Note: Descriptions are shown in the official language in which they were submitted.
~3~3~3~2
1 Field of the Inventi.on
2 The present invention relates to a method and well
3 assembly system for electrically heating the near-bore zone of
4 a well penetrating an oil reservoir, to improve the mobility of
the oil moving through the zone.
6 BACKGROUND OF THE INVENTION
7 Electrically heating an oil reservoir has been
8 investigated by many organizations and individuals. In most of
9 these instances, two electrodes or terminals were provided
downhole in spaced apart arrangement and current was caused to
11 move between the electrodes through the resistive oil-containing
12 reservoir, thereby heating the zone through which the current
13 flowed.
14 In greater detail, a typical circuit for this purpose
has involved:
16 - a source of electrical power, located at ground
17 surface;
18 - a pair of laterally spaced wells penetrating the
19 oil reservoir;
- a conductor and attached electrode positioned in
21 ` each well with the electrode usually being
22 coextensive with the vertical extent of reservoir
23 exposed by the wellbore, saia electrodes each
24 being electrically coupled with the power source
` and the reservoir; and
26 - suitable means for grounding the circuit at
27 surface.
1 In use, current was passed through the circuit, hope~ully through
2 the lateral stretch of reservoir extending between the
3 electrodes.
4 A problem with this dual well approach stems from the
fact that almost invariably the strata immediately above and
6 below the reservoir (which strata are herein referred to as the
7 "overburden" and "underburden") are far more conductive than the
8 reservoir. The current therefore has a tendency to move into and
9 through the overburden and underburden and to heat them instead
of the reservoir. This result worsens as heating of the
11 overburden and underburden progresses and increases their
12 conductivity.
13 So one objective of the research underlying the present
14 invention was to seek to ensure that the current flow would pass
through the reservoir instead of the conductive overburden and
16 underburden.
17 For another reason, the research was centered on
18 seeking to confine the electrical heating within the near-bore
l9 zone of a single well. More particularly the system to be
developed was intended to be applied to a heavy oil reservoir
~1 whose oil is inherently capable of moving only slowly toward the
22 wellbore. Such reservoirs, for example, exist in the
23 Lloydminster area of Alberta. What can happen in such reservoirs
24 is that light ends, in solution in the heavy oil in the reaches
of the reservoir remote from the wellbore, come out of solution
26 as the oil nears the low pressure sink provided by the wellbore.
27 As a result, the remaining liguid fraction increases in
28 viscosity. This can result in a blockage of oil flow in the
29 near-bore region. (The~phrase "near-bore region" is intended to
1 relate ~o that portion of the reservoir extenaing out radially
2 to about 5 meters from the wellbore.)
3 So it was another objective of the research to provide
4 a single well electrical heating system designed to heat the
near-bore region.
6 Another problem which was addressed by the applicants
7 had to do with distributing the current flow radially outwardly
8 from the wellbore through a lateral extent. If this is not done,
9 the current density may be too high and conductive water present
in the formation may form steam, thereby raising the resistivity
ll of the reservoir in the current flow zone, perhaps unduly.
12 So it was another objective of the research to provide
13 a system in which the current flow could be extended and
14 distributed laterally in a controlled fashion.
Still another problem which was considered had to do
16 with ensuring good electrical coupling between the electrodes and
17 the reservoir.
18 With this background in mind, the invention will now
19 be described.
SUMMARY OF THE I~VENTION
21 The present invention is based on a concept comprising
22 the combination of:
23 - using vertically spaced and substantially linearly
24 aligned electrodes positioned in a single
wellbore, said electrodes being located adjacent
26 the upper and lower extremities of a reservoir to
27 be heated;
1 _ said electrodes having a significant but limited
2 lateral extent; and
3 - applying low fxequency current (preferably between
4 about 6 - 60 Hz);
to thereby heat the near-bore region with a current flow of
6 controlled density.
7 When an assembly embodying this concept was tested in
8 the laboratory, it was found:
9 1. That the higher electrically conductive over
burden and underburden acted as extended
11 electrodes, whereby current flowing through these
12 regions caused little heating there, and that the
13 electrical energy conversion to heat occurred
14 mainly in the oil sand reservoir between the
extended electrodes;
16 2. That the lateral extent of heating was found to
17 be greater than other embodiments of electrical
18 heating investigated in the laboratory and
19 presented in the prior art; and
3, That the oil sand reservoir was uniformly heated
21 throughout with little heat losses to the
22 overburden and underburden. In fact, the higher
23 temperatures were observed in the middle of the
24 reservoir.
Having experimentally made these findlngs, the
26 present invention was conceived. More particularly, in
27 accordance with one aspect of the invention, a well assembly
28 system is provided comprising:
o ~ ~
1 - A source of low frequency power. This source is
2 located at ground surface. The lo~ frequency
3 power is needed to reduce electromagnetic losses
4 in the casing, and tubing (or cables) so as to
S improve the ove.rall energy efficlenoy of the
6 process;
7 - A conductive casing string extending down to about
8 the reservoir/overburden interface. The casing
9 string is electrically coupled with each of the
power source and the overburden;
11 - An electrically insulating, tubular liner
12 extending down through the balance of the wellbore
13 from the foot of the casing string, said liner
14 extending across part of the vertical extent of
the reservoir;
16 - A tubular bottom electrode forming an intermediate
17 part of the liner and being positioned in the
18 wellbore across a short reservoir interval, said
19 interval being located adjacent to but above the
reservoir/underburden interface. The bottom
21 electrode has an operative length that is only a
22 portion of the vertioal extent of thé reservoir
~3 itself. The liner has one or more ports extending
24 through its side wall at the bottom electrode,
sald ports preferably extending through the bottom
26 electrode itself;
27 - A conductive member, such as the tubing string or
28 a cable, eleotrically insulated from the casing
29 string and a-y flu d disposed in the wellbore.
:
1 The conductive member is electrically coupled with
2 each of the power source and the bottom electrode;
3 - Th,e lower most portion of the liner being
4 operative to electrically insulate the bottom
electrode from the underburden; and
6 - A body of electrolyte liquid which has been
7 injected generally radially out into the reservoir
8 through the liner ports, said liquid body being
9 electrically coupled with the bottom electrode.
From the foregoing it will be noted that:
11 - An artificially emplaced body of liquid
12 electrolyte, well coupled with the bottom
13 electrode because of use of the electrode ports
14 to emplace it, provides a lateral extension of the
bottom electrode. This relatively broad,
16 composite bottom electrode combines with the broad
17 upper composite electrode formed by the casing
18 string in combination with the overburden, to
19 yield a desirable broad pattern of current flow
through the reservoir in the near-bore region.
21 - The current fIow is substantially prevented from
22 entering the underburden by positioning the bottom
23 electrode within the reservoir interval and
24 preferably insulating it electriaally from the
underburden with insulative means, namely the
26 closed bo,ttom end of the insulating liner;
27 - The ~bottom electrode vertical length is short,
28 preferably being only a minor portion of the
29 vertical extent of the reservoir. The use of a
~, ~3 '.L ..~
1 shor~ bottom electrode assists in reducing end
2 effects normally associated with a long electrode,
3 which en~ affects might dominate the current
distributlon. In addition there is higher
electrical efficiency as the electrode resistance
6 is greater;
7 - The casing electrode is effectively electrically
8 insulated from the bottom electrode by the
9 intervening non-conductive liner;
- Because of the lateral extension of the bottom
11 electrode using liquid electrolyte, it is possible
12 to heat the reservoir with a shorter and therefore
13 more efficient electrode;
14 - Low power frequency is utilized to reduce losses
in the power delivery and ground systems; and
16 - The two electrodes are located in vertical
17 alignment in a single well, with one electrode at
18 the base of the reservoir and the other at the
19 top. As a result the current flow is vertical and
confined to the reservoir. There is no
21 opportunity for the current to move laterally and
22 widely through either the underburden or
23 overburden.
24 In accordance with another aspect of the invention, a
method is provlded for electrically heating the near-bore region
26 of an oil-containing reservoir penetrated by a well assembly
27 having a ~casing string extending to about the
28 overburden/reservoir interface an~ being electrically coupled
29 with the overburden, a tublrg ,tring exter~ding to about the
2 ~3 ~ 3 2
1 resexvoir/underburden interface, a tubular, ported, bottom
2 electrode positioned adjacent to but above the
3 reservoir/underburden interface, said bottom eleatrode having a
4 length that is only a portion of the vertical extant of the
reservoir, said bottom electrode being electrically coupled with
6 the tubing string and the reservoir, and said tubing string and
7 bottom electrode being electrically insulated from the casing
8 string and the underburden, comprising:
9 - injecting a body of liquid electrolyte radially
into the reservoir through the bottom electrode
11 at less than fracturing pressure to form a
12 . conductive lateral extension of the electrode; and
13 - then applying a low frequency current between the
14 so-extended between electrode and the casing
string to heat the reservoir.
16 The invention will now be described with reference to
17 the preferred embodiment.
18 DESCRIPTION OF THE DRAWINGS
19 Figure 1 is a diagrammatic elevational representation
of a well assembly system in accordance with the invention;
21 Figure 2 is a ssctional plan view of the electrode;
22 Figure 3 is a sectional schematic plan view of the
23 insulating liner and packer;
24 Figure 4 is a schematic perspective view showing the
experimental model used in the research; and
26 Figure 5 is a diagram illustrating the temperature
27 distribution within the reservoir along a horizontal plane, after
28 heating had been conducted in the model of Figure 4 for a period
29 of time.
,3 ~3 ~ 1 . 3 ~ ,)
1 DESCRIPTION OF THE PREFERRED EMBODIMENT
2 The well assem~ly W comprises a well bore 1 which
3 extends from ground surface 2 through the overburden 3 and oil-
4 containing reservoir 4 and penetrates into underburden 5.
The resistivity of the reservoir 4 is appreciably
6 greater than that of the overburden 3 and underburdsn 5.
7 Typically the respective resistivities might be approximately 50
8 ohm. meters for the reservoir and 1 or 2 ohm. meters for the
9 overburden and underburden.
A conventional tubular steel casing string 6 is landed
11 jus~ above the overburden/reservoir interface. The casing string
12 6 is cemented in place in conventional fashion, as indicatèd by
13 the numeral 7. The casing string 6 is conductive and is
14 inherently electrically coupled with the overburden 3. The
casing string 6 combines with the overburden 3 to create an
16 electrode assembly having vertically and laterally extending
17 segments to form the ground return system.
18 A non-conductive tubular liner 8 extends downwardly
l9 from the base of the casing string 6 to the base of the wellbore
1. A hanger 9 suspends the liner 8 from the casing string 6.
21 The liner typically is formed of fibre-glass and has perforations
22 10 for the ingress or production of oil, as identified by the
23 arrows A.
24 Between its snds and located at the base of the
reservoir 4 but above the reservoir/underburden interface 11,
26 the liner 8 carries a tubular bottom electrode 12. The bottom
27 electrode 12 has ports 13, for injecting a body 20 of liquid
28 electrolyte into the reservoir 4.
.
?~ 3
1 As shown, the liner 8 compxises a lower section 14
2 extending below the bottom electrode 12 and an upper section 15
3 extending up to the casing string 6. The liner upper section 15
4 functions to electrically insulate the electrode 12 from the
casing string 6 and the liner lower section 14 serves the same
6 function relative to the underburden 5.
7 A steel tubing string 16 is landed adjacent the base
8 of the liner 8. The tubing string 16 is connected with the
9 bottom electrode 12 by a contactor 17, so that the tubing string
provides a conductive vertical extension of the electrode. The
11 tubing string 16 is clad with an electrical insulating layer 18
12 to electrically insulate the tubing, casing, and annular fluids
13 that may be conductive, from each other.
14 The casing string 6 and tubing string 16 are suitably
connected at ground surface 2 with a source 19 of low frequency
16 alternating current.
17 The tubing string 16 is further connected at surface
18 with means, such as a storage tank O, for receiving produced
l9 fluid and means, such as a tank ~, for supplying liquid
electrolyte.
21 Conventional rods and a downhole pump (not shown) may
22 be introduced into the tubing string 16 to pump produced fluids
23 to surface as required.
24 In the course of installation of the assembly, the
lower electrode 12 is first perforated to create the perforations
26 of ports 13. Electrolyte liquid is then injected down the tubing
27 16 and through the perforations 13 into the lower portion of the
28 reservoir 4, as indicated by the arrows B, to create the
11
2 ~
1 conductive body 20. The liner 8 is then further perforated to
2 provide ports 13a for the ingress of produced oil.
3 With the previously described well assembly, it will
4 be understood that:
- Liquid electrolyte, such as brine, may be pumped
6 down the tubing string 16 and out into the
7 reservoir 4 at less than fracture pressure in a
8 generally radial direction through the electrode
9 ports 13, whereby a laterally widened bottom
electrode assembly is provided. This electrode
11 system comprises the tubing string 16, contactor
12 17, bottom electrode 12 and a thin layer or body
13 20 of liquid electrolyte;
14 - Since the electrolyte is introduced through a
point source (the ports 13) located above but
16 close to the reservoir/underburden interface 11,
17 the body 20 of electrolyte is located in the
18 reservoir 4 and at its base. The lateral extent
19 of the body 20 can be varied by varying the volume
of electrolyte injected into the reservoir in this
21 fashion;
22 - Thus low frequency alternating current (as
23 indicated in Figure 1 by the broken lines C) may
24 be applied to the reservoir via the laterally
projecting, vertically spaced, upper and lower
26 electrode assemblies, to thereby heat the near-
27 bore region (which might extend laterally in the
28 order of 5 meters) of the reservoir.
12
:
~3 ~ 'f.~3~
1The invention is supported by the following laboratory
2 example.
3 Example
4The invention was developed on the basis of laboratory
scale experiments using a model 100 shown in Figure 4.
6The model 100 comprised a wooden box inlaid with slabs
7 of rigid styrofoam, to provide thermal and electrical insulation.
8The slabs formed a chamber 103 which was packed with layers 104,
9 105, lQ6 of underburden, oil sand and overburden, as described
below. The pertinent dimensions were as follows:
11Chamber
12height: 0.71 m
13width: 0.31 m
14length: 0.41 m
15Upper and lower copper electrodes 107,108, each having
16a diameter of 0.2 meters, were positioned in the chamber 103 in
17 vertically spaced alignment. The electrodes were 0.18 meters
18 apart so as to correspond with the reservoir/underburden and
19overbu~den/reservoir interfaces 109, 110. The surfaces of the
electrodes were covered with blotting paper dampensd in an
21 electrolyte consisting of 1 mol aqueous solution of copper
22 sulphate.
23The lower electrode 108 was attached to and
24 electrically coupled with a conductive metal rod 111 having an
outer electrlcally insulative coating 112. The upper electrode
26 107 was attached to and electrically coupled with a conductive
27 metal tube 113 having an outer electrically insulative coating
28114.
2 ~ 2
1 Bituminous sand ("oil sand") from the Fort McMurray
2 region was used to simulate the reservoir. Bitumen-free sand
3 saturated with brine to a pre-determined conductivity was used
4 to simulate the overburden and underburden. The bitumen-free
sand was from tailings from a hot water process extraction plant.
6 Each of the sand batches was mixed and stones and clay
7 lumps were removed.
8 The packing of the model was performed manually with
9 the aid of an electric hammer. Densities in the range of 1.9 to
2.0 g/cm3 for the oil sand were obtained, whereas the overburden
11 and underburden densities ranged from 1.6 to 1.7 g/cm3.
12 The thicknesses of the packed layers of the chamber
13 103 were as follows:
14 underburden: 0.36 m
reservoir: 0.18 m
16 overburden: 0.17 m
17 The measured electrical and thermal properties of the
18 reservoir, underburden and overburden layers are shown in Table
19 1:
14
2 ~3 ~
1TABLE 1
2ELECTRICAL AND THERMAL PROPERTIES OF THE MATERIALS USED
3IN THE EXPERIMENTAL SIMULATOR MODEL
4 Material
Properties Unit Overburden Oil Sand Underburden
6 Electrical
7 Conductivity [103S/m] 11.3 1.21 11.3
8 Thermal
9 Conductivity [W/mc] 1.73 1.51 1.73
Heat Capacity [10 J/m C] 1.57 1.81 1.57
11 Density [10 kg/m ] 1.60 1.96 1.
12 The electrodes 107, 108 were installed in the model
13 chamber during packing.
14 The rod 111 and tube 113 were electrically coupled
with a variable transformer 115 (230 V/2300V, 7.5 kVA). The
16 transformer was operated to supply power at 60 H~.
17 The voltages, currents and powers delivered were
18 monitored by a Fluke 8000 A digital voltmeter and a Feedback
19 EW-604 electronic wattmeter and by a data acquisition system
described below.
21 The temperatures at various locations in the
22 reservoir, overburden and underburden were monitored by Typa
23 J thermocouples. The thermocouple ends were protected with
24 stainless steel sheaths 0.15 to 0.30 meters long and 1 mm in
diameter. The sheaths were electrically insulated from the
26 thermocouples and were kept at ground potential. The sheaths
27 were covered with nylon tubing to prevent shorting of the model
28 contents via the thermocouple sheaths.
3 7~ ~
1 The inputs from the thermocouples as well as the
2 voltage, current and power delivered to the model were
3 monitored and recorded by an HP3052A data logging system using
4 an HP9825A desk-top controller with 24k memory. Additional
recording support was made available with an HP7245A printer-
6 plotter and an HP9872A x-7 plotter.
7 During the period of the time the model was heated,
8 voltage, current and temperature were automatically recorded
9 at predetermined intervals. The interval between the
temperature readings was small at the beginning of a run to
11 permit finer temperature resolution of the heating during the
12 time when the temperature near the electrodes was increasing
13 most rapidly. The readings were taken at time tn = c(n+O.ln2),
14 where n=1,2, is the index of the reading and the constant
c was typically 30 to 60 seconds.
16 The data from the modelling was recorded by the
17 HP3052A data acquisition system and stored on magnetic tapes.
18 Voltage, current and power were continuously displayed on the
19 HP9852A controller. Some temperature information was also
printed out after each recording interval to enable the
21 operator to monitor the progress of the experiment. Power
22 levels wers adjustable manually at any time during the run.
23 At high power levels it might be possible to cause
24 rapid overheating adjacent to the electrode with the result
that the electrolyte would evaporate and electrical contact
26 between the electrodes, or a portion o~ the electrodes, and
27 the model formation would be lost. The onset of such a
28 condition could be detected by monitoring the changes in
29 temperature near the electrode and the changes in the
16
2~ 2
1 resistance of the formation. If an electrode were to become
2 ~boiled off', the damage would be permanent and it would be
3 necessary to repack the model. However, by suitably
4 controlling the power level, this difficulty was avoided.
The model was operated at a constant level of 777
6 volts. This provided an initial rate of heating of S00 watts.
7 In total 0.72 XWhrs of electrical energy were delivered to the
8 experimental simulator model. Approximately 75% of this energy
9 input was stored in the central oil sand layer or pay-zone
representation at the end of the heating period.
11 Heating, commencing at 22C near the bottom
12 electrode, was found to be very uniform with temperatures
13 approaching 75C to 80C throughout the central oil sand layer
14 o~ the experimental model and tapering off towards the sides
of the model. The temperature distribution in the model at the
16 end of the heating period is shown in Figure 5.
17 Most of the pay-zone within the experimental model
18 was heated to at least 50C with negligible heat losses into
l9 the overburden and underburden being observed.
Table 2 shows the parameters of the experimental
21 simulation model and two corresponding sets of predicted data
22 for commercial or field conditions using a scaling factor of
23 100 and 150, respectively.
2 ~ ) 2
1 TABLE 2
2PARAMETERS OF THE EXPERIMENTAL MODEL
3AND CALCULATED FIELD APPLICATIONS
4Field Applications
Parameter UnitModel A B
6 Scaling
7 Factor p 1 100150
8 Unit
9 Length L[m]0.41 41 62
Unit
11 Width w[m]0.41 4162
12 Oil Sand
13 Thickness t[m]0.18 1827
14 Electrode
Separation [m] 0.20 20 30
16 Electrode
17 Radius [m]0.10 1015
18 Voltage V[V]777 777777
l9 Current I[A] 0.65-1.20 65-120 98-180
Power P[kW] 0.50-0.93 50-93 75-140
21 Impedànce z[ohms] 1195-648 12-6.5 8-4.3
22 Heating
23 Time t 1 hr 1.14 yr 2.57 yr
24 Energy
Delivered [MWhr] 0.00072 720 2430
26The scope of the invention is set forth in the alaims
27 now following.
