Preinjection Reservoir Fluid Characterization at a CCS Demonstration Site: Illinois Basin – Decatur Project, USA

Abstract The Midwest Geological Sequestration Consortium is conducting the Illinois Basin – Decatur Project (IBDP), a large-scale carbon capture and storage (CCS) project in Decatur, Illinois, USA. An extensive Monitoring, Verification, and Accounting (MVA) program has been implemented, and a deep monitoring well, VW#1, was drilled to a depth of 2,201 m (7,264 ft). The well was perforated in eleven zones ranging in depth from 1,499 to 2,153 m (4917 to 7061 ft.). Fluid samples were collected during three sampling events using two different sampling methods. This paper describes the well construction, well completion, sample collection procedures, and selected preinjection sampling results from VW#1. Prior to sampling, particular care was taken to remove non-native fluids from each of the sampling zones. Density, specific conductance, bromide concentrations, and bromide-chloride ratios have been used as indicators of sample representativeness. Fluid compositional data and formation pressure data have been collected from VW#1. These high-quality, field- based data are essential to establish hydrochemical conditions in and above the storage reservoir prior to CO 2 injection, indicate the response of the reservoir to injection, provide detailed geochemical model inputs, and provide insights for on-going work at the Illinois State Geological Survey about the origin of Illinois Basin brines. Primary brine constituents are chloride, sodium, and calcium with average total dissolved solids concentrations of 190,000 mg/L in the Mt. Simon Sandstone and 65,600 mg/L in Ironton-Galesville Formation (above the injection reservoir).