Analysis and Modeling of Vertical and Horizontal Compositional Gradients in Two North Sea Reservoirs

Abstract

Abstract This paper presents compositional data from two North Sea reservoirs, one of which exhibits significant horizontal temperature gradients. At a constant depth, the methane concentration decreases with increasing temperature, whereas the C7+ concentration increases. This is attributed to a mechanism tied to molecular heat transport. The data also indicates that in vertical fluid columns there is an additional contribution to the compositional gradient caused by differences in diffusion rates between molecules of varying molecular size, which means that larger molecules take less part in the vertical convection than smaller molecules. The paper establishes equations that quantify the respective contributions to a vertical compositional gradient from gravity, molecular heat transport, and convection. The application of the presented model is exemplified through predictions of compositional variation over a depth interval of 200 meters and evaluation of fluid communication between wells.