The successful development of a light weight slurry system for cementing long intervals at high temperatures in three dimensional multiple stacked gas zone wells, has been achieved in the Gulf of Thailand. The cementing system has been developed to successfully complete progressively more difficult wells, starting with conventional large bore wells and tubing plus packer completions, and later down-sizing into single monobore wells whereby the tubing is cemented in place. The problems associated with the down-sizing of the wells are further increased by more challenging well paths and higher temperature fields. Many problems and risks were overcome to complete this task successfully. These changes have posed new challenges in primary cementing. Like the well size, the slurry system has also undergone a refinement and improvement process. The local industry has gone from dry blended slurry systems to all-liquid additive slurry systems saving considerable dollars, whilst realizing large gains in slurry flexibility, ease of mixing and improved logistics. Whilst developing and realizing these gains, the system has been refined and successfully applied to smaller, hotter and more difficult wells. This paper presents the concerns associated with cementing these challenging wells where remedial cementing has been effectively eliminated. The processes related to this success, evaluation methods and field results will be discussed. Introduction In terms of completing the well, cementing operations are arguably one of the most important operations done on a well. When performed correctly they give a well every opportunity to perform as it was designed. However, poor primary cementing can lead to lost reserves, increased water rates and expensive remedial work. The prime goal of cement is to provide adequate zonal isolation throughout the life of the well. This is essential for trouble free production. In slim hole cementing the importance of adequate zonal isolation is magnified due to the fact that remedial options are both limited and also expensive. Unocal Thailand has drilled over 1400 wells (Figure 2) in the Gulf of Thailand. During this period there have been many changes in well design. This paper will deal with the two most recent. Older completions utilized 13 3/8" casing to 1,000', 9 5/8" casing to 4,500’ TVD (+/-7,000’ MD) and 7" casing to 8,900’ TVD (11,700’ MD) with a conventional packer and sliding sleeve completion installed for production. One recent change occurred with the elimination of the 13 3/8" casing and conventional completion. The resulting wells were completed with 9 5/8" casing, 7" casing and 2 7/8" tubing being cemented in a 6 1/8" hole. The 9 5/8" casing is set at 1,000 TVD, the 7" casing at 4,500’ TVD and the tubing is run to TD at 8,900’ TVD (Figure 1). To date 627 of these wells have been completed. The most recent change has been to down-size one more level with the elimination of the 9 5/8" casing. This has been achieved by setting 7 5/8" casing at 1,000'. The 7" intermediate casing from the original Gulf of Thailand Monobore well has become 5 1/2" casing. Finally the production hole is drilled with a 4 3/4" bit and the same 2 7/8" tubing cemented in place. This ultraslim well architecture has been used on 13 wells to date. The well paths of all types are predominantly three-dimensional with 35° to 60° inclinations and 90° azimuth changes. These wells are intersecting alternating sand shale formations with numerous thin coal seams. The sandstone reservoirs contain gas condensate and high CO2 with an average thickness of 13 feet. The reservoirs themselves are broken into many smaller sections as a result of being in a complex faulted graben system. In addition to this, there are multiple gas/water contacts in the wells, making isolation essential for trouble free production.