Brian F. Towler

Numerical solution of non-linear parabolic PDEs by asymmetric finite difference formulae

Abstract A set of explicit finite difference analogues for first and second order space derivatives and for non-linear non-differential terms of a parabolic par

Underground Coal Gasification

Underground coal gasification (UCG) involves the unmined coal seams that are reacted underground, with insufficient oxygen for complete combustion, to create syngas. An oxidant, usually air, flows through an injection well and into a cavity in a coal seam. The oxygen and water within the coal seam react with the coal to produce syngas, which is withdrawn through a production well. The operator of an underground coal gasification cavity has a very limited set of process data as compared to the operation of a typical reactor in a chemical plant. The available data is generally limited to pressure, temperature, gas flow rate, and gas composition at the injection, and production wells. The reactor, which consists of the underground cavity, has unknown and constantly changing dimensions. A number of mathematical models of underground coal gasification are developed to improve the design and operation of UCG. Recent models include those of Yang and Perkins and Sahajwalla. The model developed by Perkins and Sahajwalla can be divided into two subproblems. The first concerns the description of a small section of the cavity wall and the second examines the overall changes between the injection and production wells.

Chapter 9 – Wind Energy

Wind Energy, on the other hand, is relatively cost-competitive with other energy sources, but it has the same reliability issues as solar energy. Wind does not blow all the time and cannot be turned up when the electricity demand increases. As its use becomes more widespread, the environmental impacts of wind energy production are also coming to the fore, as wind farms create noise and visual pollution such that people object to living next to them.

Plugging CSG Wells with Bentonite: Review and Preliminary Lab Results

Oil and gas wells are required to be plugged when the production of these wells is no longer economical. Cement is the current standard method for plugging wells. However, this process has limitations because cement is expensive and prone to cracking and unsealing. This paper aims to review and investigate the use of a naturally occurring clay called bentonite to plug CSG wells in Queensland as well as oil and gas wells in general. Bentonite is cheaper and easier to handle and when hydrated it creates a more reliable plug because it is malleable and self-healing when disturbed. We also experimentally and theoretically investigate the mechanisms for failure of bentonite plugs. The plug failure mechanisms can be determined by comparing the measured dislodgement pressure and the predictions of the theory developed in our group. Based on our preliminary results we found that the hydrated plugs can be made significantly stronger by restricting the expansion space. This allowed us to measure the internal swelling pressure at 8 MPa which corresponded to measurements reported in the literature at a reduced density of 1.6755 g/cm3.

Mechanistic Modelling of Counter-Current Slug Flows in Vertical Annuli

A range of mathematical models and correlations is used to estimate the pressure drop for co-current two-phase flows in vertical wells in the conventional oil and gas industry. However, in the annulus between casing and tubing of a coal seam gas (CSG) well, the upward flow of gas and downward flow of water results in counter-current two-phase flows. The flow regimes developed in such a counter-current system are noticeably different to co-current flow regimes, and thus the existing models used to predict pressure profiles in co-current wells do not adequately describe two phase flows in a (pumped) CSG well. In this study, we modified existing mechanistic models for co-current flow and counter-current flow in a pipe to predict liquid holdup and pressure profiles of counter-current flows in vertical annuli for the slug flow, which is the dominant flow regime. A model, based on the work of Taitel and Barnea (1983), was also developed to predict the transition from slug flow to annular flow in counter-current flows in annuli. Our comparison of the pressure profiles of co-current and counter-current flows in annuli for the slug flow regime indicates that the pressure loss of counter-current flows could be appreciably different to that in co-current flows under the same conditions. This highlights the need to modify the models that are currently applied in typical commercial well flow simulators to better predict the pressure drop across CSG wells.

Chapter 6 – Natural Gas

Natural Gas is a form of hydrocarbon that is strongly related to oil. In the United States it is widely used for space-heating and to generate electricity. However, even though gas production peaked in the US in 1971, it has been resurrected through the shale gas boom. Moreover, natural gas remains the third most widely used energy source in the world, ranking just below coal.

Field Trials of Plugging Oil and Gas Wells with Hydrated Bentonite

Many field trials have been conducted to explore the effectiveness of using hydrated bentonite as a sealing material for plugging and abandoning (P&A) operations of oil and gas wells. Many of those trials are reviewed here, including trials in Texas, New Mexico, Oklahoma, Wyoming and Queensland, most of which have not been previously reported. All of these trials have been successful, even though a few wells have been eliminated from the programs because they were found to be unsuitable. In most jurisdictions regulation changes are necessary to allow bentonite to be used in order to plug wells. This has been done in California, Texas and Oklahoma. In Wyoming it is currently permitted as the bottom plug in coal-bed methane wells. In Queensland a field trial has been allowed under the experimental materials clause in the regulations.

The History and Culture of Energy

Why is the supply of energy so important? Is it something we can reduce or do without? In this chapter I discuss how the most successful civilizations throughout history have been the ones that maximized their energy throughput. The energy sources have changed over time from human labor to animal labor to biomass, and we currently live in an era when hydrocarbon energy has ruled. This era of hydrocarbon energy has made energy so cheap that it has profoundly raised the standard of living for vast numbers of people. But energy use comes at a cost. Because energy is part of the environment, its use is going to have an impact on the environment. I refer to this relationship between energy use and the environment as the Towler Principle.

Chapter 4 – Environmental Issues

Whenever we capture energy for use, there are usually impacts on the environment. Hydrocarbon sources generate air pollution, particularly in the form of carbon dioxide, which is the most important of the greenhouse gases and a cause of public concern as it relates to global warming. Nuclear energy generates radioactive waste that has been the subject of intense public debate as well, but this energy source has begun to come back into favor due to the fact that it does not generate carbon dioxide. Consequently, many people are strongly in favor of renewable energy sources, but even these are not without impact. Large wind generation devices create noise and visual pollution, and there is even a perception (undeserved) that wind towers kill a lot of birds. Hydroelectric power is clean and cheap, but some people object to this form of energy because it requires dams that interrupt the migration paths of fish. Solar energy uses large tracts of land and can have a large impact on the flora and fauna wherever it is deployed. The truth is that you cannot extract energy from the environment without having an impact on the environment. We just have to work out ways to minimize that impact.

Chapter 8 – Solar Power

There is intense public interest in developing solar power for widespread use, but there are two major problems. First, the sun does not shine all the time, and, as a result, it cannot serve as a reliable energy source without reliable storage technology. More importantly, the cost of solar energy is still 10 times the cost of other energy sources, making it uncompetitive in the energy market. Surprisingly, solar power generation also has environmental impacts that are not fully appreciated. For instance, would people be happy if we had to cover the Mojave Desert with solar collectors? What if we cut down 50 million trees so that they would not shade the solar collectors on our roofs?