SEM+ EDS were performed on polished sections obtained from the core plugs after epoxy resin cementation under vacuum conditions, so that open pore spaces would be filled and clay cement immobilized, enabling it to observe the original texture in polished sections. Back scattered electron (BSE) images were obtained on a Jeol JXA 8600 Superprobe instrument (W filament, 20 kV accelerattion voltage and 20 nA prob current, carbon coating). EDS spectra were recorded in standard less mode (RemiX Si- drift detector, C- U detection range) with 60 sec collecting time (15 % dead time), quantified by PAP correction. 

Table 1: Mineralogy of selected sandstone cores

The petrophysical measurements and core flood tests were conducted at the Research Institute of Applied Earth Sciences, University of Miskolc. First, porosity measurements using He-porosimetry were performed prior to the initial absolute and effective permeability tests, and the effective permeability of the chosen samples were determined by the help of application of Hassler type core holder. Whereasthe outcome was precisely 5% of NaCl brine saturation. After that, the selected cores were placed in a Hassler- type core holder to start the core flood test (Fig. 1), and the flooding process started by the injection of selected solutions (pH 9 and pH 11). Injection rates were  (50, 100, 200 ml/h) and was done at the ambient condition (room temperature and atmospheric pressure with a sleeve- confining pressure 25 bars, no back pressure were applied during the experiments- table 2).  An alkaline solution (KOH) was prepared based on the NaCl saline, the purpose of investigating the impact of flow rate and pH on permeability reduction.

Figure 1: Experimental core flooding testing set up

 Table 2: Porosity, permeability and core flood data of the selected core samples

 3. Results and Discussion

The reduction in permeability occured due to the release of fine particles within the porous media in selected sandstone cores, then the migrated fine particles can cause the plugging of the interconnected pore channels because the pore throats are narrower than the migrated fine particles as a result reduced permeability. According to the XRD and SEM analyses the migrated fine particles are mainly composed of kaolinite, illite, chlorite and fine partciles of non- clay minerals. Wilson et al. (2014), stated that kaolinite and illite particles might reduce permeability by fine migration ⁽¹¹⁾. The obtained result shows that  detected clay minerals (kaolinite, chlorite and illite) have a positive impact on the permeability as a result of blocking the ineterconnected pore throats by dispersed fine particles within the porous media in selected sandstone cores.

As mentioned in Table 2, the result of these experiments show that selected sandstone core with a higher alkaline fluid in the porous medium and a higher flow rate in the internal clay minerals is less stable relative to low alkaline conditions, nevertheless; the degree of formation damage is more severe.  Permeability damage due to fines migration is a major concern in reservoir processes such as water flooding ⁽¹²⁾.Changes in pH and flow rate caused significant production of fines, bridging and blocking by fines. Moreover, severe damage occured when the selected sandstone core flushed with high pH and higher flow rate at pH 11 and 200 ml/ h the percentage of damage was more than 50%, while at pH 9 and the same flow rate was less severe around 14%. Priisholm et al., (1987) and Gunter (1994) observed that changes in pH can result in fine migartion^(13,14).

Magnitude of damage is believed to be a function of amount of clay minerals particularly migarted clay minerals such as kaolinite, illite and chlorite that contribute with fine migartion and permeability impairment. Mohan, et al., (1993) experimentally showed that damage due to fine particles migartions (kaolinite) does not become apparent till a high pH of  (> 9) in Steven sandstone ⁽¹⁵⁾. Tchistiakov, (2000) investigated that the declination in permeability by 20%  occured vividly after switching from 0.01 M NaCl solution to distilled water in Bentheim sandstone (Western part of Germany) ⁽⁸⁾.  Generally, several scholars have suggested that migration of fine particles of kaolinite, is caused by decreasing salinity of the injected solution (NaCl), which can drop permeability by about 90% in Berea sandstone samples ⁽¹⁶⁾.

 4. Conclusion

     This study concluded that XRD and SEM analyses show the majority of samples composed of quartz and clay minerals such as kaolinite, illite and chlorite which could generate fines migration during core flooding. In addition, mobilizing of fine particles within the porous media causes a drastic decline in reservoir permeability. Moreover, magnitude of permeability impermeant increases with the increase of flow rates. Also, the percentage of damage increases with the increase of degree of alkalanity. Furthermore, both parameters, flow rate and alkalinity have a positive impact on permeability reduction but alkalinity has a more severe influence than the flow rate.

 Acknowledgements

     GINOP-2.3.2-15-2016- 00010 “Development of enhanced engineering methods with the aim at utilization of subterranean energy resources” project of the Research Institute of Applied Earth Sciences of the University of Miskolc in the framework of the Széchenyi 2020 Plan, funded by the European Union, co-financed by the European Structural and Investment Funds. The authors wish to thank Délia Dudas for sharing her unique assistance with the XRD and SEM samples preparation.