where m and n are factors determined experimentally. Archie (1942) found the value of m varies between 1.8-2.0 for consolidated sandstone and 1.3 for clean unconsolidated sandpack. The value of n appears to be close to 2 (Archie 1942). Several studies have found more extensive range for factor m: 1.90-3.4 (Lucia 1983); 1.72-4.14 (Verwer, Eberli and Weger 2011); 1.8-2.3 (Ramakrishan et al. 1998) and factor n 0.16-1.02 (Taylor and Barker 2006).
Several studies were carried out to understand the electrical resistivity affected by fluid saturation level (Alvarez 1973; Yan, Miao and Cui 2012). Attia, Fratta and Bassiouni (2008) performed that the electrical resistivity of Berea sandstone and Limestone increases as the brine saturation decreases. Taylor and Barker (2002) presented a measurement technique for poorly cemented sandstone, and the results showed that the electrical resistivity decreases significantly in the saturation region below the critical water saturation at approximately 25%. Roberts and Lin (1997) measured the electrical resistivity of saturated Topopah Spring tuff. Water imbibition was carried out in three ways, i.e. by moisture from the air, storing samples in the humidity chamber, and adding water to the sample physically. The results of the measurements denoted that water was interconnected at approximately saturation level of 35%. Knight (1991) reported that resistivity-saturation trends were affected by ‘fluid geometry’ in which at low saturation, the resistivity decreases rapidly. On the other hand, at the highest saturation, the resistivity gradually decreasing with increasing fluid saturation.
A number of studies were also conducted to evaluate the effect of pore structure (Suman and Knight 1997; Muller-Huber, Schön and Börner 2015) and grain structure (Jackson, Smith and Standford 1978; Torskaya et al. 2014) on electrical resistivity. Muller-Huber, Schön and Börner (2015) investigated the effect of granular pore radius on formation factors (ratio of effective resistivity to fluid resistivity). Pore geometry is expressed as the ratio of pore throat radius and pore body radius. The results showed that electrical resistivity is strongly influenced by pore geometry. Jackson, Smith and Standford (1978) presented the relationship between the particle shape of sand and the electrical resistivity of rocks. Based on the result, the shape of the particle affects the formation factors, in which decreasing sphericity of the grain produces a higher formation factor. Torskaya et al. (2014) modelled rock grain shape with various rock parameters such as grain volume, grain type, grain distribution, and grain size. The formation factor of the rock grain model was calculated where the higher factor cementation of samples, the higher formation factor generates. 
A more recent work by Dong et al. (2018) modelled spatial fluid distribution in digital rock pores. There were three distribution models, i.e. adhesive type, cemented type, and scattered type. The results pointed out a different resistivity pattern for each type of distribution. Based on the results obtained by Dong et al. (2019), the spatial distribution of fluid in the pore is indicated to affect electrical resistivity. Further observations are required to analyse the effect of spatial fluid distribution on electrical resistivity. In this study, electrical resistivity measurements were conducted on fluid injected loose sand with different grain sizes in which fluid injection processes were carried out at several different locations to obtain fluid spatial distribution variations.  

2. SAMPLE DESCRIPTION AND EXPERIMENTAL PROCEDURE

 2.1 Sample description

The loose sand used in this study is unconsolidated sand from Ngrayong, Central Java, generally composed of quartz (Niyartama, Fauzi and Fatkhan 2017). The samples were sieved by the USA Standard test sieve (ASTM E11). The samples were dried in the oven first at a temperature of 120o C for 24 hours. Subsequently, the samples are contained in a cylindrical glass, with the height of 10-cm-and the diameter of 3.4-cm. Nine sand packs were used for electrical resistivity measurement. The physical properties of the samples are listed in Table 1.
There are three rock grains sizes (Samples A, B, and C) in which from each group, three relatively similar samples were made. The porosity of the samples is obtained through equation (2)