Numerical Models for Evaluating CO2 Storage in Deep Saline Aquifers: Leaky Wells and Large-scale Geological Features

Abstract

Geological storage of CO2 is one of the most promising strategies for obtaining large-scale reduction in global CO2 emissions. Deep, saline aquifers within sedimentary basins have been identified as attractive options for geologic storage with a large potential storage capacity for CO2 disposal. An important risk of geological storage is the potential for leakage along existing oil and gas wells. Wells are particularly important in North America, where more than a century of drilling has resulted in millions of wells. There is significant uncertainty surrounding the integrity of existing wells, which is due to a lack of data on the hydraulic properties of these wells. Thus, models of CO2 injection and leakage will involve large uncertainties associated with wells, and a probabilistic framework is required. These models must also be able to capture both the large-scale CO2 plume associated with the injection and the small-scale leakage problem associated with localized flow along wells. Traditional numerical methods are not suitable because grid refinement is needed to capture wellbore flow, which soon becomes computationally prohibitive because of the large number of wells and the need for multiple realizations in a probabilistic framework. However, numerical methods provide needed flexibility for handling complex geological systems. In the first part of this dissertation, we examine three techniques for handling wells in numerical models for their accuracy and computational speed. The method that performs well in both aspects is a hybrid model that uses a vertically-averaged numerical model to solve the large-scale problem on a coarse grid and a local, analytical model to capture the small-scale leakage problem. The second aspect of this dissertation addresses the lack of data regarding the integrity of existing wells. We propose a simple field test to measure the effective permeability of a wellbore section, and we analyze the feasibility of this test using numerical analysis. The final contribution of this dissertation concerns the use of vertically-averaged numerical methods to model geological CO2 storage at the basin scale. These models can be used for solving large-scale systems with complex geological features. For example, we use this approach to characterize the effect of sloping aquifers on CO2 migration for a wide range of systems and develop a dimensionless number that can be used to predict the extent of upslope migration for a given system. With these novel numerical methods for modeling CO2 injection and leakage, and a simple test design for measuring existing well parameters, this dissertation addresses some of the major challenges in the field of geological CO2 storage.