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Abstract Conventional casing design is based on single-string analysis which ignores annulus fluid heat-up loads and composite cemented string effects. This paper presents the design of a 7' production casing and 10-3/4" Ă— 9-5/8" protective casing under high burst/collapse conditions. Uniaxial and triaxial safety factors are computed and compared for single-string versus multi-string analysis with and without temperature loads. In the multi-string analysis, casing stresses are calculated for the radially composite structure assuming the elastic properties of casing, cement, and formation. Results demonstrate a significant increase in burst safety factor for cemented casing and show that burst/collapse tendency for HPHT wells is strongly dependent on multi-string top-of-cement locations. For this HPHT application, reliable triaxial design is achieved with multi-string analysis using standard weights and grades of casing, which otherwise is not possible with single-string uniaxial analysis.
Introduction In HPHT wells with subsea completions, temperatures can be critical to casing design, not only for heat-up due to production, but also for stimulation cooling and for beat-up during deeper drilling [1, 2, 3]. Additionally, these temperatures can magnify conventional loads associated with tubing leak, gas kick, and evacuation.
To determine casing temperatures and stresses, the WELLCAT tubular design system was used in this study. It consists of the five computer programs: WT-DRILL, WT-CIRC, WT-PROD, WS-CASING, and WS-TUBE. The first three codes predict temperature and pressure loads for planned operations [41, and the two stress codes provide uniaxial and triaxial analysis for casing and tubing. The codes are integrated so that predicted temperatures are automatically linked into the stress codes.
APPROACH A schematic of the HPHT well is presented in Figure 1. The 7" production string consists of section 1 from 0-9000 ft (49.5#, C-90) and section 2 from 9000-18250 ft (38#, Q-125). For the protective string, the 10-3/4" section (109#, T-95) is from 09000 ft and the 9-5/8" section (53.5", Q-125) is from 900016300 ft. Cementing practice by the operator is to fully cement these strings, but we have assumed cement short-falls to investigate possibility of collapse from annulus heatup loads. Cement top outside the 7" is 3000 ft and outside the 10-3/4" Ă— 9-5/8" is 2500 ft. The well is a subsea completion in 413 ft of water.
The analysis proceeded in four steps. First, conventional single- string analysis with standard loads (no temperatures) was performed with WS-CASING. Then, the same set of load conditions was run with multi-string analysis.
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