Note: Descriptions are shown in the official language in which they were submitted.
CA 02675160 2009-08-07
Method of Heavy Oil Production
Field of the Invention
The present invention relates to oil producing industry, particularly to heavy
oil
production methods using horizontal wells and heat treatment of formations.
Background of the Invention
A method for developing a non-uniform oil-bearing formation comprising periods
of pumping water through an injection well with simultaneous formation fluid
recovery
through producing wells when water pumping stops is know from the prior art
(patent RU
No. 2095549, IPC E21B43/22, published on 11.05.1994). Salinity of the produced
water is
periodically, once in 2-3 days, analyzed, and injecting water with
simultaneous formation
fluids recovery is carried out until stable salinity of the produced water is
achieved. The
method provides for more accurate determination of duration for the water
injection and
recovery of the formation fluid cycles.
The disadvantage of this method is low effectiveness when developing heavy oil
formations using a method of thermal-steam formation treatment through
horizontal wells.
A method of continuous viscous hydrocarbons recovery in a gravitation mode
with
heated liquid injection is known from the prior art (US patent No. 4344485,
IPC E21B
43/26, published on 26.06.1980). In the method of developing heavy oil fields
a pair of
parallel producing well and injection well located one above the other in a
vertical plane
and completed with tubing strings are used. The method comprises injection of
a heat
carrier, heating the productive formation, forming a steam chamber, recovering
products
and controlling technological parameters of the formation and well.
The method is targeted on providing mobility to usually motionless heavy oil
and
withdrawing it from a tar sand reservoir with a producing well and injection
well. Initially
a heat carrier is injected through the injection well at a high speed so that
a heat connection
is formed between the wells, and a heated penetrable (steam) chamber is
created.
At the chamber border steam is condensed, and heat is transferred to cooler
surrounding areas. Oil temperature near the chamber increases, and oil flows
down
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together with the hot steam condensate. Oil is continuously recovered at a
place that is
lower than the steam chamber.
The heat carrier expands the heated penetrable chamber while oil flows
continuously in the producing well.
The movable heavy oil is recovered through the producing well.
Steam is used as the heat carrier.
Oil production is controlled so that separate flows of oil and water are
adjusted and
excess steam breakthrough is eliminated.
Various configurations of wells are used in the invention. The following
factors are
common for all configurations: (a) a producing well is "extended" through the
tar sand
formation as a horizontal wellbore or by creating ruptures (or combination of
both); (b) a
"heat connection" between the producing well and injection well is created
before starting
oil production.
Double concentric strings are arranged inside the casing string. Inner string
is
inside the surrounding outer string of bigger diameter.
The water and heavy oil recovery are thoroughly controlled to provide optimum
oil
production without excess steam breakthrough.
Low effectiveness of the method in developing heavy oil fields due to lack of
adjustment of uniform steam chamber heating is the main disadvantage of the
method.
The closest reference in terms of technology is a method of heavy oil
production
disclosed in RU patent No. 2305762, IPC E21B 43/24, published on 10.09.07. In
the
method a pair of horizontal producing well and injection well is used,
horizontal wellbores
of the wells are parallel one above the other in a vertical plane of the
production formation.
The wells are provided with tubing strings for simultaneous injection of a
heat carrier and
product recovery, heating the production formation, creating a steam chamber,
recovering
products through the producing well tubing string and controlling
technological parameters
of the formation and wells. The trajectory of the horizontal wellbore passes
not lower than
the minimum distance to the subface of the heavy oil or bitumen or water-
bitumen contact
increasing water-free period of well operation.
Casing strings with a filter in the production interval are installed, annular
space is
cemented, and tubing strings with centralizers are arranged. The heat carrier
is injected
through the upper horizontal injection well, and simultaneously product is
withdrawn
through the lower horizontal well.
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The mode of injection is selected depending on in-place permeability, pay
thickness of the formation, oil or bitumen viscosity. The injected heat
carrier moves to the
formation top forming a steam chamber. Heavy oil is displaced by the heat
carrier over the
entire surface.
Volumes of injected steam and recovered heavy oil, an amount of water in oil
and
pressure at both well heads are periodically checked. All above parameters
characterize the
formation and the well operation, and they are technological parameters of the
formation
and the well respectively. The modes of steam injection are changed if
required, and
optimum modes of heavy oil recovery are selected.
However a breakthrough of condensate or formation water to the producing well
can happen in producing heavy oil. It is accompanied by an increase or
decrease in
temperature near the producing well and by a decrease in uniformity of
formation heating
and water-free period of the producing well operation. Selection of water
injection mode
according to this method does not allow controlling uniformity of formation
heating
accurately enough. What is more, said position of the producing wellbore
results in lesser
heat coverage of the formation.
The disadvantage of the method is significant energy consumption, low
effectiveness of heavy oil recovery due to insufficient control of uniform
steam chamber
heating.
The technical objective of the present method of heavy oil production is to
increase
heavy oil recovery through increase in coverage of the formation with thermal
action,
providing more accurate control of steam chamber heating uniformity by
adjusting modes
of heat carrier injection and product recovery. The method is an addition to
the scope of
technology methods of heavy oil production.
The technical result is achieved using the claimed method of heavy oil
production.
The method comprises providing a pair of horizontal injection and producing
wells having
horizontal wellbores arranged parallel one above the other in the vertical
plane of the
productive formation. The wells are provided with tubing strings for
simultaneous
injection of a heat carrier, production of oil, heating the production
formation through
creation of a steam chamber, and recovery of product through the tubing string
of the
producing well, at the same time controlling technological parameters of the
formation and
well. The method is novel because the ends of the tubing strings are located
at the opposite
sides of the parallel horizontal wellbores. The heating of the productive
formation starts
with steam injection through both wells for heating the interwell zone and
lowering
.. . . . . . ... . . . . _. .. . ... . . . .._ . . . __ _. .. _.... _. .. I
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viscosity of heavy oil. A steam chamber is created by injection of the heat
carrier
spreading at the top of the productive formation. The size of the chamber
increases with
product recovery. Salinity of produced water is checked regularly (2-3 times a
week), and
the relation between the salinity of the produced water and the uniformity of
steam
chamber heating is evaluated. Taking into account the change in produced water
salinity,
uniform heating of the steam chamber is adjusted by controlling the heat
carrier injection
or product recovery until stable level of produced water salinity is achieved.
Brief Description of the Drawings
Figs. 1 a, b show positions of the wells.
Figs. 2, 3, 4 and 5 show examples of particular embodiments (graphs of
relation
between recovery of heavy oil and produced water salinity).
Description of the Invention
At present heat treatment for increasing amount of recoverable oil from heavy
oil
and natural bitumen deposits has no alternative. The most common methods of
such heat
treatment are the method based of using steam or a mixture of steam and gas.
Oil extraction increases when steam is injected. It is due to a decrease in
viscosity
of oil under heating, its thermal expansion, distillation of residual oil by
steam, beneficial
effect of oil and water movement and phase penetrability of oil and water, and
effect of gas
drive.
A pair of horizontal injection and producing wells having their horizontal
wellbores
arranged parallel in a vertical plane of the formation one above the other,
the wells being
provided with tubing strings.
A heat carrier is injected in the heavy oil deposit heating the productive
formation
by creating a steam chamber, the product is recovered though the tubing string
of the
producing well, and technology parameters of the formation and the well being
controlled.
The method of heavy oil production is effective when at least one pair of one-
wellhead or two-wellhead horizontal wells is used.
When two-head horizontal wells are used, a tubing string is lowered through
each
head, and their ends are positioned at the beginning and at the end of the
parallel
horizontal wellbores (Fig. 1 a).
CA 02675160 2009-08-07
When a one-head horizontal well is used, two parallel tubing strings are
lowered,
and their ends are also positioned at the beginning and at the end of the
conditionally
horizontal portion of the well (Fig. lb). When it is difficult to arrange a
pair of parallel
tubing strings, the wells can be provided with continuous (flexible) tube.
The diagram of string positions shown in Figs. 1 a and lb include a producing
well
1 and an injection well 2 in productive formation 3 with or without rising to
the day
surface. The wells are drilled so that the horizontal wellbore 4 of the well 2
is above the
horizontal wellbore 5 of the well 1 in one vertical plane at a distance from
each other. The
well 2 is used for injecting a heat carrier in the formation, and the well 1
is used for
recovering heavy oil. The wells are provided with tubing strings 6 lowered
from different
heads of the two-head well (Fig. la) and lowered parallel in a one-head well
(Fig. lb).
Ends of tubing strings are positioned at a beginning and at an end of the
horizontal
wellbore to allow injecting a heat carrier and withdrawing production
simultaneously at
two points. Such positioning of the tubing strings provides for wider heat
action on the
formation. The well 2 is equipped with pumps 7 for lifting heavy oil to the
surface. The
upper horizontal wellbore is used for injection of the heat carrier (steam)
into the
formation and creation of a steam chamber. The process of the thermal-steam
formation
treatment starts with the step of preliminary heating when steam is circulated
in both wells.
The interwell zone (the zone between the producing and injection wells) is
heated wherein
the viscosity of heavy oil decreases, it expands, and its mobility rises. Then
in the process
of the heavy oil recovery steam is pumped in the injection well, the steam
moves to the top
of the production formation due to difference in the densities creating an
expanding steam
chamber. At the surface of the interface between the steam chamber and the
cold oil-
saturated mass there is continuous heat exchange. As a result, steam is
condensed into
water that flows under gravity to the producing well together with the heated
heavy oil.
Product is withdrawn at two opposite ends of the horizontal wellbore of the
producing well. The product comprises produced formation water of high
salinity besides
heavy oil and condensed water. The formation water salinity decreases when it
mixes with
the condensate, and the produced water salinity has an intermediate value.
Steam salinity
and correspondingly condensate salinity are equal to zero, i.e. CS<l g/l,
while the salinity
of the formation water can be Cs.,u = 100 g/l. The salinity of produced water
can vary in the
range from 1 to 100 g/l depending on the heavy oil deposit development stage.
In the mode of continuous injection and withdrawal a balance is established
between the amount of extracted oil and the salinity of the produced water.
This value of
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water salinity is called an equilibrium value of salinity. Change in the
equilibrium is
indicated by a change in salinity of the produced water. In the process of
product
withdrawal the salinity of the produced water is periodically, 2 or 3 times a
week, is
determined, and changes are reviewed and a graph of interrelation of the heavy
oil
extraction and the salinity of the produced water is plot.
An increase in the salinity of the produced water by more than 10% to the
equilibrium salinity value indicates that the withdrawal of the formation
water having a
temperature in the range of 5 to 15 C grows. As a result, the temperature near
the
producing well and in the interwell zone drops leading to uneven heating of
the steam
chamber and decreasing of the formation portion under thermal-steam treatment.
Temperature decrease near the producing well and in the interwell zone results
in a
recovered heavy oil viscosity increase in its turn leading to a decrease in
the amount of
extracted heavy oil and to a decrease in the effectiveness of the thermal-
steam treatment.
For decreasing the salinity of the produced water and increasing the
temperature
near the producing well and in the interwell zone thereby increasing steam
chamber
heating uniformity, it is required either to increase the volume of injected
steam or to
decrease the product withdrawal and consequently the volume of produced water.
When
the volume of injected steam is increased, the whole volume of the steam
chamber is more
evenly heated, and the decrease in the temperature near the producing well and
in the
interwell zone is stopped. At the same time the formation water is diluted by
the
condensate, and the salinity of the produced water decreases. After steady
heating of the
steam chamber is restored, the balance between the amount of extracted oil and
the salinity
of the produced water is restored too but not necessarily at the same level as
before, as
indicated by the graph of relation between the extraction of heavy oil and the
salinity of
the produced water.
When the salinity of the produced water falls below 10% of the balance level,
it
also indicates that the steam chamber is not heated uniformly. In this case a
steam
breakthrough to the producing well can occur. It results in steam drain
increasing energy
consumption. The steam breakthrough can also lead to some damage to the
technology
equipment because of high temperature. That is why it is important to decrease
the volume
of injected steam or increase the withdrawal of the product when the salinity
of the
produced water falls. When the product recovery increases, the volume of the
cold
formation water with high level of salinity increases too, therefore the
salinity of the
produced water also increases. As mentioned above; the temperature of the -
formation
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water is in the range of 5-15 C, and when its recovery increases, the
temperature near the
producing well and in the interwell zone decreases. The increasing recovery
continues
until the balance between the amount of extracted oil and the salinity of the
produced
water is restored. The graph of the relation between the extraction of heavy
oil and the
salinity of the produced water shows when the balance is restored.
A method of controlling uniform heating of the steam chamber using temperature
detectors is known in the prior art but it is not effective because of
frequent failures of the
detectors.
It follows from aforesaid that the method of heavy oil production allowing to
control injection of heat carrier and product recovery based on the analysis
of produced
water salinity is a very simple and effective method for controlling
uniformity of steam
chamber heating and enhancing effectiveness of oil extraction from heavy oil
fields.
Method Embodiment Examples
Example 1
At the experimental site of the Ashalchinsky heavy oil field at a depth of 90
having
non-uniform formations with a thickness of 20-30 m, a temperature of 8 C, a
pressure of
0.5 MPa, oil saturation of 0.70 units, a porosity of 30%, a permeability of
2.65 m2, and
oil with a density of 960 kg/m3 and a viscosity of 22000 mPa=s, a pair of
horizontal two-
head wells (Fig. 1 a) was drilled.
The pair included an injection well and a producing well with horizontal
wellbores
being parallel and located one above the other in the vertical plane of the
productive
formation and provided with tubing strings for simultaneous injection of a
heat carrier and
product recovery. Before putting the producing well in operation, a zone
between the wells
was heated by simultaneous circulation of steam through each of said wells. In
the process
of heavy oil recovery, steam is pumped through the injection well. Steam rises
and forms a
steam chamber growing in size. In the process of recovery salinity of the
produced water is
regularly (2-3 times a week) determined, and a graph of relation between the
extraction of
heavy oil and the salinity of the produced water is plotted (Fig. 2). At the
beginning stage
of the heavy oil formation development there is a balance between the amount
of extracted
oil and the salinity of the produced water during the period from 15.03.07 and
20.03.07
indicating uniform heating of the steam chamber. The well production of heavy
oil is 12
m3/day, and the salinity of the produced water is 2.15-2.3 g/l. The balance
(average)
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salinity is 2.2 g/1. The analysis carried out on 25.03.07 indicates that
salinity is growing
from 2.3 g/l (on 20.03.07) to 3.1 g/l, or by 34.8% while the well production
drops from 12
m3/day to 6 m3/day. It is an indication that the inflow of cold formation
water increased
bringing the temperature and mobility of heavy oil down and causing disruption
in uniform
steam chamber heating. The volume of steam injection was 44 m3/day at that
moment. On
the ground of the analysis result, it was decided to increase the volume of
injected steam. It
was increased to 60 m3/day (Fig. 2) from 25.03.07 to 30.03.07.
After that the salinity of the produced water began to decrease and was 2.28
g/l on
30.03.07; the well production increased to 11.3 m3/day. The well production of
heavy oil
stabilized at 11.3 m3/day, and later the salinity changed insignificantly in
the range from
2.28 g/1 to 2.4 g/l.
Fig. 4 represents the time graph of relation between the extraction of heavy
oil and
the salinity of the produced water. In the period from 24.08.07 to 04.09.07
the salinity of
the produced water increased from 3.7 g/l to 4.5 g/1, or by 22%. The daily
production of
heavy oil decreased from 12.7 m3/day to 10.5 m3/day indicating cooling down of
the steam
chamber. For improving the uniformity of steam chamber heating, the withdrawal
of the
produced water was decreased from 99 m3/day to 86 m3/day. After that the
salinity of the
produced water began to gradually decrease and reached the value of 3.8 g/1,
and the heavy
oil production began to grow and stabilized at 12.9 m3/day.
Example 2
At the experimental site of a heavy oil field at a depth of 90 m having non-
uniform
formations with a thickness of 20-30 m, a temperature of 8 C, a pressure of
0.5 MPa, oil
saturation of 0.70 units, a porosity of 30%, a permeability of 2.65 m2, and
oil with a
density of 960 kg/m3 and a viscosity of 22000 mPa-s, a pair of horizontal one-
head wells
(Fig. lb) was drilled.
There was a balance relation between the amount of recovered heavy oil (12-
12.8
m3/day) and the salinity of the produced water (3.58-3.45 g/l) during the
period from
07.06.07 to 22.06.07. The balance (average) salinity value was 3.52 g/l.
Analysis of the
next sample revealed that by 27.06.07 the salinity of the produced water
sharply dropped
to 2.2 g/l, the change by 37% of the balance value. It shows that a steam
breakthrough to
the producing well occurred resulting in decrease of action on the formation,
decrease of
the uniformity of steam chamber heating and drain of heat carrier. For
normalizing the
salinity and correspondingly temperature near the producing well, the
extraction of liquid
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9
was increased from 86.1 m3/day to 99 m3/day. The salinity was normalized by
07.07.07 to
3.45 g/l. The oil recovery dropped at first moment after the steam
breakthrough but then
after the extraction increase it stabilized at the level of 13.5 m3/day (Fig.
3).
Example 3
The graph of changing salinity of the produced water and heavy oil extraction
in
time is shown in Fig. 5. There was a stable relation between the heavy oil
extraction and
the salinity of the produced water up to 26.06.08. As a result of a steam
breakthrough the
salinity of the produced water and the heavy oil recovery decreased. For
restoring the
balance, the volume of steam injection was decreased from 80 m3/day to 65
m3/day. The
salinity increased to 3.1 g/l on 07.07.07 and further it was stable at this
level. The oil
recovery gradually increased up to 9.2 m3/day.
The claimed method of heavy oil production allow increasing heavy oil recovery
through the enhanced reservoir sweep efficiency and more precise control of
uniform
steam chamber heating by changing heat carrier injection and production
conditions.
