INVESTIGATING THE SUBSURFACE HETEROGENEITY OF A NATURALLY FRACTURED RESERVOIR IN RELATION TO CO2 GEOLOGICAL STORAGE AT THE KEVIN DOME SITE, MONTANA

In this study, we use data collected from the Big Sky Regional Partnership to gain a better understanding of subsurface heterogeneity at the Kevin Dome site and assess the probability of successfully injecting 1 million tons of CO₂ into the naturally fractured Middle Duperow formation. A dual-porosity, single-permeability geostatic model is constructed using seismic, well log, and core data to characterize the injection reservoir. This model builds facies distributions using a lithology analysis and rock physics based prediction workflow that integrates well logs, seismic inversion, and geological modeling to provide an estimate of the most probable lithology, and the likelihood associated with the prediction. A discrete fracture network (DFN) is developed based on core and well log observation to characterize fracture distribution and associated properties. The geologic model also incorporates 3-dimensional seismic elastic attributes to significantly improve the level of detail and guide the extrapolation of facies, porosity, permeability, and fracture properties. Matrix and fracture permeability values around the injection well (Wallewein 22-1) and production well (Danielson 33-17) are calibrated to their injection and production tests, respectively. Next, a number of CO₂ injection simulations are performed with uncertainty parameters to estimate injectivity and pressure response at the injection wellbore over 4 years. Well test results suggest 1) that the radius of investigation is relatively small due to short test duration and 2) vertical permeability is important in explaining the pressure response at the wellbore. DFN modeling shows fracture aperture and the fracture intensity have a significant impact on the calculated fracture permeability while fracture length has a relatively minor impact. Among uncertainty parameters, fracture permeability plays a major role in injectivity and pressure response. Model results suggest a high return on uncertainty reduction from characterizing these parameter distributions and properties. CO₂ injection simulation results indicate it is unlikely that Big Sky could meet its target of 1 million tons of CO₂ stored in the Middle Duperow formation, with a lower estimated probability of success compared to previous estimates based on regional parameters.