Challenges, Risks and Opportunities of Mature Field Further Development: A Niger Delta Case Study

Abstract

Identifying further development opportunities in terms of infill well drilling targets to improve recovery in matured Niger Delta fields pose significant challenges. The remaining unswept oil targets carry large uncertainties and risks associated with many sub-surface complexities and often limited and/or inconsistent surveillance data. This paper presents a case study of identifying infill opportunities in the 35-year old Niger Delta A1 field and describes how a pragmatic approach to integrated modeling and uncertainty management resulted in a robust phased further development plan which attempts to mitigate risks. Niger Delta A1 field is a vertically stacked sequence of reservoirs. Production has occurred from nine oil bearing sands, in the depth range 6,400 to 8,400 ftss, which have strong aquifer support. The average porosities vary from 23-30% and permeabilities from 1000-5800 mD. The oil densities vary from 20-27° API. The current (Dec., 2011) recovery factors range from 25-56%. Integrated static and dynamic reservoir modeling indicated further development opportunities in five of the vertically stacked reservoirs. The key uncertainties were the top structure of the reservoirs, current oil-water contacts, sand qualities and aquifer definitions. History matching of production data were carried out to narrow down the location of unswept oil, however, large uncertainties remained. Modeling indicated that the overall recovery from the field could potentially be increase by 6% from drilling four infill wells with 7 drainage targets. Risk mitigation measures include a focused data acquisition plan to calibrate the current fluid contacts and drilling wells targeting two sub-surface locations with dual or sequential phased development. Production data analysis indicates that the recovery factors and well recoverable volumes are highly correlated to average net oil pay. The correlations may be used for reserve QC.