Resistivity Logging of Fractured Basalt

Authors: V. Stefansson, G. Axelsson, 0. Sigurdsson
Conference: Stanford Geothermal Workshop Session: Geophysics and Well Logging
Year: 1982 Language: English
Abstract: The electric log has been used for about half a century as a tool for studying the formations penetrated by a borehole. A t early stage, comprehensive studies of sedimentary rock established the dependence of formation resistivity factor upon porosity. Archie (1942) pioneered this effort by suggesting his well-known empirical formula correlating the formation factor and porosity. Ever since, Archie's law has been a central point in interpretation methods for electrical logs.

Despite the simple empirical dependance of rock conductivity on porosity as expressed by Archie's law, there does not exist a simple theoretical explanation for this phenomenon. Mathematical modeling to prove the validity of an Archie type relationship has been carried out by Greenberg and Brace (1969), Shankland and Waff (1974) and Hadden (1976). However, all models depend on simplistic geometrical assumptions of pore space distributions and the degree of realism can be disputed.

During the last decade, investigations on geothermal reservoirs have accentuated the role of fractures in reservoir physics. Various types of geophysical logs have been applied in order to distinguish between fractured and intergranular reservoirs. One of the strong candidates for that is the electric log (Towle 1962, Aguilera 1974 and 1976). The main reason for that is the fact that the exponent in Archie's law seem to be 1.0 in the case of fractured rock (Brace and Orange 1968), where as a value of 2.0 seems to be valid for non-fractured rock (Brace 1965).

In this paper a simple lumped double porosity model is studied in order to Further, the results of resistivity and porosity logging in Icelandic basalt is estimate the effects of fractures on the resistivity-porosity relationship. presented, and it is shown that the distribution of porosity in these rocks are dominated by fractures.
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