Impact of Maximum Allowable Cost on CO2 Storage Capacity in Saline Formations.

Injecting CO2 into deep saline formations represents an important component of many greenhouse-gas-reduction strategies for the future. A number of authors have posed concern over the thousands of injection wells likely to be needed. However, a more important criterion than the number of wells is whether the total cost of storing the CO2 is market-bearable. Previous studies have sought to determine the number of injection wells required to achieve a specified storage target. Here an alternative methodology is presented whereby we specify a maximum allowable cost (MAC) per ton of CO2 stored, a priori, and determine the corresponding potential operational storage capacity. The methodology takes advantage of an analytical solution for pressure build-up during CO2 injection into a cylindrical saline formation, accounting for two-phase flow, brine evaporation, and salt precipitation around the injection well. The methodology is applied to 375 saline formations from the U.K. Continental Shelf. Parameter uncertainty is propagated using Monte Carlo simulation with 10 000 realizations for each formation. The results show that MAC affects both the magnitude and spatial distribution of potential operational storage capacity on a national scale. Different storage prospects can appear more or less attractive depending on the MAC scenario considered. It is also shown that, under high well-injection rate scenarios with relatively low cost, there is adequate operational storage capacity for the equivalent of 40 years of U.K. CO2 emissions.

Multiple well systems with non-Darcy flow.

Optimization of groundwater and other subsurface resources requires analysis of multiple-well systems. The usual modeling approach is to apply a linear flow equation (e.g., Darcy's law in confined aquifers). In such conditions, the composite response of a system of wells can be determined by summating responses of the individual wells (the principle of superposition). However, if the flow velocity increases, the nonlinear losses become important in the near-well region and the principle of superposition is no longer valid. This article presents an alternative method for applying analytical solutions of non-Darcy flow for a single- to multiple-well systems. The method focuses on the response of the central injection well located in an array of equally spaced wells, as it is the well that exhibits the highest pressure change within the system. This critical well can be represented as a single well situated in the center of a closed square domain, the width of which is equal to the well spacing. It is hypothesized that a single well situated in a circular region of the equivalent plan area adequately represents such a system. A test case is presented and compared with a finite-difference solution for the original problem, assuming that the flow is governed by the nonlinear Forchheimer equation.

Transient divergent flow and transport in an infinite anisotropic porous formation.

When seeking to predict plume geometry resulting from fluid injection through partially penetrating wells, it is common to assume a steady-state spherically diverging flow field. In reality, the flow field is transient. The steady-flow assumption is likely to cause overestimation of injection plume radius since the accommodation of fluid by increases in porosity and fluid density is ignored. In this paper, a transient solution is developed, resulting in a nonlinear ordinary differential equation expressing plume radius as a function of time. It is shown that the problem can be fully described by one type curve. A critical time, t(c), is identified at which the percentage error of the steady-state flow solution compared to the fully dynamic problem is less than 1%. Only for large injection rates and low permeabilities, does t(c) become greater than 1 h. Nevertheless, an improved approximate solution is obtained by a simple linearization procedure. The critical time, t(c) for the new approximate solution is 0.3% of that required for the steady-state flow solution.

Modelling radioiodine transport across a capillary fringe.

Due to its long radioactive half-life, iodine-129 is considered to be an important radionuclide in the context of underground radioactive waste disposal safety assessment. Iodine speciates as iodide (I-) in reducing conditions and iodate (IO3-) in oxidizing conditions. As iodate is more reactive, it is much less mobile than iodide. Consequently, in considering vertically upward transport within a soil profile, iodine will tend to accumulate at the top of the capillary fringe. In this paper, a model of iodine transport across a capillary fringe is developed by coupling equations for variably saturated flow, oxygen dynamics and rate-limited sorption. Model parameters are obtained by consideration of literature values, calibration on soil column data and other supporting laboratory experiments. The results demonstrate the importance of rate kinetics on the migration and bioavailability of radioiodine in the near-surface environment.