Note: Descriptions are shown in the official language in which they were submitted.
2 ~ 2 9
ELECTRICAL HEATING SYSTEMS FOR
LOW-COST RETROFITTING OF OIL WELLS
Major problems exist in producing oil in heavy-oil reservoirs because of
the high viscosity of the oil. Because of this high viscosity oil, a very
high pressure gradient builds up around the wellbore, t;hereby utilizing almost
two-thirds of the reservoir pressure in the immediate ~icinity of the
wellbore. Furthermore, as the heavy oils progress inwardly to the wellbore,
gas in solution evolves more rapidly into the wellbore. Since the dissolved
gas reduces the viscosity, this evolution further increases the v~scosity of
the oils in the immediate vicinity of the wellbore. Such viscosity effects,
especially near the wellbore, greatly impede production, and the resulting
wasteful use of reservoir pressure can reduce the overall primary recovery
from such reservoirs.
S;milarly, in light-oil deposits, dlssolved paraffin in the ai1 tends to
accumulate around the wellbore, particularly in the screens and perforations
and within the deposit up to a few feet from the wellbore. Th1s precipitation
effect is caused by the evolution of gases and volatiles as the oil progresses
into the vicinity of the wellb~re, thereby decreasing the solubility of
paraffin and causing it to precipitate. Also, the evolution of gases causes
an auto-refrigeration ef~ect which reduces the temperature, thereby decreasing
the solubility of the paraffins. Similar to paraffin, other condensable
constituents can also plug up, coagulate, or precipitate near the wellbore.
These include gas hydrates, asphaltenes, and sulfur. In the case of certain
gas wells, liquid distillates can accumulate in the immediate visinity of the
wellbore. Such accumulation reduces the relative permeability near the
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wellbore. In all such cases, such near wellbore accumulations reduce
production rates and reduce ultimate primary recoveries.
Electrical resistance heating has been employed to heat the reservoir in
the immediate vicinity of the wellbore. This has been the subject of recent
pilot tests. Basic systems are described in Bridges lJ.S. Patent No. 4,524,827
and in Bridges, et al U.S. Patent No. 4,821,798. Such systems are applicable
largely for new wells. Prior to installation, some modifications of casing
near the wellbore are usually needed to permit electrical resist~nce heating
in the reservoir near the wellbore. For a cased-hole completion, the ~-
electrode which is in the reservoir must be isolated from the casing by
fiberglass tubing above and below the electrode as discussed in Bridges et al ~
U.S. Patent No. 4,821,798. ~-In the case of open-hole completions, considerable modification of the
downhole screen and near reservoir casing and tubing is required. For
existing wells, the old gravel pack and screens must be removed and a new
gravel pack and screen syste~ installed so that an electrically isolated
electrode can be positioned ;n the deposit. Such electrode may be part of the
gravel pack and screening system.
Such near wellbore heating systems have been demonstrated to massively
heat the reservoir just outside the wellbore and to reduce or eliminate many
of the aforementioned thermally responsive flow impediments. Such elimination
can result in demonstrated flow increases of 200 to 400YO. These procedures
are used primarily in new well installations for cased-hole completions, but
can be also used for either new open-hole completions or to retrofit existing
wells with open-hole completions.
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However, open-hole modifications are largely limited to either new wel1s
or existing wells that have a very high flow rate, because the cost of
installing either a new well or repacking an existing open-hole completed well
with a new electrode assembly and gravel pack system is large.
What is desired, then, is a method of retrofitting old wells, either
cased or open-hole completions, which is inexpensive and yet heats some of the
reservoir in the immediate vicinity of the wellbore adjacent to the formation
as well as within the wellbore itself. One method of doing this has been
attempted before with a mixed degree of success. This technique employs the
use of cylindrical resistance heaters which are coaxially situated in the
wellbore and are positioned in the wellbore immediately adjacent to the
reservoir. The earliest patent in the literature on this subject matter was
issued in July of 1865 in U.S. Patent No. 48,584 which described as an
electric oil well heater~ Since then, numerous patents have been lssued whtch
have covered this type of ins;de the wellbore heating. Such past art includes
Pershing U.S. Patent No. 1,464,618, Stegemeier U.S. Patent No. 2,932,352,
McCarthy U.S. Patent No. 3,114,417, Williams U.S. Patent No. 3,207,220 and Van
Egman et al U.S. Patent No. 4,704,514. Such systems, heating inside the
wellbore, received considerable attention in the 1950s and early 1960s, with
some improvements reported in some reservoirs and other reservoirs showing
mixed results. One pr;ncipal difficulty encountered with such heaters was
that they burned out at intervals so frequent that their use could not be
justified. Though some of the causes of the failure of these resistors were
due to poor designs, some fundamental problems also exist which contr;buted to
the burn-out problem. ;
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The useful heat supplied by the cylindrical resistor flows out of the
wellbore and into the formation by thermal conduction. At the same time,
unavoidably, the flow of fluids inwardly into the wellbore removes, via
convection, transfers heat transferred by convection from the formation toward
the producing well. In the wellbore itself, the heat 1s further unavoidably
~ removed from the annular space between the heater and the screen or casing,via convection caused by the upward flow of oil in the well. Therefore, in
order to achieve a noticeable increase in temperature just outside of the
wellbore, very high heater temperatures were required. Such high heater
temperatures may also be accompanied by the deposition of scale or products of
low temperature pyrolysis on the heater. This further thermally ~solates the
heater, thereby causing requirements for even higher resistor temperatures,
wh;ch further compounds the problem. As a consequence of this fundamental
counter flow heat problem between outward thermal dlffusion and inward thermal
convection, such an approach would be effective only in slowly praducing wells
and would become decreasing less effective as the flow rate was increased much
above a few tens of barrels per day for typical installations.
One method to mitigate the aforementioned problem would be to create a
situation such that the casing itself, in the completed zone, would provide
the heat. Alternatively, for an open-hole completion, the screen and/or
gravel pack might preferably provide the heat rather than a small diameter
cylindrical resistor element coaxially located within the wellbore next to the
producing zone. By so doing, the radius of the heat producing element or
resistor could be extended from approximately 1 in. out to about 8 in.,
depending on the diameter of the wellbore or screen in the completed zone.
Such an arrangement would give at least a four-fold improvement in the amount
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of heat which could be transferred based on a given temperature of the heated
element. In additian, such an arrangement would eliminate in the annulus
convection heat losses in the annulus due to the upward thermal convection of
the fluids once they entered into the wel1bore itself.
Practical, efficient and economical methods of installing a casing
heating system in existing wells in the immediate vicinity of the producing
formation by electrical energy have not been disclosed. However, the
technique has been ineffectively addressed in two U.S. patents; 1) by A. W.
Marr in U.S. Patent No. 4,319,632 and 2) by S. D. Sprong in U.S. Patent No.
2,472,445. In either case, no system is adequately described which embodies
the use of such casing heating systems and which is combined with an efficient
downhole power delivery and control system. For example, in the case of Marr,
the electrical heating system had one electrical contact with the casing at
the surface and the other contact in the producing zone. As a consequence,
current flowed from the bottom of the casing up along the entire surface,
thereby heat;ng the entire casing string and ad~acent formations. Such a
system is qu;te inefficient, especially if high temperatures are desired. In
the case of Sprong, the system heated the casing by use of an induction eddy-
current type heating applicator. However, the applicator as described had a -
large air gap between the applicator and the casing and, as a consequence, the
reactive or industive component was large, thereby creating a low power factor
load on the power cable delivery system. Such low power factors result in
inefficient delivery of power.
For aboveground equipment, any low power factor load which has modest
power consumption (e.g., a few tens of kilowatts), and which is paired with
high power factor higher power systems does not pose a problem. However, it
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is not readily recognized that delivering power over a half mile distance to a
downholE load with a low power factor does represent a major power delivery
problem and can result in cable overheating losses, cable breakdown, and other
undesirable problems, especially if loads are in the order of tens o$
kilowatts or more. It also represents a less efficient method of power
delivery.
Marr and Sprong do not address the issue of choosing operating parameters
and the required additional subsystems or operation conditions that permit
efficient power delivery. Such operating parameters include proper selection
of the electrical waveform or frequency or proper locating and design of the
casing wall heating tool. Additional subsystems (which may include a downhole
matching network and control apparatus) are needed to prevent for~ation damage
due to deposit;on of pyrolysis prod~cts of the inco0ing liquids in the
immediate vicinity of the borehole and especially on the screens ar
perforations.
SUMMARY OF TNVENTION
The overall objective of the present invention is to describe economic
and practical methods and apparatus which can be used to economically retrofit : ~ ~
existing wells with casing or screen heating systems. ~ ~ ;
It is the technical objective of the present invention therefore to
provide a new and improved efficient pdwer delivery system which, when
combined with an improved downhole apparatus, selectively heats the casing or ~;
screen immediately adjacent to the formation. Another objective is to
describe two generic casing heating systems which are suitable to be combined
with an improved and efficient power delivery system. Such heating systems
.
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include direct ohmic heating of the casing or screen immediately adiacent to
the formation or induction heating apparatus to heat the casing or screen by
eddy-current effects. Another objective of this invention is to describe and
specify optimum design details which permit the more cost-effective
installations of a retrofit system while at the same time ~aintaininq a
reasonable power delivery efficiency. Such optimum design parameters include
the selection of the frequency and providing matching elemPnts downhole with
the type of casing or screening heating systems employed. Another objective
is to describe apparatus which will preclude possible formation damage.
BRIEF DESCRIPTION OF THE D MWINGS
Figure 1 is a simplified vertical cross-section view, partly schematic,
of one embodiment of the ;nvention comprising a casing wall ohmic current
heat~ng system which employs a matching transformer.
Figure Z is a conceptual drawing which illustrates the functions of the
downhole matching transformer and other ohmic current apparatus in the system
of Figure 1.
Figure 3 is a circuit diagram illustrating how the matching transformer ~ ~ -
functions in relation to other electrical circuit elements.
Figure 4 is a three-dimensional characterization of the downhole ohmic
current system.
Figure 5 illustrates the conceptual design of an eddy-current type
downhole casing heating system comprising another embodiment of the invention.
Figure 6 is a vertical section view of a eddy-current downhole casing
system wherein the characteristics of the eddy-current exciter are matched to
the characteristics of the cable and power source.
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Figure 7 illustrates the eddy-current heating concept and heating
patterns for moderately low frequencies.
Figure 8 illustrates how the high inductive reactance component can be
mitigated by employing movable pieces which can be moved into nearly direct
contact with the casing.
Figure 9 illustrates aboveground control and source equipment for the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figure 1 illustrates a vertical cross-section of a vertica1 oil well with
a transformer matching arrangement which matches the characteristics of the
current flowing on the casing in the vicinity of the reservoir to the
characteristics of the power delivery system. Shown here, the cross-section
of an oil well originally completed using conventlonal means and a
conventional recovery system without the cas1ng heating system. The surface
of the earth 2, the overburden 3, the reservoir 4, and the underburden 5 are
penetrated by the conventional production casing system 6. Also shown is the
surface casing 7. Conventional production tubing 8 along with the pump rod 9
are deployed from the upper part of the well system. The lower part of the -tubing 8 is modified to accommodate the transformer matching system 18, 20,
21, 23 in the lower part of the wellbore. The power is delivered via the
tubing 8 and casing 6 by exciting these from a source IO via cables 11
connecting the source to the casing 6 and the tubing 8. Non-conducting
centralizers 12 are employed to prevent the tubing 8 from contacting the
casing 6, which would otherwise short-out the circuit. The pump 15 is located
below the surface 13 of the reservoir fluids. To prevent the conducting
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reservoir fluids from shorting out the tubing with respect to the casing, the
tubing below the surface of the reservoir fluids is covered by an insulating
layer 14. Just above the reservoir 4, the tubing 8 is interrupted by a
tubular non-metallic (non-conducting) isolation section 16. The
characteristics of this isolation section are such that the normal flow of
fluid is not interrupted but the length of the isolation section serves to
iso1ate the energized tubing from the conducting packer 18. The current is
taken from the energized tubing 8 via a conductor 17 which is attached to one
of the conductors of the toroidally wound transformer assembly 20. The
current flows via conductor 17 through the primary of the toroidally wound
sections and then flows via cable 23 into the lower conducting packer 22.
Figure 2 provides conceptual details on how the toroidally wound cores
form a transformer action which drives current into the casing (or screen) 6
in the immediate vicinity of the reservoir. The voltage appearing between the
lower portion of the tubing 32 and casing 6 drives the current lnto the
toroidal winding assemblies via conductors 17 and 23. The cores are toroids
formed from thin ferromagnetic sheets (e.g., 5 mil thickness), such as
Selectron manufactured by Allegheny-Ludlum, and rolled into the form of a
toroid 31. The windings 30 on the toroid 31 are chosen to have sufficient ~;
nu~ber of turns so as to transfer the impedance of the casing wall to a value
appropriate for high delivery efficiency and design robustness. Within the
inner portion of the toroids, as shown in Figure 1, the single-turn secondary
of the transformer is formed by the highly conducting tubing such as an
aluminum tube coated with a anti-corrosion surface. This conducting tubing 32
is then in direct ohmic contact with the upper conductive packer 18 and the
lower conductive packer 22 (Figure 2). The conductive packers 18 and 22
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contact the casing 6 just below the overburden 13 and just above the
underburden 5 (Figure l). The single-turn secondary of the transformer 20 is
therefore formed by the aluminum tube 32, the conducting packers 18 and 22,
and the walls of the casing 6 in the immediate vicinity of the wellbore. The
surface electrical impedance of the casing 6 between 1:he pac~ers is larger
than the impedance of the packers and tubing, but does present a very low
impedance to the secondary winding. This low impedance must be transformed up
to an impedance in the order of a few ohms or more so as to obtain suitable
power delivery efficiency. This is done by properly choosing the number of
turns on the primary of the toroidal winding.
Figure 3 illustrates the electrical circuit equivalent for the
transformer conceptually illustrated in Figure 2. The voltage source 32, via
the conductors 17 and 23 energizes the pr~mary of the transformer, whlch is
comprised of a leakage inductance 35 and a mutual primary inductance 33 which
couples to the mutual secondary winding inductance 34 via the changing flux
36. The single-turn secondary loop is comprised of the secondary winding 34,
a leakage inductance 36, the resistance 37 of the tubing, the resistance 38 of -
the conductive packers, and the resistance 39 of the casing.
In order to obtain a proper match between the electrical characteristics
of the secondary circuit which is dominated by the impedance of the casing,
and the power delivery system, the very low impedance of the casing 6 near the ;~
reservoir 4, (Figure l) must be transformed up to a value in the order of a
few ohms or greater. This can be done by employment of silicon steel tape
wound cores 31 which have a very high permeability and a relatively high
electrical resistance; by virtue of being wound as a tape, such cores are also
laminated to ensure reduction of eddy-current losses. The use of the high
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permeabil;ty of the steel core with a small air-gap causes the ~lux that links
the primary of the transformer to link the secondary, thereby minimizing the
leakage inductances 35 and 36, (Figure 3). Should the 1eakage inductance be
too high, excessive reactance would be introduced into the ;nput leads 17 and
23, which would result in a poor power factor. However, the design, as
previously discussed, avoids the poor power factor problem by the use of high
permeability silicon type steel cares. The impedance of the casing 6, as
measured for typical installations of about ten to twenty feet, would probably
be in the order of a few tenths of a milliohm up to a few milliohms, depending
on the length of the casing to be heated and the operating frequency. ~his ~;~
low impedance has to be transformed up to something in the order of a few
ohms, at least greater than one ohm to assure an adequate power delivery
efficiency with typical commercial cables or tubing power delivery
arrangements. Since the transformed ~mpedance ~s proportlonal to the stluare
of the turns ratios, the number of turns on the primary should be
approximately twenty to five hundred turns, depending on the desired operating
impedance levels.
A system as described in Figures 1, 2 and 3 can be of retrofit into
existing wells as well as being installed in new wells of conventional design.
To retrofit a well, the existing tubing system is removed and a downhole
tubing system arrangement like that shown in Figure 1 is lowered into the
well. The system is installed by positioning the transformer assembly and
casing heating system in the immediate vicinity of the wellbore as illustrated
in Figure 1 with a conducting packer 18 near the top of the zone to be heated
and a conducting packer 22 in the immediate ~icinity of the lower portion of
the zone to be heated. These conducting packers are then installed by
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expanding the steel teeth of the tubing anchor into the steel of the casing 6.
Depending on the amount of power to be transferred and the length of the zone
to be heated~ one or more of such toroidal transformers, as shown in Figure 2
would be needed to provide the necessary energy to conduct the heating.
Figure 4 provides a three-dimensional conceptual drawing wherein a
portion of the casing 6, has been removed to show the principal downhole
portions of the system, which include the upper conducting packer 18, one of
the primary transformer assemblies 30, 31, and 20, and the lower conducting
packer 22. The tubing 8, as it enters into the immediate vicinity of the
reservoir, is insulated by an insulating sheath 14. However, as this sheath
approaches the vicinity of the wellbore, the metallic portion of the tubing
and the sheath is replaced by a non-conducting fiber-reinforced tubing 16
which is attached to the upper conducting packer 18. The conductor 17, which
is attached to the metallic portion of the tubing 8 at 17A, is routed through
the fiberglass tube 16 to attach to one of the primary leads of the toroidal
transformer. The second lead 23 from the transformer is attached to the lower
conducting packer 22. A highly conducting tube 32 is ohmically attached to
the upper conducting packer 18 and the lower conducting packer 232. The
tubing 21, the packers 18 and 22, and the casing wall 6 comprise the
components in the secondary circuit of the transformer 20.
Figures 5, 6, 7 and 8 illustrate another version of the casing wall
heating system of this invention. This version again relies on a combination
of a downhole casing wall heater system which is integrated with the pawer
delivery system such that good efficiency is realized.
Figure 5 presents a conceptua1 design of an eddy-current casing wall
heater 41. This system is comprised of a power cable delivery system
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including the cables 41 and 44, a matching system such as a capacitor 42, and
the windings 43 on a field pole 46. The field pole 46 is like the rotor from .
a sychronous motor/generator. By energizing the windings 43 on the field pol~
system 46, ~agnetic flux is created which tends to pass through the casing
wall, from one pole to the other. This creates a flow of eddy-currents in the
wall, which in turn converts the energy in the elPctrical field into thermal
energy in the wall of the casing 6.
Figure 6 is another schematic of a vertical cross-section of a conceptual
design of the eddy-current heating system as applied to a cased-hale
completion. This shows a conventional oil well which penetrates the surface 2
of the earth, through the overburden 3, into the reservoir 4, and then into
the underburden 5. This well is conventionally lnstalled with the emplacement
of the surface casing 7 and then subsaquently bor~ng a hole of suff~cient
d;ameter to lower the product~on casing 6 lnto the well. This productlon
casing is then cemented to the earth, and the well is completed by Ineans of a
perforating gun to form perforations 19 into the reservoir.
To install the retrofit system, the connventional tubing system may be
unaltered and the eddy-current heating tool slipped down the tubing as shown
in Figure 6. A source of electrical power 10 is connected via cable 11 to the
production casing 6 and to an insulated cable 41. This cable 41 is attached
to a matching element 42, usually a capacitor, which in turn is connected to
the windings 43 on a field pole 46. A space between the pole piece 46 and the
casing 6 exists to allow insertion of the tool. A conducting packer 45 is
used to terminate the well tubing 8 and to anchor it. The other winding 44
can be attached to the conducting packer 45 or, as an alternative (not shown~
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can be returned by an additional conductor in cable 41 to the surface and
grounded at the casing head.
Figure 7 illustrates how the eddy-current system interacts with the
casing wall 6 at a low frequency. Shown in Figure 7 is a source of voltage 50
as it might appear near the reservoir, a matching network 51 shown as a
capacitor, and cables 52 to the coil or winding 53 on an armature-like core
54. This creates a flux in the armature 54. The flux flows through the
casing 6 and is split into two directions as shown by 55. In addition, some
flux bypasses the casing; this is the leakage flux 58. As the current in
windings 53 vary, the flux will vary in accordance with the current flow and
cause an eddy-current which heats to create a flux polarized to oppose the
exciting flux 57. This causes eddy-currents 59 to flnw, largely on the inside
of the casing wall 6 as illustrated by the current flow patterns 59. Crosses
~nd;cate emerging currents and small circles indicate penetratin~ cl~rrents.
The system shown in Figure 7 is similar to the armature of a slngle phase
synchronous motor wherein the rotor is formed by core 54 and the stator 60
which is formed by the casing 6. However, a major difference exists, as
regards a conventional motor, inasmuch as that the stator is formed from a
single unlaminated steel oil well casing 6 whereas the rotor in this case is ;
formed from laminations of high resistance steel.
The amount of leakage flux is a function of the applied frequency and air
gap. At very low frequencies, when very little eddy-currents are introduced,
the bulk of the flux exits the pole pieces and returns to the opposite side of
the pole piece with a nominal amount of leakage flux. As the frequency is
increased, eddy-currents are created which create an opposing electromagnetic -~
flu~ which reduces the back emf voltage and thereby allo~ing more current to
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flow. This also makes the leakage flux 58 a more dominant factor. The
leakage flux is further increased by the existence of an air gap 56 which is
necessary to allow pole pieces to be inserted into the casing. The larger the
air gap, the greater the leakage flux 58. This leakage flux does not link or
penetrate the casing and contributes to a poor power factor.
As the frequency of the excitation from source 50 is increased, more and
more eddy-currents flow, which further creates an opposing flux which reduces
the amount of flux 55 penetrating the casing walls and thereby increases the
proportion of the leakage flux 58. As the frequency is increased, almost all
of the leakage flux therefore bypasses most of the steel casing via the
pathway shown for the leakage flux 58. The principal pathway of the eddy-
currents and resulting heating patterns induced into the casing now moves from
the low-frequency case shown in Figure 7 to regions adjacent the air gaps 56.
Such a shift in the position of the heating pattern is of little consequence
for the pole piece arrangement shown in Figure 5. However, it could result in
hot spots if the pole pieces are widely separated.
Means must also be available to shift the frequency of operation such
that a high power factor can be maintained by the matching network. The
reason is that the effective permeab;lity of the steel will change as a
function of the applied power. This changes the required values for the
parameters in the matching network, shown simply as capacitor 51. However, it
is not practical to make such changes downhole. Alternatively, the frequency
can be shifted somewhat to almost compensate for the change.
The presence of leakage flux cause highly reactive currents in the cable
52 which do not provide any contribution to the heating of the casing 6 and ~ -~
degrade the power factor. To improve the ,oower factor, a matching network 51
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is inserted which includes at least one series capacitor, as shown in Figure
7, or at least one shunt capacitor. More complex matching networks might also
be employed. The key, as in the previous example, that the number of turns on
the movable armature inserted into the casing have to be adjusted so th~t the
operating frequency, in combination with the matching network, at least one
ohm of resistance is exhibited free of significant reactive components and no
more than 500 ohms at the upper level. The lower limit is governed by the
series resistance of the transmitting cables such that the one ohm load shauld
be significantly larger than the series resistance of the cable and the upper
end governed by the shunt losses along the cabling system and the voltage
withstand levels for the system.
The design procedure for the system is as follows. First, a range of
frequencies is chosen which provides a suitable eddy-current pattQrn. The
turns 53 on the armature 54 are varied such that at the operatlng frequency
under matched cond;tions the lnput impedance is largely reslst~ve and has a
resistance in the order of at least one ohm but does not exceed 500 ohms. The
lower limit of resistance is determined by the series resistance of the power
delivery system which can be as small as one ohm, and the upper value is
determined by the shunt losses of the power cabling delivery system and the
voltage withstand limits of the cabling system. This system i5 optimized
empirically because of the nonlinear characteristics of the steel casing. In
practice it will be desirable to have a power source capable of varying the
frequency slightly, since the parameters of the downhole system will vary with
different values of excitation and the frequency of the system should be
adjusted so as to give optimum match with a minimum of reactive component.
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Another method of improving the performance of the eddy-current tool is
presented in conceptualized form in Figures 8A and 8B. Similar to the ohmic
contacting arrangement shown in Figures 1, 2 and 3, the pole pieces are
clamped to the casing to reduce the leakage flux and thereby improve the power
factor. The production casing 6 and a central tube or production tubing 121
are positioned in the production zone 122. Lowered over the centralizer/
tubing are "C" type cores 123 and 124. Along the longitudinal portlon o~ the
~C" cores 123 and 124 windings 125 and 126 are formed. These windings are
connected via conductors 127 and 128 to the power delivery system via matching
network 129. Retracting springs 130 withdraw the ~C" cores, when the c0il5 : ::
125 and 126 are not energized, from contact with the wall of the casing 6,
thereby allowing pos;tioning of the "C~ cores near the perforations 131, as
shown in F;gure 8A. When the windings 125 and 126 are energiz0d, the
resulting magnetic force will attract the "C" cores 123 and 12~ to the casing
wall, as illustrated in Figure 8~. Alternatlvely, a mechanism activated by
turning the tubing could be used to position the cores almost in contact with
the walls. ~-
..:
Figure 9 illustrates aboveground controls for the heating systems. As
.
-
mentioned previously, if the fluids are not flowing and power is continuously ~-~
applied, the temperature of the casing 6 will rise. The temperature rise is
limited by several factors, which includes the thermal diffusion and
convection counter-flows and the amount of water which may be evaporated in
the annulus between the tubing and the casing. However, many such wells tend
to be pumped dry and, as a result, reservoir fluids which contain water which
can provide an evaporative heat-pipe cooling function are no longer present.
If the temperature of the casing 6 becomes too high, as when the casing ~ -
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temperature exceeds 250-C, pyrolysis of the oil surrounding thè casing may
take place. As a result, the solid products of pyrolysis may damage thc
formation. This would limit the ingress of fluids to the well even after
stabilized temperature operating conditions have been restored. Another
condition can occur when the fluids in the annulus are of sufficient height
that the vaporization temperature of water exceeds 250~C.
One method to postpone or preclude excessive temperature is to position
the pump above the producing zone such that substantial quantities of water
remain in the liquid portion of the annulus but sufficiently low so that the
height of the liquids in the annulus could not cause the vaporization
temperature of the fluids near the reservoir to exceecl 250~C. If for any
reason the ingress of l;quids is stopped, evaporation of the water withln the
annulus adjacent to the producing zone will cool the heated zone. The hot
water vapors would then rise in the annulus and be condensed on the cooler
casing above the deposit. The condensed water then returns via gravity to the
heated zone to be subsequently recycled in a heat-pipe fashion. Such
positioning will not always limit the temperature buildup, but it can extend
the time re~uired to reach catastrophic temperatures. Preferably, a control
system on the surface could be used to reduce the heating as the flow of
fluids decreases.
Figure 9 shows such an above surface system. It shows a wellhead 101
near the surface of the earth 2, including the conventional casing system 6
along with the surface casing 7 emplaced in the overburden 3. The wellhead
101 is electrified and is isolated from the grounded casing 6 by means of a
circular annular insulator disk 107. The electrified wellhead 101 has an
outlet conduit 10Z for the produced fluids. These flow through a fiberglass
18 01-JEB.ID
.
2 ~ 2 ~
tubing 103 of sufficlent length to isolate the electrified wellhead 101 from
an electrically earthed metallic pipe 104. Interposed in the pipe to 104 is a
fluid flow sensor 105 which is connected to the voltage source and control 10
by means of a cable 106. A thermocouple cable 25 is at:tached, via a
continuation cable 25A outside the wellhead 101, to the voltage source 10.
The pump rod 9 includes an insulating section 108 which goes to the horsehead
pump assembly 109. The horsehead pump assembly is actuated by an electric
motor 113. Attached to the beam 116 of the horsehead assembly is a strain
gauge sensor 110 whose output is connected by a cable 112 to the source 10.
The power to the motor 113 is supplied by a cable 114 from the source 10. The
voltage power source 10 is supplied by power from a conventional ac pow~r line ~ ~'
115. AC power from the source 10 is supplied by cables 11 to the casing 6 and -~
to the electrified wellhead 101. Current flows through the electrified ~ - -
wellhead 101 and, by means of metallic grips 117, is conductively attached to
the tubing 8 which allows current to flow down the tubing and back via the
casing 6. Dur;ng nor~al operationt the output from one or more of the
sensors, such as the strain gage 110 and flow sensor 105, is monitored in
circuit 10. Should the temperature exceed a predetermined limit or should the
flow stop or should the stresses on the pump fall outside a predetermined
range, the power output from the source 10 is reduced.
Where eddy-current excitation is employed, source 10 also included a
variable frequency source. The operation of this variable frequency source is
controlled by a variable reactance having a reactance that is changed as a
function of its input voltage. The phase of the current is compared with the
phase of the applied voltage preferably by means of a phase comparator circuit
which has a dc output. The output is increasingly positive as the current
19 01-JEB.ID
,, ., ,, - , . .. .
~ , , ~ , ., ,,. . , :, ~ , . : .
209D629
leads the voltage and increasingly negative as the current lags the voltage,
or vice versa.
In the case where a capacitor is employed downhole in series with the
eddy-current system, the phase of the current will lead the voltage if the
frequency is too low and will lag the voltage if the frequency is too high.
Applying the output of the phase detector circuit to the voltage ~ariable
reactance which controls the frequency of the source 10, the frequency can be
made to shlft in a way to approach unity power factor conditions.
The range of conditions of operation can be determined from the following
relationships.
(Rload) 2 ~ ~ (R) 2+ (t~L) 2] (D) 2 (1)
(Rload) 2 ~ ~ (G) 2+ (c~C) 2] _1 (D) _2 (2)
Where:
R~o~ is the downhole load resistance
R is the series resistance of the power delivery cable per ~ter
L is the series inductance of the power cable per meter
G is the shunt conductance of the power cable per meter
C is the shunt capacitance per meter of the power cable
D is the length of the power cable in meters
w is the angular frequency, 2nf
01-JEB.ID
~' 2 ~ 2 9
These consider both the series and shunt losses of the power delivery
system. Equation (1) states that the series losses of the power delivery ;
subsystem should be less than the power absorbed in the load. For a given
cabling with R and L, the value of the load resistor R must be large enough to
meet equation (1) criterion. This can be done by increasing the turns on the
primary of the toroidal transformer of Figures 1, 2 and 3 or the number of
turns on the field poles of Figures 5, 6, 7 or 8. ~ -
The number of turns on the downhole transformer or eddy-current exciters
can be increased to reduce the I2R or ~copper~ losses in the cables by
increasing the value of the reflected load resistance R, thereby reducing the
current and increasing the voltage. However, the value of the load resistance
cannot be increased without limit as governed by the criterion in equation
(2). Another limit is the maximum voltage rating of the cable, which -~
typically will be less than 10,000 volts. The third voltage limittng
criterion for the minimum current conditions then becomes:
I=(P) (V)-'(p.f.)-' ~3
Where~
I is the cable current
P is the applied power
V is the maximum cable voltage rating
p.f. is the power factor
21 01 JE8.ID
.. ,
2~9~29
The frequency can be increased to reduce the magnetizing current in the
transformer or eddy-current exciters or also to reduce the amount of iron in
the tape wound cores of Figures 1, 2 and 3, in which case the maxlmum
frequency is governed by the criterion embodied in equation 2.
At higher frequencies, additional propagation loss criteria must also be
met as follows:
l~D < 1 (4)
20 GD < 1 (5) ;
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.
