One of the present day challenges in the oil industry is the proper characterization of in situ reservoir rock properties prior to full scale development. In this respect, experimental data acquisition requires the simulation of representative reservoir conditions in the laboratory. This paper presents details of a novel multi-sample rock testing system and some results of resistivity measurements on 8 sandstone and 4 synthetic rock samples at simulated reservoir conditions. The combined effect of pressure and temperature on formation resistivity factor (FRF) and the Archie cementation factor (m) is also reported. For the sandstone and the synthetic rock samples tested, it is concluded that resistivity increases with pressure and temperature. A pore space network model has been used to gain an insight into the mechanisms of pressure effect on rock properties.
Fatt also concluded that laboratory measurements of formation resistivity factor, in which only external pressure is varied, are sufficient to study the effect of overburden pressure. Similar results were also reported by Wyble (1958) in the study of the effects of pressure on FRF, permeability and porosity. An increase of Archie cementation factor together with formation factor with pressure was pointed out by Wyble. Glanville (1959) showed greater effect of pressure on FRF and porosity in less porous, less permeable rocks. The reports of Brace et al (1965, 1968) and Timur et al (1972) also confirmed the significant increases in FRF by net confining pressures of the same magnitude as in reservoirs. A review of the studies of the electrical properties at high pressures was given by Parkhomenko (1982) showing that the electrical resistivity at high pressures is related to other rock properties such as porosity, permeability, rock structure and pore geometry. The effect of temperature on FRF has been investigated by Hilchie (1964) using consolidated sandstone and limestone samples. It was indicated that as the temperature increased while the net confining pressure remained constant, the FRF went through a minimum after which it increased. According to Hilchie, this significant change of FRF with temperature was a function of the percent of small pores of radius less than 0.5 micron and the clay content. Helander and Campbell (1966) presented their results of resistivity measurements on six synthetic bead cores and one synthetic sand core bonded by an epoxy resin at varying conditions of pressure up to 69 MPa and temperature up to 160 °C. A definite trend of change of FRF with pressure was noticed due to the changes of rock tortuosity, pore constriction and the change of the electrochemical double layer. The effect of temperature on FRF was not as well defined as the pressure effect because of the much greater experimental difficulty. Sanyal (1971) reported results of FRF measurements at different temperatures to a maximum of 149 DC using three sandstone and one limestone samples. A decrease in apparent formation factors with increasing temperature was found experimentally by Waxman and Thomas (1974) and was attributed to the effect of clay caution exchange capacity.
