In January 1991, Petroleum Development Oman L.L.C. began a campaign to drill six medium radius horizontal wells in the Nimr field in South Oman. The wells were drilled with a 12-1/4" build up section followed by an 8-1/2" horizontal section. On the basis of this campaign, a decision was made to extend the number of horizontal wells to be drilled in the Nimr field during 1991. The first three wells of the second campaign were drilled to the same design as the original six wells, principally due to the non availability of certain tubulars needed for a radical design change. Once the required equipment was available, the wells were drilled and completed with a Composite Liner Design, i.e. the build up and horizontal sections were drilled and cased in the same hole size.

This paper discusses the original well design criteria, the results of the operational learning curve (the new design concept for Nimr wells), and the economic benefits. By the end of February 1992, four of the new design wells had been successfully drilled and completed.


The Nimr field is located in South Oman at approximately 18.50 degrees north and 56.00 degrees east. The field contains a STOIIP of 428 million cubic meters of medium gravity 21 deg API viscous oil (400 Pa.s at reservoir conditions). Oil columns are thin, less than 90 meters, and the oil is underlain by a very active aquifer. Due to a close hydrostatic reservoir pressure the oil is produced to surface with beam pumps.

The original development plan for the Nimr field consisted of drilling vertical, gravel packed, producing wells on a 600 meter grid, convertible to a seven or nine spot pattern for possible subsequent steam flooding.

In 1990, as part of a well trial of medium radius horizontal drilling in P.D.O.'s areas of Oman, it was decided to attempt a horizontal well in the Nimr field to optimize drainage. Following the success of the first well, four further wells were drilled over a ten month period to prove up the feasibility of the technique in the Nimr field.

The encouraging productivity from the four well trial led to the further Nimr Horizontal Campaigns. The first was the six well Campaign drilled in early 1991 and the second, which includes the "Composite Liner" approach to the drilling of Nimr horizontal wells, is ongoing.

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