This paper presents a methodology and a software program for mechanical design of vacuum insulated tubing (VIT) strings. It also presents findings of a technical and experimental evaluation of a VIT candidate product under conditions typical for ChevronTexaco's Tahiti project in the Gulf of Mexico.
The design-and-evaluation methodology consisted of an initial feasibility study of VIT thermal properties and service loading conditions, a closed-form solution of the VIT stress-strain state under combined loading, a parametric finite element analysis (FEA) with simulation of interactions within VIT end connections, and a full-scale test of a VIT specimen under cyclic thermal and mechanical loads that were anticipated in field service.
A complex experimental set-up was developed to apply the required combined loads to a full-length VIT joint. The test specimen was instrumented with strain gauges to measure the VIT deformation in a fully submerged environment.
The paper presents examples of data acquired in all phases of this design-and-evaluation program. The results were used for assessment of structural integrity, sealability, and thermal insulation performance of the VIT candidate product. The experimental data agreed well with analytical and numerical predictions. The paper also shows how these results were further used to develop a VIT string design program, which calculated stresses and safety factors in function of VIT geometry and loading conditions.
The conclusions of this paper include recommendations for VIT design and performance verification. Following these recommendations should result in higher reliability and more cost effective wells in completion programs utilizing VIT for deepwater applications.
The Tahiti field discovery was made in April of 2002. The field is located in the Gulf of Mexico Green Canyon Block 640, approximately 300 km (190 miles) southwest of New Orleans, Louisiana, USA. The Tahiti #1 well is under approximately 1,220 m (4,000 ft) of water. It was drilled to a depth of 8,660 m (28,411 ft), in March of 2002.
Two Tahiti appraisal wells were drilled in 2003. The appraisal wells confirmed that the reservoirs in the Tahiti #1 well area extended over a distance of 4.8 km (3 miles). One of the appraisal wells encountered more than 300 m (1,000 ft) of net pay in high quality sandstones. Combined, the two appraisal wells provided verification of ChevronTexaco's initial estimates of 65 to 80 million cubic meters (400 to 500 million oil-equivalent barrels) of ultimate recoverable oil reserves. The appraisal plan for the Tahiti field required a production well test on the Tahiti #1 well during the second quarter of 2004.
The Tahiti wells involve several challenging characteristics typical for deepwater completions: well depths exceeding 8,500 m (28,000 ft), high pressures up to 115 MPa (16,700 psi), and elevated temperatures above 100°C (212°F). The initial Tahiti completions and well tests required designing, testing, and manufacturing of several types of new equipment, which was needed to perforate, fracpack, and flow test the Tahiti discovery well.
One of the major challenges faced by ChevronTexaco in the Tahiti project was development of a subsea production tubing string that would minimize the potential for annular pressure buildup (APB). The Tahiti #1 well had several of its casing annuli sealed, and a similar completion was envisioned for other wells. In a completion with sealed annuli, APB may occur when fluids trapped between the strings of tubing and casing expand due to heat transferred from produced formation fluids1. Since compressibility of fluids in the annuli is typically low, thermal expansion of these fluids leads to high annular pressures (i.e. APB). This creates potential for burst or collapse of casing strings.
ChevronTexaco decided to utilize vacuum insulated tubing (VIT) to tackle the APB challenge. VIT keeps the heat inside the production tubing and thus reduces the APB severity. For example, fluid temperature loss is approximately 6°C (10°F) along a VIT length of 1,500 m (5,000 ft). This compares favourably to a loss of 45°C (80°F) over the same length of conventional tubing1.
Author: Nowinka, J., Xie, J., Kis, P., Hensley, R., Gonzalez, M., & Williams, D.
Publisher: SPE/IADC Drilling Conference, 23-25 February, Amsterdam, Netherlands
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