Two Saudi basaltic samples were collected from Harrat Uwayrid, a Cenozoic volcanic field in the Medina region, NW Saudi Arabia, Figure 1 (Duncan and Al-Amri, 2013). The Harrat Uwayrid comprises an elongate NW-SE oriented volcanic field of a strongly eroded basaltic plateau extending over approximately 230 km². The Harrat Uwayrid basalt overlies above the Cambrian continental sandstones of the Siq Formation (Coleman et al., 1983; Altherr et al., 2019). The age of Harrat Uwayrid basalts ranges from Miocene to Quaternary, contemporaneous with the several episodes of volcanic activities in Arabia (Kaliwoda et al., 2007). The Miocene basalt is strongly eroded and consists mainly of alkali olivine basalt, whereas the younger Quaternary basalt contains mantle xenoliths and megacrysts (Altherr et al., 2019). The samples were selected from two locations north and south of the elongated volcanic field. The investigation aims to study the efficiency of Harrat Uwayrid basaltic rocks as a seal and evaluate potential hydrogen storage in the underlying Cambrian sandstone formation for a practical period of time.

The contact angles were measured using the modified sessile drop method utilizing a KRÜSS fluid drop analyzer (DSA100 model) and a high-pressure and high-temperature (HPHT) optical cell (Figure 2). Pure H₂ gas (99.99 mol%) and a synthetic reservoir brine of 10 wt%NaCl were used for the contact angle measurements at testing conditions. First, the basalt sample was cleaned with distilled water and dichloromethane, cut into 1 cm × 1 cm × 0.5 cm dimensions. Further cleaning by purging with nitrogen was conducted prior to emplacement of the samples on a horizontal plate inside a high-temperature and pressure (HTHP) cell. Then, the H₂ is injected and pressurized using an ISCO pump (500 D) of 0.001 MPa precision. The temperature is increased using an electrical heater to reach the desired temperature (323K). Afterwards, a brine droplet of 5 μL (±2 μL) is manually introduced through a needle dispenser, controlled by another ISCO pump, and released on the substrate surface. Finally, the contact angle is measured at adiabatic conditions in a continuous increment measurement where the pressure increases gradually from 3 MPa to 28 MPa, and the contact angle is measured at each pressure step. The purpose of such modified measurement from the conventional sessile drop method is to measure the hydrogen contact angle at high pressure levels which may occur in the subsurface where a continuous pressure is applied by surrounding rocks and formation fluids when H₂ is injected into the geological formation for subsurface storage. Al-Mukainah et al. (2022) have recently adopted the same modified sessile drop method (applied in this work) to measure H₂/shale/brine contact angles and evaluate potential H₂ storage in shale formations (Al-Mukainah et al., 2022). Images were acquired using a high-resolution camera to capture the drop behaviour during the experiment. The contact angle is then analyzed using KRÜSS DSA software. The contact angle measurement was repeated three times for better accuracy. The average standard deviation for the contact angle from the two samples was approximately ±3°. Before the contact angle measurement, the surface roughness of the basalt samples was determined using the Surface Roughness Analyzer (KRÜSS GmbH) (Al-Yaseri et al., 2021c).

The measured TOC values for the analyzed basalt samples (1 and 2) were approximately 0.05 wt% and 0.03 wt%, respectively, measured by Rock-Eval pyrolysis. The XRD results provide reveal a semi-quantitative mineralogy analysis (Figure 3). The analyzed samples were collected from two different locations (south and north) to evaluate any changes in the mineral composition that could potentially influence H₂/basalt/brine wettability. The XRD analysis demonstrates a similarity in the primary minerals between the two samples, dominated by anorthite and olivine of about 44.3% and 57.1% for anorthite, 14.7%, and 24.4% for olivine in the Saudi basalt 1 and 2, respectively. Clinopyroxene (diopside) and nepheline were also found only in Saudi basalt 1 (24.8% and 16.3%, respectively), while Saudi basalt 2 contains magnesioferrite (∼17.6%) and traces of albite and vermiculite that were missing in Saudi basalt 1.

A mineralogical comparison between the investigated Saudi basalts and other basaltic rocks tested for underground gas storage (Table 2) reveals significant differences, which may influence the wettability measurement and storage efficiency of H₂ beneath the basalts. Relative to other basalt samples (Iranian and Icelandic basalts), Saudi basalts are enriched in olivine relative to plagioclase and alteration products (e.g., clays). Such variation in mineralogy typifies a differential alteration (Harnois, 1988; Weltje et al., 1998; Dessert et al., 2003; Leila et al., 2018; Leila et al., 2021). For example, olivine minerals are more susceptible to chemical weathering and alteration into other phases such as serpentine; thus, enrichment of olivine phases would infer into fresh and un-altered samples. Therefore, the Saudi basalts are fresh samples, whereas other basalt samples were subjected to more severe chemical alteration. Such variation in chemical alteration may impact the wettability measurements and hence the storage potential. Notably, nano-capillary pores are more common in clays and these nanopores may become saturated with hydrogen, and hence reduce the storage capability of the basaltic rock. Additionally, the occurrence of specific clay phases that are susceptible to hydro-swelling (e.g., Montmorillonite) would decrease pore throat size and pore system connectivity (Aksu et al., 2015; Jiu et al., 2021). Thereby, variation in mineralogical composition would significantly impact the capillary behaviors and sealing capacity of the naturally-impervious rocks.

The FTIR measurement indicates the dominant chemical functional groups on a rock surface, significantly contributing to the wettability status (Madhurima et al., 2011; Cheng et al., 2014; Pillai et al., 2018). The FTIR analysis of the Saudi basalt samples demonstrated that SiO₂ is the primary chemical group on the samples’ surface (Figure 4). Visible wave numbers in the range of 1,000 and 1,200 cm⁻¹ correspond to the Si-O-Si bond (OH and Choi, 2010; Waman et al., 2011; Liu et al., 2019). The interpreted Si-O-Si bond is attributed to the presence of silica and silicate minerals in the studied basalt samples. The more pronounced Si-O-Si bond in Saudi basalt 1 is most likely related to the occurrence of nepheline which is absent in basalt 2. Weak absorption at 800 cm⁻¹ corresponds to Si-H₂/(Si-H₂)_n band. This band is more significant in the Saudi basalt 2 sample typifying a more hydrophilic characteristics relative to basalt 1 sample (Liu et al., 2019). FTIR is also corroborated by XRD and XRF which confirmed a silica-rich samples. According to XRF data, basalts 1 and 2 contain 36.16% and 37.50% SiO₂, respectively.

The SEM analysis focuses on the basalt samples’ surface morphology and micro-scale characteristics, such as the fracture and matrix porosity, which can significantly influence the wettability measurement (Rucker et al., 2019). Micro, irregular, stylolite-like fractures were found with more intensity in Saudi basalt 1 (Figures 5A, B) compared to the Saudi basalt 2 sample (Figures 5C, D). Both samples showed low matrix porosity in the micro- and nano-scales. However, matrix porosity is relatively more significant in Saudi basalt 2 sample (Figure 5D). Notably, the fracture and matrix pores are scattered and non-connected, suggesting that the studied Saudi basalts are impervious with nano-scale pore throats. The surface roughness analyzer indicated a root-mean-square (RMS) from different vertical locations of approximately 29.74 and 25.14 µm surface roughness for Saudi basalt 1 and Saudi basalt 2, respectively (Figure 6). The RMS roughness is a measure of the standard deviation in the surface heights values, thus reflecting the extent of surface irregularities (Gadelmawla et al., 2002). The surface topography of the samples induces a paramount impact on the rock-fluid contact angle. Irregular, rough surfaces with high RMS values often enhance the wetting behavior of the sample (Johnson and Dettre, 1964). Additionally, irregular surfaces usually show water-wet behavior. Nevertheless, the influence of the surface roughness becomes insignificant below 1,000 µm (Al-Yaseri et al., 2016). Thus, it does not impact the wetting characteristics of storage formation rocks (Al-Mukainah et al., 2022).

The static H₂/brine/basalt contact angles of the two samples were measured at different pressures up to 28 MPa and a temperature of 323K (Figure 7). The Saudi basalt 2 displayed slightly higher contact angles than Saudi basalt 1. The H₂ wettability of Saudi basalt 2 demonstrated a more hydrophilic behavior than Saudi basalt 1, mainly attributed to the Si- H₂ bond FTIR peak, which is higher in Saudi basalt 2 than Saudi basalt 1. However, the variation in contact angle measurements could be attributed only to surface roughness differences (AlRatrout et al., 2018; Mehmani et al., 2019). Nevertheless, both samples displayed a strong to intermediate water-wet behaviour and demonstrated a slight decrease in pressure, which is linked to the decline of interfacial between the brine and H₂ gas (Figure 8). Images of drop profiles are shown in Figures 9, 10 for the different pressure steps confirming the observation of the contact angle measurements. These profiles demonstrate intermediate to strongly water-wet behaviour.

Overall, the contact angles for both basalt samples displayed an initial steep drop as pressure increased from 2.5 MPa to 10 MPa, which started to level up at 10 MPa pressure. The observed decrease in the contact angles with pressure differs from some of the gas/basalt/brine measurements, for example, (Iglauer et al., 2020; Al-Yaseri et al., 2021a; Hosseini et al., 2022b), where the contact angle increase with pressure. In this work, the modified sessile drop is applied, where the contact angle is measured for each pressure step in a continuous pressure increment approach. Moreover, the decrease in contact angle with pressure for the H₂/basalt/brine system is related to the decline of H₂-brine interfacial tension (IFT) with increasing pressure (Figure 8) and the very low density of H₂ compared to CO₂, where the cohesive forces between gas molecules and rock matrix increase at higher pressures (Yekeen et al., 2021; Al-Mukainah et al., 2022).

The present work measured the static contact angle of H₂/basalt/brine. While the only available work of H₂/basalt/brine wettability in the literature by Hosseini et al. (2022b) was conducted using the tilted plate method, first introduced by Lander et al. (1993), and provides dynamic contact angles. The tilted plate method theoretically attempts to represent the imbibition (wetting phase displacing non-wetting) and drainage (non-wetting phase displacing wetting) processes by correlating them to tail contact angles of a brine drop on a tilted rock surface, the receding contact angle ( θ r ) and advancing contact angle ( θ a ).

In the literature, there is extensive experimental work applying the tilted plate method to investigate underground H₂ storage using different rock types, where the dynamic contact angles are related to the intrinsic contact angle or the equilibrium contact angle ( θ e ) (Morrow, 1975; Chow et al., 2018; Liang et al., 2020; Hashemi et al., 2021; Ali et al., 2022b). The equilibrium contact angle can be calculated using Tadmore’s empirical approach (Tadmor, 2004) based on Young’s equation and Neumann’s equation of state (Li and Neumann, 1992; Kwok and Neumann, 1999) as a function of the receding ( θ r ) and advancing ( θ a ) contact angles as shown in Eq. 1: θ e = cos − 1 ℵ a cos θ a + ℵ r cos θ r ℵ a + ℵ r (1)

Where θ a , θ r represent advancing and receding contact angles, and ℵ a and ℵ r are correlation parameters (receding and advancing), calculated as shown in Eqs 2, 3: ℵ a = sin 3 θ a 2 − 3 cos ⁡ θ a + cos 3 θ a 3 (2) ℵ r = sin 3 θ r 2 − 3 cos ⁡ θ r + cos 3 θ r 3 (3)

The above correlation was used to calculate θ e from the dynamic contact angles measured at two different temperatures (308 K and 343 K) by Hosseini et al. (2022a). Then, it was used for comparison with H₂/basalt/brine static contact angles in this study, which was conducted at a temperature of 323 K (Figure 11). The comparison reveals a relatively close approximation, particularly at low pressures (less than 10 MPa). The trend of the measured contact angles differs due to the different applied experimental techniques.

The modified sessile drop used in this work depends on maintaining an isolated gas environment and measuring the change in drop contact angles at each pressure point via a subsequent increase of the pressure inside the high-pressure, high-temperature cell. In contrast, the titled-plate method measures the dynamic contact angles in a stepwise procedure involving using multiple rock samples and depressurizing the cell each step to load the new sample. Moreover, the mineralogy comparison based on XRD analysis listed in Table 2 demonstrates that the Saudi basalts have different mineral compositions than the Iranian basalt. The Iranian basalt composition contains more lizardite and plagioclase; thus, it can exhibit a wettability behaviour similar to clay-rich rocks (Abramov et al., 2019; Al-Yaseri et al., 2021c).

The sealing efficiency of a caprock is evaluated by studying capillary characteristics at the interface between the formation brine in the caprock and the injected gas. A non-wetting phase (e.g., CO₂ and H₂) cannot invade the rock until the pressure drop exceeds a certain threshold, which is known as the capillary entry pressure p c e and determined by the Laplace equation (Espinoza and Santamarina, 2017), as follows: p c e = 2 γ lg cos θ g r (4) where γ lg is the brine/gas interfacial tension, and θ g is the contact angle of the gas, and r is the effective pore throat radius of the rock.

The sealing capacity and storage efficiency of the studied basalt samples at 323K is obtained by calculating the static height of the H₂ column that can be safely trapped beneath the basaltic rocks using Eq. 5 (Dake, 1978): h g max = 2 γ lg cos θ g r g ρ l − ρ g (5) where h g max is the maximum static height of the column that can be safely trapped beneath the seal (Iglauer, 2022), g is the gravitational acceleration, ρ l and ρ g are the density of the liquid and gas phases, respectively.

The capillary entry pressure ( p c e ) and maximum static gas column height ( h g max ) was evaluated using the contact angle measurements and density differences following Eqs 4 and 5, Figures 12A, B. In addition, theoretical pore throat radii values in the range of 10 and 25 nm were used to evaluate the impact of the pore throat radius on the sample wettability. Basaltic rocks’ average pore throat size can range from 6 to 32 nm (Al-Yaseri et al., 2021a; Abdulelah et al., 2021; Hosseini et al., 2022a). The mean pore throat radius of r = 6 nm (Hosseini et al., 2022a) was utilized to calculate the column height for H₂/Iranian basalt and CO₂/Icelandic basalt studies. Thus, the H₂ column height for the Saudi basalts was recalculated using r = 6 nm to present an equivalent comparison. While for the CO₂/WA basalt, r = 29 nm was used as obtained by Al-Yaseri et al. (2021b). The H₂ column height is used to calculate the hydrogen storage capacity or maximum mass of H₂ that can be safely stored in basaltic rock (Shah et al., 2008).

The calculations in this study using H₂/basalt/brine contact angles demonstrate an insignificant influence of pressure on p c e and h g max , while the pore throat radius demonstrated the highest impact on the sealing efficiency and H₂ storage capacities (Figure 13). The calculated p c e values vary significantly with the pore throat radius with approximately a two-fold decrease in p c e with increasing the pore throat radius from 10 to 25 nm (Figure 13A). The Saudi basalt samples with their pore throat radii less than 10 nm will be able to store a hydrogen column higher than 1,000 m (Figure 13B).

Notably, the p c e in Saudi basalt 1 was always higher than in Saudi basalt 2, consistent with the contact angle measurements, as the elevated hydrogen wettability of Saudi basalt 2 would result in a lower p c e . Moreover, the p c e values of the Saudi basalt samples did not vary significantly with increasing pressure, which can be interpreted as the semi-constant density of H₂ at elevated pressures. The strong water-wet status and less variation in p c e with pressure suggests the good sealing efficiency of Saudi basalt for underground H₂ storage.

The obtained gas column height versus pressure trend of the Saudi basalts varies from that reported in the literature for the H₂/Iranian basalts (Hosseini et al., 2022a) and CO₂/basalts (Iglauer et al., 2020; Al-Yaseri et al., 2021b) (Figures 14A, B). The CO₂/brine/basalt behaviour could be attributed to the sharp increase in CO₂ density with pressure; therefore, a rapidly increasing pressure is expected. On the other hand, we hypothesize that the wide difference in H₂ column height versus pressure between Saudi and Iranian basalts is most likely attributed to their different mineralogy and the presence of some silicate-rich phases (e.g., lizardite) in the Iranian basalt and, therefore, it behaves like clay-rich rocks and become fully-hydrogen wet as pressure increases.