Experimental Design (ED) methods are becoming a wellestablished alternative to traditional uncertainty analysis due to the fact that it offers a more efficient and consistent approach to identify and assess the impact of the key subsurface uncertainties on field development decision. This paper describes a case study of applying ED to assess development risks in a gas reservoir in the Niger Delta. The reservoir is areally very extensive but relatively thin and underlain by aquifer. Key development and recovery risks were recognised to be access to pressure support and water breakthrough at the production wells. The objective of the study was to define realistic P10-P50-P90 subsurface realisations and test various development concepts against this range of models to generate an optimal field development strategy.A multi step ED approach was used to understand and manage the key subsurface uncertainties. Alternative linear screening designs and multi-level response surface designs were used in this study.Key outcomes of the study were:-Successful demonstration of the application of Experimental Design technology and alternative screening designs -Generation of Response Surface models for In-Place and recoverable volumes. -Definition of a realistic range of sub-surface realisations for the reservoir and identification of P10-P50-P90 sub-surface models using Probabilistic (Monte-Carlo) analysis -Definition of key sub-surface uncertainties to be targeted for appraisal program -Generation of a robust range of production profiles and reserves estimates.

Many gas reservoirs critical to providing a reliable supply of gas to the Nigeria Gas market have observed or potential oil rims where only a gas-down-to (GDT) has been logged. Development of these oil rims must be considered as part of the overall hydrocarbon maturation plans for the reservoir. Maturation studies of this type can take a long time and may lead to restrictions on the availability (quantity and timing) of the gas volumes to the market. This poses a serious challenge to maintaining gas supply and meeting contractual obligations. Field performance of selected oil rim reservoirs in the Niger Delta have been analysed and presented in this work. The main factors that influence oil rim performance are highlighted and oil recovery trends have been established. In addition, a generic simulation model has also been developed to analyse oil rim dynamics and assess the impact on oil and gas recovery for a range of sub-surface uncertainties. A range of alternate development strategies has been considered. Experimental design was used to obtain oil recovery correlations for each development strategy which has proven to be a useful first pass screening tool to make assessments of the likely oil recovery range from a specific oil rim reservoir. The oil recovery trends obtained from the simulation study have been found to be consistent with actual oil rim performance. The result of this work is therefore useful for quick screening of oil rims for technical and economic feasibility to support key business decisions before embarking on costly detailed studies, if eventually required, subject to the complexity of the situation and decision required. Introduction Many gas reservoirs critical to providing a reliable supply of gas to the Nigeria Gas market have observed or potential oil rims where only a gas-down-to (GDT) has been logged. Development of these oil rims must be considered as part of the overall hydrocarbon maturation plans for the reservoir. Maturation studies of this type can take a long time and may lead to restrictions on the availability (quantity and timing) of the gas volumes to the market. This poses a serious challenge to maintaining gas supply and meeting contractual obligations.

This paper presents the results of the study that was initiated to investigate the incremental benefit of gas and/or water injection in the production of oil rim reservoirs. Due to the nature of the project, emphasis had been on maximizing oil recovery. Consequently, only the incremental oil recovery was used to screen development concepts with a view to optimizing the overall economic value of the concept. This paper includes reservoir simulation on oil rim development with gas and/or water injection with sensitivities on: 1) Permeability anisotropy, Kv/kh 2)GOR relaxation policy 3) Aquifer strength 4) Oil Column Thickness (Ho) 5) Well placement. Results of this study suggest that simultaneous gas and water injection (voidage replacement maintained) could increase recovery factors by upto 15% of STOIIP.

A pilot Gravity Assisted Water (Dumpflood) Injection scheme was executed in 1997 in X1 reservoir of a Niger Delta field 'Alpha'. Post performance analysis showed that the injection has been successful in maintaining good pressure support and in supporting future recovery from the reservoir. However due to X1 reservoir uncertainities and variabilities in Water Dumpfllod injection rates, expected range of future recoveries could not be quantified.

One of the most conspicuous geological features of the Niger Delta is its growth fault pattern. Growth faults are a particular type of normal fault that develops during ongoing sedimentation, so the strata on the hanging wall side of the fault tend to be thicker than those on the footwall side. In this study, a detailed treatment is given of the transient pressure behaviour of a well located near a leaky boundary with unequal formation thickness on opposite sides of the barrier. These types of barriers are typical features of growth fault. The results of this study generalize previously published works which assumed the thickness index for media on different sides of the fault was zero, one, or infinity. These assumptions are not always valid for growth fault reservoirs commonly found in the Niger Delta. One application of this work would be to facilitate detection of formation thickness on the other side of the fault without drilling a well in that location, this means reduced capital development cost thereby maximizing NPV. The range of variables examined includes thickness index from zero to one and from one to infinity, also, the effect of transmissibility factor ranging from zero to infinity was included. Based on this study, it was observed that for high values of the thickness index, the response is similar to that of a constant pressure boundary, while the behaviour resembles the sealing boundary characteristics for low values of the index.