Electrical resistivity measurments of ch 4 hydrate-bearing sandstone during formation

Resistivity measurements are acquired during and after drilling wells through hydrate‐bearing intervals and are often used in correlation with other logs to determine gas hydrate concentrations. In order to properly interpret the acquired data reliable calibration data are needed. Resistivity changes during hydrate formation can also give insights into how and where the hydrate forms in the pore space, and can be used to build permeability correlations. Electrical resistivity measurements were performed on sandstone cores during methane hydrate formation under differing conditions of initial water saturation and brine composition. A two-electrode sample holder was designed to fit inside a magnetic resonance imager in order to take advantage of its monitoring capabilities. Hydrate saturation was determined from the quantity of liquid water still present in the core. A four-electrode system was used without imaging possibilities, and the agreement between the two setups was good. R0 was measured on this system using a range of salinities. The two-electrode system worked well during hydrate formation with little variation in phase angle that result from changes in electrode contact with the sample. In all cases, an initial decrease in resistivity was observed at the beginning of hydrate formation. This was likely due to ion exclusion during hydrate formation that initially reduced the brine’s resistance. This effect was less pronounced at higher initial water salinity values. As hydrate continued to form, reduced pore connectivity and increased tortuosity through the core led to a significant increase in resistivity. Resistivity values were higher than what have been reported by others. The interpretation of Archie’s Formation Factor and Resistivity Index was problematic. The saturation exponent was found to be lower than expected in all of the experiments performed. Dynamic Rw and R0 values were incorporated into Archie’s equations to account for changes in brine composition during hydrate formation. Based on this study we propose that Archie’s basic model needs to be modified to account for chemical changes during hydrate growth.