. Much
greater flow rates are promoted to the toe region of the
5 well, and the injection ratio from the two ends of the
interval is reduced by a factor of 10. Further improvements
can be made with additional increases in the radial flow
resistance, and the method also enhances the improvements
associated with increased liner diameters.
11~~1.~1a~1W W n m~~ll~moiw
r1
_
7
slot
density
(slotsim)
6
t
---
130
~d ~ 20
Inja~tton 4
Rate
8
2
1
---
-
0
700 600
500
400
300
200
100
0
Dbtant~
from
Tos
(m)
10
In this patent document, the word "comprising" is used
in its non-limiting sense to mean that items following the
word are included, but items not specifically mentioned are
not excluded. A reference to an element by the indefinite
article "a" does not exclude the possibility that more than
one of the element is present, unless the context clearly
requires that there be one and only one of the elements.
It will be apparent to one skilled in the art that
modifications rnay be made to the illustrated embodiment
CA 02418195 2003-O1-31
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without departing from the spirit and scope of the invention
as hereinafter defined in the Claims.
