Common use of General context Clause in Contracts

General context. Although the existence of shale gas has been known for decades, the technology for the economically viable extraction of shale gas was made available only recently [1]. The experience that obtained during the years of shale gas exploitation, together with the information that has been accumulated, have revealed a number of issues associated with shale gas extraction. The environmental impact of key technologies necessary for shale gas extraction (such as hydraulic fracturing) is poorly understood, with evidence that they may be related to important issues such as pollution of potable water [2] and seismic phenomena [3]. Furthermore, it is currently established that each shale field is unique, therefore requiring adaptation of the available methods for optimal yield. These facts pose a pressing demand for scientific research that would help understand the underlying physical phenomena. In this way it will be possible to progressively tackle the various open issues that will ultimately enable an undoubtedly green exploitation of proven available energy sources in a sustainable and beneficiary for the society way. The study of the phenomena taking place at the atomistic level in a shale rock is of immense importance. It is related to the transportation of the shale gas, the fracturing fluid, and the naturally occurring radioactive materials (NORM) [4]. The understating of the factors controlling the migration of these substances would have profound implications in the design of processes and methods for gas extraction, and elimination of any potential environmental cost in an economically viable and sustainable manner. Computational modeling methods are expected to play an important role towards the understanding of the complex physical and chemical phenomena, tacking place in the highly confined environment of shale rocks. For these reasons, it is important to have realistic models of the primary materials constituting shale gas reservoirs. It is currently assumed that shales of interest are composed primarily of clays (kaolinite, muscovite, smectites) and other minerals (quartz, calcite) with dispersed nodules of organic matter. The extractable shale gas is predominantly located in the nodules or organic matter. The insoluble (in common organic solvents) part of the organic matter found in a shale rock is usually defined as kerogen [5]. In shale formations, shale gas (typically a mixture of methane, ethane and propane with a very small amount of butane and heavier hydrocarbons as well as carbon dioxide and nitrogen) is stored as free gas, adsorbed gas, and dissolved gas. The free gas gathers in the fractures and pores of a shale rock, the adsorbed gas occurs on the surfaces of both the organic material and clay minerals, and the dissolved gas enriches the organic matter. The adsorbed and dissolved gas is in equilibrium with a homogeneous free gas phase in an interconnected shale pore structure. If some shale pores are not interconnected, there may be a departure from the equilibrium between the adsorbed and dissolved gas, and the free gas. The amount of free gas is relatively easy to estimate based on the temperature and formation pressure, its porosity, and the fraction of porosity which is filled by the gas. The contribution of adsorbed gas to the total gas in place (GIP), estimated to reach up to 60%, is still poorly understood, mainly due to complexity of the gas adsorption on shale. Generally, most of the adsorption area is located in the organic material and the contribution of the adsorbed gas to the total GIP is less significant in the inorganic matrix [6]. Kerogen is a complex organic material with an amorphous porous carbon skeleton and exhibits significant pore-shape and pore-connectivity variations. Depending on its geographic origin, maturity and sedimentary history, kerogen displays a broad range of density and, chemical composition in terms of atomic contents and chemical functions, porosity and tortuosity. The van Krevelen diagram [7,8] (Figure 1), a plot of hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) atomic ratios, provides a tool to distinguish different types of kerogen in terms of depositional origin: type I (lacustrine), type II (marine), type III (terrestrial), and type IV (originating from residues) and maturity. Increasing the maturity of kerogens due to exposure to high temperature and pressure over geological time scales leads to a shift of the kerogen H/C and O/C atomic ratios in the van Krevelen diagram from the top-right to bottom- left corners. Figure 1: The van Krevelen diagram, a plot of hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) atomic ratios. 1) Kerogen is an amorphous solid composed of a mixture of different not very well defined macromolecules of varying size. 2) Not only kerogen in every shale field is different, but also the characteristics of kerogen in different depths of the same well vary significantly. 3) There are limited tools available for detailed characterization of the porosity of amorphous materials and their performance for the analysis. 4) Large size structures needed to study amorphous material is problematic. 5) Clay mineral structures are very diverse as they incorporate a significant degree of compositional and structural disorder. 6) In most previous molecular simulations, the structure of clay particles are represented by semi-infinite layers, i.e., they do not have any edges. This simplification is acceptable to a certain extent, but real clay particles always have a finite size and should be terminated by lateral surfaces or edge surfaces. 7) Edge surfaces of clay particles can exhibit adsorption sites highly different from the ones on the basal surfaces of clay layers, which is extremely important for the interaction of clay particles in shale rocks with kerogen and with naturally occurring radioactive materials (NORM).

General context. The recent developments of upstream sector technology has rendered possible the extraction of oil and gas from unconventional sources. The term “unconventional sources” refers to natural formations of low permeability that hold substantial quantities of oil and/or gas. Hydraulic fracturing and extended horizontal drilling are two such new technologies developed over the past couple of decades, that allow industry to access oil/and gas that was otherwise inaccessible. Shale gas is one of the most promising unconventional sources of energy. It is natural gas, which is found in shale formations, an unconventional energy source. Currently it is only being commercially exploited by a handful of countries, with USA being the major producer. Although the existence of shale gas has been known for decadesis advantageous over other fossil fuels such as coal, the technology for the economically viable extraction of shale gas was made available only recently [1]. The experience that obtained during the years of shale gas exploitation, together with the information that has been accumulated, have revealed there are a number of issues associated with both its environmental impact. During the hydraulic fracturing technique, fracturing fluid – which is essentially a water solution – is injected to the shale formation in order to fracture it. This is necessary in order to access the gas of the practically impermeable source rock. This process has been related to increased seismic activity in regions neighboring the shale gas extraction sites. The subject of other investigations has been the possibility of contamination of potable water with CH4. Lastly and more importantly, there is great concern about the possible migration of radioactive materials that are naturally trapped in the shale, the so-called NORMs. Molecular Dynamics (MD) simulation technique has been proven extremely useful in acquiring quantitative and qualitative (i.e. physical properties) information about phenomena happening in length and time scales inaccessible to experimental techniques. The phenomena occurring at the atomistic scale may have a determining effect on the macroscopically observed behavior of the systems of interest. In the context of shale gas technology, MD simulations can provide substantial information of the relevant fluid systems at subsurface conditions that can be used to optimize the extraction process with reduced environmental hazard. A prime example of use of MD in this context is the study of the transport properties of fracturing fluid in the shale rock, which can provide essential information that will allow the assessment of possible risks associated with the migration of unrecovered fracturing fluid and NORMs from the shale rock to nearby regions (that are not related to the shale gas extraction. The environmental impact of key technologies necessary for shale gas extraction (such as hydraulic fracturing) is poorly understood, with evidence that they may be related to important issues such as pollution of potable water [2] and seismic phenomena [3]. Furthermore, it is currently established that each shale field is unique, therefore requiring adaptation of the available methods for optimal yield. These facts pose a pressing demand for scientific research that would help understand the underlying physical phenomena. In this way it will be possible to progressively tackle the various open issues that will ultimately enable an undoubtedly green exploitation of proven available energy sources in a sustainable and beneficiary for the society way. The study of the phenomena taking place at the atomistic level in a shale rock is of immense importance. It is related to the transportation of the shale gas, the fracturing fluid, and the naturally occurring radioactive materials (NORM) [4]. The understating of the factors controlling the migration of these substances would have profound implications in the design of processes and methods for gas extraction, and elimination of any potential environmental cost in an economically viable and sustainable manner. Computational modeling methods are expected to play an important role towards the understanding of the complex physical and chemical phenomena, tacking place in the highly confined environment of shale rocks. For these reasons, it is important to have realistic models of the primary materials constituting shale gas reservoirs. It is currently assumed that shales of interest are composed primarily of clays (kaolinite, muscovite, smectites) and other minerals (quartz, calcite) with dispersed nodules of organic matter. The extractable shale gas is predominantly located in the nodules or organic matter. The insoluble (in common organic solvents) part of the organic matter found in a shale rock is usually defined as kerogen [5]. In shale formations, shale gas (typically a mixture of methane, ethane and propane with a very small amount of butane and heavier hydrocarbons as well as carbon dioxide and nitrogen) is stored as free gas, adsorbed gas, and dissolved gas. The free gas gathers in the fractures and pores of a shale rock, the adsorbed gas occurs on the surfaces of both the organic material and clay minerals, and the dissolved gas enriches the organic matter. The adsorbed and dissolved gas is in equilibrium with a homogeneous free gas phase in an interconnected shale pore structure. If some shale pores are not interconnected, there may be a departure from the equilibrium between the adsorbed and dissolved gas, and the free gas. The amount of free gas is relatively easy to estimate based on the temperature and formation pressure, its porosity, and the fraction of porosity which is filled by the gas. The contribution of adsorbed gas to the total gas in place (GIP), estimated to reach up to 60%, is still poorly understood, mainly due to complexity of the gas adsorption on shale. Generally, most of the adsorption area is located in the organic material and the contribution of the adsorbed gas to the total GIP is less significant in the inorganic matrix [6]. Kerogen is a complex organic material with an amorphous porous carbon skeleton and exhibits significant pore-shape and pore-connectivity variations. Depending on its geographic origin, maturity and sedimentary history, kerogen displays a broad range of density and, chemical composition in terms of atomic contents and chemical functions, porosity and tortuosity. The van Krevelen diagram [7,8] (Figure 1), a plot of hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) atomic ratios, provides a tool to distinguish different types of kerogen in terms of depositional origin: type I (lacustrine), type II (marine), type III (terrestrial), and type IV (originating from residues) and maturity. Increasing the maturity of kerogens due to exposure to high temperature and pressure over geological time scales leads to a shift of the kerogen H/C and O/C atomic ratios in the van Krevelen diagram from the top-right to bottom- left corners. Figure 1: The van Krevelen diagram, a plot of hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) atomic ratios. 1) Kerogen is an amorphous solid composed of a mixture of different not very well defined macromolecules of varying size. 2) Not only kerogen in every shale field is different, but also the characteristics of kerogen in different depths of the same well vary significantly. 3) There are limited tools available for detailed characterization of the porosity of amorphous materials and their performance for the analysis. 4) Large size structures needed to study amorphous material is problematic. 5) Clay mineral structures are very diverse as they incorporate a significant degree of compositional and structural disorder. 6) In most previous molecular simulations, the structure of clay particles are represented by semi-infinite layers, i.e., they do not have any edges. This simplification is acceptable to a certain extent, but real clay particles always have a finite size and should be terminated by lateral surfaces or edge surfaces. 7) Edge surfaces of clay particles can exhibit adsorption sites highly different from the ones on the basal surfaces of clay layers, which is extremely important for the interaction of clay particles in shale rocks with kerogen and with naturally occurring radioactive materials (NORM).