Downhole Material Selection for Clyde Production Wells: Theory and Practice

Abstract

SPE Member Abstract The Clyde oilfield lies within North Sea block 30/17b. First oil was produced from the field in March 1987. Wells drilled during field appraisal indicated a potentially, highly corrosive downhole environment. Although high chromium content (Duplex) steels appeared to be the technically preferred completion material a decision was taken to use carbon steel tubulars. Historical data has shown that carbon steel gives acceptable performance in the large majority of production wells. Failures have occurred but these have been highly specific and related to high fluid velocities and increasing watercuts. A "mixed" string completion employing carbon steel, 13 Cr and plastic coated tubing has been successful in controlling downhole corrosion in these few problem wells. This paper reviews the downhole corrosion control strategy used on the field and shows how carbon steel may be used successfully in a severe downhole environment. Introduction The Clyde Field lies within block 30/17b and is situated some 290 km South east of Aberdeen (Figure 1). The field is operated by BP (51% equity) on behalf of the other partners, Shell UK Ltd (24.5%) and Esso Exploration (24.5%) The discovery well for the prospect was drilled in 1978. Further appraisal drilling led to the approval for field development being received in 1982. First oil was produced in March 1987 and a plateau production rate of 50,000 bopd was achieved in 1988. Current field production rates are approximately 30,000 bopd (75,000 bpd Gross). The Clyde reservoir is comprised of a complex sequence of Fulmar type sandstones with widely differing poroperm characteristic. Three discrete production intervals can be identified, these are classified as A, B and C sand intervals. There are 3 zone C high rate natural flow production wells (production>20,000 bpd). The remaining 12 wells are A and B zone producers and are generally of much lower productivity (<6,000 bpd). These wells typically require the use of artificial lift (either jet-pump or gaslift) and/or fracturing to maintain production rates at economic levels. A more comprehensive review of the individual zonal characteristics is given in Figure 2. The presence of CO2 and H2S, coupled with a high bubble point pressure, high chlorides concentration in the formation water and a high BHT (297 Deg F) make Clyde produced fluids, potentially, highly corrosive. Clyde production wells also have a high propensity for downhole scale formation (both carbonate and sulphate) and asphalt(ene) deposition. These additional mechanisms further complicate the downhole environment. This paper reviews the initial design work and rationale behind the original material selection philosophy for the Clyde downhole production completions. Subsequent downhole corrosion history from these wells is used to illustrate the accuracy, or otherwise, of these predictions. P. 105^