Managing Uncertainties in Hydrocarbon-in-place Volumes in a Northern Depobelt Field, Niger Delta, Nigeria

Abstract

Abstract Petroleum resources represent a significant part of a company's upstream assets and are the foundation of its current and future upstream activities. Often times, at the discovery of a new field or extension of an existing field, there are uncertainties associated with quantifying the amount of hydrocarbons in place. These uncertainties may be related to the structure, aerial extent of the accumulation, unseen fluid contacts to delineate the vertical extent, internal architecture of the reservoir and the characteristics of the resident fluid(s). In some cases, companies may complete and produce discovery wells before they can fully appraise the structure or may be forced by other considerations such as community disturbances to abandon appraisal drilling and continue to produce from existing well(s). All these and much more, have an impact on the evaluation of in-place hydrocarbon resources and consequently recoverable hydrocarbons. In Field Development Planning, it is routine to identify and quantify the impact of major subsurface uncertainties such as the in-place volumes and their distribution. This paper presents the methodology and results of an integrated disciplinary effort at translating uncertainties into a range of static (in-place) volumes for the purpose of field development. Erratic sand development, paucity of biostratigraphic control coupled with a complex structure make the G1.0 complex of the EGBM field one of the least understood hydrocarbon reservoirs of the Northern depobelt - Onshore, Niger Delta, Nigeria. Lack of PVT samples and analyses also add to the uncertainty in fluid properties. The erratic distribution of the petrophysical parameters especially from the G sand core also contributes to the petrophysical uncertainties. The construction of 3-D static reservoir models based on the understanding of facies and their relationships, through the integration of all available data have been used to enhance the understanding and quantification of the uncertainties. Standard evaluation of uncertainties in the spread of petrophysical parameters like porosity, hydrocarbon saturation and Net-to-Gross ratio was carried out and compared with the multiscenario concepts incorporated in the geological models. PVT parameters were derived for the reservoir based on analogy and correlations constrained with production and test data. Efforts were also made to comply with definitions of proved reserves by Securities and Exchange Commission (SEC) while still evaluating expectation volumes for internal purposes. An attempt has also been made in comparing results from the probabilistic volumetric evaluation of this reservoir and the deterministic (best estimate) method.