Fighting Lead and Zinc Sulphide Scales on a North Sea HP/HT Field

Abstract

Lead and Zinc sulphides have recently become a concern in some HP/HT gas fields. The Elgin/Franklin Field (Central Graben North Sea), started production early 2001. The wells produce a condensate-rich gas from the Fulmar and Pentland reservoirs, with initial temperature of 200 degC and pressure of 1100 bar. Mid-2002, the first Calcium Carbonate obstructions appeared downhole on several wells and resulted in a progressive production decrease. Moreover, lead and zinc sulphides were identified on well G6. The heavy scaling from its surface-controlled subsurface safety valve (SCSSV) to the Christmas Tree caused the well to be shut-in from end 2004 until an appropriate programme of remediation and prevention was implemented. Lead and zinc sulphides had not been predicted during the initial scaling studies. While several publications mention these on HP/HT fields, little information is available on downhole inhibitor squeeze. Scale removal lab studies included dissolution of the recovered scales and corrosion tests. A different fluid package was selected for downhole and SCSSV conditions. Scale remediation using downhole acid washes proved efficient by increasing wells production. The upper part of the completion on well G6 was cleared from scales by a coiled tubing operation in mid-2005 and the integrity of the SCSSV was restored. To prepare for scale prevention, a benchmarking of inhibitors from four suppliers was performed in a third party lab over a period of more than two years. Appropriate equipment was put in place and the methodology was optimised. The tests were conducted in anaerobic conditions with thermally aged chemicals. Two scale inhibitor squeezes were deployed, although progress is still to be made for "exotic" scale prevention. This paper presents the Elgin/Franklin scale-control strategy from a thorough fluids selection to the field deployments. Results and optimisation are discussed. Introduction Calcium Carbonate (CaCO3) is the most common "self-scaling" specie found in oil and gas fields. Scale deposition is a widespread problem that causes production deferment, necessitates costly intervention, and can compromise safety systems. The situation can be more severe in high-pressure, high-temperature (HP/HT) fields since the changes in pressure and temperature are potentially greater and because the formation water in such fields is often of very high salinity. Initial scale control philosophy. The Elgin/Franklin Field is a platform development in the Central Graben area of the North Sea's UK sector (Block 22/30b, 22/30c and 29/5b), approximately 240 kilometers East of Aberdeen. Production started in March 2001, from 10 wells. The Fulmar and Pentland reservoirs in Elgin/Franklin produce a retrograde condensate gas and are characterized by very harsh conditions - temperature close to 200°C, initial pressure up to 1100 bar - and a high salinity formation water, close to 300,000 mg/l. The hydrocarbons also contain around 3 mol% CO2, and traces of H2S in insufficient quantity to be measured in produced fluid. The potential for scale deposition - primarily sodium chloride (NaCl) and Calcium Carbonate (CaCO3) - was recognized early in the project phase. Preliminary studies were performed in 1995 and 2000, based on water samples taken during DST in 1991 and 1995. Initial scale prediction studies identified a low risk for calcium carbonate scaling in the upper tubing of wells with less than 100m3 of water per day. Of more initial concern was the NaCl, prone to forming not only in topsides, but also downhole, early in the field life, due to vaporisation of water when in small amount.