Emerging CO₂ Sequestration Techniques in Bakken Unconventional Reservoirs: An Enormous Opportunity Yet Materialized

Abstract

Widely-distributed organic-rich shales are being considered as one of the important carbon sequestration targets owing to three differentiators compared with conventional reservoirs and saline aquifers, making CO₂ storage in organic-rich shales attractive because shale in US are in the vicinity of existing power/chemical plants. As a result, there are large savings for infrastructure and inventory, e.g., CO₂ purification facility and pipelines, for plant operators as compared with CCUS in depleted conventional reservoirs/saline aquifers. Storage in Bakken allows reduced expenditures on infrastructures, and would welcome a better power plant participation to further mitigate climate change challenges. The incapability to model capillarity with the consideration of imperative pore size distribution(PSD) characteristics using commercial software may lead to inaccurate results for CCUS. We developed a novel algorithm to examine how PSD would alter phase and flow behavior under nano-pore confinements, and incorporate the new model to a 3D dynamic simulator. The assumptions in Langmuir isotherm in commercial software are not held for shale, and CO₂ storage purpose in particular. Our adsorption model uses a local density optimization algorithm and is designed for multi-component interactions to adsorption sites for a full spectrum of reservoir pressures. This feature allowed us to fully understand the storage and sieving capabilities for CO₂/N₂ flue gas system with remaining reservoir fluids. Our adsorption model revolved from monolayer at low pressure to more complex multilayer model spontaneously and consistently. With our advances in modeling of two key features of adsorption and capillarity, our simulator allows a realistic representation that differentiates CO₂ storage in unconventional reservoirs. Some of physical mechanisms that control CO₂ migration in subsurface at various stage of project were examined and analyzed in details. Results show that should capillarity be consistently formulated with PSD, difference in retaining and migration of CO₂ plume is captured. Due to the effect of molecular sieving and higher affinity to CO₂ by shales, the application of flue gas(mostly N₂ and CO₂) for CCUS shows a great economic advantage for large-scale CCUS projects. The results indicate that the cost for CCUS could be reduced by 20% because of much less rigid requirement for CO₂ capture thanks to selectivity of shale formations for CO₂/N₂ mixtures.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016