This paper describes kinetic experiments to determine the rate at which Ba ions are mobilized from crystalline barium sulfate into aqueous solution by a solvent in which the active ingredient is a strong chelating agent for Ba ions (DTPA - diethylene triamine pentaacetic acid). The experiments define the temperature dependence and hence the activation energy of the dissolution process, the surface area dependence and most importantly the dependence on the concentration of the DTPA in solution. In a 0.05M DTPA solution the dissolution rate of barium sulfate is highly temperature dependent with an activation energy of 45kJ mol-1. This value suggests that the rate is controlled by the desorption of a Ba-DTPA surface complex. Surface complexation is further identified as the key to barium sulfate dissolution by the observation that over the concentration range 0.5M to 0.05M DTPA the initial dissolution rate is inversely related to the DTPA concentration. The activation energy remains unaltered but the absolute dissolution rate is increased. In other words, a 0.05M DTPA solution is more efficient as a solvent than a 0.5M solution. This unexpected result is interpreted in terms of a passivation of the barium sulfate surface by the formation of a surface complex layer at high DTPA concentrations. Introduction During secondary oil recovery from offshore oil-bearing reservoirs, pressure is maintained by injecting the well with seawater. A common problem with this procedure is that the formation water in the reservoir often contains high concentrations of alkaline-earth metal ions (Ba2+, Ca2+ and Sr2+). When the injected seawater, which contains sulfate ions (SO42-) comes into contact with the formation water in the region of the wellbore, the resulting barium, calcium and strontium sulfate precipitation blocks the production tubing and reduces the porosity of the surrounding reservoir rocks. This is a world-wide problem and causes considerable production losses. An associated problem is that the formation water may also contain a significant concentration of radioactive isotopes (uranium and thorium decay products) which also precipitates as the sulfate and may render the scale sufficiently radioactive to be a potential health problem. One possible approach to avoid the scaling problem is to attempt to remove the sulfate from the seawater by reverse osmosis prior to injection. However, the large volume of seawater to be treated would require significant capital expenditure. P. 773

Mobil has developed a new solvent to dissolve oilfield scales, including sulfates of barium, strontium and calcium, as well as calcium carbonate. Naturally occurring radioactive material (NORM), which is usually co-precipitated with barium and strontium sulfates, is removed by this solvent. Performance features of the solvent and case histories of field tests are reviewed. Materials compatibility with the solvent and disposal options are also briefly discussed. Introduction Considerable production losses are associated with scaling in many oil and gas producing areas of the world. Scale inhibitors can often be used to control precipitation of calcium carbonate and calcium sulfate. If scale inhibition fails or is not applied, these scales can be removed using acid washes or combination treatments of chemical converters and hydrochloric acid. Due to the low solubility of barium sulfate, 2.4 mg/l in water at 20C, formation of this scale in wells, production tubing and surface equipment is an unwelcome event. Once formed, these scales are nearly impossible to remove from tubing, valves and surface equipment except by mechanical means. Co-precipitates of radioactive isotopes (uranium and thorium decay products) are usually present in barium/strontium sulfate scale mixtures. Activities of these naturally occurring radioactive materials (NORM) can sometimes be high enough to present a potential health problem to workers. Ordinarily though, the problems are confined to appropriate disposal. This quandary has renewed interest in developing new solvents, particularly solvents with high specific activity for barium and strontium sulfate scales. Laboratory Processes A limited number of solvents are listed in the literature for use on barium and strontium sulfates. However, the patent literature is well stocked with different schemes for removing barium sulfate scale. Most of the literature concentrates on the use of some type of chelating agent, most often the amino polycarboxylic acids: for example, ethylenediaminetetraacetic acid (EDTA) and similar reagents. Other compounds suggested for dissolving barium sulfate include crown ethers, cryptands and other exotic and expensive materials. Although these chelating agents have been applied for removing oilfield scales, our experience with them was not wholly successful. P. 845^

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SurnrnaryThe reactions of pyridine with excess of alkyllithium compounds generally give excellent yields of 2,6-dialkylpyridines.

An extracellular polysaccharide produced by Alcaligenes bacteria has been crosslinked to form firm gels that are stable for long periods at elevated temperatures. The biopolymer can be gelled with multivalent ions such as Cr + 3 ; it can also be gelled at the 2,000-to-4,000-ppm level without Cr +3 in high-salinity [ -20% total dissolved solids (TDS)] brines. Gel samples have been stable for 2 years at 74 and 93°C. Gels could be formed over a wide pH range, but best long-term stability was achieved in the pH range of 7 to 8. A flow test of Alcaligenes biopolymer gel in Berea sandstone at 74 °C showed that the gel gave a brine permeability reduction factor of 320.