Читать книгу Synthesis Gas - James Speight G., James G. Speight - Страница 12

The major sources of energy have been, and continue to be, the various fossil fuels of which the major component of the group are (i) natural gas, (ii) crude oil, and (iii) coal with tar sand bitumen and oil shale available in considerable quantities for use on an as-needed basis. However, the Earth contains a finite supply of fossil fuels – although there are questions about the real amounts of these fossil fuels remaining. The best current estimates for the longevity of each fossil fuel is estimated from the reserves/production ratio (BP, 2019) which gives an indication (in years) of how long each fossil fuel will last at the current rates of production. Thus, estimates vary from at least 50 years of crude oil at current rates of consumption to 300 years of coal at current rates of consumption with natural gas varying between the two extremes. In addition, the amounts of natural gas and crude oil located in tight sandstone formations and in shale formations has added a recent but exciting twist to the amount of these fossil fuels remaining. Peak energy theory proponents are inclined to discount the tight formations and shale formation as a mere aberration (or a hiccup) in the depletion of these resources while opponents of the peak energy theory take the opposite view and consider tight formations and shale formations as prolonging the longevity of natural gas and crude oil by a substantial time period. In addition, some areas of the Earth are still relatively unexplored or have been poorly analyzed and (using crude oil as the example) knowledge of in-ground resources increases dramatically as an oil reservoir is exploited.

Energy sources have been used since the beginning of recorded history and the fossil fuel resources will continue to be recognized as major sources of energy for at least the foreseeable future (Crane et al., 2010; World Energy Council, 2008; Gudmestad et al., 2010; Speight, 2011a, 2011b, Khoshnaw, 2013; Speight, 2014a; BP, 2019). Fossil fuels are those fuels, namely natural gas, crude oil (including heavy crude oil), extra heavy crude oil, tar sand bitumen, coal, and oil shale produced by the decay of plant remains over geological time represent an unrealized potential, with liquid fuels from crude oil being only a fraction of those that could ultimately be produced from heavy oil and tar sand bitumen (Speight, 1990, 1997, 2011a; 2013d, 2013e, 2014).

In fact, at the present time, the majority of the energy consumed worldwide is produced from the fossil fuels (crude oil: approximately 38 to 40%, coal: approximately 31 to 35%, natural gas: approximately 20 to 25%) with the remainder of the energy requirements to come from nuclear and hydroelectric sources. As a result, fossil fuels (in varying amounts depending upon the source of information) are projected to be the major sources of energy for the next 50 years (Crane et al., 2010; World Energy Council, 2008; Gudmestad et al., 2010; Speight, 2011a, 2011b, Khoshnaw, 2013; BP, 2014; Speight, 2014; BP, 2019).

Fuels from fossil fuels (especially the crude oil-based fuels) are well-established products that have served industry and domestic consumers for more than 100 years and for the foreseeable future various fuels will still be largely based on hydrocarbon fuels derived from crude oil. Although the theory of peak oil is questionable, there is no doubt that crude oil, once considered inexhaustible, is being depleted at a measurable rate. The supposition by peak oil proponents is that supplies of crude oil are approaching a precipice in which fuels that are currently available may, within a foreseeable short time frame, be no longer available. While such a scenario is considered to be unlikely (Speight and Islam, 2016), the need to consider alternate technologies to produce liquid fuels that could mitigate the forthcoming effects of the shortage of transportation fuels is necessary and cannot be ignored.

The best current estimates for the longevity of each fossil fuel is estimated from the reserves/production ratio (BP, 2019) which gives an indication (in years) of how long each fossil fuel will last at the current rates of production.

Alternate fuels produced from a source other than crude oil are making some headway into the fuel demand. For example, diesel from plant sources (biodiesel) is similar in performance to diesel from crude oil and has the added advantage of a higher cetane rating than crude oil-derived diesel. However, the production of liquid fuels from sources other than crude oil has a checkered history. The on-again-off-again efforts that are the result of the inability of the political decision-makers to formulate meaningful policies has caused the production of non-conventional fuels to move slowly, if at all (Yergin, 1991; Bower, 2009; Wihbey, 2009; Speight, 2011a, 2011b, Yergin, 2011; Speight, 2014a).

This is due in no small part to the price fluctuations of crude oil and the common fuel products (i.e., gasoline and diesel fuel) and the lack of planning and associated foresight by various levels of government. It must be realized that for decades the price of crude oil produced in the crude oil-exporting nations has always been maintained at a level that was sufficiently low to discourage the establishment of a domestic synthetic fuels industry in many of the crude oil-consuming countries (Speight, 2011a). However, in spite of additional supplies of crude oil and natural gas coming from tight formations and shale formation, the time will come when the lack of preparedness for the production of non-conventional fuels may set many a national government on its heels. It is not a matter of “if the lack of preparedness come to fruition” but “when will the lack of preparedness come to fruition?”

In the near term, the ability of conventional fuel sources and technologies to support the global demand for energy will depend on how efficiently the energy sector can match available energy resources (Figure 1.1) with the end user and how efficiently and cost effectively the energy can be delivered. These factors are directly related to the continuing evolution of a truly global energy market. In the long term, a sustainable energy future cannot be created by treating energy as an independent topic (Zatzman, 2012). Rather, the role of the energy and the interrelationship of the energy market with other markets and the various aspects of market infrastructure demand further attention and consideration. Greater energy efficiency will depend on the developing the ability of the world market to integrate energy resources within a common structure (Gudmestad et al., 2010; Speight, 2011b; Khoshnaw, 2013).

World petro-politics are now in place (Bentley, 2002; Speight, 2011a) for the establishment of a synthetic fuels (including a biofuels) industry and, without being unduly dismissive of such efforts, it is up to various levels of government not only to promote the establishment of such an industry but to lead the way recognizing that it is not only a matter of supply and demand but of the available and variable technology. Unfortunately, although there may be sufficient crude oil remaining to maintain the Crude Oil Age (or the Petroleum Age, that is, the age in which the developed countries of the world operate) for another 50 years (Speight, 2011a, 2011b), the time to prepare is now. The world is not yet on the precipice of energy deficiency (as many alarmists claim) but it is necessary that the politicians in the various levels of (national) governments of oil-consuming nations look beyond the next election with an eye to the future. It should also be the focus of the proponents of biofuels production and use to ensure that sufficient feedstocks are available to successfully operate a biofuels refinery thereby contributing alternate fuels to the gradual (but not drastic) reduction of crude oil-based fuels (Speight, 2008; Giampietro and Mayumi, 2009; Speight, 2011a, 2011b). However, it is time for procrastination to cease, since delay will not help in getting beyond the depletion of crude oil and natural gas resources, and various levels of government must start being serious in terms of looking to the future for other sources of energy to supplement and even replace the current source of hydrocarbon fuels.

Figure 1.1 Types of energy resources.

In addition, and in keeping with the preferential use of lighter crude oil as well as maturation effect in the reservoir, crude oil available currently to the refinery is somewhat different in composition and properties from those available approximately 50 years ago (Parkash, 2003; Gary et al., 2007; Speight, 2008; Siefried and Witzel, 2010; Speight, 2011a, 2014a, 2015b; Hsu and Robinson, 2017; Speight, 2017). The current crude oils are somewhat heavier insofar as they have higher proportions of non-volatile (asphaltic) constituents. In fact, by the standards of yesteryear, many of the crude oils currently in use would have been classified as heavy feedstocks, bearing in mind that they may not approach the definitions that should be used based on the method of recovery. Changes in feedstock character, such as this tendency to more viscous (heavier) crude oils, require adjustments to refinery operations to handle these heavier crude oils to reduce the amount of coke formed during processing and to balance the overall product slate (Speight, 2011a, 2014a).

As the 21^st century matures, there will continue to be an increased demand for energy to support the needs of commerce industry and residential uses – in fact, as the 2040 to 2049 decade approaches, commercial and residential energy demand is expected to rise considerably – by approximately 30% over current energy demand. This increase is due, in part, to developing countries, where national economies are expanding and the move away from rural living to city living is increasing. In addition, the fuel of the rural population (biomass) is giving way to the fuel of the cities (transportation fuels, electric power) as the lifestyles of the populations of developing countries changes from agrarian to metropolitan. Furthermore, the increased population of the cities requires more effective public transportation systems as the rising middle class seeks private means of transportation (automobiles). As a result, fossil fuels will continue to be the predominant source of energy for at least the next 50 years.

However, there are several variables that can impact energy demand from fossil fuels. For example, coal (as a source of electrical energy) faces significant challenges from governmental policies to reduce greenhouse gas emissions, and fuels from crude oil can also face similar legislation (Speight, 2013a, 2013b, 2014a) in addition to the types of application and use, location and regional resources, cost of energy, cleanness and environmental factors, safety of generation and utilization, and socioeconomic factors, as well as global and regional politics (Speight, 2011a). More particularly, the recovery of natural gas and crude oil from tight sandstone and shale formations face challenges related to hydraulic fracturing. Briefly, hydraulic fracturing is an extractive method used by crude oil and natural gas companies to open pathways in tight (low-permeability) geologic formations so that the oil or gas trapped within can be recovered at a higher flow rate. When used in combination with horizontal drilling, hydraulic fracturing has allowed industry to access natural gas reserves previously considered uneconomical, particularly in shale formations. Although, hydraulic fracturing creates access to more natural gas supplies, but the process requires the use of large quantities of water and fracturing fluids, which are injected underground at high volumes and pressure. Oil and gas service companies design fracturing fluids to create fractures and transport sand or other granular substances to prop open the fractures. The composition of these fluids varies by formation, ranging from a simple mixture of water and sand to more complex mixtures with a multitude of chemical additives. Hydraulic fracturing has opened access to vast domestic reserves of natural gas that could provide an important stepping stone to a clean energy future. Yet questions related to the safety of hydraulic fracturing persist and the technology has been the subject of both enthusiasm and increasing environmental and health concerns in recent years, especially in relation to the possibility (some would say reality) of contaminated drinking water because of the chemicals used in the process and the disturbance of the geological formations (Speight, 2015a).

The danger revealed by the peak energy theory is that the world is approaching an energy precipice in which (apparently) crude oil that is available one year will not be available the next year. On the other hand, the peak energy opponents take a more realistic view in that the depletion of fossil fuels will occur gradually and, and with the current trends in considering other sources of energy, the concept of the energy precipice is not logical (Speight and Islam, 2016).

The most unrealistic variable in the peak energy scenario arises from the misuse of data that supposedly indicate that the world is approaching the energy precipice in which fossil fuel will no longer be available for use as energy sources – the date of the energy precipice is not only wildly speculative but, in many cases, totally unrealistic. Fossil fuel energy sources will undoubtedly reach a depletion point in the future when these energy sources are no longer available – but not at the moment or even in the present century. At the same time, new gas-fired generating units use highly efficient technologies and are supported by abundant gas supplies. As a result, gas is increasingly viewed as the most economical fossil fuel choice for electricity generation for the United States. Finally, a word on reserve estimation. There are a number of different methods by which crude oil and natural gas reserves can be calculated. These methods can be grouped into three general categories: (i) volumetric methods, (ii) materials balance method, and (iii) the decline curve method or production performance method.

The methods designated as volumetric methods represent attempts to determine the amount of oil-in-place by using the size of the reservoir as well as the physical properties of the reservoir rock(s) and the reservoir fluids. In the calculation process, a recovery factor is assumed, using data (and assumptions) from other crude oil and natural gas fields with similar characteristics to the field under evaluation. Based on these assumptions, the estimated amount of crude oil or natural gas in-place is multiplied by the recovery factor (derived from the other (similar) fields to arrive at an estimate of the reserves in-place. Current recovery factors for oil fields around the world typically range between 10 and 60% v/v of the crude oil and natural gas in-place while some recovery factors are in excess of 80% v/v of the crude oil and natural gas in place. The wide variance is due largely to the diversity of fluid and reservoir characteristics for different deposits. The method is most useful early in the life of the reservoir, before significant production has occurred. However, site specificity, which arise because of the differences in reservoir character (for example reservoir mineralogy, porosity, permeability) and the character of the reservoir fluids must also be given serious consideration, otherwise the estimation of the reserves in-place may be in error (by an order of magnitude above or below the real amount in-place. Such is the difficulty of estimating the reserves.

In addition, the materials balance method for a crude oil field or natural gas field uses an equation (or derivation thereof) that relates (in the case of a crude oil reservoir with associated natural gas) the volume of crude oil, water and gas that has been produced from a reservoir, and the change in reservoir pressure, to calculate the remaining crude oil or natural gas. The calculation uses the assumption that as fluids from the reservoir are produced, there will be a change in the reservoir pressure that depends on the remaining volume of oil and gas. The method requires extensive pressure-volume-temperature analysis as well as an accurate pressure history of the field. If the pressure history of the field is not available, the calculation requires some production to occur (or to have occurred) (typically 5% to 10% v/v of ultimate recovery), unless reliable pressure history can be used from a field with similar reservoir rock characteristics as well as the characteristics of the reservoir fluids.

The decline curve method (also known as the production performance method) uses known production data to fit a decline curve and estimate future oil production – the three most common forms of decline curves are exponential, hyperbolic, and harmonic. The decline curve analysis is a long-established tool for developing future outlook for crude oil and/or natural gas production from an individual well or from an entire oilfield. Depletion has a fundamental role in the extraction of finite resources and is one of the driving mechanisms for oil flows within a reservoir and the depletion rate can be connected to decline curves. Consequently, depletion analysis is a useful tool for analysis and forecasting crude oil and natural gas production.

In the calculation, it is assumed that the production will decline on a reasonably smooth curve, and so allowances must be made for wells shut in and production restrictions. The curve can be expressed mathematically or plotted on a graph to estimate future production. It has the advantage of (implicitly) including all reservoir characteristics. However, the method requires a sufficient well or reservoir history to establish a statistically significant trend, ideally when production is not curtailed by regulatory or other artificial conditions.

Generally, the initial estimates of the size of newly discovered oil fields are usually too low. As crude oil and/or natural gas production continues, successive estimates of the ultimate recovery of fields tend to increase. The term reserve growth refers to the typical increases in estimated ultimate recovery that occur as oil fields are developed and produced. Reserve growth has now become an important part of estimating total potential reserves of an individual province or country. As the worldwide crude oil reserves continue to decline, there is the need for the reserve estimates to be more precise through application of the reserve-growth concept. In fact, the concept could be applied even to the so-called undiscovered resources with some qualifications as to the inherent risk (Speight, 2011a, 2014a).