Overcoming Shallow-Gas Drilling Difficulties in the Gulf of Thailand

Abstract

Abstract 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 solutions-oriented 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. Introduction Bongkot field is in the Gulf of Thailand approximately 600 km south of Bangkok and 180 km off the coast of Songkhla province (Fig. 1). 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 comprised of PTT Exploration and Production Plc (PTTEP), 40%; Total Exploration and Production Thailand (TEPT), 30%; British Gas Thailand Ltd., 20%; and Statoil Thailand Ltd., 10%. The field was operated by TEPT from the beginning until mid-1998, when the operatorship was transferred to PTTEP. Since 1998, Statoil Thailand Ltd. has farmed out its interest to the rest of joint venture partners. In 1995, the shallow gas anomaly was discovered in the Tonsak south area of Bongkot field. (Fig. 2) Drilling through the shallow gas was proven 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 (LWD) information. This well was abandoned safely. In 1998, a decision was made to set a platform over shallow gas anomaly. The motivation was a projected cost saving of approximately U.S. $20 million, in part because draining of the gas could then be accomplished with a single wellhead platform instead of three small wellhead platforms outside the shallow gas anomaly. (Fig. 3) Background The detailed work started in early 2001. The preparation included risk analysis, mitigating measures, personnel training, exercises, procurement of special equipment, and additional safety equipment. In early 2002, cement jobs performed across the shallow gas sand in three wells with various drilling and cementing techniques failed to isolate the gas-bearing sand. Because of the concern of gas breakout to surface, which would jeopardize the platform stability, one pilot hole was drilled to the upper layer sands and a pressure monitoring system was installed to monitor the pressure development of these sands. Then, two wells were drilled and completed to final depth. These two wells were subsequently perforated and produced. Sustained annulus casing pressures from the shallow gas zone were monitored and controlled.