A Systematic Approach To Remove Iron Sulphide Scale: A Case History

摘要

A number of Khuff reservoir gas condensate wells have been on production for more than twenty years in Saudi Arabia. Many of these wells are classified as sour gas producers with hydrogen sulfide levels upwards of 10 mol% and carbon dioxide of 4-5 mol%. Most wells are completed with C-95 tubulars to withstand high stimulation treatment pressures. Condensed or formation water production rates range from 2 to 100 BWPD. The condensate to gas yield ranges from 30 to 250 BBLS/MMSCFD depending on the area of the field. Recent extensive lab work indicated that the iron sulfide scale deposits observed in the production tubulars of a number of wells comprised various types of iron sulfide, iron carbonate and calcium carbonate compounds. Removing iron sulphide scale is a complex process, especially at downhole temperature and pressure conditions (285°F and up to 7,000 psi. in Khuff gas producers). Inorganic acids cannot dissolve all types of iron sulphide scale. As iron sulphide scale (FeS) ages, it is converted into pyrite (FeS2) which is insoluble in mineral acids. Moreover, iron sulphide is oil-wet, a condition which creates a protective organic coating that minimizes contact with acids and chelating agents. Furthermore, the hydrogen sulphide released during the acid dissolution of acid-soluble scale may cause serious problems, because the spent acid may redeposit iron sulphide and elemental sulphur. This paper describes a systematic approach pursued to identify the optimum methodology to effectively remove complex iron sulphide scale in Khuff reservoir gas producers ahead of field trials. A discussion on iron sulphide scale dissolvers and an optimized mechanical scale removal using fluidic oscillation technology is included. Details of a significant number of dissolution tests conducted are shared.