Abstract

Best Practices for cementing are well established in the industry however there remains the continuous challenge of effectively employing these best practices in balancing the need for drilling performance and the need for delivering the best possible well to production. For multi-zone monobore completions where the cemented tubing annulus is the single means for zonal isolation, this becomes a critical goal. Such is the case with the majority of wells drilled in the Gulf of Thailand. Desire to meet this challenge led to the creation of a full-circle process that both provides the methodology and initiates the inter-disciplinary communication necessary to improve zonal isolation and the added value it will bring. Components of the full-circle process employed in Thailand include: creating a cementing "scorecard" to focus operations on cementing best practices; forming a cross-disciplinary team of drilling, completion; production and contractor personnel to review bond logs, adjusting drilling practices; and communicating to all involved in the process.

An integrated team approach developed within the context of the full-circle process has successfully linked the goals of the Subsurface Operations and Production Departments resulting in the creation of more production opportunity and maximized recovery rates. The CBL Review Team (CBLRT) created in late 2002 consists of drilling, completions and reservoir engineers, along with logging and cementing experts from the representative service providers. In many cases members of the Asset Management team also attend the regular weekly meetings. To date, the team has reviewed over 450 wells.

This paper presents the full-circle process model and shows how improving zone isolation, evaluation methods and field results can be adapted for application in other operating environments.

Background

Unocal comenced drilling in Thailand during the 1960's with first production coming in 1981. As of April 2004, the company has produced 5,527 bcf of gas and 198 mm barrels of condensate from several large offshore gas fields. First oil production occurred in July 2001 and as of April 2004, 14.1 mm barrels of oil has been produced. The company now has over 2,000 wells on 95 wellhead platforms. Current daily production averages are ~1.2 bcf gas, ~42,000 bpd condensate and ~18,000 bpd oil1.

In general, a batchdrilled, slimhole, mono-bore completion is adopted for the GOT. The surface 9–5/8" casing is set in 12–1/4" hole at approximately 1,000 ft. TVD, followed by the 7" casing set at the pressure ramp near 5,000 ft TVD in 8–1/2" hole. During this section most of the directional work is accomplished. The final phase is a 6–1/8" deviated tangent section that intersects as many as 30 pay sands. Porosity, permeability and water saturation in theses sands vary tremendously as well as the reservoir pressures, which range from 1,600 to over 5,000 psi. A 2–7/8" completion string with TRSV is run into the 6–1/8" hole and is cemented in place. Simplifying operations via batch drilling and the use of the slimhole, mono-bore concept has resulted in substantial cost savings. This has been the driver for more development wells and higher production rates, which in turn has allowed further drilling into ever more challenging environments. Several drilling speed records have been set by Unocal in the GOT. In 2002, 253 wells were drilled at an average of 5.3 days per well. For 2005 Unocal's goal is to drive the drilling time down to 4 days per well in the GOT. To achieve this, several technical challenges will need to be overcome.

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