The Zatchi B Heavy Oil Reservoir Development: The Unique Challenges of the TAML6 Multilateral Well ZAM-408ML

Abstract

Abstract 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.