Methane sorption and storage characteristics of organic-rich carbonaceous rocks, Lurestan province, southwest Iran

https://doi.org/10.1016/j.coal.2017.12.005Get rights and content

Highlights

  • Methane excess sorption isotherms on carbonaceous rocks measured

  • Effects of different parameters on gas sorption and storage capacities were investigated.

  • Total organic content is the most pivotal parameter controlling gas sorption capacity.

  • There is no coloration between total organic contents and measured porosities.

Abstract

High-pressure-high-temperature methane sorption isotherms have been measured on nineteen samples from the Jurassic Sargelu and the Cretaceous Garau formations in Lurestan province, southwest Iran. Measurements were performed on dry and moisture-equilibrated samples. The study aimed at investigating the effects of pressure, temperature, organic matter and water content on sorption and gas storage characteristics.

On the dry samples sorption isotherms were measured between 45 and 130 °C at pressures up to 25 MPa. Isotherms for the moisture-equilibrated samples were measured at 45 °C. An excess sorption function based on the Langmuir model was used to fit the experimental data.

The total organic carbon (TOC) contents of the Garau samples range between 0.18 and 5.41 wt% and those of the Sargelu samples vary between 0.23 and 15.91 wt%. Carbonate is the dominant mineral in both sample sets, followed by quartz and clay minerals. No clear correlation was found between TOC content and porosity of the samples, indicating multiple factors controlling the abundance and volumes of both organic and inorganic pores.

A linear correlation between sorption capacity and TOC value was found for both sample sets. Due to the larger variance in TOC values this relationship was more obvious for the Sargelu samples. Clay minerals constitute only a minor component of these carbonate-rich rocks. Therefore, as expected, no correlation was observed between sorption capacity and clay content. Organic matter content thus is the pivotal factor controlling methane sorption capacity.

With increasing temperature the excess sorption capacity decreases while the Langmuir pressure increases, as evidenced by decrease in the initial slope of the isotherms.

A negative correlation was observed between water content and sorption capacities and a positive correlation between water content and the Langmuir pressure (PL).

The total gas storage capacity of the two sample sets was estimated as a function of depth based on all measured data and representative temperature and pressure gradients. Sorption generally tends to dominate the total storage capacity of shale gas systems in the low-depth/low-pressure range (as high as 90% depending on TOC content and specific pore volume), whereas in the great-depth/high-pressure range the volumetric storage capacity prevails.

Introduction

The successful development of shale gas exploration and production in the USA has stimulated the research on unconventional hydrocarbon resources in other countries. Alongside Canada, China and Australia, Iran has also decided to initiate a multi-disciplinary research project on shale gas systems. The Jurassic Sargelu and the Cretaceous Garau formations in the Lurestan province, southwest Iran have been chosen as target formations for shale gas exploration.

The assessment of gas in place is one of the most challenging and crucial task economic evaluation of shale gas systems. Unlike conventional reservoirs, where gas is stored as a free, compressed phase, appreciable amounts of the gas in carbonaceous rocks are adsorbed on the surfaces of organic matter and - to some extent - clay minerals (Curtis, 2002). In both, conventional and unconventional systems, gas will also be dissolved in formation water and oil/bitumen.

The free gas storage capacity can be estimated from porosity measurements, ideally under defined/controlled stress. The assessment of the amounts of adsorbed and dissolved gas in shale systems is challenging and requires additional information. In common experimental procedures it is not possible to distinguish between “adsorbed”, “absorbed”, and “dissolved” gas. Therefore the term “sorption” is used to denote any state of the gas other than “free” gas. Sorption isotherms (more precisely “excess sorption isotherms”) are determined by measuring the gas uptake by a solid as a function of pressure at a constant temperature and comparing it to the corresponding reference measurement conducted with a “non-adsorbing” gas (typically helium).

Sorption capacity is a function of pressure and temperature and is controlled by the composition and properties of the shale (organic carbon content and type, mineral phases, specific surface area, moisture content).

The total organic carbon content is generally recognized as the main controlling factor affecting the sorption capacity of carbonaceous rocks (Lu et al., 1995, Chalmers and Bustin, 2007, Chalmers and Bustin, 2008, Ross and Bustin, 2008, Ross and Bustin, 2009, Weniger et al., 2010, Zhang et al., 2012, Gasparik et al., 2014). Furthermore, the composition and thermal maturity of the organic material play an important role. Thus, Zhang et al. (2012) found that sorption capacity increased with increasing aromaticity from kerogen Type I to kerogen Type III. They also report a negative correlation between maturity and Langmuir pressure.

The contribution of clay minerals to the sorption capacity of low-TOC shales has been investigated by Ross and Bustin (2009), Ji et al. (2012) and Gasparik et al. (2012). Gasparik et al. (2012) reported the significant impact of clay mineral on sorption capacities of low-TOC shales from the Netherlands. Sorption capacities of low-TOC, clay-rich shales were found to be of the same order and even higher than those of organic rich shales. Ji et al. (2012) studied the effects of clay mineral composition on methane sorption capacity. They report a decrease of the sorption capacity in the following order: montmorillonite  illite/mectite mixed layer > kaolinite > chlorite > illite. Ross and Bustin (2009) find a considerable contribution of clay minerals such as illite to the sorption capacity of shales, which they attribute to the micropore structure of these clay minerals.

Most of the published sorption data for shales have been measured on dry samples. To investigate the impact of water on sorption capacitiy, high-pressure sorption isotherms were measured in this study on samples moisture-equilibrated at different relative humidity levels. Results of previous studies showed a linear decrease in sorption capacity with increasing in moisture content (Gasparik et al., 2014, Merkel et al., 2015, Yang et al., 2016b).

In the present study we investigated systematically and comprehensively the effects of different parameters such as pressure, temperature, organic matter and water content on gas sorption and storage capacity of carbonaceous shales. The samples were taken from the Jurassic Sargelu and the Cretaceous Garau formations in SW Iran. High-pressure/high-temperature excess sorption isotherms were determined on dry and moisture-equilibrated samples. These results were then to be combined with porosity and specific pore volume measurements to estimate the total storage capacity of the samples. The results of this study contribute to a better and more quantitative understanding of volumetric and sorptive storage capacity and the relative importance of sorption in shale gas reservoirs.

Section snippets

Samples

Nineteen samples were collected from the lower Cretaceous Garau (eleven samples) and middle to upper Jurassic Sargelu (eight samples) formations in Lurestan province, located in southwest Iran. The samples were collected either from well cores or outcrop sections at different locations (undisclosed due to confidentiality reasons).

Total organic carbon (TOC)

Total organic carbon (TOC) contents of the samples were determined with a LiquiTOC II analyzer (Elementar Analysensysteme GmbH, Germany). Nearly 100 mg of fine-ground powder were combusted in an oxygen current, following a two-step protocol: 1.) heat to 550 °C (with approximately 300 °C/min) and hold for 600 s. 2.) heat to 1000 °C and hold for 400 s. The amounts of CO2 released during this procedure were recorded continuously and used to assess the Total organic carbon content, TOC (stage 1) and the

Mineral and TOC contents

The results of the XRD and TOC analyses are listed in Table 1. The total organic carbon contents of the Garau samples range between 0.18 and 5.41 wt% and TOC contents of Sargelu samples vary between 0.23 and 15.91 wt%.

The XRD results show that carbonate (12%–90% wt%; mean value 66 wt%) is the dominant mineral in both, Garau and Sargelu samples, followed by quartz (2%–26% wt%; mean value 10 wt%) and clay minerals (3%–23% wt%; mean value 8 wt%). Most Sargelu samples contain large amounts of dolomite

Effect of organic matter content and specific pore volume on sorption and storage capacity

Fig. 7 illustrates the relationship between TOC content and sorption capacity of the samples analyzed. The diagram shows a clear linear correlation between sorption capacity and TOC value for both sample sets. Due to the larger variance in TOC values this relationship is more obvious for the Sargelu samples. No clear relationship was observed between sorption capacity and the amount of clay minerals in the samples analyzed. Such a relationship has been reported by some authors for organic-lean

Conclusion

The total organic carbon (TOC) contents of the Garau samples range between 0.18 and 5.41 wt% and those of the Sargelu samples vary between 0.23 and 15.91 wt%. Carbonate is the dominant mineral in both sample sets, followed by quartz and clay minerals.

It could be documented that organic matter content is the pivotal factor controlling the methane sorption capacity of these carbonaceous rocks (“gas shales”). No clear relationship was observed between sorption capacity and the amount of clay

Acknowledgments

We thank the Iranian Research Institute of Petroleum Industry (RIPI) and the Exploration Directorate of National Iranian Oil Company (ED-NIOC) for providing funds (Grant number: 97481113) and samples for this study. M. S. thanks the Institute of Geology and Geochemistry of Petroleum and Coal (GGPC), RWTH Aachen University for financial support during his research visit. The Department of Geosciences of RIPI provided information on the mineralogical composition and maturity of the samples

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