Resistivity logs are electric logs that measure a formation’s resistivity, that is, resistance to the passage of an electric current. The key important use of resistivity logs is the determination of hydrocarbon vs. water-bearing zones of a formation. There are three categories of resistivity measurements: Laterologs, Induction logs, and micro-resistivity (Rxo) measurements. The resistivity logs are usually presented in a logarithmic scale. The induction and laterologs both measure the resistivity of the uninvaded zone of the formation while the micro-resistivity logs measure the resistivity of the formation very close to the borehole, in the zone that has been flushed by the drilling fluid.
Because the rock’s matrix or grains are non-conductive, the ability of the rock to transmit a current is almost entirely dependent on water in the pores. And because hydrocarbons, like rock’s matrix, are non-conductive, as hydrocarbon saturation of the pores increases, the rock’s resistivity also increases (fig.4).
With the knowledge of the formation’s water resistivity, porosity, the formation’s water saturation (Sw) can be determined from the Archie equation.
Sw = (a*Rw/Øm*Rt)1/n
Once the water saturation is known the hydrocarbon saturation can be determined thus:
Shc = (1.0-Sw)
It is worthy of note to state here that resistivity tools behave differently in reservoirs and non reservoirs.
Because non reservoir rocks like shale, anhydrite, salt like permeability, there is no invasion of mud filtrate in the formation. Consequently, all the three resistivity devices will read the same resistivity.
For porous and permeable reservoirs, mud filtrate () will invade the zone close to the wellbore, thereby replacing all the water resistivity and hydrocarbons, if present.
The resistivity logs are generally classified on the basis of the depth of investigation which they measure, and are presented below.
The LaterologThe Laterolog is an electrode log that measures the true formation resistivity (Rt) in boreholes filled with saltwater muds (where Rmf~= Rw). Laterolog was actually introduced to cope with conditions of salty mud and high formation resistivity. A current from the surveying electrode is forced into the formation by focusing electrodes. This focuses the measuring current into a sheet to obtain the best tool resolution (fig.). The focusing current can be adjusted so that the tool measures both the deep resistivity and the shallow resistivity (fig.)
The effective depth of Laterolog investigation is controlled by the extent to which the surveying current is focused. Deep reading laterologs are therefore more strongly focused than shallow reading laterologs.
Laterolog can be influenced by invasion, but because resistivity of the mud filtrate is approximately equal to the resistivity of the formation water when a well is drilled with saltwater-based mud, invasion does not strongly affect Rt values derived from laterolog.
The Induction LogFocused induction logs have proven to be the best method for obtaining formation resistivity in wells drilled with fresh mud, air or oil base mud.
Induction logging instruments are composed of transmitter-receiver coil pairs. The number of coils and spacing of these coils determine the depth of investigation, borehole response and the resolution of the instrument.
The modern induction log is called the Dual Induction Focused and it can measure both deep and shallow resistivity reading. The Dual Induction Focused Log is used essentially in formations that are deeply invaded by mud filtrate. Because of deep invasion, a deep reading induction log (RILd) may not accurately measure the true resistivity of the formation. The Dual Induction logs should be corrected for borehole, bed thickness and invasion effects if three curves are present. Resistivity values obtained from the three curves on a Dual Induction Focused Log are used to correct deep resistivity (RILd) to true resistivity (Rt) from a Tornado chart (Fig.).
The induction system works best where the undisturbed formation has lower resistivity than the invaded zone (this is typical of logging in a fresh mud system).
The Micro-resistivity Logs (MFSL)Both the Laterolog and the Induction logs give two of the three independent resistivity measurements, a deep and an intermediate reading. A shallow resistivity reading is normally provided by the micro-resistivity Logs. The most widely used is the micro-spherically focused log (MSFL). The Micro-resistivity Logs (MFSL) are essentially used to measure resistivity of the flushed zone (Rxo), and to delineate permeable beds by detecting the presence of mudcake. The Micro-resistivity logs (MFSL) are characterized by short electrode distances (a few inches), which permit a shallow depth of investigation, subsequently providing a value of the resistivity of the flushed zone (Rxo).
INTERPRETATION OF RESISTIVITY LOGSThe resistivity readings should be corrected for borehole effects and thin bed effects, if required. Charts are always available to determine the true formation resistivity from the three resistivity measurements at different depths of investigation (e.g. the Schlumberger chart book) (fig. resistivity readings over oil and water bearing intervals).
In hydrocarbon –bearing intervals there is a clear separation between the curves. Close to the borehole the resistivity is low due to the presence of mud filtrate. But deeper into the formation the resistivity increases, which clearly suggests the presence of hydrocarbons. Below the hydrocarbon-water contact no separation is normally observed between the curves. They all read low resistivities. This indicates that even deep in the formation where the mud filtrate did not penetrate the resistivity is still low. This confirms that the interval is water-bearing. For quick look evaluation, the deep resistivity reading is usually taken to be the true formation resistivity. This ignores the influence of the invaded zone, which tends to make the reading of the deep resistivity tool to read lower than the true formation resistivity.
Asquith G.A. & Gibson C.R. (1983): Basic Well Log Analysis for Geologists
Dresser Atlas (1982): Well Logging and Interpretation Techniques
Krygowski D.A. (2003): Guide to Petrophysical Interpretation
Rider, Malcolm (1996): The Geological Interpretation of Well Logs
Schlumberger (1998): Log Interpretation Principles/Applications
Shell Nigeria Graduate Training Programme: Petrophysics
American Association of Petroleum Geologists (AAPG) www.aapg.org
Society of Exploration Geophysicists (SEG) www.seg.org
Society of Sedimentary Geologists (SEPM) www.sepm.org
Nigerian Association of Petroleum Explorationists (NAPE) www.nape.org.ng
Nigerian Mining and Geosciences Society (NMGS)