Lead zinc and iron sulphide scales are known to be particular issues with gas production fields, particularly those producing from HP/HT reservoirs. The Elgin/Franklin Field is located 240 kilometres east of Aberdeen in the Central Graben Area of the North Sea, blocks 22/30b, 22/30c and 29/5b. With initial temperatures of 200°C and pressures of 16,000psi this is one of the highest pressure and temperature developments ever undertaken. The fields began production in Q1 2001. Preliminary scaling studies identified a risk for calcium carbonate scaling with an increased scaling risk as the wells mature. In May 2002 the first obstruction occurred and was identified as CaCO3 resulting in a programme of remediation and treatment as discussed previously in SPE 94865. In late 2004 however zinc and lead sulphide scale deposits were also identified. These had not been predicted during the initial scaling studies. This resulted in the well being shut in and the squeeze treatments designed for carbonate scales (see SPE 94865) being delayed to allow further chemical selection.

It was recognised however that the prediction of sulphide scale and the methodologies available for their laboratory assessment, especially in brines containing high dissolved iron concentrations, are not as well developed as those for the more conventional sulphate and carbonate scales. This paper therefore focuses on the detailed assessment and methodology development required in order to assess the problem in the Elgin/Franklin Field. Scale prediction identified that the major scales that could be formed were calcium carbonate, iron carbonate, iron sulphide, lead sulphide and zinc sulphide. Given the predicted oversaturation of various minerals, preliminary laboratory tests were conducted in order to ensure that the scale formed was representative of the field scale. This showed that small variations in brine composition can have a relatively significant impact on the type of scale formed in the laboratory tests and more significantly that this did not completely tie up with predictions conducted using commercial thermodynamic models. The paper describes the importance of proper test methodology and describes more sophisticated test protocols and processes for ensuring that the laboratory tests replicate field production conditions. These techniques were then used to select an appropriate chemical for field treatment. Field treatments have now been conducted by downhole "squeeze" application using a treatment package similar to that identified in SPE 94865 and the results from these field treatments are discussed.

Details of the developed test methodologies and chemical selection process undertaken for these more exotic scales together with results from the field trial are described. In addition discrepancies between the thermodynamic scale prediction models and the laboratory data are also discussed indicating that further model tuning may be required for these less common scales.

You can access this article if you purchase or spend a download.