Total operates an offshore oil field approximately 60 Km West of Pointe-Noire, Republic of Congo. To date, the Albian reservoir has contributed most of the oil produced. However, a significant part of the oil in place is in the Cenomanian, a low permeability sandstone formation with poor quality. As of early 2007, only two of the initial 50 wells in the field had been completed in the Cenomanian. It was desired to evaluate the potential of hydraulic fracturing stimulation treatments to more fully comprehend the production capability of this reservoir. In early 2007, a third well was drilled and completed in this structure, penetrating the Cenomanian at a 60 degree inclination for this purpose. Proppant fracturing from a wellbore with a 60 degree inclination in a hard-rock formation has always been difficult. Also adding another level of uncertainty was the fact that the Cenomanian has never been fracture stimulated. This paper will detail the planning process from both the operator and the service provider perspectives with specific well control issues related to an over-pressured reservoir. Review of the well conditions and operational constraints indicated the best approach would be a multi-stage pinpoint stimulation method incorporating hydrajet perforating and sand plug isolations using a stimulation vessel especially designed for such work. Additionally, a comprehensive review of the actual three-stage stimulation treatments will detail the perforating, fracturing, and sand plug isolation after each stage. With BH memory gauges just below the jetting tool assembly, post-frac reviews of downhole pressure data is compared with the live annulus data used for real time decisions during the stimulation stages. Post-frac well cleanup and early production data will be included. Background Of the initial 50 wells in the field, only two producers were completed in the Cenomanian instead of the Albian reservoir. Although this reservoir constitutes a major part of the oil reserves of the field, its low permeability (10mD) and poor formation quality (argillaceous siltstones) always result in a low production index in perforated cased hole configuration. To show that a field development on such a reservoir could be economic, it was needed to prove that effective fracturing operations could be performed on the third well, drilled early in 2007, in this structure. No similar operation had been undertaken in West Africa on a hard low perm rock where the core samples appear to come from a well in Texas, instead of offshore West Africa. Furthermore, a fines stabilizer chemical agent was pumped to provide a treatment solution to lock the migrating fines in place to minimize the potential damage to the formation and maintaining the production flow rates of the well. This offshore field is currently operated from two primary platforms set in about 140 m of water. Most of the oil production has been from the 48 wells completed in the Albian reservoir. This reservoir, below the Cenomanian, is a moderate permeability carbonate formation with a varied degree of natural fracturing present. It is typically completed using a large volume HCl acid-frac stimulation after perforating. These wells account for more than 97% of the pre-2007 production of the field What enhances the value of the Cenomanian formation as a producing reservoir (while increasing the complexity of drilling and completion operations) is that this reservoir is significantly overpressured. The reservoir pressure in these zones required the use of a 1.40 sg completion fluid (mixed CaCl2 / CaBr2 brine) to maintain 8 bar over-pressure above reservoir pressure.

fax 01-972-952-9435. AbstractN'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.