Tubular Design Optimization For Arco China's Yacheng 13-1 Development
Authors: Payne M.L. , Zerbi J.L. , Sims D.C. 
Published: December 2007

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Abstract
In 1981, ARCO executed the first offshore development contract ever between China and a Western oil company. In 1983, ARCO discovered the Yacheng 13-1 gas reservoir during exploration drilling in the South China Sea. Following several years of delineation drilling, reservoir evaluation, and commercial negotiations, a full field development program was initiated in 1992 to produce the field's estimated 3 TCF of gas reserves. This paper describes design processes, special engineering tasks, procurement decision making, and field procedures which were implemented to optimize cost and performance of oil country tubular goods (OCTG) for the development drilling program. Structural drive pipe casings were designed in an integrated manner with the drilling platforms. Conductor, surface and intermediate/production casings were optimized by identifying constraining loads and selecting specialized OCTG products to improve tubular design efficiency for those loads. Production liners and tubings were designed to provide metallurgical corrosion protection, standardized well programs, completion flexibility, and minimum cost. To further optimize liners and tubings, empirical collapse ratings were derived from a focused testing program. Connection qualification testing was conducted to maximize confidence in the premium connections and field running procedures. These engineering efforts resulted in significant reductions in tangible drilling costs and substantial increases in the field reliability of the OCTG.

General Design Considerations
As shown in Figure 1, the Yacheng gas development is located 60 miles south of Hainan Island and 480 miles southwest of Hong Kong in 300' water depths, The development, overviewed in [1], was optimized to minimize the number of wells, thus making production capacity and reliability of each individual well critical to meeting production obligations. Due to this remote location and the importance of production deliverability, wells are designed for long producing periods and minimum intervention. Figure 2 shows the development scenario involving two well platforms and a total of 12 wells, with the second platform planned some years after initial production is established from the first platform. The two independent development platforms allow for optimized reservoir drainage, reduced costs by limiting well departures, and greater flexibility for responding to future drilling needs in the life of the field. The final number and placement of wells will be determined based on ongoing refinement of reservoir and production information.
Pore pressure and fracture gradients in the Yacheng area have been established through the eight (8) previously drilled exploration and delineation wells. Figure 3 shows the pore pressures, fracture gradients, and mud weights estimated for the Yacheng development wells. Normal pressures were predicted to depths of 8,000' to 9,000'. A transition zone is then encountered with geopressuring resulting in shale pore pressures as high as 13.8 ppg in the interval from 9,500' - 10,500' and mud weight is increased accordingly. A pressure reversal below 11,500' occurs in the reservoir section to 8.5-9.0 ppg equivalent. Geothermal temperatures in the Yacheng area, and the South China Sea in general, are high relative to other operating areas. Figure 4 shows the estimated geothermal gradient based on log data from the various wells drilled in the area. Downhole temperatures reach 400 F near 15,450', resulting in a linear geothermal gradient of 2.27/100' with a 50 F sea floor temperature. Normal production rates range from 40-60 MMSCFPD with gas chemistry of 84% methane and 11% CO2 with up to 50 ppm H2S. Flowing wellhead temperatures range from 200 F to 302 F. Water production was estimated to be less than 50 BPWD, but could increase to 1,200 BWPD with Chlorides of 11,000 ppm. These conditions required the use of 13Cr metallurgy for production liners and tubings.
Based on the described pressure regime and previous drilling experience, casing setting depths were chosen. Drive pipe would be driven to refusal in the range of 300'. Conductor casing would be set near 1,400' TVD. The 13.3/8" surface casing would be set in in the range of 5,800' TVD.
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