Abstract

A narrow hydraulic window was the main challenge for a Dutch operator drilling various slimhole wellbores into the Carboniferous-aged Rotleigend reservoir formation in the North Sea, offshore Holland. The targets were reached by first drilling through the Zechstein salt, Silverpit, and Lower Slochteren formations. This paper discusses the application of a high performance organo-clay-free invert-emulsion fluid (OCF-IEF) with low equivalent circulating density (ECD) characteristics to help drill well targets through depleted formations.

The objective, referred to as ZeRoOne, was to drill the Zechstein and Rotliegend formations in one section. The decision to drill ZeRoOne was based on a risk and value assessment for extending field life and maximizing production in the maturing asset. A primary factor in the basis of design was selection of a fluid system capable of delivering low ECD margins, thus mitigating risks involved in drilling a 1200 m (3,937 ft) long section through the overburden and into the reservoir. Design mud weight was set at 1.62 SG with an expected fracture gradient (FG) of 1.86 SG (15.49 lbm/gal). Though a relatively wide margin, there was a marked downside risk—it is difficult to quantify a lower fracture gradient in overburden-reservoir transition zone because of depletion.

The OCF-IEF system delivered ECDs well within the critical mud-weight window, minimizing risks through the 6-in. section. ECDs ranged from 1.70 to 1.72 SG (14.20 to 14.33 lbm/gal), resulting in a 0.14 SG (1.2 lbm/gal) margin below FG.

The operator was able to successfully drill the well and penetrate the depleted reservoir, overcoming the challenge through Lower Slochteren formation. In contrast, offset wells in the same field development, drilled with conventional invert-emulsion fluids, showed much higher ECD values ranging from 1.79 to 1.84 SG (14.91 to 15.33 lbm/gal), with a margin of only 0.2 SG below FG; some of these wells did not reach target total depth (TD) because of the restrictive (minimal) drilling margin.

The successful trial application of the OCF-IEF while drilling the ZeRoOne objective allows more challenging, longer stepouts, directional, and extended-reach drilling (ERD) wells to be drilled as part of the ongoing extended-life field development. The technical objectives of the well would not have been achieved without the use of this system.

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