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^

TX 75083-3836, U.S.A., fax 01-972-952-9435.

Mobil North Sea Limited (MNSL) have been conducting treatments to prevent both Carbonate and Sulphate scale formation in the Beryl Field since the early 1980's. In order to effectively prevent scaling without frequent treatments, a novel method of increasing the squeeze life of the scale inhibitor has been developed. This method is based on the use of a proprietary extending agent. This results in greater retention of the scale inhibitor within the formation and substantially reduced inhibitor backflow during the initial production following the squeeze. The inhibitor is then released from the formation at a low but effective concentration over a long period of time. Following extensive laboratory studies, the first Mobil Long Life Scale Inhibitor (MLLSI) squeeze was performed on Beryl Well 9/13a-B16 in January, 1989. A squeeze life of fifteen months was obtained compared to an average life of about eight months from commercial treatments. Other wells in the Beryl Field have been treated subsequently and show improved treatment lives. The benefits of the longer inhibitor life resulting from MLLSI include reduced deferred production and substantially lower inhibitor treatment costs.

No abstract