Note: Descriptions are shown in the official language in which they were submitted.
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TS 6041
OIL PRODUCTION WELL AND ASSEMBLY OF SUCH WELLS
The invention relates to an oil production well and
to an assembly of such wells.
It is known in the art to stimulate the recovery of
viscous oil from underground oil bearing formations by
injecting a hot fluid, such as steam, into the formation,
thereby reducing the in-situ viscosity of the crude oil.
Thermally enhanced oil recovery processes where steam
is employed to reduce the in-situ viscosity of the crude
oil within the pores of a oil bearing formation are
disclosed in US patent specification Nos. 3,994,340;
4,344,485; 4,465,137 and 5,046,559.
A problem encountered with steam drive methods is
that the injected steam has a much lower viscosity than
the crude oil, even after heating thereof, so that once
there is a break-through of steam into the production
well, this well will produce mainly steam and water
stemming from condensed steam together with very little
crude oil.
Steam has a lower density than crude oil. Therefore,
steam injection will create a pocket of steam at the top
of the oil bearing formation. Hence, if oil is produced
via a conventional vertical well the break-through of
steam will typically occur at the top of the inflow
section of the well and the operator will plug off the
upper part of the inflow section such that fluids will
only flow into the lower part thereof.
US patent specification No. 4,344,485 discloses in
Figure 6 a steam injection method where steam is injected
into a tar sand deposit above a horizontal inflow section
of a production well such that there are separate steam
and oil flow paths within the pores of the formation and
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mobilized heavy oil is induced to drain from the formation
towards the horizontal inflow section of the well.
It is an object of embodiments of the present
invention to provide an oil production well in which oil
production is stimulated by injection of a hot fluid, such
as steam, but which is less sensitive to the detrimental
effects of a break-through of the injected hot fluid than
the prior art production systems.
Accordingly, in on aspect of the invention, there
is provided an oil production well comprising an upwardly
dipping or substantially horizontal inflow section via which
crude oil flows from an oil bearing formation into the well
and an injector well or conduit for injecting a hot fluid in
the region of a toe of the inflow section such that the
injected hot fluid is induced to flow through at least part
of said inflow section, thereby transferring heat to the
crude oil which is present in said inflow section and the
surrounding formation.
In a preferred embodiment of the well according to
the invention the well is substantially J-shaped and the
inflow section forms a foot of the well.
In the well according to the invention the flow of
the hot injection fluid together with the produced crude oil
through the inflow section of the well has the advantage
that primarily the inflow section and surrounding formation
are heated which will stimulate the inflow of crude oil into
and through the well. A J-shaped well configuration is
attractive since the updip orientation of the toe of the
well will provide a natural barrier against a steam
break-through.
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In many wells for the production of viscous crude
oils a pump is present for pumping the crude oil to the
surface.
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In that case the pump in the well according to the
invention will be present in the region of a heel of said
inflow section and the hot fluid injection means comprise
a steam injection system which is adjustable to inject
steam in such a quantity and at such a pressure and
temperature that the steam condenses before the entry of
the pump and the resulting mixture of hot water and crude
oil has a pressure below the steam flash point at the
conditions at the entry of the pump.
In order to allow heating of the inflow section along
its whole length it is furthermore preferred that the hot
fluid is injected via an injection well in the region of
the toe of the inflow section of the production well.
The invention also relates to a well assembly for
producing viscous oil from an underground oil bearing
formation.
The assembly comprises a hot fluid injection well and
a plurality of oil production wells. The toes of the
inflow sections of these production wells are located at
substantially equal distances from the injection well and
extend in a substantially radial direction away from the
injection well. This star-shaped well configuration
ensures an uniform distribution of the injected hot fluid
over the horizontal sections of the production wells and
creates an efficient stimulation system. In the system a
single injection well is able to warm-up the inflow
sections of a plurality of production wells that extend a
substantial distance away from the injection well.
These and other objects, advantages and embodiments
of the well and well system according to the invention
will become apparent from the accompanying claims,
abstract and drawing in which
Fig. 1 shows a schematic vertical cross-section of a
well system and wells according to the invention.
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In Fig. 1 there is shown a central injection well 1
for injecting hot fluid in the form of steam into an
underground viscous oil bearing formation 2 as
illustrated by arrows I. Apart from the injection well 1
there are two J-shaped production wells 3 according to
the invention which comprise substantially horizontal or
inclined inflow sections 3A. The sections 3A are
perforated or equipped by a slotted liner to allow the
inflow of fluids from the surrounding formation as
illustrated by arrows II.
The injected steam will form a pocket of steam
against the top of the oil bearing formation 2 thereby
heating up the formation 2, decreasing the viscosity of
the crude oil within the pores of the formation 2 and
driving the thus mobilized crude oil towards the inflow
sections 3A of the production wells 3.
The present invention is based on the insight that if
steam or still hot condensate is permitted to flow
together with the mobilized crude oil into the regions of
the toes 4 of the inflow sections 3A it will warm-up the
inflow sections 3A and stimulate the inflow of crude oil
into the wells 3.
The flow rate and pressure of the injected steam are
adjusted such that at the entry of the downhole pump 5,
which is located in the elbow of each well 3, the
produced mixture of crude oil and condensate has a
pressure below the steam flash point in order to avoid
that the condensate would re-evaporate at the entry of
the pump and create a vapour lock.
In the event that steam would tend to bypass the
crude oil in one of the wells 3 and this well would
produce a mixture with a too high water cut and/or at a
too high temperature the toe of the inflow section 3A is
plugged to reduce the steam intake. Instead of or in
addition to such plugging back of the toe 4 of a well 3
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the quantity and pressure of injected steam may be
reduced in order to reduce the water cut of the produced
mixture.
It will be understood that the presence of an
upwardly dipping toe 4 in each well 3 will considerably
reduce the risk of a steam break-through in the event
that steam or condensate is permitted to flow into the
well 3.
However, if necessary, substantially horizontal
inflow sections could be employed if these sections are
of sufficient length to ensure that the steam will
condensate within these horizontal sections and the
pressure of the produced mixture of crude oil and
condensate has a pressure below the steam flash point at
the intake of the downhole pump.
