Many West Africa Offshore Fields are maturing and operators are completing secondary targets in their wells to maintain the economic operation of their valuable assets. However, off-shore environment makes the capital expenditure associated to this kind of interventions of critical importance. It follows that the selection of the right and most remunerative activities is crucial. In the Kitina Field, offshore Congo, deeper sands have been produced to economic depletion and reservoir studies allowed the determination of alternative production intervals for production maintenance. Large quantities of reserves can be found in low permeability, consolidated, formations as well as in very deep and remote culminations. During the first semester of 2007, the Kitina field production increased of 160% reaching a production level lost since early 2004. This was achieved with a variegate set of actions on different reservoirs:infilling the Kitina South culmination with the long reach and ultra deep well KTM-SM5,a massive multistage hydraulic fracturing campaign carried out on the three wells draining the low permeability 3A reservoir and 3rd) with the sweep optimization of the reservoir 1A. Eight propped hydraulic fractures were placed in three re-completed, cased-hole wells with very significant production improvements. These represented the first applications in Congo of different technologies opening a wide range of further applications in similar environments. The paper describes the 2007 and 2008 Kitina rejuvenation campaign with an eye to all the disciplines involved, from reservoir engineering and modeling, to operation geology, drilling and completion, production. The papers focuses with more detail on the successful multi-stage hydraulic fracture campaign from the preliminary design and production forecast pre-job to the reservoir model history match and forecast phase post-job. Interesting reservoir engineering overviews of the future development of the field via improved and enhanced oil recovery techniques are also presented. Introduction Existing oil and gas fields are maturing and new finds are more complex to discover and produce. In today's oilfields portfolio, mature reservoirs production maintenance and increase represent the biggest challenge to face over the next decades to meet the continuously increasing demand for hydrocarbons. Technology research, development and innovation have been the recent answer to sustain the world's oil and gas production and will continue to be so. However, new developments in technology need professionals who are taking the risk of testing them keeping in mind that the failure can be sometimes only a temporary and/or necessary stop towards the success. Mature fields can represent the "working ground" and "technological gyms" where to test new techniques with the final aim of accelerating and increasing reserves. Mature fields have to be seen today as opportunities for improvement rather than declining assets. Accurate candidate selection, optimized treatment design, sound reservoir modeling of production forecast represent crucial and interdependent factors for successful economic evaluations.

Many West Africa Offshore Fields are maturing and operators are completing secondary targets in their wells to maintain the economic operation of their valuable assets. However, the capital expenditure associated to this kind of interventions of critical importance. It follows that the selection of the right and most remunerative well activities is crucial. In the Kitina Field, offshore from Pointe Noire, Congo, deeper sands have been produced to economic depletion and reservoir studies allowed the determination of alternative production intervals for production maintenance. Large quantities of reserves can be found in low permeability, consolidated, formations as well as in very deep and remote culminations. From the Kitina field production increased of 80% reaching a production level lost since early 2004. This was achieved by infilling the Kitina South culmination with the long reach and ultra deep well KTM-SM5 and via a massive multistage hydraulic fracturing campaign carried out on the three wells draining the low permeability 3A reservoir. These represented the first applications in Congo of such technology. Eight hydraulic propped fractures were placed in three re-completed, cased-hole wells in the with very encouraging production improvements. A stabilized production increase ranging from 2 to 3 times was achieved. The paper describes the unique reservoir modeling, operation geology, and drilling, completion and production challenges encountered in the 2007 Kitina successful rejuvenation campaign. In particular, it focuses on the successful multi-stage hydraulic fracture campaign carried out on four wells of the Kitina 3A reservoir from the preliminary design and production forecast pre-job to the reservoir model history match and forecast phase postjob.

The Zatchi field, located in the Lower Congo Basin offshore, is a multi-layer reservoir of Cenomanian/Albian age operated by Eni Congo in partnership with Total Congo. The " Zatchi B?? reservoir is 30 m thick sand characterized by a large accumulation of heavy and highly viscous oil (15° API, 1000 cP) trapped in the marine-transgressive sands of the " Gres de Tchala?? formation. Three aspects make the successful development of such reservoir an extreme challenge: the presence of both bottom water and a gas cap, the very low reservoir pressure, the very high viscosity of the oil. As a matter of fact, over the 27-year life of the field, only three wells were put in production from the " B?? layer, with not satisfying results due to the heaviness and viscosity of the oil and the severe problem of gas coning and cresting. The multilateral technology generally allows increasing the reservoir exposure with fewer wellbores, reducing and spreading the drawdown along the drains reducing the potential for coning. This technology has been proven effective in several heavy oil deposits recovery worldwide. For this reason, in order to improve the reservoir drainage reducing times and costs, the layer B was selected as the optimal candidate for the first multilateral well in the Congo basin: well ZAM 408 ML. This paper will review the Zatchi B reservoir history and development challenges with a focus on the multilateral well reservoir modeling, the TAML6 completion and artificial lift design. Furthermore, the challenges encountered during the operations in terms of reservoir properties sampling, operations geology, drilling, completions and production will be described in detail. Introduction Heavy oil has become an important theme in hydrocarbon industry with an increasing number of operators getting involved or expanding their plans in this market around the world. A huge number of " non conventional?? oil reservoirs have been discovered worldwide, but only a small percentage of them is producing or is under active development. Heavy oil represents a massive world resource, but the great challenge is to find the best way to produce, transport and process it. Eni Congo is taking part to this challenge, concentrating the efforts on the heavy oil of the Zatchi B reservoir. The Zatchi field is located in the Congo offshore basin, with water depth ranging from 55 to 57 meters and an areal extension of about 34 km2 (fig.1) The field is characterized by multiple stacked reservoirs (from A down to E) belonging to the " Gres de Tchala?? and " Carbonates de Sendji?? formations (Cenomanian/Albian age). Like many other commercial reservoirs in offshore Congo, the Zatchi fied is characterized by sandy and dolomitic rocks deposited during the early drift phase in the mid-Cretaceous, with structural-stratigraphic traps created by movement of the transition-phase Aptian salts (fig. 2 and 3). The Zatchi field was discovered in 1980 with the first exploratory well ZAM-1, which found oil accumulations in the Cenomanian/Albian section.

The Zatchi field, located in the Lower Congo Basin offshore, is a multi-layer reservoir of Cenomanian/Albian age operated by Eni Congo in partnership with Total Congo. The "Zatchi B" reservoir is a 30 m thick sand characterized by a large accumulation of heavy and highly viscous oil (15° API, 1000 cP) trapped in the marine-transgressive sands of the "Gres de Tchala" formation. Three aspects make the successful development of such reservoir an extreme challenge: the presence of both bottom water and a gas cap, the very low reservoir pressure, the very high viscosity of the oil. As a matter of fact, over the 19-year life of the field, only three wells were put in production from the "B" layer, with not satisfying results due to the heaviness and viscosity of the oil and the severe problem of gas coning and cresting. The multilateral technology generally allows increasing the reservoir exposure with fewer wellbores, reducing and spreading the drawdown along the drains reducing the potential for coning. For this reason, in order to improve the reservoir drainage reducing times and costs, the layer B was selected as the optimal candidate for the first multilateral well in the Congo basin: well ZAM 408 ML. This paper will review the Zatchi B reservoir history and development challenges with a focus on the multilateral well reservoir modeling, the TAML6 completion and ESP artificial lift design. An innovative system for the ZAM-408ML ESP PCP efficiency improvement will be also presented. Furthermore, the challenges encountered during the operations in terms of reservoir properties sampling, operations geology, drilling, completions and production will be discussed in detail. Introduction Dwindling oil supply, high energy prices and the need to replenish reserves are encouraging oil companies to develop all the assets that were temporarily parked for years either because too technological challenging or because not economically interesting. A huge number of "non conventional" oil reservoirs have been discovered worldwide, but only a small percentage of them is producing or is under active development. Most of the world's oil resources are heavy, viscous hydrocarbons that are costly and difficult to produce and refine. In particular, heavy oil (density lower than 22.3 °API), extra heavy oil (density lower than 10° API) and bitumen make up about 70% of the world's total oil resources of 9 to 13 trillion bbl [Ref. 1]. Heavy oils have become recently an important theme in hydrocarbon industry with an increasing number of operators getting involved or expanding their plans in this peculiar market. Many countries are moving now to increase their production, revise reserves estimates, test new technologies and invest in infrastructure to ensure that their heavy oil resources are not left behind. Eni Congo, operator of the offshore Congo Zatchi field participates to this challenge. The case history presented in this paper deal with both technological and economical challenges discussed above: a pilot TAML6 multilateral well in the complex heavy oil Zatchi B reservoir located in the offshore Congo.

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