Note: Descriptions are shown in the official language in which they were submitted.
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TEC~NIQUE FOR ELECTRICALIJY HEATING FORMATIONS
This invention relates to a technique for electrically
heating subterranean formations, particularly those bearing
liquid hydrocarbons.
5There are many oil producing regions in the world where
liquid hydrocarbons in the ground do not flow at a desired
rate because the viscosity is too high at formation tempera-
ture. Often, these liquid hydrocarbons are in association
with saline formation water and the relatlve viscosity of the
oil is much too high relative to the water. In these situa-
tions, heating of the formation and consequently heating of
the oil and water causes viscosity reductions in both, but
primarily in the oil. Because the viscosities of typical
crude oils are much more temperature dependent than the
viscosity of typical saline waters, a modest increase in
formation temperature often creates a much more favorable
viscosity ratio. For instance, a 10 Fahrenheit increase in
the temperature of most oils will cut the viscosity in half,
which in turn will double the oil flow rate through the heated
section. Often this change in viscosity ratio is sufficient
to cause the production of much more oil and change an
uneconomic situation into a profitable one. This situation is
common in heavy crucle producing areas of the world where
produced crudes show API gravities of 18 or less.
25A number of different heating techniques have been
attempted in such areas where the producing formations are
relatively thick. These techniques include steam injection,
in-situ combustion, hot water floods and electrical heating.
Where the formations are relatively thin, no heating technique
is very successful because of excessive heat loss to the over-
lying and underlying formations. If the criteria of success
of heating techniques is the frequency with which they are
used, the only substantially successful technique is steam
injection because it has been used in more producing fields to
produce more incremental oil than all other heating techniques
combined.
The reason heating techniques are unsuccessful is that
more money has to be spent, in the form of equipment, manpower
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and energy, than is justified by increased oil production. In
all heating techniques, a substantial improvement in thermal
efficiency, i.e. the ratio of formation and oil heated to
energy expended, would dramatically change the efficiency of
the process and thereby dramatically affect the economic
viability of any formation heating project.
Electrically heating oil bearing formations is old and
well known in the art. Developments in which the present
inventors had a significant part are found in U.S. patents
3,507,330, 3,547,193, 3,605,888, and ~,642,066. This approach
includes delivering an alternating current into the well and
transmitting it through saline connate water in the oil
bearing formation. Resistive heating occurs in the saline
water. When the water heats up, heat is transmitted to the
adjacent oil. Disclosures relevant to this invention are
found in U.S. patents 4,010,799; 4,135,579; 4,140,179;
4,140,180; 4,193,451 and 4,320,801.
Another situation where it is desirable to heat oil
producing formations is where the crude oil has unusual pour
point characteristics. For example, in much of the Uinta
Basin of eastern Utah, crude oil from two quite distinct
formations suffer the same unusual property of flowing easily
like conventional crude oils at 120F but look like shoe
polish at 75F room temperature. This phenomenon is well
known and is caused by a high wax content in the crude oil.
This high wax content requires that flow lines, gun barrels,
tanks, tank trucks and the like be heated. Unfortunately, the
reduction in flowing temperature often occurs before the
produced oil reaches the surface thereby plugging production
tubing well below the depth where conventional heating
techniques can be used.
This sounds like the old and well known oil field
problem where paraffin from paraffin based crude oils often
settles out of crude oil inside the tubing strings where the
flowing temperature falls below some predetermined tempera-
ture, often opposite some relatively shallow fresh water
aquifer. Oil producing companies are well acquainted with
paraffin problems and have devised many different techniques
for dealing with it. The problem of high pour point crudes is
quite different and, for present purposes, it will su~fice to
say that these techniques have not been successful in main-
taining production of Uinta Basin wells to anything like their
potential.
If there is any free gas in the formation of a high
pour point crude, such as that of the Uinta Basin, it is
conceivable that the expanding gas can cause local cooling of
the formation adjacent the well bore where velocities are
fastest and the greatest pressure drop occurs. This, of
course, is catastrophic because production dwindles off and
stops. Tripping the tubing and cleaning it out may seem to
cure the problem because production begins again, only to
dwindle off and stop again. To begin with, these problems are
difficult to diagnose because one normally tries those
solutions that have always worked to a well which has stopping
producing - you see if the pump is working, the tubing is
plugged up or the perforations are plugged up. In addition,
very little can be easily done where the problem occurs in the
formation.
Electrical heating of Uinta Basin type formations is
very desirable for a variety of reasons. First, not much
heating is required - only enough to keep the material liquid
- which may not be more than an additional 30 F. Second, it
is easy to control electric heating because the amount of
energy delivered into the ground can be closely monitored and
changed. This means that excessive heating can be avoided
thereby minimizing energy costs, electricity having to be
purchased, usually directly from a utility or indirectly by
the consumption of capital and fuel. Even in these situations
where electrical heating appears to be very desirable, thermal
efficiency remains paramount because an inefficient operation
will shortly be driven from the market. Thus, an improvement
in thermal efficiency would dramatically improve the economics
of Uinta Basin type or other high pour point oil production.
During the conduct of laboratory experiments involving
electrically heating oil bearing formations, an unusual effect
has been noted. As shown in Figure 1, a test cell 10 com-
prises a closed container 12 having a quantity of rock 14
representing an oil productive formation containing a liquid
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representative of the hydrocarbon-water mixture in the yround.
The rock 14 is preferably obtained by grinding up cores from
the productive formation. The liquid in the formation 14 is
mixed from produced oil, gas and water to create a liquid as
representative as possible of formation liquid. A pair of
electrical conductors 16, 18 represent wells through which
electricity is delivered to the formation 14. A test probe 20
between the conductors 16, 18 comprises a thermocouple 22 for
measuring the temperature of the formation 14 and a pressure
transducer 24 for measuring the pressure of the liquid in the
formation 14. In such tests, electrical energy is delivered
to the formation 14 through the conductors 16, 18 at a
predetermined amperage, voltage and frequency. Referring to
Figure 2, a normal test uses commercially available 110 v, 60
Hz alternating current. A normal time - temperature response
curve 26 starts at an initial point 28 at ambient temperature
and gradually increases in a more-or-less linear fashion
through a region 30 until temperature losses through the
container 12 causes the temperature rise to slow down until
equilibrium is ultimately reached in a region 32.
Many hundreds of such tests have been run. Once in a
while, the response curve produced an anomalous result shown
by the curve 34 where the temperature gradually increases from
an initial point 36 at ambient temperature in a more-or-less
linear fashion throuyh a region 3~ until an abrupt change in
slope occurred in a region 40 and temperature at the ther-
mocouple 22 increasecl abruptly. Typically, the region 40 is
of relatively short duration. The recorded temperature con-
tinued to increase, but in a region 42 roughly parallel to the
upper end of the region 30. Ultimately, thermal equilibrium
was reached in a region 44.
It is now believed there are at least two heating
mechanisms operating in the test cell 10. Heating in the
regions 38 and 42 are the normal type of heating seen when
heating any material, i.e. ohmic or resistive heating. Heat
and consequently temperature are, of course, a manifestation
of molecular motion. What happens in the region 40 is that
the applied electric current is at a frequency which is some
harmonic of a significant component of the formation rock
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molecules or of the formation liquid molecules. This increase
in the temperature in the region 40 can be used to dramatical-
ly increase the effectiveness of formation heating because the
necessary temperature rise can be achieved with an expenditure
of far less energy. This increase in thermal efficiency has a
dramatic effect on the economics of any heating project.
One might properly as~ why increased temperature rise
in the region 40 may be a function of a match between the
frequency of the applied alternating current and some harmonic
of the formation or liquid therein because, after all, the
frequency applied during the test was constant 60 Hz alternat-
ing current. It is believed the harmonic response is a
function of the formation composition, the formation liquid
and gas composition, the formation temperature and the
formation pressure. As presently advised, formation pressure
is believed to be significantly important only when free gas
is present in the formation and an increase in pressure acts
to drive the free gas back into solution and thus change the
composition of the formation liquid. An increase in tempera-
ture causes gas to break out of solution. The exact effect ofincreasing temperature and pressure in the reservoir will
depend on the composition of the formation fluid and perhaps
the composition of the formation. The harmonic response in
the region 40 is temperature dependent in the sense that when
the temperature of the formation rises, the applied current
frequency that creates the harmonic response rises. Thus, in
the abnormal tests exemplified by the curve 34, the tempera-
ture of the formation rose enough that the harmonic frequency
rose to a value of 60 Hz. After the formation temperature had
driven the harmonic frequency to a value above 60 Hz, harmonic
heating in the region 40 ceased and normal ohmic heating was
again the dominant heating mechanism operating. In a sense,
the normal curve 26 fails to show the harmonic response
because the original formation temperature was too high or the
test cell 10 did not get hot enough to reach the harmonic
response.
In accordance with this invention, field operations are
conducted to heat a subterranean oil bearing formation with a
form of alternating current at a selected frequency which
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corresponds to the frequency eliciting harmonic heating of the
formation. Initially, scale model tests are conducted in the
test cell 10 to determine an approximate value for the
harmonic frequency at the existing formation temperature.
Wells are then equipped with suitable insulators to deliver
electricity into the formation. The frequency of current
delivered into the wells is changed from conventional 60 Hz AC
into the desired wave shape and frequency desired and deliv-
ered down the wells. After heating starts, the frequency may
be varied in a range including the laboratory determined
harmonic frequency in an attempt to insure that at least some
of the electrical energy delivered into the formation is at
the actual harmonic frequency of the formation which elicits
harmonic heating as evidenced in the region 40. In elaborate
installations, a temperature sensor is placed near the bottom
of the well to deliver a readout at the surface on a more-or-
less continuous basis. Thus the applied frequency and
temperature rise can be monitored on a continuous basis. Not
only can one thus be sure that harmonic heating occurs, but
one can also change the applied frequency to fine tune the
operation.
Oil is produced from the wells and its temperature is
measured and recorded~ When the temperature of the oil rises
above that believed to be due solely to resistive heating, a
manifestation of harmonic heating is seen, either at the
surface or from the bottom hole sensor. As heating continues
and the formation temperature increases, the frequency
eliciting the harmonic heating effect increases. Accordingly,
the frequencies applied to the well is increased, i.e. the
range is moved slightly up the scale.
It is accordingly an object of this invention to
provide a new and improved technique for heating a subter-
ranean oil bearing formation.
Another object of this invention is to provide an
improved electrical heating technique exhibiting improved
thermal efficiency.
These and other objects of this invention will become
more fully apparent as this description proceeds, reference
being made to the accompanying drawings and appended claims.
5 IN THE DR~WINGS:
Figure 1 is an isometric view of a test cell used to
determine the thermal response of a simulated oil bearing
formation in response to electrical energy input;
Figure 2 is a time-temperature chart showing a typical
and an atypical formation temperature response to electrical
energy input; and
Figure 3 is a schematic view of a well equipped to
deliver alternating current of variable frequency into an oil
productive formation.
Referring to Figure 3, there is illustrated a well 46
equipped to deliver alternating current into the formation at
a frequency corresponding to a thermal harmonic ~requency of
the formation or the formation contents. The well 46 illus-
trates a typical open hole completion in which a bore hole 48
extends downwardly from the surface toward an oil bearing
formation 50. A casing string 52 has been cemented in the
bore hole 48 above or in the top of the formation 50 and the
bore hole 48 deepened into or through the formation 50.
A string 54 of insulated tubing provides an electrode
25 56 in electrical communication with the formation 50. The
tubing string 54 extends upwardly from the electrode 56 to a
tubing hanger 58 supported in and insulated from a well head
60 on the upper end of the casing string 52. The well 46 is
illustrated as a flowing well having an insulated, but
otherwise conventional tree 62 for delivering produced
formation fluids to a separation and storage facility (not
shown). Electricity is delivered to the tubing string 54
through a connection 64 and electrical cable 66. Those
skilled in the art will recognize the well 46 as a convention-
al electrically heated well shown in U.S. patents 3,507,330,
3,547,193, 3,605,888, and 3,642,066.
The current delivered to the cable 66 is some form of
alternating current in the sense that it may be of convention-
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al sine wave type, square wave type, pulse~ dc, or the like.
To this end, there is provided a controller 68 to receive 60
Hz alternating current from a source 70, preferably three
phase power lines of an eleetric utility, and deliver variable
frequency alternating current of some description to the
outputs 66, 72, the output 72 being grounded in any suitable
manner. The controller 68 may be of any suitable type, such
as a Model Accutrol 150 eommereially available from Westing-
house Electrie Corporation. Such eontrollers 68 are typically
organized to include an ac-to-dc converter 74 delivering dc
current through a pair of conductors 76, 78 to a chopper 80
which converts the de eurrent to a series of reversed polarity
single phase pulses in the outputs 66, 72. A eontrol meehan-
ism 82 is used to eontrol the ehopper 80 and thereby vary the
frequency of the alternating current in the outputs 66, 72.
In use, tests are preferably run in the laboratory
using the test cell 10 to determine a thermal harmonic
frequency of the formation 50 and/or formation contents at
existing formation temperature and/or formation pressure.
This is done by first determining, either by calculation or by
measurement, the thermal response of the test cell 10 to ohmic
heating, i.e. in response to constant frequency current.
Next, the frequency of the power is ehanged and applied to the
conduetors 16, 18, measuring the temperature at the thermo-
eouple 22 and determining the frequeney at whieh the tempera-
ture rises at a rate greater than that of ohmie heating. At
the well 46, the eontrol meehanism 82 is manipulated to
deliver alternating eurrent at the thermal harmonie frequeney
to the outputs 66, 72 and thereby delivering alternating0 eurrent at the thermal harmonie frequency to the formation 50.
It is appreeiated that the value for a thermal harmonic
frequency obtained in the test cell 10 will not always be the
same as the thermal harmonic frequeney of the formation 50
and/or the contents thereof. Thus, it is preferred that the
control mechanism 82 be of a type that will eyclically
manipulate the ehopper 80 to produee a range or band of
frequeneies that inelude the thermal harmonic frequency
determined in the laboratory. The Aceutrol series of adjust-
able frequeney controllers available from Westinghouse
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Corporation is capable of accepting programmable controllers
which can manipulate the controller 68 to deliver any frequen-
cy or frequencies within the capability of the device in
substantially any sequence desired. In the event further
information is needed, reference is made to the appropriate
technical publications of Westinghouse Corporation.
From experience it is presently believed that many, if
not most, formations will show a thermal harmonic response,
evidenced by the curve 40, at applied frequencies in the range
of 12-20 Hz. In a simplified version of this invention, the
control mechanism 82 may be manipulated to deliver alternating
current in the range of 12-20 Hz, knowing that the thermal
harmonic frequency of the formation 50 and/or its contents is
within this range and that some harmonic heating will occur.
Another technique that may be used in the field to fine
tune the frequency applied from the controller 68 relies on
the ability to calculate how much the temperature of the
produced formation fluids should rise for a given input of
energy. Typical ohmic or resistive heating is in the range of
30% efficient while the combined effect of harmonic and
resistive heating is considerably higher. If the formation
fluids produced from the tree 62 are substantially hotter than
a predicted predetermined value assuming only ohmic heating,
it is apparent that harmonic heating is occurring. Thus, if
the range of applied frequencies achieves harmonic heating of
the produced fluids, the range may be restricted, e.g. divided
in half, and heating continued to determine if the harmonic
response lies inside or outside the new restricted range. If
the temperature of the produced fluids declines, the conclu-
sion is that the harmonic response lies outside the new rangeand the controller 68 is adjusted accordingly.
Rather than rely on surface temperature measurements,
it is preferred to provide a downhole temperature sensor 84
adjacent the bottom of the well. Conveniently, the sensor 84
and its communication wire 86 may be installed in a conven-
tional manner, as by strapping them to the tubing string 54.
The upper end of the wire 86 exits through a port 88 in the
well head 60. A sealing assembly 90 closes the port 88 and
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allows the wire ~6 to connect to a temperature gauge or
recorder 92.
There is another ancillary benefit from the practice of
this invention. The standard electrical hookup of electrical-
ly heated wells has only two electrical paths, one path downthe insulated tubing and one path to ground. Thus, the
standard electrical hookup cannot use commercially available
three phase power because there are three power lines and no
place to hook up the last line. Because electrically heated
wells consume a substantial amount of power, some small
electric utilities may not allow the operator to take single
phase power off the three phase line - this will create an
imbalance in the power left on the line and disrupt other
customers. Thus, the operator may have to take and pay for
three phase power, usually putting two of the lines to yround.
Thus, a third of the power purchased in such a situation is
wasted. It will be appreciated that the use of the ac-dc-ac
converter 74 converts all of the three phase ac input into dc
which is in turn chopped into some form of alternating current
by the chopper 80. Admitting there are some power losses
because of the inefficiencies of the converter 74 and chopper
~0, these are minor compared to the gain from using al~ of the
three phase power purchased from the utility comprising the
source 70.
Although this invention has been disclosed and des-
cribed in its preferred forms with a certain degree of
particularity, it is understood that the present disclosure of
the preferred forms is only by way of example and that
numerous changes in the details of operation and in the
combination and arrangement of parts may be resorted to
without departing from the spirit and scope of the invention
as hereinafter claimed.
