Surface and Subsurface Geochemical Monitoring of an EOR-CO2Field: Buracica, Brazil

Abstract

This paper presents a surface and subsurface geochemical survey of the Buracica EOR-CO<sub>2<sub/> field onshore Brazil. We adopted a methodology coupling the stable isotopes of carbon with noble gases to investigate the adequacy of geochemical monitoring to track deep fluid leakage at the surface. Three campaigns of CO<sub>2<sub/> flux and concentration in soils were performed to understand the CO<sub>2<sub/> variability across the field. The distribution of the CO<sub>2<sub/> soil contents between 0.8 and 14% is in great part controlled by the properties of the soil, with a first-order topographic dependency. These results, together with a <i>δ<i/><sup>13<sup/>C<sub>CO<sub>2<sub/><sub/> between –15 and –23‰, suggest that the bulk of the soil CO<sub>2<sub/> flux is biological. The gas injected and produced at numerous wells across the field showed a great spatial and somewhat temporal heterogeneity with respect to molecular, <i>δ<i/><sup>13<sup/>C<sub>CO<sub>2<sub/><sub/> and noble gas compositions. This heterogeneity is a consequence of the EOR-induced sweeping of the petroleum fluids by the injected CO<sub>2<sub/>, producing a heterogeneous mixing controlled by the production scheme and the distribution in reservoir permeability. In light of the <i>δ<i/><sup>13<sup/>C<sub>CO<sub>2<sub/><sub/> found in the reservoir, the stable isotopic composition of carbon was insufficient to track CO<sub>2<sub/> leaks at the surface. We demonstrate how noble gases may be powerful leak discriminators, even for CO<sub>2<sub/> abundances in soils in the bottom range of the biological baseline (~1%). The results presented in this study show the potential of geochemical monitoring techniques, involving stable isotopes and noble gases at the reservoir and soil levels, for tracing CO<sub>2<sub/> in CCS projects.