Fields in the Niger Delta Region in Nigeria typically comprise of stacked multiple reservoirs, sometimes with more than one culmination in a particular sand unit. Due to the occurrence of numerous hydrocarbon-bearing intervals in these stacked reservoirs, it is often unlikely that an appraisal well downdip of the structures will provide fluid contacts information at all levels in the field. Based on this fact, some gas-bearing reservoirs with relatively small Hydrocarbon In-Place volumes cannot be economically developed owing to the requirement of a dedicated appraisal well to establish the presence or absence of an oil-rim. The presence of an oil rim in a predominantly gas reservoir can go a long way in influencing the long term development philosophy. For example, the presence of an oil rim could lead to delay in gas cap production, and thus affect an operator's ability to meet gas demand contracts. An alternative approach that can verify the presence of oil-rim without a dedicated appraisal well will aid early development of these reservoirs.
In this paper, the use of compositional gradient model as an alternative way of predicting presence of oil-rim in the Niger Delta reservoirs is presented. The model is calibrated with data from several reservoirs with known fluid contacts, and then applied to gas reservoirs where fluid contacts are unknown. The results show good comparison with log, core and amplitude data when the fluid column is continuous and not interrupted by fluid infusion or geological barrier.
The theory of using compositional gradients to establish the presence of oil-rim has been widely investigated around the world. The theory is based on compositional variation along hydrocarbon column where the forces at play are majorly arising from gravity, chemical and thermal forces. In a typical hydrocarbon bearing column under gravitational forces, the mole fraction of lighter components decreases while that of heavy fractions increases from top to base of the column. As a result, fluid composition and properties also vary. This is the basic theory underlying the use of compositional gradient model to predict oil rim presence.
In the Niger Delta where the CAPEX for a new well is in the region of $25 million for an onshore well, this approach becomes a cost-effective way of confirming the presence of oil-rim, and therefore aid the profitable and speedy development of the asset.