Abstract

The maturation and development of hydrocarbons in partially appraised fields (PAFs) is often threatened by the high degree of subsurface uncertainty resulting from limited well penetration and paucity of subsurface data in such fields. The uncertainties ranges are sometimes very wide and the resultant cost of further appraisal is so prohibitive that the value and economic indices of carrying out development projects in these fields are severely eroded. For PAFs which are gas-bearing, the challenge is further underscored by the relatively lower price of natural gas and associated higher cost of infrastructure compared to oil. Thus, if not adequately managed, the subsurface uncertainties can go a long way in defining the economic success or failure of planned development projects in PAFs. For this reason, geoscientists and petroleum engineers are tasked with the responsibility of integrating and analysing all available data in the field with the aim of assessing, managing and reducing these uncertainty ranges as much as possible.

The OZ field, which is discussed in this paper, is located in the Niger Delta and has a maximum of 6 well penetrations across sixteen (16) reservoirs in a predominantly gas field. Comprehensive data acquisition (electrical surveys and formation pressures and samples) from the last well drilled in the field in 2012, helped eliminate the fluid typing and contact uncertainties in most of the reservoirs.

However, for the potentially largest reservoir in the field, the actual fluid contacts (Gas Oil Contact or Hydrocarbon Water Contact) were not logged rather a Gas-Down-To (GDT) and Water-Up-To (WUT) were logged in this reservoir at 100ft apart. With a 100ft column of undifferentiated fluid, the resource volumetric uncertainties varied substantially and if the entire 100 ft column contained hydrocarbon then depending on the type (gas or oil) and ratio, the planned development of the reservoir could easily change from primarily gas to an oil development with a gas-cap blowdown in the future. Hence, the fluid typing and contact delineation emerged as one of the major uncertainties associated with the development of the reservoir and the field at large. To reduce this uncertainty, systematic field reservoir pressure analysis coupled with the integration of other electrical surveys and regional knowledge were applied to significantly minimize the fluid type and contact uncertainties.

This paper showcases details of the analysis and its implication in cost reduction and project value enhancement.

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