Shale Gas Composition Research Articles

Carbon isotopic composition of shale gas in the Silurian Longmaxi Formation of the Changning area, Sichuan Basin

Abstract A comprehensive analysis was carried out on the geochemical characteristics of 15 shale gas samples from the Changning area to study the carbon isotopic composition features of shale gas and the reversal cause of carbon isotopic composition in post-mature shale gas in the Silurian Longmaxi Formation of the Changning area, Sichuan Basin. Through the analysis of alkane gas component and carbon isotopic composition, combining with the research on carbon isotopic composition from the Longmaxi Formation of the Fuling and Weiyuan areas in Sichuan Basin, the methane from the Longmaxi Formation shale gas accounts for 97.11% to 99.45%, the average gas wetness is 0.49% representing typical dry gas, abnormal average δ13C1 value as −28.2‰ and the average of δ13C2 values is −33.2‰, in view of sapropel-type kerogen, the Longmaxi Formation shale gas belongs to the typical oil-associated gas. With the increasing degree of thermal evolution, the wetness of shale gas decreases gradually, and carbon isotopic composition of methane becomes heavier, and the carbon isotopic composition of ethane and propane will reverse, but the carbon isotopic composition of ethane and propane in the post-mature shale gas of the Changning area stays in the post-stage of reverse and will not get continuously heavier. The abnormal heavy carbon isotopic composition of methane and the reversal phenomenon of carbon isotopic (δ13C1>δ13C2>δ13C3) mainly generate from the secondary cracking effect in the post-mature stage and reactions between ethane with ferrous metals and water under Reileigh fractionation situation. Furthermore, the high temperature is also one of the important influence factors.

Characteristics of stable carbon isotopic composition of shale gas

A type Ⅱ kerogen with low thermal maturity was adopted to perform hydrocarbon generation pyrolysis experiments in a vacuum (Micro-Scale Sealed Vessel) system at the heating rates of 2 °C/h and 20 °C/h. The stable carbon isotopic compositions of gas hydrocarbons were measured to investigate their evolving characteristics and the possible reasons for isotope reversal. The δ13C values of methane became more negative with the increasing pyrolysis temperatures until it reached the lightest point, after which they became more positive. Meanwhile, the δ13C values of ethane and propane showed a positive trend with elevating pyrolysis temperatures. The carbon isotopic compositions of shale gasses were mainly determined by the type of parent organic matter, thermal evolutionary extent, and gas migration in shale systems. Our experiments and study proved that the isotope reversal shouldn't occur in a pure thermogenic gas reservoir, it must be involved with some other geochemical process/es; although mechanisms responsible for the reversal are still vague. Carbon isotopic composition of the Fayetteville and Barnett shale gas demonstrated that the isotope reversal was likely involved with water–gas reaction and Fischer-Tropsch synthesis during its generation.

Variation in the carbon isotopic composition of alkanes during shale gas desorption process and its geological significance

Taking Chang 7 shale of the Yanchang Formation in the southeastern area of the Yishan slope of the Ordos Basin as our research subject, desorption experiments were performed on more than 30 shale samples that helped this study focus on the variation of carbon isotopic composition of alkanes during the desorption process of shale gas, the possible causes of this phenomenon and its geological significance will also be discussed as follows. It was found that carbon isotopic composition became higher by 9.2‰ (from −50.1‰ to −40.9‰) for methane desorbed gas (δ13C1), and it also became higher by 2.8‰ (from −35.5‰ to −32.7‰) for ethane (δ13C2), but there's barely any differences for propane during the desorption process. At room temperature or constant temperature, carbon isotopic composition for both methane and ethane (δ13C1 & δ13C2) increased continuously. The values of δ13C1 and δ13C2 were low at first and then it gradually became high whilst the desorption process of shale gas that was optimized with the aid of the increasing temperature. The reason for this phenomenon is most likely due to the main adsorption/desorption of shale gas and diffusion migration fractionation of isotope. Additionally, these variation characteristics of methane and ethane carbon isotope composition of shale gas may be a vital reason for the relatively low carbon isotopic composition of methane compared to δ13C calculated by means of the vitrinite reflectance (RO), and this may be applied to evaluate the remaining amount of shale gas resources.

To What Extent Could Shale Gas “Fracking” Contaminate Ground Water

The possibility to economically produce gas from unconventional natural resources (shale gas) reservoirs has been made possible recently through horizontal well and hydraulic fracturing application. Hydraulic fracture or ‘fracking ‘is a technique that use in combination with horizontal drilling to stimulate or improve extracting natural gas from shale formation. However, this operation requires large volume of fluid to be pumped into the well at high pressure and flow rate in order to split the rock. This fluid is mainly water and mixed with sand and some chemical materials such as proppant. (Serve to prop open the fracture). Injection large volume of fluid into subsurface is not without risk. The recent report have highlighted the following issues: (earthquake induced by slip on nearby fault, emission of CO2 into atmosphere) However, this project focus on the effect of fracking operation on ground water and aquifer due to chemical additives and methane which is the main composition of shale gas. According to this report the gases and chemical additives may escape and flow into overlying ground water ‘aquifer’ in three possible pathways. Loss of wellbore integrity disposal of flow back water natural fracture network. This paper also looks at two case studies, North-East Pennsylvania, New York and Wyoming in the USA, which have the same geological shale formation. The study focuses on finding the potential impact of fracking technique that is used to extract natural shale gas on ground drinking water systems.

Optimization and Selection of Reforming Approaches for Syngas Generation from Natural/Shale Gas

Reforming of natural or shale gas is the primary industrial method for the production of syngas. There are four major approaches to reforming: steam reforming, partial oxidation, dry reforming, and combinations of these reforming reactions. The selection of the reforming approach is not a straightforward task. It is highly dependent on process objectives and availability of material and energy resources. Furthermore, the reformer selection has a significant impact on: process yield, energy requirement, CO2 emissions, and wastewater generation. It also has operational implications including: catalyst life, process safety, and control. The purpose of this work is to develop a systematic tool to aid in the modeling and selection of appropriate reformers to achieve an objective. This can include particular process or economic objectives and constraints such as H2:CO ratio. An equilibrium model is developed and solved using optimization software (LINGO). Economic, energy, and environmental constraints are included in the optimization formulation. Top-level economic scenarios were investigated including the inclusion of carbon tax, natural gas, and energy price fluctuations. The work is also extended to shale gas reforming and shows that the composition of the shale gas has a significant impact on potential yields. Given the potential variability in shale gas composition, the optimization model is also used in selecting a gas reservoir (feedstock) for specific objectives and constraints of the reforming process. The inclusion of strict energy and environmental constraints can favor some reforming options over others.