Abstract:
A method is provided for recovering oil from a subterranean oil-bearing formation. One or more pairs of electrodes are inserted into the ground in proximity to a body of oil in said formation. A voltage difference is then established between the electrodes to create an electric field in the oil-bearing formation. As voltage is applied, the current is manipulated to induce oxidation and reduction reactions in components of the oil. The oxidation and reduction reactions lower the viscosity in the oil and thereby reduce capillary resistance to oil flow so that the oil can be removed at an extraction well.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Patent Application No. 60/335,701, filed Oct. 26, 2001, the entire disclosure of which is incorporated by reference herein. 

   FIELD OF THE INVENTION 
   The present invention relates generally to oil production, and more particularly to an improved method for recovering oil from subterranean oil reservoirs with the aid of electric current. 
   BACKGROUND OF THE INVENTION 
   When crude oil is initially recovered from an oil-bearing earth formation, the oil is forced from the formation into a producing well under the influence of gas pressure and other pressures present in the formation. The stored energy in the reservoir dissipates as oil production progresses and eventually becomes insufficient to force the oil to the producing well. It is well known in the petroleum industry that a relatively small fraction of the oil in subterranean oil reservoirs is recovered during this primary stage of production. Some reservoirs, such as those containing highly viscous crude, retain 90 percent or more of the oil originally in place after primary production is completed. Oil recovery is frequently limited by capillary forces that impede the flow of viscous oil through interstitial spaces in the oil-bearing formation. 
   Numerous methods have been proposed for recovering additional oil that remains the in oil-bearing formations following primary production. These secondary recovery techniques generally involve the expenditure of energy to supplement the expulsive forces and/or to reduce the retentive forces acting on the residual oil. A summary of secondary recovery techniques may be found in U.S. Pat. No. 3,782,465, the entire disclosure of which is incorporated by reference herein. 
   One secondary recovery technique for promoting oil recovery involves the application of electric current through an oil body to increase oil mobility and facilitate transport to a recovery well. Typically, one or more pairs of electrodes are inserted within the underground formation at spaced-apart locations. A voltage drop is established between the electrodes to create an electric field through the oil formation. In some processes, electric current is applied to raise the temperature of the oil formation and thereby lower the viscosity of the oil to facilitate removal. Other methods use electric current to move the oil towards a recovery well by electroosmosis. In electroosmosis, dissolved electrolytes and suspended charged particles in the oil migrate toward a cathode, carrying oil molecules with them. These methods typically use a DC potential source to generate an electrical field across the oil-bearing formation. 
   Oil recovery methods that utilize electrodes frequently encounter problems affecting the quantity and quality of the recovered oil. Systems using straight DC voltage typically operate under high voltages and currents. In addition, systems using DC current consume relatively large amounts of electricity with corresponding large energy costs. 
   SUMMARY OF THE INVENTION 
   With the foregoing in mind, the present invention provides an improved method for stimulating oil recovery from an oil-bearing underground formation through the use of electric current. Electric current is introduced through a plurality of boreholes installed in the formation. In systems using only two boreholes, a first borehole and a second borehole are provided in the proximity of the underground formation. The boreholes are located at spaced-apart locations in or near the formation. A first electrode is placed into the first borehole and a second electrode is placed into the second borehole. A source of voltage is then connected to the first and second electrodes. The second borehole may penetrate the body of oil in the underground formation or be located beyond the oil body, so long as some or all of the oil body is located between the second borehole and the first electrode. The first and second boreholes may penetrate the body of oil to be recovered, or they may penetrate the formation at a point beyond but in proximity to the body of oil. 
   The first and second electrodes are installed in an electrically conductive formation, such as a formation having a moisture content sufficient to conduct electricity. A DC biased current with a ripple component is applied through the electrodes under conditions appropriate to create an electrical field through the oil formation. The current is regulated to stimulate oxidation and reduction reactions in the oil. As redox reactions occur, long-chain compounds such as heavy petroleum hydrocarbons are reduced to smaller-chain compounds. The decomposition of long-chain compounds decreases the viscosity of the oil compounds and increases oil mobility through the formation such that the oil may be withdrawn at the recovery well. Electrochemical reactions in the formation also upgrade the quality and value of the oil that is ultimately recovered. The system can be used with a multiplicity of cathodes and anodes placed in vertical, horizontal or angular orientations and configurations. 

   
     DESCRIPTION OF THE DRAWINGS 
     The foregoing summary as well as the following description will be better understood when read in conjunction with the accompanying figures, in which: 
       FIG. 1  is a schematic diagram of an improved electrochemical method for stimulating oil recovery from an underground oil-bearing formation; 
       FIG. 2  is a schematic diagram in partial sectional view of an apparatus with which the present method may be practiced; and 
       FIG. 3  is an elevational view of an electrode assembly adapted for use in practicing the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the Figures in general, and to  FIG. 1 , specifically, the reference number  11  represents a subterranean formation containing crude oil. The subterranean formation  11  is an electrically conductive formation, preferably having a moisture content above 5 percent by weight. As shown in  FIG. 1 , formation  11  is comprised of a porous and substantially homogeneous media, such as sandstone or limestone. Typically, such oil-bearing formations are found beneath the upper strata of earth, referred to generally as overburden, at a depth of the order of 1,000 feet or more below the surface. Communication from the surface  12  to the formation  11  is established through spaced-apart boreholes  13  and  14 . The hole  13  functions as an oil-producing well, whereas the adjacent hole  14  is a special access hole designed for the transmission of electricity to the formation  11 . 
   The present invention can be practiced using a multiplicity of cathodes and anodes placed in vertical, horizontal or angular orientations and configurations. In  FIG. 1 , the system is shown having two electrodes installed vertically into the ground and spaced apart generally horizontally. A first electrode  15  is lowered through access hole  14  to a location in proximity to formation  11 . Preferably, first electrode  15  is lowered through access hole  14  to a medial elevation in formation  11 , as shown in FIG.  1 . By means of an insulated cable in access hole  14 , the relatively positive terminal or anode of a high-voltage d-c electric power source  2  is connected to the first electrode  15 . The relatively negative terminal on the power source or cathode is connected to a second electrode  16  in producing well  13 , or within close proximity of the producing well. Between the electrodes, the electrical resistance of the connate water  4  in the underground formation  11  is sufficiently low so that current can flow through the formation between the first and second electrodes  15 ,  16 . Although the resistivity of the oil is substantially higher than that of the overburden, the current preferentially passes directly through the formation  11  because this path is much shorter than any path through the overburden to “ground.” 
   To create the electric field, a periodic voltage is produced between the electrodes  15 ,  16 . Preferably, the voltage is a DC-biased signal with a ripple component produced under modulated AC power. Alternatively, the periodic voltage may be established using pulsed DC power. The voltage may be produced using any technology known in the electrical art. For example, voltage from an AC power supply may be converted to DC using a diode rectifier. The ripple component may be produced using an RC circuit. Once the voltage is established, the electric current is carried by captive water and capillary water present in the underground formation. Electrons are conducted through the formation by naturally occurring electrolytes in the groundwater. 
   The electric potential required for carrying out electrochemical reactions varies for different chemical components in the oil. As a result, the desired intensity or magnitude of the ripple component depends on the composition of the oil and the type of reactions that are desired. The magnitude of the ripple component must reach a potential capable of oxidizing and reducing bonds in the oil components. In addition, the ripple component must have a frequency range above 2 hertz and below the frequency at which polarization is no longer induced in the formation. The waveshape of the ripple may be sinusoidal or trapezoidal and either symmetrical or clipped. Frequency of the AC component is preferably between 50 and 2,000 hertz. However, it is understood in the art that pulsing the voltage and tailoring the wave shape may allow the use of frequencies higher than 2,000 hertz. 
   A system suitable for practicing the invention is shown in FIG.  2 . In this system, borehole  13  functions as an oil producing well which penetrates one region  17  of underground oil-bearing formation  11 . Well  13  includes an elongated metallic casing  18  extending from the surface  12  to the cap rock  23  immediately above region  17 . The casing  18  is sealed in the overburden  19  by concrete  20  as shown, and its lower end is suitably joined to a perforated metallic liner  24  which continues down into the formation  11 . Piping  21  is disposed inside the casing  18  where it extends from the casing head  22  to a pump  25  located in the liquid pool  26  that accumulates inside the liner  24 . Preferably the producing well  13  is completed in accordance with conventional well construction practice. The pump  25  is selected to operate at sufficient pumping head to draw oil from adjacent formation  11  up through metallic liner  24 . 
   Access hole  14  that contains first electrode  15  includes an elongated metallic casing  28  with a lower end preferably terminated by a shoe  29  disposed at approximately the same elevation as the cap rock  23 . The casing  28  is sealed in the overburden  19  by concrete  30 . Near the bottom of hole  14 , a tubular liner  31  of electrical insulating material extends from the casing  28  for an appreciable distance into formation  11 . The insulating liner  31  is telescopically joined to the casing  28  by a suitable crossover means or coupler  32 . Although shown out of scale in  FIG. 2 , liner  31  preferably has a substantial length and a relatively small inside diameter. 
   Below the liner  31 , a cavity  34  formed in the oil-bearing formation  11  contains the first electrode  15 . The first electrode  15  is supported by a cable  35  that is insulated from ground. The first electrode  15  is relatively short compared to the vertical depth of the underground formation  11  and may be positioned anywhere in proximity to the formation. Referring to  FIG. 2 , first electrode  15  is positioned at an approximately medial elevation within the oil-bearing formation  11 . The first electrode may be exposed to saline or oleaginous fluids in the surrounding earth formation, as well as a high hydrostatic pressure. Under these conditions, first electrode  15  may be subject to electrolytic corrosion. Therefore, the electrode assembly preferably comprises an elongate configuration mounted within a permeable concentric tubular enclosure radially spaced from the electrode body. The enclosure cooperates with the first electrode body to protect it from oil or other adverse materials that enter the cavity. 
   Referring now to  FIG. 3 , a preferred assembly for the first electrode  15  is shown. The assembly comprises a hollow tubular electrode body  15  electrically connected through its upper end to a conducting cable  35  and disposed concentrically in radially spaced relation within a permeable tubular enclosure  16   a  of insulating material. The first electrode  15  is preferably coated externally with a material, such as lead dioxide, which effectively resists electrolytic oxidation. The assembly preferably includes means to place the internal surfaces of the first electrode  15  under pressure substantially equal to the external pressure to which the first electrode is exposed, thereby to preclude deformation and consequent damage to the first electrode. The enclosure  16   a  is closed at the bottom to provide a receptacle for sand or other foreign material entering from the surrounding formation. 
   Referring again to  FIG. 2 , the first electrode  15  is attached to the lower end of insulated cable  35 , the other end of which emerges from a bushing or packing gland  36  in the cap  37  of casing  28  and is connected to the relatively positive terminal of an electric power source  38 . The other terminal on the electric power source  38  is connected via a cable  42  to an exposed conductor that acts as a second electrode  16  at the producing well  13 . The second electrode  16  may be a separate component installed in the proximity of producing well  13  or may be part of the producing well itself. In the embodiment shown in  FIG. 2 , the perforated liner  24  serves as the second electrode  16 , and the well casing  18  provides a conductive path between the liner and cable  42 . 
   Thus far, it has been presumed that electrodes  15 ,  16  are located in a formation with a suitable moisture content and naturally occurring electrolytes to provide an electroconductive path through the formation. In formations that do not have adequate capillary and captive groundwater to be electrically conductive, an electroconductive fluid may be injected into the formation through one or both boreholes to maintain an electroconductive path between the electrodes  15 ,  16 . Referring to  FIG. 2 , a pipe  40  in borehole  14  delivers electrolyte solution from the ground surface to the underground formation  11 . Preferably, a pump  43  is used to convey the solution from a supply  44  and through a control valve  45  into borehole  14 . Borehole  14  is preferably equipped with conventional flow and level control devices so as to control the volume of electrolyte solution introduced to the borehole. A detailed system and procedure for injecting electrolyte solution into a formation is described in the aforementioned U.S. Pat. No. 3,782,465. See also, U.S. Pat. No. 5,074,986, the entire disclosure of which is incorporated by reference herein. 
   Referring now to  FIGS. 1-2 , the steps for practicing the improved method for stimulating oil recovery will now be described. An electric potential is applied to first electrode  15  so as to raise its voltage with respect to the second electrode  16  and region  17  of the formation  11  where the producing well  13  is located. The voltage between the electrodes  15 ,  16  is preferably no less than 0.4 V per meter of electrode distance. Current flows between the first and second electrodes  15 ,  16  through the formation  11 . Connate water  4  in the interstices of the oil formation provides a path for current flow. Water that collects above the electrodes in the boreholes does not cause a short circuit between the electrodes and surrounding casings. Such short circuiting is prevented because the water columns in the boreholes have relatively small cross sectional areas and, consequently, greater resistances than the oil formation. 
   As current is applied across formation  11 , electrolysis in the capillary water and captive water takes place. Water electrolysis in the groundwater releases agents that promote oxidation and reduction reactions in the oil. That is, negatively charged interfaces of oil compounds undergo cathodic reduction, and positively charged interfaces of the oil compounds undergo anodic oxidation. These redox reactions split long-chain hydrocarbons and multi-cyclic ring compounds into lighter-weight compounds, contributing to lower oil viscosity. Redox reactions may be induced in both aliphatic and aromatic oils. As viscosity of the oil is reduced through redox reactions, the mobility or flow of the oil through the surrounding formation is increased so that the oil may be drawn to the recovery well. Continued application of electric current can ultimately produce carbon dioxide through mineralization of the oil. Dissolution of this carbon dioxide in the oil further reduces viscosity and enhances oil recovery. 
   In addition to enhancing oil flow characteristics, the present invention promotes electrochemical reactions that upgrade the quality of the oil being recovered. Some of the electrical energy supplied to the oil formation liberates hydrogen and other gases from the formation. Hydrogen gas that contacts warm oil under hydrostatic pressure can partially hydrogenate the oil, improving the grade and value of the recovered oil. Oxidation reactions in the oil can also enhance the quality of the oil through oxygenation. 
   Electrochemical reactions are sufficient to decrease oil viscosities and promote oil recovery in most applications. In some instances, however, additional techniques may be required to adequately reduce retentive forces and promote oil recovery from underground formations. As a result, the foregoing method for secondary oil recovery may be used in conjunction with other prior art processes, such as electrothermal recovery or electroosmosis. For instance, electroosmotic pressure can be applied to the oil deposit by switching to straight d-c voltage and increasing the voltage gradient between the electrodes  15 ,  16 . Supplementing electrochemical stimulation with electroosmosis may be conveniently executed, as the two processes use much of the same equipment. A method for employing electroosmosis in oil recovery is described in U.S. Pat. No. 3,782,465. 
   Many aspects of the foregoing invention are described in greater detail in related patents, including U.S. Pat. Nos. 3,724,543, 3,782,465, 3,915,819, 4,382,469, 4,473,114, 4,495,990, 5,595,644 and 5,738,778, the entire disclosures of which are incorporated by reference herein. Oil formations in which the methods described herein can be applied include, without limitation, those containing heavy oil, kerogen, asphaltinic oil, napthalenic oil and other types of naturally occurring hydrocarbons. In addition, the methods described herein can be applied to both homogeneous and non-homogeneous formations. 
   The terms and expressions which have been employed are used as terms of description and not of limitation. Although the present invention has been described in detail with reference only to the presently-preferred embodiments, there is no intention in use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof. It is recognized that various modifications of the embodiments described herein are possible within the scope and spirit of the invention. Accordingly, the invention incorporates variations that fall within the scope of the following claims.