CA2043092A1 - Electrical heating of oil reservoir - Google Patents
Electrical heating of oil reservoirInfo
- Publication number
- CA2043092A1 CA2043092A1 CA 2043092 CA2043092A CA2043092A1 CA 2043092 A1 CA2043092 A1 CA 2043092A1 CA 2043092 CA2043092 CA 2043092 CA 2043092 A CA2043092 A CA 2043092A CA 2043092 A1 CA2043092 A1 CA 2043092A1
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- CA
- Canada
- Prior art keywords
- reservoir
- bottom electrode
- electrode
- underburden
- overburden
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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Abstract
"ELECTRICAL HEATING OF OIL RESERVOIR"
ABSTRACT OF THE DISCLOSURE
A well is completed with the steel casing landed at the overburden/reservoir interfacing said casing being conductively coupled with the overburden. An insulated tubular liner, suspended from the foot of the casing, extends through the oil reservoir and comprises a ported, short electrode positioned just above the reservoir/underburden interface. Insulated tubing is coupled with the electrode. Electrolyte is pumped into the reservoir through the prot of the electrode, to emplace a conductive body of liquid that laterally extends the electrode.
Low frequency current is supplied to the enlarged electrode through the tubing, passed upwardly through the near-bore region of the reservoir and returned through the casing, to thereby heat the near-bore region of the reservoir and returned through the casing, to thereby heat the near-bore region of the reservoir.
ABSTRACT OF THE DISCLOSURE
A well is completed with the steel casing landed at the overburden/reservoir interfacing said casing being conductively coupled with the overburden. An insulated tubular liner, suspended from the foot of the casing, extends through the oil reservoir and comprises a ported, short electrode positioned just above the reservoir/underburden interface. Insulated tubing is coupled with the electrode. Electrolyte is pumped into the reservoir through the prot of the electrode, to emplace a conductive body of liquid that laterally extends the electrode.
Low frequency current is supplied to the enlarged electrode through the tubing, passed upwardly through the near-bore region of the reservoir and returned through the casing, to thereby heat the near-bore region of the reservoir and returned through the casing, to thereby heat the near-bore region of the reservoir.
Description
~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.
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.
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 ~ ,) 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 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.
:
~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:
2 ~3 ~
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 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 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.
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.
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 ~ ,) 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 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.
:
~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:
2 ~3 ~
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 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 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.
Claims (4)
1. A well assembly system for electrically heating heavy oil, present in a subterranean reservoir and into which liquid may be injected at less than fracturing pressure, said reservoir being contiguous with a relatively conductive overburden and underburden, comprising:
(a) a source of low frequency alternating current power located at ground surface;
(b) a wellbore penetrating the reservoir to about the reservoir/underburden interface;
(c) a conductive casing string extending to about the reservoir/overburden interface, said casing string being electrically coupled with the power source and the overburden;
(d) a tubular bottom electrode positioned in the wellbore adjacent to but above the reservoir/underburden interface, said bottom electrode having at least one port extending through its sidewall and a length that is only a portion of the vertical extent of the reservoir;
(e) means extending through the wellbore, for electrically coupling the power source and the bottom electrode;
(f) means, extending through the wellbore between the bottom electrode and the casing string, for electrically insulating the former from the latter;
(g) means for electrically insulating the means recited in paragraph (e) from the casing string;
and (h) a body of electrolyte liquid having been injected into the reservoir through the bottom electrode port at an elevation adjacent to but above the reservoir/underburden interface, said body being operative to electrically couple the bottom electrode and the reservoir.
(a) a source of low frequency alternating current power located at ground surface;
(b) a wellbore penetrating the reservoir to about the reservoir/underburden interface;
(c) a conductive casing string extending to about the reservoir/overburden interface, said casing string being electrically coupled with the power source and the overburden;
(d) a tubular bottom electrode positioned in the wellbore adjacent to but above the reservoir/underburden interface, said bottom electrode having at least one port extending through its sidewall and a length that is only a portion of the vertical extent of the reservoir;
(e) means extending through the wellbore, for electrically coupling the power source and the bottom electrode;
(f) means, extending through the wellbore between the bottom electrode and the casing string, for electrically insulating the former from the latter;
(g) means for electrically insulating the means recited in paragraph (e) from the casing string;
and (h) a body of electrolyte liquid having been injected into the reservoir through the bottom electrode port at an elevation adjacent to but above the reservoir/underburden interface, said body being operative to electrically couple the bottom electrode and the reservoir.
2. The system as set forth in claim 1 wherein:
the means (g) comprises a non-conductive tubular member extending from the base of the casing string to the base of the wellbore, said tubular bottom electrode being carried by the member between its ends, whereby the upper section of the member electrically insulates the bottom electrode from the casing string and the bottom section of the member electrically insulates the bottom electrode from the underburden.
the means (g) comprises a non-conductive tubular member extending from the base of the casing string to the base of the wellbore, said tubular bottom electrode being carried by the member between its ends, whereby the upper section of the member electrically insulates the bottom electrode from the casing string and the bottom section of the member electrically insulates the bottom electrode from the underburden.
3. A method for heating the near-bore region of an oil-containing reservoir penetrated by a well assembly having a casing string extending to about the overburden/reservoir interface and being electrically coupled with the overburden, a tubing string extending to about the reservoir/underburden interface, a tubular, ported bottom electrode positioned adjacent to but above the reservoir/underburden interface, said bottom electrode having a length that is only a portion of the vertical extent of the reservoir, said bottom electrode being electrically coupled with the tubing string, and said tubing string and bottom electrode being electrically insulated from the casing string, comprising: injecting a body of liquid electrolyte radially into the reservoir through the bottom electrode port at less than fracturing pressure to form a conductive lateral extension of the electrode, said body functioning to electrically couple the electrode with the reservoir; and applying a low frequency current between the so-extended electrode and the casing string to heat the reservoir.
4. The method as set forth in claim 3 wherein:
the frequency of the current applied is between about 6 and 60 Hz.
the frequency of the current applied is between about 6 and 60 Hz.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2043092 CA2043092A1 (en) | 1991-05-23 | 1991-05-23 | Electrical heating of oil reservoir |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2043092 CA2043092A1 (en) | 1991-05-23 | 1991-05-23 | Electrical heating of oil reservoir |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2043092A1 true CA2043092A1 (en) | 1992-11-24 |
Family
ID=4147643
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2043092 Abandoned CA2043092A1 (en) | 1991-05-23 | 1991-05-23 | Electrical heating of oil reservoir |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2043092A1 (en) |
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-
1991
- 1991-05-23 CA CA 2043092 patent/CA2043092A1/en not_active Abandoned
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|---|---|---|---|---|
| US8172335B2 (en) | 2008-04-18 | 2012-05-08 | Shell Oil Company | Electrical current flow between tunnels for use in heating subsurface hydrocarbon containing formations |
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| US8162405B2 (en) | 2008-04-18 | 2012-04-24 | Shell Oil Company | Using tunnels for treating subsurface hydrocarbon containing formations |
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| US8881806B2 (en) | 2008-10-13 | 2014-11-11 | Shell Oil Company | Systems and methods for treating a subsurface formation with electrical conductors |
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| US8752623B2 (en) | 2010-02-17 | 2014-06-17 | Exxonmobil Upstream Research Company | Solvent separation in a solvent-dominated recovery process |
| US8684079B2 (en) | 2010-03-16 | 2014-04-01 | Exxonmobile Upstream Research Company | Use of a solvent and emulsion for in situ oil recovery |
| US8833453B2 (en) | 2010-04-09 | 2014-09-16 | Shell Oil Company | Electrodes for electrical current flow heating of subsurface formations with tapered copper thickness |
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| US8899321B2 (en) | 2010-05-26 | 2014-12-02 | Exxonmobil Upstream Research Company | Method of distributing a viscosity reducing solvent to a set of wells |
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| FZDE | Dead |