METHOD FOR ELECTROKINETIC PREVENTION OF SCALE DEPOSITION IN OIL PRODUCING WELL BORES

Muhammad Haroun, Ph.D.

J. Kenneth Wittle, Ph.D.

George V. Chilingar, Ph.D.

Bisweswar Ghosh, Ph.D.

FIELD OF THE INVENTION

The present invention relates generally to the prevention of mineral scale deposition in a well bore, and more particularly to a method for electrokinetically preventing mineral scale deposition in oil well bores with the aid of DC electric current.

BACKGROUND OF THE INVENTION

The waterflood, a secondary enhanced oil recovery process/ ¹¹ is a simple, low cost, and proven approach for pressure maintenance and for driving oil towards a production well.

Though initial waterflooding attempts were used to rectify plugged wells or casing leaks, the apparent benefits led to broader applications ^pl . Waterflood efficiency depends on oil viscosity, permeability, wettability, structural considerations, uniformity of reservoir rock, and type of flood ^[2] . The volume of liquid produced partly determines the volume of water required for injection ¹¹¹ . For economic reasons nearby seawater is commonly used, where available, as the injection water type to save money on water transportation. The mixing of incompatible injection seawater and formation water frequently produces mineral scale deposits, one of the most significant and costly problems encountered in oilfield operations ^[3 Water flooding operations conducted in the Abu Dhabi oilfields often result in the formulation of BaS0 ₄ , CaS0 ₄ and SrS0 ₄ deposits. The S0 ₄ ^2" and Ba ²⁺ ion content in both seawater and formation water, respectively, can easily reach the solubility product (K _s ) causing accumulation of BaS0 ₄ scale on surface and subsurface equipment. This is recognized as a major cause of formation damage in production or injection wells. Inorganic scale contributes to wear, corrosion, and flow restriction, resulting in a decrease of oil and gas production. This scale also deposits in downhole pumps, tubing, casing, flow lines, heaters, treaters, tanks and other production equipment and facilities ¹³¹ . Barium sulfate (BaS0 ₄ ) scale is among the toughest to remove either by mechanical or chemical means. BaS0 ₄ is typically removed by mechanical tools that involve abrasion, such as gauge cutters, nipple brushes and spinning wash tools. Chemical removal methods utilizing ethylenediaminetetraacetic acid (EDTA) are also available ^[3

Unfortunately, current scale inhibitor applications incur high costs due to conventional chemical dissolution. Scale inhibitor treatment is limited by its "squeeze efficiency" into the formation, which results in limited penetration as well as quick consumption in the reservoir. A squeeze usually involves the application of pump pressure to force a treatment fluid or slurry into a planned treatment zone (Schlumberger Oilfield Glossary). The problem is that scale inhibitors do not move deeply into the reservoir, hence only a small volume can be squeezed before being rapidly consumed. A need exists for a new methodology to prevent scale formation which is both economical and effective.

While electrically enhanced techniques for promoting oil recovery are known, including those described in United States Patent Nos. 5,614,077 and 7,325,604, such techniques have not been applied in the fashion described herein for preventing mineral scale deposits.

SUMMARY OF THE INVENTION

The method of the invention involves the application of electrokinetics for mitigating mineral scale formation. In one embodiment, the present invention provides an electrokinetic method for preventing mineral scale deposition in an oil well, having a well bore in fluid communication with an oil-bearing formation in which water and positively and negatively charged scale-forming species are present. The method comprises the steps of:

a) positioning at least one first electrode adjacent to a well bore;

b) positioning at least one second electrode at a location spaced apart from the first electrode and within electrical current conducting proximity of the first electrode;

c) applying a potential difference between the first electrode(s) and the second electrode(s) using a direct current (DC) power source, the potential difference producing an electrical current flow between the first electrode(s) and the second electrode(s), whereby the positively charged scale-forming species are caused to migrate toward one of the first and the second electrode(s), and the negatively charged scale-forming species are caused to migrate toward the other of the first and second electrode(s). In an aspect of this method, the potential difference is applied such that the first electrode(s) serves as one or more cathodes and the second electrode(s) serves as one or more anodes.

In another aspect of this method, at least one of the positively and negatively charged scale-forming species is introduced into the formation from an external source such as waterflooding. When waterflooding is the source of the scale-forming species, the method may be performed using an electrically conducting aqueous solution, e.g., a prepared or manmade aqueous salt solution, or alternatively, an aqueous solution selected from the group consisting of seawater, groundwater, surfacewater, and wastewater.

In a further aspect of the method, the positively and negatively charged scale-forming species include at least one alkaline earth metal ion and sulfate or carbonate ions.

In still a further aspect of the electrokinetic method, multiple cathodes are positioned in the vicinity of the well. Additionally, multiple anodes may be positioned at locations spaced apart from the cathodes and beyond the well, and in preferred installations the number of anodes exceeds the number of cathodes.

In yet another embodiment, the present invention provides an electrokinetic method for preventing mineral scale deposition in an oil well, and the vicinity of the well, with the well having a well bore in fluid communication with an oil-bearing formation in which water and positively and negatively charged scale-forming species are present, the method comprising the steps of:

a) positioning at least one cathode adjacent to the well bore;

b) positioning a plurality of anodes at a location spaced apart from the at least one cathode and within electrical current conducting proximity of the at least one cathode, wherein the number of anodes exceeds the number of cathodes;

c) applying a potential difference between the at least one cathode and each individual anode of the plurality of anodes; whereby electrical current flow between the at least one cathode and each individual anode of the plurality of anodes causes the positively charged scale-forming species to migrate toward the at least one cathode, and the negatively charged scale-forming species to migrate toward each individual anode of the plurality of anodes.

In another aspect, the method further comprises the step of providing a switch between the at least one cathode and each individual anode of the plurality of anodes, wherein the switch is adapted to be opened to interrupt application of the potential difference between the at least one cathode and each individual anode of the plurality of anodes, or closed to apply the potential difference between the at least one cathode and each individual anode of the plurality of anodes.

In a further aspect of the method of the invention, the step of applying a potential difference between the at least one cathode and each individual anode of the plurality of anodes further comprises the step of providing a DC power source between the at least one cathode and each individual anode of the plurality of anodes.

The method described herein is believed to be the first use of direct current to prevent scale deposition in a well bore in fluid communication with an oil bearing formation. The electrokinetic method for preventing scale deposition described herein may be categorized as a green technology, since there is no water consumption, and no air, water, or formation pollution. The technology can be applied without depth limitations in situ, thereby making it an attractive option in remote or environmentally challenging operating locations.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description will be more easily understood when read in conjunction with the accompanying figures in which:

FIGURES 1A-C are circuit diagrams representing cathode and an anode configurations where the number of anodes exceeds the number of cathodes.

FIGURE 2 is a graphical representation of the pressure across the core versus time in experiment 1 of Example 1.

FIGURES 3A-C are a set of graphs showing the barium concentration profiles of the experiments in Example 1 ; Figure 3A is a graphical representation of the concentration profile of barium found in the tested electrode configuration (++--) for all tested salinity and

composition waters against the blank at the five strategic sampled positions across the 18 cm sand specimen of Example 1; Figures 3B-C are graphs representing the concentration profile of barium remaining after application of DC current; where Figure 3B includes the average of experiments 1 and 10 in addition to experiments 2, 5, 8 - seawater/formation composition water (SW/FW) and 11 of Experiment 1; and Figure 3C includes experiments 5, 8, 9 and 12 of Example 1. FIGURE 4 is a graph of the current across the core as a function of time for experiment 2 of Example 1.

FIGURES 5A-C are a set of graphs showing current as a function of time across the core for several experiments of Example 1 ; Figure 5A is a graph of the current across the core as a function of time for experiment 3 of Example 1; Figure 5B is a graph of the current across the core as a function of time for experiment 5 of Example 1 ; and Figure 5C is a graph of the current across the core versus time for experiment 8 of Example 1.

FIGURES 6A-C are a set of graphs showing pressure as a function of current across the core for several experiments of Example 1; Figure 6A is a graph of the pressure versus current for experiment 2 of Example 1 ; Figure 6B is a graph of the pressure as a function of current for experiment 3 of Example 1 ; and Figure 6C is a graph of the pressure as a function of current for experiment 8 of Example 1.

FIGURE 7 is a graph of the standardized concentration profile of barium with and without DC current - No salinity and actual seawater/formation composition water (SW/FW) of Example l(see Table 3).

FIGURE 8 is a graphical representation of the change in permeability with respect to the pore volume in the blank experiment of Example 2.

FIGURE 9 is a schematic illustration of a consolidated sand cell shown, in cross-section, with an electrode positioned at each of the production water outlet and the sea water inlet.

FIGURES 10A-B are schematic illustrations of the electrokinetic cell utilized in

Example 2; Figure 10A is a schematic illustration of a consolidated sand cell showing, in cross- section, the distribution of anodes and cathodes in a first configuration (AAACC), and Figure 10B is a schematic illustration of a consolidated sand cell shown, in cross-section, a distribution of anodes and cathodes in the second configuration (AAAAC).

FIGURE 11 is a graphical representation of the effect of pH on BaS0 ₄ solubility.

FIGURES 12 A-F are a set of graphs showing permeability as a function of pore volume for several experiments of Example 2; Figure 12A is a graphical representation of permeability reduction with respect to the pore volume in experiment 5 of Example 2; Figure 12B is a graphical representation of permeability reduction with respect to the pore volume in experiment 6 of Example 2; Figure 12C is a graphical representation of permeability reduction with respect to the pore volume in experiment 7 of Example 2; Figure 12D is a graphical representation of permeability reduction with respect to the pore volume in experiment 8 of Example 2; Figure 12E is a graphical representation of permeability reduction with respect to the pore volume in experiment 9 of Example 2; and Figure 12F is a graphical representation of permeability reduction with respect to the pore volume in experiment 10 of Example 2.

DETAILED DESCRIPTION OF THE INVENTION

Electrokinetics is a term applied to a group of physicochemical phenomena involving the transport of charges, action of charged particles, effects of applied electric potential and fluid transport in various porous media to allow for a desired migration or flow to be achieved ¹⁴¹ . These phenomena include electromigration, electrophoresis, electroosmosis, enhanced chemical reaction, and joule heating. Electromigration occurs due to the movement of anions and cations between the anode and the cathode across spatial distance in both directions. Electrophoresis induces movement of the negatively charged colloidal and surface charged particles that are free to migrate in formation pores towards the anode. This mechanism, which is typically used to dewater clays at rates several orders of magnitude higher than hydraulic rates, can effectively increase apparent reservoir permeability and oil production ^[5] .

Another important mechanism is electroosmosis, which is the preferable movement of electrolytes caused by an imposed potential difference, involving an electric double layer (also called Helmholtz double layer), and consisting of two sub regions: mobile and immobile. The potential difference between this interface and the bulk liquid is the zeta potential ^t6

Electrochemically enhanced reactions are effective to induce "cold-cracking" of heavy crudes, which results in their breakdown into lighter components, with a significant increase in the flow rate. Reactions between the pore fluids and matrix materials are enhanced by E _SHE PH changes caused by the passage of DC; this mechanism lowers the viscosity of heavy oil. Finally Joule heating is the process by which the passage of an electric current through a conductor releases heat ^[4] .

Scale deposits occur in a well bore of a producing oil well, typically at the well bore interface due to accumulation of insoluble minerals, such as barium and calcium sulfate. To find a long term solution for this common problem, electrokinetics can be utilized to counteract scaling. In practicing the method of this invention it is demonstrated that this scale deposition process may be mitigated or alleviated by using electrokinetics via application of DC current. This treatment stabilizes the system by moving anions towards the anode and cations towards the cathode, thus separating the scale-forming ions.

As used herein, the term "electrode" includes either of two electrically conductive elements having different potential activity that enables an electric current to flow in the presence of an electrolyte. Electrodes can also be referred to as plates or terminals, and require at least one cathode (the negative electrode to which positively charged ions migrate) and at least one anode (the positive electrode to which negatively charged anions migrate). The electrodes can be fabricated from metallic and non-metallic electrically conductive material. Metallic conductive materials can be selected from the group which includes, but is not limited to, zinc, aluminum, copper, iron, manganese dioxide, nickel, cadmium, titanium, platinum, or an alloy thereof. Preferably, the electrodes are fabricated from a non-metallic material. More preferably, the non-metallic material is graphite. However, the ordinary artisan would understand that the material utilized in the fabrication of the electrodes is dependent upon the environment and conditions in which it will be utilized.

The potential difference may be applied across the electrodes such that the first electrode(s) serves as one or more cathode(s) and the second electrode(s) serves as one or more anodes. In a preferred embodiment of the invention multiple cathodes are positioned in the vicinity of the well, and multiple anodes are positioned at locations spaced apart from the cathodes and beyond the well, with the number of anodes exceeding the number of cathodes. For example, where the number of anodes exceeds the number of cathodes, the cathode/anode configuration may be as shown in Figures 1A-C. In the more preferred embodiment of the invention, the first electrode is a cathode and the second electrode serves as multiple anodes, with the cathode positioned in the vicinity of the well and the multiple anodes positioned at locations spaced apart from the cathodes and beyond the well.

In practicing the method of the invention, the potential difference applied across the electrodes should provide a potential gradient of at least about 0.01 to 100 volts/cm. Preferably, the potential gradient is at least about 1 to 10 volts/cm. In the most preferred method of the invention, the potential gradient is at least about 2 volts/cm. It is also preferred that the applied voltage produces an electric current density in the range of at least about 0.5 to 250 mA/cm ² .

As used herein, the term "mineral" includes inorganic substances, salts and compositions. As used herein, the term "mineral scale" refers to precipitated insoluble inorganic salts that are composed of positively and negatively charged scale-forming species. Such species comprise at least one cation and at least one anion, respectively, that can form insoluble salts that become deposited in and around the well bore. The cationic scale-forming species include alkaline earth, alkali, and transition metals that precipitate out of an aqueous solution when combined with the appropriate anion. Typically, the cations are alkaline earth metals, including, but not limited to, barium (Ba ²⁺ ), calcium (Ca ²⁺ ), and strontium (Sr ² *). The anions often found in scale deposits include, but are not limited to, sulfate (S0 ₄ ^2" ) and carbonate (C0 ₃ ^2" ) ions.

As used herein, the term "well bore", refers to any elongated hole or shaft drilled in or in fluid communication with a reservoir for exploring or extracting natural resources therefrom and also to any such opening drilled in or in fluid communication with a reservoir for the purpose of introducing a fluid into a reservoir. In instances where the well bore is used for the extraction of natural resources, those resources include water, oil, gas or a mixture thereof, and may be extracted for an extended period of time. In instances where the well bore is used to introduce a fluid, the preferred method of fluid introduction is water flooding. The fluid introduced may include electrically conducting aqueous solutions or gas. Electrically conducting aqueous solutions include aqueous salt solutions, seawater, groundwater, surfacewater, and wastewater. Groundwater is water located beneath the ground surface and may include formation water from an oil field, water from a geological strata apart from the strata containing the oil well, and water from an aquifer. Surfacewater includes brackish water that generally has more salinity than fresh water but has less salinity than seawater and includes water from an estuary, lake, or marsh. Wastewater includes residual water from a water treatment facility and residual water from a reverse osmosis facility. In the preferred method of the invention, a production well bore may be drilled in or adjacent to a reservoir composed of sand which contains oil and at least some formation water. Formation water, which may be connate water, is an electrically conducting aqueous solution containing at least some positively or negatively charged scale forming species. In the preferred method of the invention at least one introduction well bore may be drilled in or adjacent to a reservoir penetrated by a production well and fluid is introduced into the reservoir. Preferably, the fluid introduced is an electrically conducting solution containing sodium chloride at a concentration greater than about 10,000 ppm. More preferably, the sodium chloride concentration is at least about 10,000 - 40,000 ppm. In a preferred embodiment of the method of the invention the waterflooding fluid is seawater.

As used herein, the term "oil well vicinity" includes the zone generally surrounding the well bore, through which oil flows to the well and in which mineral deposition occurs, which is associated with the oil containing reservoir and includes the entirety of the oil containing reservoir. As used herein, the term "beyond the well" signifies that, in connection with the positioning of anodes, such anode(s) are placed at a distance from the producing well which is greater than that of the cathode(s).

As used herein, the term "waterflood", refers to the introduction of fluid through an introduction well bore in or adjacent to a reservoir containing oil, gas or a mixture thereof, to create an edge water drive flooding the oil, gas or a mixture thereof, towards a production well bore by displacement. Waterflooding provides pressure maintenance and operates as a secondary process for oil recovery enhancement.

The following examples describe the invention in further detail. These examples are provided for illustrative purposes only, and should in no way be considered as limiting the invention.

EXAMPLE 1

An electrokinetic (EK) model was developed to study BaS0 ₄ scale mitigation in a sand- packed glass cell with variable electrode position and fluid injection options. The mitigation of scale deposition was determined by measuring pressure build up within the cell with and without the application of a DC electric field. Results demonstrate that under a DC electric field, the BaS0 ₄ scaling rate is reduced by up to 60% at the production outlet. Chemical analysis of the scale deposits at various points showed high localized concentration distribution throughout, but 50% or less deposition near the producing end. Electrokinetic Model

The basic apparatus used in this study was adapted from an electrokinetic cell as shown in Wittle et al., U.S. Patent No. 5,614,077. Graphite electrodes were connected to an adjustable DC power supply and DC power was applied at a fixed 2 V/cm potential gradient using a constant electrode configuration in all experiments. The injector sites, the locations of sulfate anion introduction, were used as anode locations while the simulated production outlet was used as the cathode location. Saline solutions were provided as simulated formation fluid and varied in sodium chloride concentration from 0 to 40,000 ppm NaCl. The rate of formation fluid flow was set at 1 mL/min.

Two pumps injected a Ba ²⁺ solution and a S0 ₄ ^2" solution at the inlet portion of an 18 cm electrokinetic sandpack at the constant rate of 1 mL/min. All experiments were performed for 830 minutes. A single electrode configuration was tested, with an anode at the inlet end and a cathode at the producing end. At the production end, the produced water was collected for chemical analysis. A digital pressure gauge calibrated for a low pressure range was used to measure real-time pressure across the core. At the completion of each experiment, samples from five different locations along the core ( 1 - inlet-first anode, 2 - second anode, 3 - center of the core, 4 - first cathode, 5 - outlet-second cathode) were collected and analyzed for Ba ²⁺ and S0 ₄ ^2" content.

Sand Preparation

Sand used in the sand packs was obtained from Al Ain, U.A.E., 120 km inland from the shore. The sand was washed and graded to obtain uniform grain size of 125 μΜ for use in all the experiments. The sand was washed several times with deionized water to remove dust and dissolvable salts, dried and graded for use.

Solution Preparation

Two injection solutions were prepared: one with 500 ppm of Ba ²⁺ simulating formation water, and one with 500 ppm of S0 ₄ ^2' simulating seawater. Five saline solutions were prepared with various NaCl concentrations: 0, 10,000 ppm, 20,000 ppm, 30,000 and 40,000 ppm. The final solution used was actual Abu Dhabi formation water and seawater having the specific composition shown in Table 1. Formation Water Seawater

Well No.: TH-1 from ADMA-OPCO Abu Dhabi

Concentration, ppm Concentration, ppm

Na ²⁺ 60,962 13,000

Ca ²⁺ 14,085 531

Mg ²⁺ 1,072 1,530

Fe ²⁺ 2

Ba ²⁺ 5

Sr ²⁺ 500

cr 124,573 23,100

S0 ₄ ^2" 526 3,130

HC0 ₃ - 70 154

TDS 201,795 41,445

Table 1. Concentration of ions in formation water and seawater.

Results and Discussion

To measure scaling in the absence of DC current, Experiments 1 and 10 were performed at zero potential to determine the average deposition concentration and distribution of BaS0 ₄ along the core length upon the mixing of the incompatible waters. Experiments 2-9 and 11 were performed, using solutions of simulated formation water, with a final experiment (Experiment 8) utilizing actual formation water and seawater.

In the absence of DC electrical current there was a steady build up of pressure across the core (Figure 2). The concentration profile of the core is presented in Table 2. Figures 3A-C show the concentration profile. Position No.

Concentration

Exp Conditions ofBa ²⁺ in 1 2 3 4 5 No. Sand, ppm

1 0 V/cm, no EK 26.47 12.98 15.01 1518.25 2173.16 1610.23

10 0 V/cm, no EK 11.54 11.18 171 1.52 907 893.94

Avg. of Exp. Nos. 1 and 10 12.26 13.1 1614.9 1540.1 1252.1

Table 2. Concentration of Ba at positions 1 , 2, 3, 4, and 5 along the core (in blank experiments 1 and 10).

The results of experiment 2, in which a DC field was applied with zero salinity, demonstrated accumulation of Ba ²⁺ ion at all locations rather than migrating out of the system. This resulted in localized increased concentration and precipitation at the electrode locations. In this case electroosmosis occurred at a very low rate due to an average of 0.5 mA of DC current achieved through the test duration as shown in Figure 4.

The results of experiments 4-9 and 1 1 , in which a DC field was applied, with a salinity of 10,000 to 40,000 ppm, demonstrated accumulation of Ba ²⁺ ion at all locations with varying migrating fractions of Ba ²⁺ ion out of the system. This resulted in localized increased concentration and precipitation at the electrode locations. In this case electroosmosis and electromigration where the dominant migration mechanisms due to an average of 50 - 250 mA of DC current achieved throughout the test duration as can be seen in Figures 5A-C. The pressure drop recorded was a significant reduction following the application of DC current, which indicated a sustained fluidic flow though the core as a result of high mobility of the Ba ²⁺ out of the system.

Figures 6A-C demonstrate a correlation between recorded real-time pressure drop as well as current achieved across the core for all the experiments. The correlation coefficient ranged from as low as 0.54 for the case of experiment 2 with 0 ppm NaCl concentration to 0.89 for the case of experiment 5 with 30,000 ppm NaCl concentration.

Figures 3A and 7 demonstrate the Ba ²⁺ concentration through the core for all

experiments 1 - 11. With the application of a DC potential gradient at 2 V/cm all the tested specimens with salinity greater than 10,000 ppm demonstrated a 60% average reduction of Ba ⁺ concentration, with a 71% maximum reduction using a simulated formation water having a NaCl concentration of 30,000 ppm. A real-time pressure transducer recorded a reduced pressure drop of approximately 50% across the sample, with a 90% maximum when compared to the blank results (experiments 1 and 10).

Experiment 8 was carried out using Abu Dhabi seawater and formation water having the ion concentrations listed in Table 1. The results demonstrated a 52% reduction of Ba ²⁺ at the outlet, with an average accumulation of Ba ²⁺ at other locations across the sample. Tables 3 and 4 present the final concentration of Ba ²⁺ (in ppm) remaining across the core as well as the fraction of initial concentration as generated from the blank experiments. Both experiments 5 and 9 at 30,000 and 20,000 ppm NaCl concentration, respectively, generated the best flow results.

Formation water and seawater

Table 3. Concentration of Ba ²⁺ at positions 1, 2, 3, 4, and 5 along the core.

Position No. (Ba ²⁺ Concentration, >pm)

Exp No. Concentration of Salinity

1 2 3 4 5

Ba ²⁺ in Sand, ppm (ppm)

Avg. of

(1 + 10) 12.26 13.10 1614.90 1540.10 1252.10 Zero

2 12.68 113.49 147.35 132.51 171.49 176.66 Zero

9 0.85 1.21 1.26 0.83 0.36 20K

5 7.71 30.47 0.24 0.21 0.29 30K

11 35.10 24.20 0.59 1.51 0.43 40K

8 0.92 5.22 1.30 2.65 0.48 t

Formation water and seawater

Table 4. Fraction concentration of Ba remaining after DC application for 830 minutes along five positions across the core.

Optimum results were achieved with a 20,000 ppm NaCl formation saline solution that generated the maximum reduction at most locations between the inlet end and the producing end. This allowed more Ba ²⁺ to be produced out of the outlet and less accumulation of Ba ²⁺ in situ across the core length. Application of the DC field resulted in the reduction of Ba ²⁺ ion concentration from the center to the producer of the tested core when the salinity was greater than 10,000 ppm NaCl.

Application of a DC electric field mitigates BaS0 ₄ scale formation in a simulated oilfield experiment. In essence, application of DC electrical current destabilizes the system by moving

Ba 2+ and S0 ₄ 2 ^" ions away from each other. On application of DC electrical current maintaining 2 V/cm potential gradient all the tested specimens with salinity greater than 10,000 ppm NaCl showed a 60% average reduction of Ba ²⁺ concentration at the outlet (producing end), with a 71% maximum reduction when using a simulated formation water (30,000 ppm NaCl). Pressure drop reduction across the sample was 50% on average, with a maximum of 90%. The 20,000 ppm NaCl solution generated the maximum reduction of Ba ²⁺ content at most locations between the inlet end and the producing end. Thus, applying a direct current reduced Ba ²⁺ from the center of the core to the outlet when the salinity of the water was greater than 10,000 ppm NaCl. EXAMPLE 2

An electrokinetic (EK) model was developed having a net spatial distance of 46 cm between the injection and production wells. The reservoir model consists of a sample tube containing 125 μπι uniform sand particles consolidated to a net pressure of 30 psi in order to achieve a homogeneous state. The EK-reservoir model contained eight openings; three holes for 500 ppm S0 ^2" solution injection, 500 ppm Ba ²⁺ solution injection and water outlet production respectively. The other five were allocated for testing electrode configurations allowing comparisons of anode and cathode combinations covering the range of spatial distance. Salinity was altered in the range of (0 ppm to 40,000 ppm), a 2 ml/min flowrate was maintained and 2 V/cm of voltage gradient was applied. On a real time basis; the current, pressure, temperature, and pH of produced water were all monitored.

The electrode configuration was varied in the five positions by changing the anode and the cathode locations along the injectors' inlets and the production outlet. Results demonstrated that EK has an impact on scale mitigation due to improved electroosmosis and electromigration allowing for increased efficiency in arresting the precipitous ions.

An experiment was conducted using an electrolytic cell to study the effect of applying DC current on a solution prepared from 4000 ppm of BaCl ₂ . Barium accumulated in the cathode vicinity while chlorine gas was produced at the anode. Barium concentration at the cathode is due to arresting while the barium being repelled at the anode side is due to deposition.

Barium and chlorine gas are produced according to these reactions:

(Cathode) Ba ²⁺ + 2 e ^" → Ba

(Anode 2C1 ^" → Cl ₂ + 2 e ^"

A blank experiment was also conducted, in which pure deionized water was used. The objective was to check the regular permeability trend before adding saline components. Figure 8 provides the permeability trend of change with pore volume number. In this case it was observed that permeability was stable under an average permeability of 5700 mD (millidarcy). Sand Preparation

Different sand samples, simulating the formation, were tested. The sand sample was obtained from Al Ain, U.A.E., which is 120 km away from the shore. The sand grain size was non-uniform. A sieve process was performed to achieve a uniform sand particle size of 125 μιη. The sand was washed several times with tap water and eventually with de-ionized water, to clean it from dust and dissolvable salts. Finally the sand was dried in an oven before loading into the EK cell.

Solution Preparation

Since the experiments depend on the electrolytes and the distribution of cations and anions in the formation, the electrolyte concentration required careful control. Two solutions were prepared having concentrations of 500 ppm S0 ₄ ^2" and 500 ppm Ba ²⁺ to simulate the sea and formation water, respectively. Temperature, pressure, and salinity were monitored during the experiment to control the parameters of scale formation.

Cell Design and Consolidation

Figure 9 is a schematic representation of the comprehensive apparatus used for conducting the experiments of Example 2. The dried sand mixed with 35% of deionized water was prepared for consolidation; sand was compressed to 30 psi in the 46 cm cylinder. The apparatus was designed to simulate real reservoirs, including both injection and production paths. It also contains graphite electrodes to conduct the electric current. Two pumps were used to inject the barium and sulfate solutions. Two thermometers were added to measure temperature change. A receptacle was placed at the production end to collect effluent solution. A direct current power source supplied electric current allowing measurement of current change through the apparatus on a real time basis. A digital pressure gauge was connected to measure the pressure across the EK cell and a 2 V/cm voltage gradient was applied while the flow rate was fixed at 2 ml/min. Each experiment was conducted for 1000 min. The study was designed to understand the effect of varying electrode configuration on scale mitigation. Different configurations were tested, where anode and cathode electrodes were distributed in this order: (Anode, Anode, Anode, Cathode, Cathode) and (Anode, Anode, Anode, Anode, Cathode). After each experiment solid samples were analyzed using ICP-MS. Figure 9 shows the position of the solid collected samples.

Results and Discussion

A first electrode configuration was prepared to provide the following order: Anode, Anode, Anode, Cathode, Cathode (Figure 10A), and a second electrode configuration was prepared to provide the following order: Anode, Anode, Anode, Anode, Cathode (Figure 10B). Three gases were produced by the electrochemical reaction: chlorine, oxygen and hydrogen.

The chemical reactions at the electrodes are as follows:

(Anode) 2H ₂ 0 - 4e ^" → 0 ₂ + 4H ⁺

2Cr - 2e→Cl ₂

(Cathode) l/20 ₂ + H ₂ 0 + 2e ^" → 20H ^" (Alkaline solution)

2e ^" + 2H ⁺ → H ₂ (Acidic solution)

Table 5 provides the results of experiments conducted and ICP-MS results for the solid samples extracted from the five electrode placement positions in two tested configurations.

Table 5. Experiment configurations and ICP-MS results.

The pH of the effluent stream was greater than 10 during all the experiments due to the cathode's location near the production well. It can be concluded that the chlorine gas generated might cause a stimulation effect due to possible acidizing. Figure 11 demonstrates the effect of pH on barium sulfate solubility. Barium sulfate dissolves at low pH because SO4 ^"2 converts to HSO4 ^" High saline brines produce greater BaS0 ₄ complications due to the greater initial mass in the brine and since the solubility of an ionic compound decreases in the presence of a common ion. A common ion is any ion in the solution that is common to the ionic compound being dissolved. As a result, complications due to greater scale deposition were expected with more concentrated saline solutions. Table 6 demonstrates the anode arresting barium and barium sulfate depositing at the cathode near the simulated production well.

Table 6. Barium arresting and barium sulfate deposition.

Electrode Configurations

Experiment 5 vs. Experiment 6. When the configuration AAACC was applied, an increase of up to 135.63% in barium arresting was observed, due to the increase in barium arresting locations (Cathodes). A decrease of up to 41.50% in barium sulfate deposition was observed since most of the barium was arrested by the cathode specific surface area before reacting with the sulfate anions. Figures 12A-B demonstrate a decrease in permeability with the volume injected in the two experiments. This significant decrease can be attributed to the increase of barium arresting and the deposition of barium sulfate. Alternatively, it could be posited that the increased chlorine gas generated was reducing the EK effect due to starvation of the conductive media. The slope is another indication for deposition; it can be noticed that slope (rate of deposition) is lower in the case of AAACC.

Experiment 7 vs. Experiment 8. When the configuration AAACC was applied, an increase up to 462.80% in barium arresting was observed due to an increase in barium arresting locations (Cathodes). A decrease of up to 40.78% in barium sulfate deposition was due to reduced contact of barium and sulfate ions. Figures 12C-D demonstrate a decrease in permeability with the numbers of pore volumes injected in the two experiments. This decrease is due to the increase of barium arresting and the deposition of barium sulfate, which is promoted even more by the volumes of chlorine gas generated due to the increase in anode stations. The slope is another indication for deposition; it is noted that the slope (rate of deposition) is lower in the case of AAACC.

Experiment 9 vs. Experiment 10. When the configuration AAACC was applied an increase up to 356.90% in barium arresting was observed, due to the increase in barium arresting locations (Cathodes). A decrease of up to 43.44% in barium sulfate deposition was observed since most of the barium is being arrested by cathode before meeting sulfate. Figures 12E-F demonstrate the decrease in permeability with the volume injected in the two experiments. This decrease is due to the increase of barium arresting and the deposition of barium sulfate. The decreasing trend in EK process efficiency over time can be attributed to chlorine gas generated which affects the conductivity. The slope is another indication for deposition; it can be noticed that slope (rate of deposition) is lower in the case of AAACC.

These experiments indicate a possible need to modify the process to include chlorine gas suppressing solutions.

Application of a direct current electric field provides an effective method to alleviate mineral scale formation in oilfields. Application of direct current destabilizes the system by moving barium and sulfate ions away from each other. Electrode configuration has an appreciable influence on the arrest of barium with a maximum observed increase of up to 462.80% and a maximum deposition reduction up to 41.50%. Upon DC application, an increase in liquid flowrate has been observed allowing EK to simulate or enhance water injectivity.

A number of patent and non-patent publications are cited in the foregoing specification in order to describe the state of the art to which this invention pertains. The entire disclosure of each of these publications is incorporated by reference herein.

While certain embodiments of the present invention have been described and/or exemplified above, various other embodiments will be apparent to those skilled in the art from the foregoing disclosure. The present invention is, therefore, not limited to the particular embodiments described and/or exemplified, but is capable of considerable variation and modification without departure from the scope of the appended claims.

Furthermore, the transitional terms "comprising", "consisting essentially of and

"consisting of, when used in the appended claims, in original and amended form, define the claim scope with respect to what unrecited additional claim elements or steps, if any, are excluded from the scope of the claim(s). The term "comprising" is intended to be inclusive or open-ended and does not exclude any additional, unrecited element, method, step or material. The term "consisting of excludes any element, step or material other than those specified in the claim and, in the latter instance, impurities ordinary associated with the specified material(s). The term "consisting essentially of limits the scope of a claim to the specified elements, steps or material(s) and those that do not materially affect the basic and novel characteristic(s) of the claimed invention. All methods for preventing mineral scale deposits in and around an oil well via electrokinetic phenomena that embody the present invention can, in alternate embodiments, be more specifically defined by any of the transitional terms "comprising", "consisting essentially of and "consisting of.

REFERENCES

[1] http://www.waterforlife.gov.ab.ca/docs/water_oil_info_bookle t.pdf.

[2] Evaluation of Water flood prospects; F, H Callaway. (Member aim).

[3] Prediction the amount of barium Sulfate Scale formation in Siri Oilfield Using OLI

ScaleChem Software; M. Amiri and J. Moghadasi; Department of Petroleum engineering, Petroleum University of Technology, Ahwaz, Iran.

[4] Haroun, M. H. 2009. "Feasibility of In-situ Decontamination of Heavy Metals by

Electroremediation of Offshore Muds." Doctoral Dissertation in Environmental

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[5] Application of Direct Current Potential to Enhancing Waterflood Recovery Efficiency;

Emad Waleed Al Shalabi, Bisweswar Ghosh, Muhammad Haroun and Sibel Pamukcu

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