Reducing Contaminant Uncertainty from First Principles

Abstract

Abstract Contaminants in gas reservoirs such as CO2 and H2S have large impact on field development. Although the contaminant concentrations are usually determined during the appraisal of a field, their distribution across the field often remains a key uncertainty with significant impact on the development cost (e.g. through the choice of materials for wells and facilities.) In this paper we present an innovative approach to determine the distribution of two gaseous contaminants (CO2 and H2S) in different reservoir zones using the fundamental equations for diffusion. In the offshore carbonate gas field, that we describe in this paper, strong variations of CO2 and H2S levels in two of its three reservoirs zones were measured during appraisal of the field. The contaminants originated from thermo-chemical sulfate reduction deep below the reservoir and migrated through the aquifer into the reservoir build-up. The paper addresses two key uncertainties, which have great impact on the development planning:the contaminant levels of the third (deepest) reservoir zone which were expected to be higher than the levels encountered in the other zones;the risk of a contaminant level increase as a result of the liberation of dissolved contaminants from the aquifer when the reservoir pressure drops Making use of the contaminant generation model and Fick’s 2nd law we determined the contaminant concentrations in the third reservoir zone. On the basis of that result we also show that an increase of contaminant levels as a result of the decrease of the contaminants solubility in the aquifer is no risk in this field. These results allowed us to tie down these key uncertainties so that a significant cost saving for our wells and facilities could be obtained.