The B24 well in the Fidaa Field was the first dual lateral underbalanced horizontal well drilled in Libya and the second well of Zueitina Oil Company's (ZOC) underbalanced drilling campaign.

The targeted reservoir "Facha" is partly depleted after 23 years of production to a 7.2 ppg equivalent mud weight. Nitrified diesel, a two-phase drilling fluid, was used to drill the reservoir section of the B24 underbalanced to avoid circulation losses and formation damage. Real time LWD was crucial for geosteering the well path in a complex reservoir structure and real time annular pressure measurement was also necessary to ensure the underbalanced condition was maintained throughout the entire UBD drilling phase.

Two 6" horizontal drains were drilled successfully with 2053' and 1517' drain lengths. Integrated efforts of ZOC and service companies of various disciplines during planning and operation stages of the well led to the success of the project. No QHSE incidents were reported and the productivity of the well was enhanced significantly.

The B24 well's production alone contributes to over 30% of the total Fidaa field production.


The Fidaa field in the north of Muelah Graben at the western edge of Sirt Basin was discovered in 1979. The structural map of the Facha reservoir (Figure 1) shows the structure to be northwest - southeast trending with approximately 9.5 by 2.5 km size and a pay zone thickness of 28 feet. A total of 23 vertical wells were drilled to date in the Fidaa field. Well B24 was the last one drilled horizontally with two lateral legs. It was completed in May 2005.

Facha reservoir "C" layer is dolomite with vuggy porosity of 23% on average and logs indicate extensive fracturing. The reservoir pressure of the upper Facha ranges between 1600 psi to 2330 psi and the average pressure along the planned drain hole in the Facha "C" was 1900 psi at 6085' TVD (6 ppg EMW). The Facha reservoir is an oil bearing pay zone with a stock tank oil gravity of 41.7ΒΊ API, a gas oil ratio of 68 SCF/STB, and water cut ranging from 6% to 70%.

Due to a partially depleted pressure of the Fidaa field, lost circulation and other drilling problems were encountered in the field.

Challenging Problems

To cover the un-drained areas of the field, drilling new wells with conventional drilling fluids was very difficult. Severe losses and differential sticking especially in the horizontal reservoir section(s) were experienced. The invasion of foreign fluids and cuttings into the reservoir led to formation damage that accounted for low flow rates after the wells were put into production. Therfore, another drilling methods or techniques were needed to be considered by Zueitina Oil Company to solve the problems of the Fidaa field.


The Drilling Engineering Department of Zueitina Oil Company investigated many possible alternatives to solve the problems encountered which could also be applied economically and effectively in the field. Finally, the solution found was to employ the underbalanced drilling (UBD) technique, using a two-phase drilling fluid utilizing Fidaa's native crude and nitrogen gas lift in order to drill two horizontal drains in the Facha reservoir. It was determined that this nitrified crude oil must:

  • Create a bottom hole circulating pressure (BHCP) of less than the reservoir pressure,

  • Prevent lost circulation and invasion of foreign fluids into the reservoir that cause formation damage,

  • Increase the productivity of the field.

This technology was available and the UBD equipment had already been successfully utilized on Zueitina's well G55 in the Sabah Field. Zueitina Oil Company drilling team had gained underbalanced Drilling experiences and many lessons were learnt from the G55 well.

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