N'Kossa is an offshore field located 60 km west of the coasts of Congo in water depths of 170 m. The field is producing light sweet oil from an Albian age reservoir buried between 3100m and 3400m TVD. In order to access reserves located in the southernmost compartments of the reservoir, Extended Reach Drilling (ERD) was implemented. Six ERD wells have been drilled to date with lateral extensions close to 6500 m leading to total depths sometimes in excess of 8600m. In addition to the challenges pertaining to the drilling itself, the completion also carried its own ones as the formation would require effective acid-stimulation (not only an acid wash) to reach the desired levels of productivity. Stimulation of long intervals, and how to ensure full coverage of treatments is a recurrent topic of debate, several approaches have been discussed in the literature. In the particular case of N'Kossa, this issue was not only rendered difficult by the length of the perforated intervals (up to 1200m) but also derived from the combination of lithology and permeability contrasts existing in the formation: indeed the reservoir is an alternation of rather tight carbonates (with permeabilities as low as 1 mD) and porous sandstones (which permabilities sometimes reach up to 400mD). The contrast in permeability is unfavorable as the high permeability layers are often encountered at the heel of the drains. Finally, the reservoir temperature is 150°C (300°F), leading to the need for retarded acid systems.

Building on the experience acquired from the successive treatments performed on N'Kossa, the methodology and treatment design, fluids and diversion have been continuously evolved. The treatments currently involves two phases: An injectivity initiation is performed via Coiled-Tubing creating an artificial thief zone at the toe of the well; then a massive treatment based on emulsified acid and ball-sealer diversion is bullheaded from a stimulation vessel.

This paper will discuss design considerations and operational aspects of the acid treatments performed of these ERD wells. We will discuss some of the observations made and present lessons learnt from such treatments.

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