To meet increasing gas demands while drawing from the shrinking reserves of a mature field, CTEP (Chevron Thailand E&P) is drilling as many wells as economically feasible. Wells have to be drilled and completed very quickly to minimize rig time and reduce the "per well" drilling cost. In 2005, the average time to drill and complete a slimhole well was 6 days; the target for 2006 is 4.5 days.

During 2005, CTEP drilled and completed over 300 wells while 450 wells are planned for 2006. In addition to exceptional drilling performance in the Gulf of Thailand, value creation is enhanced by performing a high number of IFDP (in-fill drilling projects) whenever possible. This reduces cost by minimizing new platform construction, reusing slots, and reducing installation of new surface casing. The IFDP project can be categorized into two phases:

  1. slot recovery; and

  2. drilling.

This paper focuses on phase 1 of the project, where CTEP uses HWO (hydraulic workover) systems along with cementing, slickline, and wireline services to perform the slot-recovery operations before rig arrival. Topics of discussion include current operational equipment, methods and procedures, logistical challenges, lessons learned, and new development plans intended to further enhance the operation.

Statistically, it is estimated that for each single platform 6-well abandonment campaign, CTEP gain 7 to 8 days of drilling time by not utilizing rig time for the Phase I operation. The drilling rig focus is, therefore, maintained on, doing what it does best, drilling new wells safely and economically. Additional wells are drilled with the time saved supplying early gas to meet Thailand's expanding industrial demands.


Before the acquisition of Unocal Corporation by Chevron, Unocal Thailand (UTL) had over three decades of successful energy-development history in the country. With gross natural gas production averaging more than 1.2 Bcf/D from over 100 platforms in the central Gulf of Thailand (GOT), Unocal supplied natural gas to generate over 30 % of the nation's total power demand. Unocal has continued to increase natural gas and condensate production in Thailand since 1981 to meet current and future demands while effectively replenishing reserves. This has been achieved by using advanced drilling and three-dimensional seismic technologies in conjunction with a substantial reinvestment of capital.

The small, stacked reservoirs over large areal extent means that well life averages only 2 years and continual drilling and redrilling are required to maximize recovery from shrinking reserves in a mature field. This paper focuses on redrilling or IFDP, where depleted wells are abandoned and slots are reused to drill new wells. This has proved to be a very cost-effective way of gaining more wells without having to build new offshore wellhead platforms and support facilities for these new wells.

In the past, the process of making slots of depleted wells available for new wells was done completely by drilling rigs with the exception of cement squeezing off the perforations, where only cement pumping equipment is used. This takes a significant amount of rig time to cut and pull tubings, casings and conductors, set kickoff cement plugs, run splitter conductors, etc. This "non-drilling" period could be better used to drill more wells. This paper discusses details of UTL's process of "well preparation for redrill" and the benefits in terms of rig-day-equivalent (RDE).


Before 1995, most of the wells in the GOT were completed conventionally with 26-in. driven conductors, 13–3/8-in. to ±1,000 ft, 9–5/8-in. casing to ±4,500 ft total vertical depth (TVD), and 7-in. casing extended to an average depth of 12,000 ft total depth (TD). The 2–7/8-in. tubings are held in place by permanent packers, equipped with sliding side doors (SSD's) for zonal isolation.

As the reserves in the ground diminish, the return on investment for new wells also diminishes. New wells have to be drilled in a more cost-saving manner resulting in the emergence of economically attractive slimhole wells. These wells are completed with 9–5/8-in. surface casing to ±1,000 ft, and 7-in. casing to ±4,500 ft TVD. The 2–7/8-in. tubing is then run inside the 6.5-in. open hole (OH) and cemented in place. After these wells are completed, the productions are commingled. Figs. 1 and 2, respectively, show diagrams of typical conventional and slimhole wells.

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