CO2 Sequestration: Understanding the Plume Dynamics and Estimating Risk
Geological sequestration of CO2 in deep saline aquifers is one of the ways to remove combustion emissions in sufficient volumes to mitigate the greenhouse effect. The sequestration efficiency can be defined as maximizing trapping of CO2 and minimizing CO2 leakage. The leakage risk is parameterized into three response variables: time the plume takes to hit the top seal, maximum lateral extent after certain time and total mobile gas. Extensive compositional reservoir simulations were carried out to quantify these risk parameters for different reservoir and operating parameters including permeability, porosity, aquifer thickness, perforation interval, dip, permeability anisotropy, depth and type of well. A database has been generated which can be used for predicting the risk parameters for any aquifer using interpolation between the database cases or by using simplified models. A general methodology for such predictions is discussed and illustrated for two filed cases. Several simplified analytical or semi-analytical models are developed to understand some of the characteristics of CO2 plume in saline aquifer. The models include plume migration in dipping aquifer, approximating shale barrier by equivalent permeability anisotropy in homogeneous medium, optimizing perforation interval or injection rate in order to utilize all the perforations. The behavior of plume and effect of horizontal well length on sequestration efficiency is also discussed. The various reservoir and operating parameters can be grouped in dimensionless gravity number which is the ratio of gravity forces and viscous forces. It is shown that higher gravity number lowers the CO2 sequestration efficiency. One of the risk parameter (time to hit the top seal) is studied in detail and a correlation is developed between dimensionless time to hit the top seal and gravity number. The importance of using appropriate relative permeability curves is shown. The laboratory measured relative permeability needs to be extended to account for drying region near wellbore. An analytical model of predicting the pressure profile away from injector is explained. The simulation database and simplified models are useful in ranking the prospective aquifers in terms of risk of CO2 storage. These models can act as screening tools for commissioning and decommissioning the geological sites for CO2 storage. Thus it reduces the time and money to do full scale simulation for each individual aquifer.