Formation Factor Determinations byIn-SituResistivity Logging

Abstract

ABSTRACTMatrix diffusion in bedrock is traditionally studied by laboratory liquid diffusion experiments, which are time consuming and expensive. A new way of studying matrix diffusion is to measure the electrical resistivity of the rock. This could be done either in laboratories orin-situ. A fast method of obtaining a formation factor log, later used in matrix diffusion calculations, for an entire borehole is proposed.It is a standard procedure in geophysical well logging to measure rock resistivity and there are well-developed tools for this in the oil industry. The SKB, Sweden, uses boreholes with a small diameter (56 mm) and this reduces the options in choosing resistivity tools. Therefore they have so far relied on the Normal log that only gives quantitative measurements in special cases, after corrections are made. Modern tools, such as the slimhole Dual-Laterolog, are accurate with a high vertical resolution.The pore fluid resistivity is required when obtaining a formation factor log. In previous work the borehole fluid resistivity has been used. A new method measures the resistivity in groundwater from local fractures. These values seem more appropriate to use as newin-situmeasurements show that there may be local groundwater resistivity differences.A preliminary study shows that the conductivity could be obtained in saline Swedish groundwater by measuring the chloride concentration only.Anomalies in the formation factor log have to be sorted out by using supporting core logging and non-electricalin-situmethods.