Fractured limestone formations present a great challenge to the Well Engineer in the designing of oil/gas wells. In the upper hole sections it may be acceptable to cure severe lost circulation by judicious use of cement or other non-reversible treatments, but in the case of a fractured reservoir the use of such treatments is not acceptable due to the damaging nature of the treatment. In really severe cases of lost circulation the only option may be to drill ahead, accepting losses, and having in place a floating mud cap. This procedure, however, is far from ideal due to the many uncertainties that prevail during such operations.

This paper deals with experiences gained while drilling wells in a highly fractured, vugular limestone reservoir offshore Italy. The reservoir in this case is dominated by very low matrix porosity rock but has highly fractured and karst related vugular porosity. These fractures and vugs are the main producing and stock tank oil originally in place (STOOIP) containing elements. Therefore the permanent plugging of the fracture/vugular porosity is not an acceptable option in the cure of lost circulation. Previous operations were unsatisfactory; when a conventional drilling fluid was used to drill the reservoir lost circulation resulted which rapidly gave way to total losses. Attempts to cure the losses were unsuccessful and the well was drilled ahead blind with seawater with no returns.

The problem was addressed by developing and implementing a loss control strategy for a subsequent well. The approach used an unconventional highly shear thinning fluid as the ‘drill-in’ fluid, the intention being to use the highly thixotropic nature of the fluid to control the rate of loss in the first instance. As losses became more severe, two component, cross-linking polymer pills were pumped to seal the loss zones and restore circulation.

This staged approach for controlling the losses was highly successful. Several incidences of total lost circulation were encountered and cured with the result that 258 m of reservoir section were drilled and successfully evaluated.

The techniques developed and the lessons learned during this operation will be of interest to engineers working in areas such as the Timor Sea, the Australian NW shelf and various regions of Indonesia, where similar lost circulation problems are common.

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