Overcoming Shallow-Gas Drilling Difficulties

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 87179, "Overcoming Shallow-Gas Drilling Difficulties in the Gulf of Thailand," by Pornpoch Tiraputra, SPE, Pattanapong Thiravutpinyo, SPE, and Sunchai Pongorapin, PTT E&P; Bernard Fraboulet, SPE, Total E&P; and Mohammed Dooply, SPE, Walter Friedl, SPE, and Youssef Elmarsafawi, Schlumberger, prepared for the 2004 IADC/SPE Drilling Conference, Dallas, 2-4 March. 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. Use of conventional lightweight gastight cement slurries, two-stage cementing operations with conventional gas-migration-control slurries, and injection of sodium silicate to damage the shallow-gas sand failed to control the gas migration through permeable upper sand layers. The full-length paper presents case histories describing the solutions-oriented approach to this problem that resulted in changes to the drilling program and cementing operations. Introduction Bongkot field is in the Gulf of Thailand approximately 600 km south of Bangkok and 180 km off the coast of Songkhla province. Following discovery of the field in 1973, it was delineated by drilling 23 wells. In 1990, the concession development rights were transferred to a joint venture group comprising PTT E&P plc (PTTEP), Total E&P Thailand (TEPT), British Gas Thailand Ltd., and Statoil Thailand Ltd. The field was operated by TEPT from the beginning until mid-1998, when the operatorship was transferred to PTTEP. In 1995, the shallow-gas anomaly was discovered in the Tonsak south area of Bongkot field. Drilling through the shallow gas was proved technically feasible and safe after a pilot hole, Tonsak SG, was drilled in 1996. The shallow-gas-anomaly characteristics were verified with logging-while-drilling information. This well was abandoned safely. In 1998, the decision was made to set a platform over the shallow-gas anomaly. The motivation was a projected cost saving of approximately U.S. $20 million, in part because the gas could be drained with a single wellhead platform instead of three small wellhead plat-forms outside the shallow-gas anomaly.