**Abbreviations**


#### **References**


**Pavel Afanasev 1,\*, Evgeny Popov 1, Alexey Cheremisin 1, Roman Berenblyum 2, Evgeny Mikitin 3, Eduard Sorokin 3, Alexey Borisenko 3, Viktor Darishchev 4, Konstantin Shchekoldin <sup>4</sup> and Olga Slavkina <sup>4</sup>**


**Abstract:** Hydrogen can be generated in situ within reservoirs containing hydrocarbons through chemical reactions. This technology could be a possible solution for low-emission hydrogen production due to of simultaneous CO2 storage. In gas fields, it is possible to carry out the catalytic methane conversion (CMC) if sufficient amounts of steam, catalyst, and heat are ensured in the reservoir. There is no confirmation of the CMC's feasibility at relatively low temperatures in the presence of core (reservoir rock) material. This study introduces the experimental results of the first part of the research on in situ hydrogen generation in the Promyslovskoye gas field. A set of static experiments in the autoclave reactor were performed to study the possibility of hydrogen generation under reservoir conditions. It was shown that CMC can be realized in the presence of core and ex situ prepared Ni-based catalyst, under high pressure up to 207 atm, but at temperatures not lower than 450 ◦C. It can be concluded that the crushed core model improves the catalytic effect but releases carbon dioxide and light hydrocarbons, which interfere with the hydrogen generation. The maximum methane conversion rate to hydrogen achieved at 450 ◦C is 5.8%.

**Keywords:** hydrogen production; steam methane reforming; in situ hydrogen generation

### **1. Introduction**

The growing demand for clean energy resources stimulates the development of unconventional and alternative energy. Renewable energy is a promising and developing field, but hydrogen has a number of benefits as an energy source. According to the world's long-term programs for developing hydrogen technologies, hydrogen can ensure 12% of the world's total primary energy demand in 2050 [1]. Besides that, hydrogen is a valuable chemical product required for the refining industry and the production cycle of ammonia, methanol, and others. However, there is no cheap way for sustainable hydrogen production without greenhouse gas emissions.

Hydrogen can be obtained from natural gas through catalytic steam methane reforming (SMR), partial oxidation, autothermal reforming, and methane cracking. It also can be produced from water through electrolysis of water or from coal through coal gasification [2]. However, all these hydrogen production methods are very energy consuming. In addition, energy for hydrogen production is usually produced by burning hydrocarbons with carbon dioxide emissions. Most hydrogen is produced mainly via the SMR process, which also produces up to 10 kg of CO2 per kg of generated hydrogen [3]. Greenhouse gases are produced during energy generation, as direct products of the chemical reactions, as well as during stages of compression and transportation of reagents and products. It is essential to

**Citation:** Afanasev, P.; Popov, E.; Cheremisin, A.; Berenblyum, R.; Mikitin, E.; Sorokin, E.; Borisenko, A.; Darishchev, V.; Shchekoldin, K.; Slavkina, O. An Experimental Study of the Possibility of In Situ Hydrogen Generation within Gas Reservoirs. *Energies* **2021**, *14*, 5121. https:// doi.org/10.3390/en14165121

Academic Editor: Bahman Shabani

Received: 21 July 2021 Accepted: 16 August 2021 Published: 19 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

realize carbon capture and storage (CCS) technology to make synthesized hydrogen "blue". These actions increase the cost of hydrogen significantly [4,5].

A promising, energy-efficient, and cost-effective technology for producing low-emission (almost "green," or without any greenhouse gas emissions) hydrogen is an in situ hydrogen generation in hydrocarbon reservoirs. Hydrogen can be generated in situ in oil/bitumen fields (for example through bitumen gasification) [6–10], coal deposits (underground coal gasification) [11–13] or gas fields [14–16]. These feedstock types imply different hydrogen generation mechanisms: oil aquathermolysis and thermolysis, coke gasification, methane cracking, steam methane reforming, and water-gas shift reaction.

The chemical transformations occurring in the gas reservoir include mainly steam methane reforming, water-gas shift reaction, and methane cracking (at temperatures higher than 500 ◦C) in the presence of a metal-based catalyst [17,18], according to the following forward reactions:

$$\text{CH}\_4 + \text{H}\_2\text{O} \leftrightarrow \text{CO} + 3\text{H}\_2 - 206 \text{ kJ/mol},\tag{1}$$

$$\text{CO} + \text{H}\_2\text{O} \leftrightarrow \text{CO}\_2 + \text{H}\_2 + 41 \text{ kJ/mol}, \tag{2}$$

$$\text{CH}\_4 \leftrightarrow \text{C} + 2\text{H}\_2 - 75 \text{ kJ/mol.} \tag{3}$$

At the same time, side reactions take place that consume generated hydrogen. These reactions mostly include methanation reactions: reverse reactions (1) and (3) and forward reaction [7]:

$$2\text{CO} + 2\text{H}\_2 \leftrightarrow \text{CH}\_4 + \text{CO}\_2 + 59.0 \text{ kcal/mol.} \tag{4}$$

The generated hydrogen can be stored underground and produced at any time. Moreover, it is expected that hydrogen will rise to the geological uplifts of the reservoir under the influence of gravitational forces. Simultaneously, the environmentally undesirable greenhouse gases, such as carbon and nitrogen oxides, having a higher density than hydrogen, will sink to the bottom of the field under the influence of gravity. These gases are also more soluble in water, compared with hydrogen. In addition, carbon oxides also can react with rocks, forming insoluble compounds such as carbonates. So, greenhouse gases may not be produced at all during hydrogen production from the gas reservoir [16].

Technology considered in this study, implies pure hydrogen production from gas fields with simultaneous CO2 storage [14,15]. It can be implemented even in depleted or abandoned fields or fields in a late stage of exploration because the main process proceeds with an increase in the amount of gaseous components (up to four volumes of hydrogen can be generated from one volume of methane). The existing infrastructure (wells, pipeline network) can be used in hydrogen production, leading to a significant decrease in the produced hydrogen cost. For example, the produced hydrogen can be transported using a modern gas pipeline through mixing with natural gas in concentrations up to 20 and even 70% (for the Nord Stream) [19].

In this research, the idea of in situ hydrogen generation within gas fields supposes the implementation of the CMC (catalytic methane conversion) in the porous medium of the reservoir. The technology implies the injection of a catalyst precursor (aqueous solution of Ni-containing salt) or an active catalyst (particles of Ni-based catalyst) into a hydrocarbon-containing zone on the first stage. Since the reducing conditions are in the reservoir, active phase of catalyst can be formed from the precursor in situ. Then the temperature in the reaction zone should be raised to a temperature, at which catalyzed SMR and methane cracking occur.

The study introduces the results of laboratory experiments performed in an autoclave reactor at initial conditions the same as in the Promyslovskoye gas field, using core material taken from this target field. The Promyslovskoye gas field is located 96 km southwest of Astrakhan city, Russia. It contains about 1700 mln m3 of natural gas, the reservoir temperature is 48 ◦C, the initial pressure is 8.9 MPa, and the current reservoir pressure is 2.3 MPa. The porosity of the target layer is about 29%, residual gas saturation is 77%, and residual water saturation is 23%. The depth of gas-bearing layers is about 730 m.

The temperature range from 300 to 450 ◦C was discovered during the experiments to estimate the possibility of hydrogen generation from methane in situ within the target field. These temperatures can be achieved in a porous medium of rock due to steam/overheated water injection into the reservoir (up to 350 ◦C) or due to in situ combustion of saturating hydrocarbons (up to 700 ◦C for oil combustion) [20,21]. The effects of different forms of catalyst and steam/methane ratios on CMC were also investigated during the experiments. The obtained data can help conclude the expediency of the new stage of field exploration and manage the process of CMC to intensify and speed up of in situ hydrogen generation processes.

This is the first publication from the planned series of publications devoted to the in situ hydrogen generation from methane under gas reservoir conditions. The concept, feasibility, and regularities of the considered process are investigated in the current study. The results of experiments performed at more favorable conditions (higher temperature and dynamic mode) will be presented in further publications.

#### **2. Materials and Methods**

Experiments were designed to study the possibility of methane conversion into hydrogen at relatively low temperatures in the presence of different types of catalysts: in situ synthesized (precursor is nickel nitrate hexahydrate) and ex situ synthesized Ni-based. The influences of the type of porous medium and steam/methane ratio on the process were also investigated.

#### *2.1. Porous Medium*

Several different porous media were investigated, varying from crushed alumina to crushed ceramics, to river sand and crushed core. These types of porous medium simulated different types of reservoir rock, including the target gas field. Industrial alumina (Al2O3) pellets, Alumac 5D® (Salindres, France), were used as an inert porous medium. Alumac 5D® has a high specific surface area of about 335 m2/g, is very hydrophilic, inert to most liquids and gases and, is stable at temperatures up to roughly 2000 ◦C. Granules of Al2O3 were crushed to 0.8–1.2 mm before use. River sand with granules size 0.8–1.2 mm was used as filler in some experiments to model sandstone rock samples. Its composition can be roughly approximated as SiO2. One more option for the porous medium was crushed ceramics. The mineralogical composition of this filler is presented in Table 1.

**Table 1.** Mineralogical composition of crushed ceramic filler.


In other experiments, non-extracted core (rock) samples from the Promyslovskoye gas field were used to recreate reservoir conditions and investigate the influence of the real core on the process of hydrogen generation. The average content of total organic carbon determined with the rock-eval method [22,23] is 1.35 wt.% Data for the averaged composition of the mineral matrix is demonstrated in Table 2.


**Table 2.** Averaged mineralogical composition of the core for laboratory experiments.

Experimental studies observe slightly overestimated results of the methane conversion since the packed model may not exactly repeat the properties of the consolidated core. For example, the porosity and permeability of the consolidated core cannot be reproduced with high accuracy.

#### *2.2. Catalyst Preparation Procedure*

There were two types of monometallic Ni-based catalysts used in the experiments. The first one was an in situ prepared catalyst, which can be delivered into the reservoir in the form of a water solution of the catalyst precursor, then obtained through chemical transformations directly at the reservoir [24]. So, this catalyst was prepared in the reactor during experiments from catalyst precursor solution. The second one was the ex situ prepared catalyst which was nickel oxide particles supported on alumina. This catalyst can be delivered into the reservoir in the form of suspension together with steam or overheated water. In this case, the catalyst was prepared in advance and loaded into the reactor before the experiments.

The catalyst precursor, used for in situ prepared catalyst, was water-soluble nickel nitrate hexahydrate (Ni(NO3)2·6H2O, chemically pure), which had to be decomposed under high temperature according to the summary equation [25,26]:

$$2\text{Ni(NO}\_3\text{)}\_2\cdot6\text{H}\_2\text{O}=2\text{NiO}+\text{O}\_2+4\text{NO}\_2+12\text{H}\_2\text{O}.\tag{5}$$

This salt solution in deionized water was put into the reactor before the experiment with other reactants (water and methane). The catalyst here is the particles of nickel oxide, which have a catalytic effect themselves or can be reduced to a more active metallic phase by interaction with hydrogen or a mixture of steam and methane at a high temperature [27–30] according to the equation:

$$\text{NiO} + \text{H}\_2 = \text{Ni} + \text{H}\_2\text{O}.\tag{6}$$

The second type of catalyst was the ex situ prepared catalyst by wet impregnation of α-Al2O3 (granular size of 0.5–1.0 mm, the specific surface area of 174 m2/g) with a water solution of nickel salt. This catalyst was obtained through heat treatment of the carrier, and soaked in 31.42% nickel nitrate solution in a muffle furnace. Catalyst's preparing procedure included treating 100 g of α-Al2O3 particles with 150 g of the catalyst precursor solution (soak period-2 h), drying the carrier with the precursor solution in the air at 110 ◦C, while water was not evaporated. Next, the heat treatment of the carrier particles coated with precursor salt particles was necessary. Heat treatment was carried out in a muffle furnace, in the air atmosphere, for 3 h at 150 ◦C and then 3 h at 450 ◦C. The decomposition of Ni(NO3)2·6H2O occurred and nickel oxide particles formed on the substrate's surface because of the last operation. The catalyst can be used in the experiments after this procedure. Such supported catalyst contains 16.16% of the active component, calculated in terms of nickel oxide.

#### *2.3. Experimental Setup*

An autoclave reactor used for static experiments is a reactor by Parr (USA), fabricated of Inconel 600 alloy, designed for experiments at max temperature 600 ◦C and max pressure ~408 atm. The reactor volume is 1 L. It has a control block, external heaters, magnetic stirrer, thermocouple, check valve, manometer, and bursting disc (as a safety measure). High-pressure, high-temperature tubes with the dimensions 1/8" and 1/16", and vessels by Swagelok were used for connection lines. The appearance of the reactor and hydrodynamic scheme of installation used in the experiments are shown in Figures 1 and 2, respectively.

**Figure 1.** (**a**) The appearance of autoclave installation and (**b**) sampling system used in experiments.

**Figure 2.** The scheme of autoclave installation used in experiments: 1—computer; 2—pump (Quizix); 3—piston column (*V* = 1 L, with gas); 4—autoclave (reactor); 5—digital pressure gauge; 6 thermocouple; 7—bursting disc; 8—check valve; 9—manometer; 10—vacuum pump; 11—condenser (cooler); 12—back pressure regulator; 13—separator (*V* = 0.25 L); 14—gas meter (0.5 L); 15—gas chromatograph; 16—ventilation system with gas afterburning.
