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.
Introduction
Scale buildup and deposition of corrosion products in tubulars are the result of a combination of factors and practices which create operational hazards and a negative financial impact on a company's bottomline. Some of the triggering factors include production of aqueous fractions, changing temperature and pressure, high dissolved ion concentrations into the produced water, changing conditions on pH and the presence of mineral deposits in the sandface.
During the reservoir diagenesis process a complete equilibrium is achieved in the reservoir by the formation rock, the fluids and gases present, chemical ions, capilar effects and segregation. When this equilibrium is disturbed by drilling a new well, for example, significant changes occur which may create the right conditions for scale buildup, formation of organic deposits and accumulation of corrosion products.
This paper describes a new systematic approach to remove the iron sulphide scales that have precipitated in the Ghawar Field gas producers. This approach is designed to reduce operational risk, increase the dissolution ability of the fluids used, avoid unwanted chemical reactions, and reduce the risk of creating corrosion problems during stimulation treatments.
Field Description
The Ghawar field is located in the eastern province of the Kingdom of Saudi Arabia, and it produces rich gas condensate from a large structure spanning an area 225 Kms. long by 25 Kms. wide. Figure 1 shows a cross-section depicting the field's reservoirs.
