Significant and dangerous drilling problem is created when suddenly facing massive lost returns. Consequently, conventional means of controlling the well through circulation down the drill pipe and returning up the annulus becomes impossible. Due to their heterogeneous nature, carbonate rocks typically present drilling challenges. In M offshore field it is common to encounter tight formations, highly fractured rocks, and vugular textures. Accordingly, alternate means of controlling the well and safely drilling ahead must be used.

This paper reviews and discusses the application of mud cap drilling (MCD) which was used to drill a well in a highly fractured formation offshore Iran in Persian Gulf. The target formation in 12 ¼″ section of an appraisal well of Iran's M gas field shared with Saudi Arabia was a carbonate reef. This section contains highly fractured rocks and dissolution channels created by large caverns that partially collapsed with later sedimentation. When these dissolution channels were penetrated by the well path, the formation completely yielded to any small amount of pressure overbalance and complete loss of returns was experienced. Seismic analysis was not successful in clearly identifying these areas. All wells drilled into the target reservoir had the potential for this loss. All attempts at curing this lost circulation with conventional techniques failed. The logical way of approaching the problem and achieving total depth was to drill with MCD technique.

PETRPARS has recently implemented a MCD solution on one well offshore Persian Gulf, Iran, which experienced total losses. Once total losses are encountered, a series of procedures are executed that result in converting the well from a conventional drilling operation to a MCD operation. While drilling Well M03, PETROPARS encountered total lost in Surmeh and Neyriz formations from 7904 to 10184 feet. The consequent actions include trying to cure losses with lost-circulation materials, cementing, waiting on mud materials and enduring other drilling problems associated with massive losses encountered in the lower portion of the well. Full penetration of these formations requires the use of a MCD and consequently well cost was minimized due to application of this method. A comparison for the cost of drilling mud in 12 ¼″ section in well M03 shows 43.7% reduction when drilling shifted to MCD technique.

This paper discusses the operational and technical problems encountered including well control, optimizing mud composition and usage, and formation evaluation on an appraisal well which was drilled on this field. The geology, lithology and causes of loss also will be discussed at this paper.

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