Reserves Estimation and Classification Challenges in a Mature Oil Field

Abstract

AbstractEstimation of hydrocarbon reserves is governed by natural, technological and economic factors. Re-estimations and reviews continue almost till the end of exploitation in many instances. Estimation of oil reserves, when a field is in its initial stages of development is a relatively easy task. The estimation will not invite critical or close scrutiny, as the oil production at that stage is far lower than the reserves calculated. However as the field matures and the cumulative oil production approaches the estimated value of reserves, the accuracy in the estimation of reserves becomes crucial and more important. Also the estimation process itself is a challenging task, in the light of uncovering geological complexities and reservoir dynamics during the development and the exploitation methods employed to produce the oil.Bahrain field, the first discovery in the Arabian Gulf region, has been on production since 1933. It is a multi-stacked field with 16 oil and 4 gas reservoirs. Most of them are carbonate reservoirs. The nature of the fluids varies from tarry oil in the Aruma formation occurring at shallow depths to dry gas in the deep Khuff zones.The geology of the field is complex with a large number of faults on the structure. The major oil zone Mauddud – an oil wet system - has been on gas injection from 1938. The gas injection has lead to the transfer of oil and gas into layers above and below through the faults. Though many reservoirs of the field have reached mature stage, there are some zones containing oil resources still unexploited due to the heterogeneities, complex nature of the fluid distribution, unfavorable rock-fluid characteristics, compartmentalization, thin oil rim overlain by gas and underlain by water, etc.This paper brings out the challenges posed by the above situations while attempting to estimate and classify oil reserves.