Patent Number: 056132425
Section: summary

FIELD OF INVENTION The present invention relates to a solid waste disposal system for disposing solid waste containing naturally occurring radioactive material in a safe and economic manner. More specifically, this invention relates to a solid waste disposal system that utilizes geothermal means and/or naturally available hydraulic power to dissolve radioactive deposits which accumulate typically during petroleum production operations, and inject the resultant solution, which could be a dilute solution or concentrated sludges, containing such radioactive material into a subterranean geological formation in an economical, safe and environmentally acceptable manner. BACKGROUND OF THE INVENTION In oil and gas production operations, water often is produced concurrently with oil and/or gas. The-rate of water production relative to oil and/or gas is determined by the relative permeability characteristics of the reservoir rock and the relative saturation of water contained therein. In many oil and gas production operations, it is not uncommon to have the percentage of water production, or water cut, in the range between 50% to 90% of the total fluids produced, or more. High percentage of water cut is often observed during the mid- or later-stage of the primary production after water breakthrough. A substantial increase in the water cut of the produced fluids is often observed during the so-called secondary recovery operation processes, in which large amounts of water are injected via a pumping means or a naturally occurring mechanism such as pressure differential or gravity heads from the surface into the subterranean formation to maintain reservoir pressure and sustain oil and gas production. Most subterranean waters contain large amounts of alkaline earth metal ions, such as barium, strontium, calcium, and magnesium. Water injection during the secondary oil recovery operations also dissolves such ions from the reservoir rocks and brings them to the surface. Under the reservoir conditions, these alkaline earth metal ions can co-exist in a thermodynamically stable state with many anions, such as sulfate, bicarbonate, carbonate, phosphate, and fluoride, etc. However, when the subterranean waters are brought up to the surface during the production of oil and gas, the stable state may no longer be maintained, mainly due to temperature and/or pressure changes. Such a change of solution condition often causes the alkaline earth metal ions to form inherent deposits, or scales, with many of the anions. The presence of barium sulfate often represents a unique and particularly troublesome problem because barium sulfate has a very low solubility. At room temperature, or about 25 degrees Celsius, the solubility of barium sulfate is only 2.3 milligrams per liter. Another problem associated with the formation of the barium sulfate scales, or any other alkaline earth scales, is that radium, another member of the alkaline earth group of metals, also tends to be deposited at the same time. Disposal of such radioactive solid wastes becomes a serious problem in the oil and gas production operations. Such radioactive waste may be referred to as naturally occurring radioactive material (NORM). Using the example of a typical oil production field which produces about 100,000 barrels of oil per day at a water cut of 50% at the surface, the amount of barium scale produced can be as high as 60 pounds per day. Continued oil production operation inevitably results in higher water cut and a greater amount of barium sulfate scale. Although only a very small amount of radium is deposited with the barium scale, the entire solid waste mass must be considered radioactive, as far as solid waste disposal is concerned. The expense to be incurred to dispose such radioactive solid waste is enormous, if one is fortunate enough to find a site willing to accept its disposal. Scale, including NORM, forms in wells and production facilities as a function of the temperature and pressure changes associated with producing hydrocarbons and/or the mixing or commingling of incompatible waters, e.g. waters high in barium with waters relatively high in sulfate. As shown in FIG. 3, barium sulfate becomes more soluble when heated. In addition, it also becomes more soluble when the ionic strength (salt content) of the solution is increased. Both of these conditions can be obtained by using the produced waters from a hot salt water producing well. Typically, these wells would be producing wells in the oil and gas field that produce significant amounts of associated water. The heat energy of the well is used to increase the solubility of the barium sulfate in the presence of salt water. Steam is not a viable alternative since solids are not practically soluble in steam. Various proposals have been made in the prior art for the removal of barium sulfate scales using chemical scale removal compositions. Examples of barium sulfate scale removal techniques can be found in U.S. Patent No. 2,877,848; U.S. Pat. No. 3,660,287; U.S. Pat. No. 4,708,805; U.S. Pat. No. 4,190,462; U.S. Pat. No. 4,215,000; U.S. Pat. No. 4,288,333; U.S. Pat. No. 4,973,201; and U.S. Pat. No. 4,980,077. All these prior art technologies are designed to remove scales from equipment or tubular goods, such as meters, valves, tubing strings, surface pipes, etc. None of the prior art addresses the issue of the disposal of the radioactive solid waste, nor is any prior art known. Also, the use of chemical scale removing agents is subject to a large number of variables. They usually require a right combination of environmental variables in order to work, and yet even under the right conditions they do not always work. Furthermore, since a large amount of solution is required to dissolve the scale, it is essentially economically prohibitive to use such chemical means in the waste disposal process. The techniques proposed in the prior art are to be used for spot-wise dissolution of scales formed on pipes or other equipment, but they are not suitable for handing solid waste disposal. U.S. Patent No. 4,973,201 discloses a method for decontaminating surface layers of the earth which are contaminated with precipitates of alkaline earth metal sulfates including radium sulfate. The method includes applying an aqueous chemical composition comprising a chelating agent and a synergist to the surface layers in situ to bring the precipitates into dissolved form after which the dissolved precipitates are leached into lower layers of the earth by percolation with water. U.S. Pat. Nos. 4,980,077 and 5,049,297, which are assigned to the same entity as the '201 patent described above, generally disclose a method and composition, both utilizing a chelating agent, for removing barium sulfate scale deposits from oil field articles. U.S. Pat. No. 5,022,787 discloses a method for disposing of noncondensing and toxic geothermal gases wherein the gases are returned to the underground. U.S. Pat. No. 4,632,601 discloses a system for disposing of non condensable gases from geothermal wells wherein the non-condensable gases are dissolved into geothermal waste water. U.S. Pat. No. 4,429,740 discloses a gas producing well wherein waste water is disposed of in an earth formation underlying the gas-producing earth formation. U.S. Pat. No. 4,400,314 discloses a method for disposing of high level radioactive water wherein an aqueous solution is diluted with formation water recovered from a subsea reservoir in a porous geological formation and the dilute solution is injected into the geological formation. U.S. Pat. No. 4,738,564 discloses a method for disposal of nuclear and toxic wastes wherein the wastes are rendered harmless by dilution into a huge mass of molten lava. Finally, U.S. Pat. No. 4,844,162 discloses a method of treating a flow of hot, pressurized, hydrogen sulfide-containing geothermal steam. This method teaches disposal of condensate in a disposal well but offers no suggestion or even consideration of the difficulties involved in the disposal of solid NORM. SUMMARY OF THE INVENTION The primary object of this invention is to provide a method utilizing geothermal means to dispose solid waste containing radioactive material. More particularly, the primary object of this invention is to provide a method by which solid wastes containing radioactive materials--mainly barium sulfate scale with radium ions deposited thereon, which are accumulated during the oil and gas productions, can be disposed in an economic, safe and environmentally acceptable manner. Further, the present invention is particularly applicable to the disposal of waste that is produced at a site away from the disposal site. Another object of this invention is to provide an economic, safe, and environmentally acceptable method for the disposal of radioactive solid waste which utilizes geo-pressure source to reduce the processing cost. Yet another object of this invention is to provide an economic, safe and environmentally acceptable method for the disposal of radioactive solid waste utilizing a geo-water source which contains large amounts of dissolved cations to reduce the processing cost. Yet another object of this invention is to provide an economic, safe and environmentally acceptable method for the disposal of radioactive solid waste which requires little or no pumping means by utilizing a naturally occurring hydraulic head. Further yet another object of this invention is to provide a leak-proof and essentially maintenance-free process, which is also operable as a closed system at least at the surface, for the disposal of radioactive solid waste that has been accumulated during the oil and gas production operations. This invention relates to a waste disposal technique by which solid wastes containing naturally occurring radioactive material can be safely and economically disposed utilizing geothermal means. A preferred embodiment of this invention is to operate the entire solid waste disposal process in a closed system at the surface to provide a leak-proof system requiring essentially no or little effort for maintenance. During oil and gas production operations, barium sulfate often forms as a scale due a change in the thermodynamic environment. Other scales nay also precipitate from cations such as barium, strontium, calcium, and magnesium and anions such as sulfate, bicarbonate, carbonate, phosphate, and fluoride, etc. While the formation of the scales may cause some operational difficulties, the removal of which have been discussed in the prior art, the major problem involves the deposition of radioactive radium-containing ions on such scales. The presence of the trace amount of radioactive radium makes the entire solid to be classified as radioactive waste. The problem can worsen during deeper wells, usually involving gas producing operations, where the reservoir temperature is higher and the produced water contains higher concentrations of barium or other earth metal ions. In a preferred embodiment of this invention, the solid waste will be placed in a central waste processing chamber, preferably but not necessarily under a closed condition to allow the maintenance of high pressure. Water is produced from a subterranean formation, preferably at a very deep formation so that the water produced is at elevated temperature. The water produced or fresh water is directed into such a waste processing chamber to dissolve the solid waste to form soluble ions. Since the solubility of barium is extremely small, very large amounts of water will be required. The produced water, after picking up dissolved ions including radioactive ions, is injected into a subterranean formation, preferably another subterranean formation at a reservoir pressure lower than the pressure of the subterranean formation from which the water is produced. In addition, chemical additions may be required to allow faster and/or more economic dissolution of the barium sulfate scale. These chemical additions might include acids, chelating agents and/or chemicals to convert the barium sulfate scale to a more soluble chemical solid such as barium carbonate. Research has been done to determine effective agents and is shown in FIG. 4. There are several advantages of using subterranean water to dissolve the solid wastes. First, the produced water is usually readily indictable without further processing such as filtration. Second, as mentioned hereinabove, the subterranean water can be produced from a formation at elevated temperature. For example, at 300 degrees Fahrenheit, the solubility of barium sulfate is increased by about 100-fold compared to room temperature. The amount of water that will be required to treat the solid waste is reduced by a similar factor. Third, the subterranean water is often produced at very high pressure, which further promotes the dissolution of barium sulfate. Fourth, the subterranean water often contains significant amounts of cations such sodium, ferric, ferrous, potassium, magnesium, etc. The presence of such cations reduces the activity of barium ions and further increases the solubility of barium sulfate. Fifth, the preferred embodiment calls for the production of water from a high-pressure subterranean formation and the injection of the treated water into a low-pressure formation. Utilization of such naturally available hydraulic power allows the elimination of a pumping unit and other necessary control implementations. Certain geologic structures, however, may dictate the use of some pumping force. This invention not only substantially saves energy cost for processing the solid waste, it also eliminates many possible leaks which are a major concern in treating radioactive waste. It may be required to use fresh or city water in the process for efficient dissolution. If this were the case, the dissolution tanks would be jacketed to take advantage of the hot produced water while keeping the chemistry isolated. The present invention further provides for the disposal of solid NORM as a solution or a slurry or sludge material. In this embodiment, the NORM is ground to a size compatible with the pore space and permeability of the receiving formation and is disposed into the formation via the disposal well. This technique of the present invention does not rely on the formation of a partial vacuum in the disposal well, but relies on the pressure of the production system to drive the disposal process. A partial vacuum in the disposal well due to the high brine density (i.e., the weight of the fluid column) in the disposal well and the high permeability of the disposal reservoir merely enhances the performance of this system. Pumps may be used to supplement the disposal process. These and other features and advantages of the present invention will be apparent to those of skill in the art from a review of the following detailed description.