The ability to drill many wells at absolute minimum cost is one of the most significant factors for an operator to have long-term financial success in the Gulf of Thailand (GOT). In partnership with PTTEP and MOECO, UNOCAL has been exploring for gas in the GOT since 1962, with first production occurring in 1981. During this period, the partnership has set 73 platforms, drilled more than 900 wells, and produced over 2.6 Tcf of gas and 96 MM bbls of condensate. Development drilling and completion design has changed incrementally many times, with an average well going from 60 days and $5 MM in 1980 to 9 days and $1.3 MM in 1994 (Figs. 1 and 2). The well design was completely overhauled and a slimhole design with monobore completion was implemented in 1994. By the end of 1996, completed wells were averaging 6 days and $750 M with time and cost still coming down. Many technical challenges have been overcome to achieve the slimhole drilling time and cost savings, including optimizing hole size, casing depth, drill string design, PDC bit design, drilling tools, BHA performance, and wellhead equipment. Additional challenges to produce and manage the wells have also been overcome, including perforating, wireline, and coil tubing tools. Increased reserves recovery is expected as a result of improved zone accessibility and better liquid lifting capability. Redesign of the wellhead platforms is also in progress as a result of the smaller wells. Slimhole development has completely transformed the future of GOT operations and has paved the way for operators to continue providing a long-term economic supply of gas to the Kingdom of Thailand.


Unocal's slimhole development in the GOT has significantly reduced development capital cost and improved recovery. This paper presents a case history of the complete implementation of slimhole development from the initial trials to present. Many of the technical drilling, production, and reservoir management hurdles which have been overcome to date and the thought process behind many of the decisions are presented in this paper. Table 1 provides the GOT operating environment in which this work has been carried out.

The initial idea behind slimhole was based on designing a minimum well bore around the required 2-7/8" production tubing and eliminating the costly completion. A key assumption made in the initial thought process was the smaller the hole, the faster the well and the less drilling problems incurred. Although potentially reducing the well cost by an estimated 40-50% was the key driver to move ahead, other extremely important benefits were thought to be inherent to the design including (1) much simpler completions without downhole jewelry (2) improved recovery factor with complete access to all zones (3) increased reserves by lowering economic threshold (4) significant reduction in abandonment cost (5) smaller platforms and (6) reduced logistics creating a safer operation. Unocal's management readily agreed in 1994 to several trial wells in order to prove the drilling cost savings were possible and that the other inherent benefits would be realized.

The following concerns or assumptions existed prior to the initial slimhole trial wells:

Drilling Concerns or Assumptions

  • time and cost baseline

  • less time required to run and pull smaller tubulars

  • improved ROP by reducing hole size

  • tubing buckling and sticking

  • casing design and well control

  • torsional strength of standard NC-38 tool joint

  • increased ECD and potential for lost circulation

  • PDC bit baseline, ROP and durability

  • ability to achieve directional profiles and total depth

  • small BHA tool reliability

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