1. Introduction
Shale gas reservoirs are located in multiple Paleozoic and Mesozoic formations from the Cambrian to the Cretaceous. These formations have given rise to reservoirs with different properties, depending on the geological environment in which they sedimented.
Shale gas is stored in shales, marls, or gas siltstones, all of which are fine-grained sedimentary rocks rich in organic matter, capable of generating and retaining gas, which can be exploited through unconventional methods. These rocks act both as a source and reservoir of gas, with no traps or seals, which generally gives them a regional distribution. In these reservoirs, methane is present in two different forms, adsorbed on the surface of organic components or in the clay minerals of the shales. They also have very low permeability (micro-nano darcy), making gas extraction difficult and requiring the use of stimulation methods such as hydraulic fracturing.
Shale gas is usually found in very large areas that laterally present highly variable petrophysical properties, requiring significant “in situ” volumes due to their low recovery factor [
1]. The advances in horizontal drilling techniques and in well stimulation have made it possible to obtain economically profitable production and to place the significant resources of unconventional gas reservoirs in the reserves category.
As an alternative to traditional sources of natural gas, depending on their geological characteristics, quality and volume, as well as other factors (accessibility, ease of extraction, location, cost, and production technology), there are four types of unconventional gas [
2,
3] (
Figure 1).
The hydraulic fracturing is a method used to increase the flow rate of oil and gas wells. The production wells are drilled to a depth of 8000–10,000 ft [
4,
5], and may have horizontal or directional sections. It is performed by pumping into the geological formation the fracturing fluid at high pressure, commonly composed of water (more than 90%), proppant with sand and chemical additives [
6]. The injection rate must be sufficient to increase the downhole pressure such that the fracture gradient/pressure gradient equilibrium of the surrounding rock is broken [
7,
8].
The width of the fractures created is maintained by introducing a proppant into the fluid to prevent them from closing when injection stops and pressure is reduced. The permeability generated allows the flow of formation fluids into the wellbore [
9].
The internal pressure of the geological formation causes the injected fracturing fluid to rise to the surface and can be stored in tanks and wells prior to disposal or recycling. The recovered fluid is known as return flow or produced water, which can be managed by discharge to surface water or by underground injection [
4,
5,
6,
7,
8,
9,
10].
Worldwide, shale gas resources are estimated at 16,000 Tcf (450 Tcm), of which 510 Tcf (14 Tcm) would be found in Europe: Germany, France, Holland, Austria, and Poland; and about 2000 Tcf (57 Tcm) in North America: USA and Canada. Considering that world conventional gas reserves are on the order of 6400 Tcf (180 Tcm), the potential for shale gas is enormous. On the other hand, they require significant investments, mainly due to the high number of wells that are necessary, usually horizontal, and may be multilateral, requiring multi-stage fracturing. U.S. conventional natural gas production peaked in early 21st century. The supply-side solution seemed to lie in the development of LNG in the Middle East, Africa, Australia, and Russia for import to the U.S. market. The IEA expected that the U.S. would need a high level of LNG imports to meet gas demand [
11]. However, shale gas production in the U.S. grew significantly in the 2000–2010 period, from 0.39 Bcf to 4.87 Bcf, representing a 22% increase in dry gas production. The forecast increase in shale gas production to 13.6 Tcf by 2035 would account for 49% of total U.S. gas production [
12]. This gas is currently the main driver of natural gas independence in the US [
13], and with a significant influence on the world price system of the sector [
11,
13,
14].
On 8 March 2022, the European Commission proposed the outline of a plan to make Europe independent of Russian fossil fuels well before 2030. This plan would transform the European energy system and address the climate crisis [
15]. The European Union has included natural gas in the E.U. Taxonomy [
16] of clean energy. The “Plan Nacional Integrado de Energía y Clima” 2021–2030 of the Spanish “Ministerio para la Transición Ecológica y el Reto Demográfico” [
17] foresees a constant use of natural gas as a primary source of electricity, making it the only hydrocarbon whose consumption is not expected to decline.
The Spanish electricity system relies heavily on the use of natural gas (total Spanish consumption is about 100 MMm
3/d) to compensate for the intermittency of wind and solar generation plants. Practically all of the natural gas consumed in Spain is imported (99%) [
18]. The countries that supplied natural gas to Spain in 2020 were Algeria (41.0%), the United States (16.9%), and Nigeria (11.0%) [
18], with the United States becoming the main supplier in 2022 and an increase in imports of Russian gas in 2022 [
19]. None of these countries belongs to the European Union, nor to the so-called Schengen Area. The supply of natural gas is then subject to the bilateral agreements to be established with the supplying countries, affected by the international geopolitical situation and the USD–EUR ratio.
The exploitation of shale gas in Europe is strongly opposed because of its potential environmental and public health impacts, which have been endorsed in the form of protests and bans in several countries and regions [
20]. In Spain, any exploration and subsequent production of hydrocarbons is illegal since the entry into force of Law 7/2021, of 20th May, on climate change and energy transition [
21].
The absence of exploration in Spain for more than thirty years, unsuccessfully attempted by several companies, which have renounced to dozens of research permits, is the result of media campaigns against fossil fuels and fracking technology [
22,
23].
The development of exploitation activities in previously unconsidered areas must be supported by consistent studies based on the extensive experience developed in many countries.
This work aims to estimate the existence of enough natural gas reserves in the Jaca Flysch formation, located between the provinces of Huesca and Navarra, by means of the analysis of geological and geophysical logs and the volumetric method. In addition, the rate of production and the economic impact that its possible exploitation would have on the internal demand for natural gas has been analyzed.
The results and conclusions obtained could be taken as a reference for a complete analysis of the rest of the Spanish basins where the presence of shale gas is recognized. It would thus contribute to the European REPowerEU plan in response to global energy market disruptions with the “Recovery and Resilience Mechanism” (RRM) and the “E.U. Energy Platform” (a unified procurement platform) [
15].
2. Materials and Methods
2.1. Geology. Discovery of the Jaca Flysch
The exploration and development of shale gas fields requires the integration of numerous disciplines such as geology, geophysics, hydrogeology, geochemistry, and petrophysics [
24].
Although Spain’s geography has been little explored with respect to its hydrocarbon reserves, it has favorable depositional environments and pressure regimes similar to other countries. The prospective possibilities of Shale Gas are located in the Basque-Cantabrian, Pyrenean, Ebro, Guadalquivir, and Betic basins, which correspond to Paleogene (Eocene), Cretaceous (Upper and Lower), Lower Jurassic (Lias), and Carboniferous (Westphalian-Eastphalian) areas.
Effective shale volumes are conditioned by organic matter richness, reservoir quality, and adsorption capacity. These parameters are affected by age, depth, organic content, maturity, and mineralogy, with great variation among different formations worldwide. Major shale geological formations include Goldwyer and Shahejie (Paleogene), Vaca Muerta (Cretaceous-Jurassic), Barnett, Bakken and Bowland (Carboniferous), Marcellus, Muskwa, Otter Park and Evie (Devonian), Qiongzhusi, Llandovery, and Longmaxi (Cambrian and Silurian).
Geographically, the area studied covers the pre-Pyrenean zone, framed between the meridians of Pamplona to the W and Boltaña to the E, to the S it is limited by the Ebro depression (Somontano de Huesca and Ribera de Navarra) and to the N by the loamy depression of the Canal de Berdún-Val Ancha and the Aoiz area. Important features include the Sierras Oscenses, which form a continuous elongated front from ESE to WNW. To the north of these mountain ranges, the Tertiary Continental folding gives rise to a moderately rugged relief in which the conglomeratic masses of Canciás, Peña Oroel, San Juan de la Peña, and Peña Izaga stand out due to their elevation [
25]. The Jaca Flysch is located in the eastern part of the Pamplona-Jaca Basin (
Figure 2).
The producing units are the fractured carbonate megabreccias of Aurín and Jaca, intercalated in the Flysch units (Eocene) that reach a total thickness of 2500 m, and are divided into (
Figure 3) [
26]:
Lower carbonate Flysch. Carbonate breccias intercalated with sands and clays, 150–300 m thick. Recognized as the source rock of the Serrablo Field.
Sandy Flysch. Submarine turbiditic sands with intercalations of carbonate mega-layers (Gallego, Aurín, Suprajaca and Jaca), 10–100 m thick.
Upper Flysch. Similar to sandy Flysch, but without megalayers.
Blue marls. They are interfingered with the Flysch acting as a seal.
The Serrablo field is a case of a gas trap in carbonate megabasins formed by a conglomerated and brecciated basal zone, where gas is produced mainly through fractures. Each producing megabasin has an independent gas-water contact. The marl and clay deposits along with other levels of the Flysch sequence constitute the source rock for the generated hydrocarbons, which migrated and were trapped in structures created during the later stages of thrust [
27].
The field began production in the 1980s, when several exploratory boreholes were drilled near the town of Sabiñánigo (Huesca) [
28]. The Flysch and the carbonate series were identified, both with the presence of natural gas with an approximate methane content of 98%.
The Serrablo 1–5 wells allowed estimating Flysch reserves of at least 25 Bsm3. Twenty three drillings were carried out to characterize the reservoir, with companies such as Hispanoil and Eniepsa interested in its possible exploitation.
At the beginning of 1984, the area that was put into production was the carbonate rocks. The Zaragoza-Serrablo gas pipeline was built to connect the field with the existing gas pipeline network managed by Enagás [
29].
At the end of the 1990s, the field ceased to be exploited because the reserves proved to be lower than expected. These old fields have recently been used as a natural gas storage facility managed by Enagás.
Drilling was also carried out in the region of Navarra where traces of natural gas were found when crossing the aforementioned Flysch.
The Jaca Flysch extends from the region of Navarra to approximately the border between Aragón and Cataluña. The area highlighted in red dots in the stratigraphic column of
Figure 3 shows the location of the Flysch, with no evidence of gas found along its entire extent. (
Figure 4).
From the analysis of the data collected by a previous study of the Petrophysical Institute Foundation (IPf) [
26], the wells that did identify natural gas were Pamplona-2, Pamplona-3, Aoiz-1, San Vicente-1, Serrablo-13, Serrablo-7, and Ayerbe de Broto-1. These drillings are compiled in the IGME/CSIC (Spanish National Research Council) lithological database [
28], where the column of the different lithologies crossed by each one of them is found.
In the Serrablo-13 well, a production test was carried out in the Flysch, giving a ¼ choke flow for about 500 h over a period of 31 days. At 25 h, a flow rate of 13,000 sm
3/d was measured and after ten days it dropped to 5300 sm
3/d and 117 psi at the wellhead [
26].
2.2. Estimated Volume of Flysch Reserves
The volumetric method has been used to estimate the amount of gas in the reservoir (Flysch), taking into account the part of the formation susceptible to host gas (NTG), porosity, and gas saturation (Equation (1)).
Geochemical studies on samples from several wells indicate that the Eocene sediments have a low organic matter content (TOC 0.6%) with a degree of maturity (Ro 1.0–1.3%) in the dark shale interval, formed by clayey limestones intercalated with marls [
30]. These values can be compensated by the large accumulation of shales whose volume can be deduced from the thickness of the layers of the drillhole columns [
31,
32].
2.2.1. Flysch Thickness
The thickness of the Flysch varies depending on the geographical area in which it is located. We can know its thickness with the existing wells and extrapolate the results in areas where there are no logs (
Table 1).
Since exact data on which part of the Flysch contains gas and which does not are unknown, an estimate will be made that the NTG is of 50%.
2.2.2. Porosity
The Flysch porosity has been calculated from sonic logs using the Wyllie equation. [
33] (Equation (2)).
The transit time has been taken for the Aoiz-1 well, which is the one with logs in the Flysch formation. The data extracted from this well are as follows (
Table 2).
An average value is obtained from the logs Δt of 58.5 μs/ft. In turn, the values of Δtma and Δtf can be approximated, respectively at 46.5 μs/ft and 500 μs/ft for a mostly gas content. A formation porosity value of 2.65% is obtained.
2.2.3. Water Saturation
Water saturation was calculated using the formula of Archie et al. [
34] (Equation (3)).
The formation resistivity (Rt) 235 ohm/m has been obtained directly from the electrical logs. The Pamplona-3 well has been used, as it is the only one for which these logs are available, estimating that the rest of the formation has the same resistive parameters. The resistivity of water (Rw) at the formation temperature (about 90 °C) is 0.03146 ohm/m.
Taking the value 2.0 for the saturation (n) and cementation (m) exponents, and 1.0 for the tortuosity factor (a), we obtain with the formation porosity (2.65%) a water saturation value of 35.3%.
2.2.4. Gas Formation Initial Volume Factor
The volumetric method estimates the reserves in the reservoir and requires a factor (Bgi), which relates these reserves in the reservoir to their volume at surface.
The MHA-P3 software developed by the company Malkewicz Hueni Associates, Inc (Golden, CO, USA) was used to estimate it [
35]. By entering the reservoir pressure/temperature data 3600 psi/90 °C, we have obtained the results of
Figure 5.
As can be observed, for a pressure close to 3670 psi, the volumetric factor is 0.004632 (rm3/sm3).
2.2.5. Reserves (OGIP)
An estimate of the gas reserves in the field is made with the data previously obtained (
Table 3). For this estimation, it has been considered that the radius of influence of each well studied is 3 km, and therefore, the area of influence of each vertical well (considered within the Flysch containing reserves) is 28.27 km
2.
In total there are an estimated of 498 Bsm3, applying a recovery factor of 5% and 10% of the reserves, this results in a respective OGIP of 24.88 Bsm3 and 49.77 Bsm3.
5. Conclusions
The recent increase in energy prices is making European consumers vulnerable. In order to face the new international scenario of the gas sector, it is convenient to estimate its own exploitable reserves, including hydraulic fracturing technology.
Spain has the capacity to use hydraulic fracturing technology to supply itself with natural gas. Although consumption would still be much higher than existing reserves, a complementary domestic natural gas extraction industry can be generated, reducing foreign dependence on other countries.
The Jaca Flysch site is one of the possible sites that can be exploited in Spain. Its possible exploration and exploitation are restricted by Law 7/2021, of 20th of May, on climate change and energy transition, which expressly prohibits the exploration and extraction of hydrocarbons [
21].
The average production proposed in this study would make a significant contribution to the energy mix and would represent between 10% and 20% of national consumption for several years, highlighting the potential in areas similar to the Jaca basin to increase natural gas production by around 48% through the use of hydraulic fracturing.
This technology is in question due to the consequences on the environment if the environmental risks involved are not effectively managed [
56]. However, there are procedures and best practices that can help manage and minimize these risks [
20], even if it means increasing the cost of a typical well by 7%.
This study proposes a series of actions to ensure sustainable development of Flysch exploitation as it is considered that public confidence in the safety of hydraulic fracturing would be greatly improved by more frequent microseismic monitoring and public dissemination of the results. The establishment of an independent Well Examiner to guarantee the integrity of the wells, verifying through reports the correct design and drilling system, is also proposed.
It is concluded that it is possible to carry out a profitable and sustainable production of natural gas with hydraulic fracturing technology in Spain, limiting the study to the Jaca Flysch, so that it is possible to continue using natural gas as part of the energy mix, contributing to the transition and the reduction of the energy dependence on other countries.