Effect of chemical clogging on the permeability of weakly consolidated sandstone due to reinjection at different temperatures

With the progress of human civilization and the rapid development of social economy, people are urgently looking for renewable energy to optimize the energy structure. Geothermal energy has attracted more and more attention because of its huge reserves, stable temperature, and low carbon dioxide emissions. China is actively developing and utilizing geothermal resources (Zhao et al. 2022). Also, Chinese geothermal resources account for 8% of the whole world (Zhao &Wan 2014).

China is rich in geothermal resources, but only 2.3% of shallow geothermal resources have been exploited (Wang et al. 2017). Medium–low temperature hydrothermal geothermal resources are rich in northern Shandong (Wang et al. 2015; Kang et al. 2021), Kaifeng basin (Lin et al. 2007), Qinghai Xining (Zhang et al. 2021, 2022a), Guanzhong Basin (Xv et al. 2019), and Tianjin (An et al. 2016). Most of them occur in sandstone thermal reservoirs, which are loose porous media. In the process of wastewater reinjection, clogging reduces the permeability, and even causes damage to the reservoir irreversibly (Ungemach 2003; Cui et al. 2021a, 2021b, 2022; Zhang et al. 2022a, 2022b). Bouwer (2002) proposed that the causes of geothermal reinjection clogging can be mainly divided into suspended solids, microorganisms, bubble clogging, and chemical clogging; Brehme et al. (2018) subdivided clogging processes into physical, chemical, and biological processes. Among them, chemical clogging is caused by the change of the original hydrogeochemical equilibrium after wastewater enters the thermal reservoir. To reduce the impact of clogging, it is vital to optimize geothermal reinjection technology (Allow 2013; Diaz et al. 2016). At present, all hydrothermal geothermal projects in China require reinjection operations, and the rate of reinjection depends on provincial regulations (Kong et al. 2017). However, in actual projects, most geothermal wells cannot achieve continuous reinjection due to clogging. For example, the average reinjection rate of the Neogene Guantao Formation in Tianjin is 34.7%, and 6.3% for the Minghuazhen Formation (Wang 2014). Typical minerals of chemical clogging include carbonates, silicates, sulfates, metal oxides, and clay minerals (Scheiber et al. 2012; Haklidir & Haklidir 2017; Ma et al. 2017).

The main precipitation in the chemical clogging of geothermal reinjection was the carbonate scale in Xianyang, NW China (Ma et al. 2012). Pokrovsky et al. (2009) studied the dissolution of calcite and dolomite under different pH, temperature, and CO₂ partial pressure environments rate. The study by Eppner et al. (2017) showed that the increase in temperature would reduce the solubility of calcite, which is beneficial to the precipitation of calcite. Moreover, the increase of temperature would also reduce the solubility of CO₂, leading to the precipitation of calcium carbonate. Yang et al. (2022a, 2022b) found that the precipitation of carbonate minerals (calcite, dolomite) caused clogging at the back of the column. The concentration of Mg²⁺ can determine the form of carbonate minerals precipitation. Boyd et al. (2014) found the higher the Mg²⁺ concentration, the slower the rate of calcite formation, but the faster rate of aragonite formation.

Silicate is prevalent in wastewater and reservoirs, especially in geothermal fields with moderate mineralization and pH (Gallup 1997). Gunnarsson & Arnorsson (2005) stated that the silica scale mainly originates from the monomer amorphous silicas in the process of high-temperature geothermal water recovery and reinjection. Kunan et al. (2021) used hydrogeochemical modeling software Phreeqc to reproduce the scaling and the results indicated that silicate precipitation is strongly controlled by kinetic. Potassium feldspar is a kind of silicate. Ngwenya et al. (1995) confirmed by SEM, XRD, and ion assay that potassium feldspar can precipitate during reinjection resulting in lower permeability.

Sulfate minerals consist of and cations. The excess of Ca²⁺ and ions can lead to the formation and precipitation of sulfate scales (Cobos et al. 2021). Hastie et al. (2011) found that the polysulfide can be oxidized to sulfate during the reinjection due to the oxidation of water. Bedrikovetsky et al. (2006) combined experimental and mathematical analysis methods to simulate sulfate mineral scaling patterns. Wagner et al. (2005) found that gypsum precipitated in warmer regions of the reservoir. Brehme et al. (2019) stated that gypsum is more prone to precipitation than barite. Pape et al. (2005) investigated the precipitation mechanism of anhydrite. Their results implied that for the same super-saturation anhydrite crystals did not nucleate in small pores, but formed preferentially in large pores.

Metal oxides, especially iron oxides will cause serious clogging if Fe²⁺ is oxidized to Fe³⁺. Ni et al. (2018) found that the presence of both nitrate and oxygen caused Fe³⁺ precipitation. Cobos & Sogaard (2020) stated that reducing iron concentration did not seem to be an efficient method to reduce precipitates if oxygen had not been removed from the fluid. Yin et al. (2019) observed that Fe²⁺ was oxidized to Fe³⁺ to generate Fe(OH)₃ colloids and adsorbed them in the seepage channels, causing chemical clogging.

Clay minerals are very common in sandstone. Liu et al. (2017) stated that the composition of clay minerals is the most important factor that affects the permeability of bine aquifer. They detected that the montmorillonite led to the largest permeability reduction, followed by kaolinite and illite in their further work. Temperature can influence the precipitation of clay minerals. Rosenbrand et al. (2014, 2015) discovered heating caused permeability reduction in sandstone formations containing kaolinite clay particles. And a permeability reduction due to heating from 20 to 80 °C was largely reversible with cooling.

Researchers have conducted studies on various aspects of chemical clogging. The reasons for clogging during reinjection depended on the geothermal reservoir and wastewater types. However, nearly all precipitation was affected by temperature, and geothermal wastewater reinjection will form the temperature gradient. Therefore, in this study, we took the weakly consolidated sandstone thermal reservoir of the Guantao Formation in Lubei as an example, simulating the reinjection process at different temperatures. Compared to previous studies, we minimized the impact of physical clogging by setting very low flow velocity. Our study aims to determine the influence of the temperature of wastewater on the chemical clogging in the Lubei area. The influence of mineral dissolution and precipitation caused by the change of ion concentration of displacement water on the permeability of the reservoir is emphatically discussed. The results from this study can be used for reducing chemical clogging during wastewater reinjection in the Lubei area.

In the Lubei area, only a small part of bedrock is exposed, and most of them are covered with quaternary sediments, which play a role in the thermal insulation of the lower geothermal reservoir. At present, the main geothermal resource reservoir in this area is the Guantao Formation of Neogene in Cenozoic The lower part of the Guantao Formation is siltstone, conglomerate, and sandy aquitard. The upper part is mainly siltstone, fine sandstone, and medium sandstone, belonging to fluvial facies (Zhang et al. 2019; Xv et al. 2021).

Guantao Formation sandstone is weakly consolidated fine-grained lithic feldspar sandstone, which is mainly composed of albite, microcline, quartz, Illite/montmorillonite mixed layer, montmorillonite, tremolite, kaolinite, and illite. The particle size ranges from 1.88 to 454 μm and the pore diameter ranges from 20 to 200 μm. The geothermal wastewater is weakly alkaline, and the main ion concentration from high to low is ⁠, Na⁺, Cl⁻, ⁠, ⁠, K⁺, Ca²⁺, Mg²⁺, which is classified as Bicarbonate-sodium water-B type according to the Schukalev classification method (MGMR 1983).

To exclude the effects of physical and biological clogging on the results, desanders were used to remove most of the microorganisms, large particles, and suspended particles. The pH of the reinjection water was 8.45, the TDS was 5,237.41 mg/L, and the results of ion analysis are shown in Table 1. The wastewater in the Lubei area has a lower TDS compared with others (Regenspurg et al. 2010; Öner et al. 2011; Tomaszewska et al. 2017; Sasaki et al. 2021; Yu et al. 2021). A control group of deionized distilled water was also set up to exclude the effects of clogging caused by particle transport, suspended particle transport, water absorption, and swelling of clay.

Table 1

Results of chemical analysis of reinjection water

Ion type .	Na+ .	K+ .	Ca2+ .	Mg2+ .	Cl− .	.	.	.
Mass concentration（mg/L）	470.9	7.66	16.5	2.34	409.3	294.9	3,900.4	135.41

Ion type .	Na+ .	K+ .	Ca2+ .	Mg2+ .	Cl− .	.	.	.
Mass concentration（mg/L）	470.9	7.66	16.5	2.34	409.3	294.9	3,900.4	135.41

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Considering the different temperature gradients caused by low-temperature water and long-term heat extraction, the temperature was set to 25, 35, 45, 55, and 65 °C in this experiment. The depth of the reservoir was about 1,100 m, therefore the surrounding pressure was set to 10 MPa. To fully investigate the effect of water chemistry on clogging during reinjection, and to let the fluid fully contact with the rock sample, the reinjection time was set to 100 h, and the reinjection flow rate was set to 0.5 ml/min (0.0083 cm³/s). The low flow rate was used to avoid the effect of physical clogging caused by particle migration. The experiments were conducted in constant flow mode, and the pressure values at both ends of the cores were collected in real-time to calculate the permeability. The data were collected every 20 s for each group of experiment, and a total of 120,000 data were collected for 100 h. The experimental conditions were set as shown in Table 3.

From Figure 3, it can be found that the permeability of each group showed a downward trend over time. The permeability of the control group decreased rapidly in the first two days and then decreased slowly. The permeability of the control group decreased by 7.69, 14.28, 33.33, 80, and 84.46% from 63.82 mD at 1, 6, 12, 48, and 100 h, respectively.

The permeability of the control group did not change within the first 25 min of reinjection displacement. This indicates that the physical clogging and water absorption expansion clogging of the rock sample are very weak when it begins to contact with the wastewater. After 25 min, the physical clogging gradually grew, which indicates that the physical clogging and water absorbing expansion clogging developed gradually through the continuous action of the fluid, and the size of the particles became larger and larger. In the first two days, the pore and throat structure changed and caused the clogging, after that, the pore and throat structure was relatively stable and the permeability was maintained at a low state.

On the whole, k_c in all reinjection groups had similar patterns, increasing rapidly in the first 4 h, and the increasing rate gradually decreased. Then k_c started to decline rapidly. The rate of decline gradually decreased, and then it stabilized. In the rapid descent stage, the descent speed first increased and then decreased. The k_c of 25, 35, 45, 55, and 65 °C reinjection groups reached their maximum value at 3.7, 3.9, 4, 4.2, and 4.3 h, respectively. In the first 20 min of reinjection, the higher the temperature of the wastewater, the more drastic the k_c changed. Moreover, in the first 10 min, the k_c of 45, 55 and 65 °C reinjection groups showed negative values, indicating that chemical clogging improved permeability. While the k_c of reinjection groups at 25 and 35 °C was positive, that is, chemical clogging reduced permeability. After 2 days of reaction, k_c of each reinjection temperature group decreased to a relatively stable value. Also, the higher the temperature, the greater the k_c.

Taking the 45 °C reinjection group as an example, it is not difficult to find that if we collected displacement water during the whole reinjection process, only the concentrations of Na⁺, Cl⁻ and will be higher than the original wastewater. Although the concentration of K⁺ increased by 26.5% in 1 h. It was lower than original wastewater for the next 80 h. From the aspect of mineral composition, the minerals containing Na⁺, Cl⁻ and were dissolved during the reinjection. XRD results proved that the contents of albite and montmorillonite decreased by 19.64 and 28.61%, respectively, after reinjection, and both of them contained Na⁺. Similarly, minerals containing K⁺, Ca²⁺, and Mg²⁺, such as potash feldspar and illite, increased by 31.19 and 12.31%, respectively, after reinjection. The calcite was not detected in the original rock samples, but the content increased to 1.13% after reinjection. In general, when the wastewater at different temperatures was reinjected, the reaction inside the rock sample is roughly the same. The increase in temperature will accelerate the rate of chemical reaction, resulting in more precipitation blocking pores.

The chemical clogging is due to the change in hydrogeochemical balance after the reinjection water enters the geothermal reservoir (Brehme et al. 2018). The composition and structure of water quality and reservoir are changed by water chemical reactions such as dissolution and precipitation. There are many influencing factors of chemical clogging. At present, it is considered that the composition of reinjection water, formation water, mineral composition of geothermal reservoir, temperature, pressure, structure, time, and other factors are the controlling conditions of chemical clogging. (Brehme et al. 2019; Song et al. 2020; Zhang et al. 2021; Gan et al. 2022a).

The precipitation of minerals is affected by the temperature, especially the carbonate precipitation. Its solubility decreases with increasing temperature (Pokrovsky et al. 2009; Ma et al. 2017). On the contrary, the solubility of sulfate minerals such as gypsum (CaSO₄) increases with increasing temperature (Pape et al. 2005).

To reduce the impact of chemical clogging, low-temperature wastewater should be used for reinjection (He et al. 2018; Kamila et al. 2021; Gan et al. 2022a). High-temperature wastewater can only dissolve some minerals in the reservoir at the beginning, temporarily improving the permeability of the reservoir. In the end, due to higher temperature, the chemical reaction in the reservoir is accelerated, which will reduce the reservoir permeability more. In addition, Ca²⁺ and Mg²⁺ can be removed before reinjection, which can avoid the decline of permeability caused by the production of potash feldspar, illite, and calcite.

In this paper, the permeability evolution of weakly consolidated sandstone in the process of wastewater reinjection at different temperatures was analyzed through reinjection and hydrochemical experiments. The main conclusions are as follows:

• The permeability of the rock samples was 63.821 mD, and the permeability decreased due to chemical clogging at the end of reinjection. It decreased faster in the first 2 days. After 100 h of reinjection, the permeability of rock samples C1–C6 decreased to 15.39, 13.64, 11.68, 10.71, 7.87 and 5.68%, respectively.

• In this paper, we used the permeability of wastewater reinjection group minus the permeability of the control group at a certain moment as chemical clogging permeability k_c to express the degree of chemical clogging during reinjection. If the wastewater temperature reached 45 °C or higher, the chemical clogging permeability will be negative in the first 10 min, so that the reservoir permeability can be temporarily improved. After that, the chemical clogging permeability became positive, and the higher the temperature, the more clogging will eventually be caused.

• Compared with the original wastewater, the concentration of Na⁺, Cl⁻ and in the displacement water increased, while the concentration of K⁺, Ca²⁺, and Mg²⁺ decreased finally. It is suggested that the chemical clogging of reinjection is mainly caused by potash feldspar and illite, calcite, and other carbonate precipitation. As the temperature increased, the chemical clogging in rock samples became worse. To reduce the influence of chemical clogging, it is recommended to adopt low-temperature reinjection for the low TDS of wastewater in the Lubei area, and reducing the concentration of Ca²⁺ and Mg²⁺ in wastewater before reinjection is also beneficial for reducing chemical clogging.

This research work was supported by National Key R&D Program of China (Grant Number 2019YFB1504201).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.