Viscous oil recovery using high electrical conductive layers

A selectively electrically insulated, cemented and perforated tubular electrode provides a more effective system for electrically heating formations comprised of interbedded high and low electrical conductivity layers. The tubular electrode is located opposite the formation and is exteriorly insulated at an upper part of the formation and perhaps in low part of the formation. A central part of the tubular electrode is left free of electrical insulation. The tubular electrode is cemented in place and perforated at vertically spaced apart points into oil-bearing layers of the formation. The electrode may be a part of a casing string and the casing string specially designed to reduce alternating current hysteresis losses and current losses to the overburden.

BACKGROUND OF THE INVENTION 
This invention relates to recovery of oil from an interbedded, hydrocarbon 
bearing subterranean formation while electrically heating the producing 
strata. More specifically, this invention pertains to selective electrical 
resistance heating of a layered oil-bearing formation wherein power input 
and production outlet are controlled to selectively use the 
characteristics of the layered formation. 
It has been proposed, for example, in U.S. Pat. Nos. 3,642,066; 3,874,450; 
3,848,671; 3,948,319; 3,958,636; 4,010,799 and 4,084,637 to use electrical 
current to add heat to a subsurface pay zone containing tar sands or 
viscous oil to render the viscous hydrocarbon more flowable. Two 
electrodes are connected to an electrical power source and are positioned 
at spaced apart points in contact with the earth. Currents up to 1200 
amperes are passed between the electrodes. The effectiveness of the 
electrical heating process depends on effective utilization of electrical 
power. 
Certain formations, for example, the Ugnu formation in Alaska, are 
compresed of alternating relatively thin layers. Geologically such 
formations are called interbedded formations. In some interbedded 
formations, the conductivity of oil-bearing strata have an electrical 
conductivity which is much lower than the electrical conductivity of 
non-oil-bearing strata. For example, the Ugnu formation is comprised of 
alternating layers of sand containing oil and siltstone. The electrical 
conductivity of the siltstone layers is much higher than the electrical 
conductivity of the oil layers, for example, the siltstone conductivity 
may be ten times that of the oil-bearing sand. It is the primary object of 
this invention to provide a more efficient method of utilizing electrical 
power to apply heat to the oil-bearing layers of an interbedded formation. 
SUMMARY OF THE INVENTION 
In accordance with this invention, more effective electrical power 
utilization in an interbedded hydrocarbon-bearing subsurface formation is 
achieved with a tubular electrode arrangement that tends to focus the 
electrical current within the formation and to limit power dissipation in 
the overburden and underburden above and below the formation. An 
interbedded hydrocarbon-bearing formation is comprised of layers. In this 
invention, the hydrocarbon-bearing layers have a relatively low electrical 
conductivity in comparison to the other layers. In the essential 
embodiment of this invention, a tubular electrode is located opposite 
layers of the interbedded formation. All of the outer exterior surface of 
the tubular electrode in the upper part of the formation is electrically 
insulated. At least some of the outer exterior surface of the tubular 
electrode opposite the formation is left insulation-free. The tubular 
electrode is cemented in place throughout the part of the formation 
traversed by the electrode. The electrode and cement are perforated at 
vertically spaced apart points into the hydrocarbon-bearing part of the 
formation. The tubular electrode is electrically connected to an 
alternating current power source. When power is applied, this tubular 
electrode arrangement causes the electrical current to flow mostly into 
the high conductivity layers. The temperature of these layers rises and 
their conductivity increases several fold and the region of high power is 
dissipated farther into the formation. Since the high conductivity layers 
are relatively thin, the heat is readily conducted into the adjacent 
oil-bearing layers. As the hydrocarbon heats, its mobility is increased 
significantly and it flows along the heated boundaries toward the 
perforated tubular electode. Heat transfer to the moving oil cools the 
high conductivity layers, with the maximum cooling occurring near the 
electrode just where it is needed for effective power application. 
In one of the principal embodiments of this invention, the tubular 
electrode is a part of a casing string. Both the exterior and interior 
surface of the upper part of the casing is electrically insulated. This 
upper part acts as an electrical conductor. The insulation reduces power 
losses to the overburden and corrosion. The upper part of the casing is 
made of nonmagnetic metal, for example aluminum. This reduses hysteresis 
losses in the conductor leading to the electrode and thereby increases 
effective use of the alternating current electrical power. An inner string 
of tubing provides a means of producing oil that flows into the tubular 
electrode through the perforations therein. 
In a further embodiment, a packer and an electrically nonconducting packer 
liquid provide inner electrical insulation for the upper part of the 
casing string. In a still further embodiment, a lower portion of the 
exterior surface of the tubular electrode or casing is electrically 
insulated starting at a point opposite the formation and below the 
insulation-free portion of the tubular electrode or casing.

DESCRIPTION OF PREFERRED EMBODIMENTS 
In the drawing, there is illustrated a well completion system for 
selectively transmitting alternating current power into layers of a 
subsurface interbedded hydrocarbon-bearing formation comprised of 
hydrocarbon-bearing relatively low electrical conductivity layers and 
relatively high conductivity nonhydrocarbon-bearing layers. The power is 
used to apply heat to the hydrocarbonaceous material and thereby to 
stimulate production of oil from the formation. 
More specifically, metal casing string 11 extends from near the surface of 
the earth downwardly at least to subsurface interbedded formation 12 where 
an electrode is to be located so that alternating current of up to 1200 
amperes may be passed through the formation to another electrode or 
electrodes (not shown). An interbedded formation has been previously 
defined. This invention is useful to an interbedded formation containing a 
hydrocarbonaceous material whose flowability is increased by heat. For 
ease of description, this disclosure will refer to the Ugnu formation in 
Alaska. Accordingly, the formation shown is comprised of 
hydrocarbon-bearing sand layers 13, 14 and 15 and high electrical 
conductivity siltstone layers 16, 17, 18 and 19. There could be more or 
less than the number of layers shown. The electrical conductivity of the 
hydrocarbon-bearing layers is significantly lower than the electrical 
conductivity of the other layers. For example, in the Ugnu formation the 
electrical conductivity of the siltstone layers may be ten times the 
electrical conductivity of the oil-bearing shale layers. 
For purposes of illustrating further embodiments of this invention, casing 
11 is shown extending beyond lower layer 19; but it is to be understood 
that casing need not extend beyond this layer. Casing 11 is comprised of 
casing sections and is divided into lower casing part 22 and upper casing 
part 23. The lower casing part contains the portion of the casing that is 
used as a tubular electrode. The upper part is used as an electric 
conductor for the tubular electrode. In order to reduce the overall 
impedance of the transmission system and reduce magnetic hysteresis 
losses, upper casing part 23 is comprised of a nonmagnetic metal, such as 
for example, stainless steel or aluminum. Aluminum is preferred because of 
its high conductivity and availability. But aluminum is very susceptible 
to corrosion and metal loss due to current leaving the casing. Corrosion 
and premature loss of power to the overburden above formation 12 are 
effectively prevented by electrically insulating the upper casing part 
with inner electrical insulation 26 and outer electrical insulation 20. It 
is important that the outer electrical insulation covering the exterior 
surface of upper casing part 23 extend downward to a first preselected 
point. This point is selected so that insulation 20 traverses a part of 
formation 12 and lies opposite a part of the formation. This point is 
selected to focus current into layers of the formation, preferably the 
central layers of the formation. An insulation-free portion of the lower 
casing part serves as the current emitting surface of the tubular 
electrode. The current applies heat to the formation. Insulating the 
casing down to this point has a further advantage in that it is below last 
coupling 27 which connects the upper casing part to the lower casing part. 
This also assures that the outer insulation extends over a part of lower 
casing part 22. The outer electrical insulation thereby prevents loss of 
current to the overburden and prevents corrosion of the nonmagnetic 
casing. This outer insulation may be comprised of coatings, pipe wrapping, 
extruded plastic, heat skrinkable sleeves, or other similar insulating or 
nonconductive corrosion protection materials. Some of the insulation may 
be pre-applied. 
It is highly desirable that inner electrical insulation 26 also extend 
downward below last coupling 27 which connects the upper casing part to 
the lower casing part and into lower casing part 22. This prevents 
interior corrosion of nonmagnetic metal casing 23. The inner electrical 
insulation may be comprised of a pipe liner, an extruded liner, coatings 
or other similar internal insulating or nonconductive corrosion protection 
materials; but the preferred inner insulation is shown as packer 28 and 
nonconductive packer fluid 26 which is placed in the annulus above the 
packer between casing 11 and tubing 29. Packer fluid may be any standard 
nonconductive or oil base fluid or thixotropic substance. Tubing string 29 
extends downward from the surface of the earth inside casing 11 through at 
least a part of lower casing part 22. The tubing may extend beyond the 
casing. The tubing string is adapted to be used for production between the 
surface and a predetermined subsurface point so that it will conduct 
fluids produced into lower casing part 22. 
Casing 11 is shown connected to typical metal christmas tree 30 represented 
schematically. The christmas tree is shown electrically connected via 
conductor 31 to alternating current power source 32 which is also 
connected to one or more other electrodes (not shown). Power source 32 is 
capable of generating voltages up to several thousand volts. Typically, a 
well may include a larger concentric surface casing string (not shown) 
which extends through fresh water zones to a predetermined point and is 
sealed in place with cement. If this surface casing were made of ordinary 
steel casing, it would cause hysteresis losses in upper casing part 23. 
Accordingly, if there is a surface casing, the surface casing string will 
also be comprised of a nonmagnetic metal and will also be covered with 
exterior surface electrical insulation. 
It was previously mentioned that lower casing part 22 may or may not extend 
below formation 12. If the casing extends below the formation, it is 
preferred that the exterior surface of a lower portion of the casing be 
electrically insulated. This enhances the focusing characteristics of the 
electrode. Accordingly, as shown, second outer electrical insulation 21 
covers the exterior surface of a lower portion of casing 11. This 
electrical insulation extends upward traversing a part of formation 12. 
Between the upper end of a second outer electrical insulation 21 and the 
lower end of outer electrical insulation 20 is a portion of lower casing 
part 22 that is left free of electrical insulation. As shown, this 
insulation-free portion is opposite layers 13, 14, 15, 17 and 18 of 
formation 12 and acts as a tubular electrode for passing alternating 
current into the formation. The annulus between lower casing part 22 and 
the borehole has been filled with enough cement to at least cover the 
exterior of the insulation-free portion of lower casing part 22 and the 
part of first electrical insulation 20 which traverses a part of the 
formation, for example it is shown traversing high conductivity layer 16 
and part of underlying low electrical conductivity layer 15. Lower casing 
part 22 and cement 24 have been perforated at vertically spaced apart 
points with perforations 25 in a manner such that there are flow passages 
extending into a hydrocarbon-bearing part of the formation, for example, 
any combination of layers 13, 14 and 15. 
In application, a borehole large enough for casing string 11 is drilled to 
a predetermined depth. This borehole traverses at least some layers of 
interbedded formation 12. Casing 11 is lowered downward into the borehole. 
The part of casing 11 that is to be used as a tubular electrode is lowered 
exposed so that its outer surface can contact the formation opposite the 
predetermined point where the tubular electrode will be used to apply heat 
to a subsurface formation. Thereafter, as the tubular electrode is 
lowered, the exposed exterior surface of the tubular electrode above the 
portion left free of electrical insulation is covered with electrical 
insulation 20. Part of the tubular sections may be preinsulated so that, 
for example, only the ends and couplings need to be insulated. The 
insulation-free portion of the tubular electrode and at least a part of 
the upper insulated portion of the tubular electrode is lowered to a 
preselected first point. This first preselected point has been selected so 
that the insulation-free portion lies opposite layers of formation 12 and 
so that the insulated upper portion traverses a part of the formation, for 
example high conductivity layer 16 and lies opposite a part of underlying 
low electrical conductivity layer 15. As previously mentioned, optionally 
it may be desirable in some situations to have the casing traversed the 
entire formation or the entire group of layers of interest and extend 
deeper into the earth. In such situations, the method could include the 
step of covering the exposed exterior surface of a lower portion of the 
tubular electrode with electrical insulation as the tubular electrode is 
being lowered into the borehole. In this case, the first preselected point 
will also be selected so that the insulated lower portion traverses a part 
of the formation, for example high electrical conductivity layer 19 and 
lies opposite a part of overlying low electrical conductivity layer 13. In 
addition, it was optionally mentioned that the tubular electrode could be 
a part of a casing string so that the upper part of the casing string 
could act as a conductor for alternating current to flow into the tubular 
electrode. In this situation, the method could include the step of 
covering the exposed exterior surface of the casing string above the 
insulated portion of the tubular electrode as the casing string is lowered 
into the borehole. Preferably, in this latter optional situation, casing 
11 would be divided into upper casing part 23 and lower casing part 22. As 
the lower casing part is lowered and the time comes to connect it to the 
upper casing part, the method would include the step of connecting the 
lower casing part to a nonmagnetic metal upper casing part. After the 
casing has been installed, the method could include the step of lowering a 
tubing string with a packer downward from near the surface of the earth 
into and through upper casing part 23 and into lower casing part 22. The 
packer is thereafter be set at a point inside the lower casing part below 
the lowest point of the upper casing part. An electrically nonconductive 
liquid is added to the annulus between the tubing and the casing string 
above the packer. This effectively insulates the interior surface of the 
nonmagnetic upper casing part and thereby prevents corrosion to the 
interior surface of the nonmagnetic part of the casing string. 
Alternating current from power source 32 is caused to be flowed from the 
insulated part of the tubular electrode into the layers of formation 12 
adjacent the uninsulated part of the electrode. The high electrical 
conductivity layers carry most of the current and the overlying and 
underlying low electrical conductivity hydrocarbon layers can thus be 
viewed as electrically insulating. The insulated perforated tubular 
electrode and characteristics of the formation thereby selectively combine 
to focus the current within the formation and limit power dissipation in 
the overburden and underburden. As time passes, the high electrical 
conductivity layers increase in temperature with the increase occurring 
more swiftly near the electrode well. As the temperature increases, the 
conductivity of these layers increases (for example, three to fivefold). 
This moves the region of high power dissipation farther from the tubular 
electrode or producing well. The high conductivity layers are relatively 
thin. Heat, therefore, is readily conducted into the adjacent 
hydrocarbon-bearing layers. As the hydrocarbonaceous material heats, its 
mobility increases significantly and it can thus flow along the heated 
boundaries towards the producing tubular electrode well. The flowable oil 
in the hydrocarbon-bearing layers is forced towards the heated boundaries 
by thermal expansion, decompression (that is, pressure expansion), 
comparison, solution gas drive and gravity drainage. Heat transfer to each 
moving layer of oil cools the overlying and underlying high electrical 
conductivity layers with the maximum cooling occurring near the tubular 
electrode well where the cooling is needed most for more effective 
application of the alternating current. Oil is produced through the 
perforated tubular electrode to the surface of the earth through tubing 
29. 
From the foregoing, it can be seen that specially insulated, located and 
perforated tubular electrodes provide far more effective use of 
alternating current power in an interbedded hydrocarbon-bearing formation. 
Reasonable variations and modifications are possible within the scope of 
this disclosure without departing from the spirit and scope of the 
invention.