Shallow gas-bearing formations in the Gulf of Thailand present numerous drilling and cementing challenges. In the first three wells drilled on platform WP11 in Bongkot field, initial cementing of surface casing did not achieve effective zonal isolation of the shallow-gas zone. Several techniques were unsuccessful, including the use of conventional lightweight gas-tight cement slurries, two-stage cementing operations with conventional gas migration control slurries, and injection of sodium silicate to damage the shallow gas sand. All methods failed to control the gas migration through permeable upper sand layers. This paper presents case histories describing the solutionsoriented approach to this problem, which resulted in changes to both the drilling program and the cementing operations. There is discussion of the extensive laboratory testing performed to formulate the lightweight gas-tight cement slurry at low bottomhole temperature, the best drilling practices learned, and changes to selection of drilling fluid. Results of post-job cement logs are also included. The new approach involved consolidating and damaging the formation with drilling fluid and using low-temperature, lightweight, gas-tight cement slurry. This water-reduced formulation required optimization of the particle size distribution, gas migration control, and short transition time. Modifications were made to drilling techniques and cementing practices, and an openhole external casing packer (ECP) was employed as an additional barrier. Successful zonal isolation was achieved in seven wells drilled from WP-11 Platform in the Gulf of Thailand with excellent cement coverage above and below the gas sands and no increase in wellhead pressure.
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