[20/03/2007] RESISTIVITY MEASUREMENTS - GENERAL

The ability of a material to conduct or resist the flow of electrical current is an extremely useful property in evaluating well formations. Most reservoir rocks consist of non-conductive minerals. These matrix pore space, if any, will the filed with non-conductive hydrocarbons and / or conductive formation water. As a result, formation resistivity is related to the amount of water or fluid in the formation pore space.

Unfortunately measurement of the true formation resistivity is complex Resistivity measurements, which are taken from the borehole, are affected by the invasion of drilling mud filtrate into permeable formations. This invasion can be very shallow to very deep. There for, it is necessary to use several resistivity tool measurements, with different depths of investigation to accurately determine the true resistivity of the formation. As a result, most resistivity tools are designed to take several resistivity measurements simultaneously.

By measuring formation resistivity at several different depths of investigation a correction may by made to the deepest measurement to correct for the effect of mud filtrate invasion. This corrected measurement is close to the actual resistivity of the undisturbed formation resistivity. An estimate of the depth invasion may also be obtained. The adjacent chart was constructed to apply this principle to the resistivity measurements obtained with the Dual Induction Laterolog (DIL) instrument. Other charts are available to correct for invasion effect with the Dual Laterolog MSFL instrument as used in borehole containing a saline mud system.
 

Dual induction tools are usually selected for low resistivities and fresh mud while the Dual Laterolog is selected for high resistivities and salt mud. When the ratio of mud filtrate resistivity to formation water resistivity is 2.5 or greater, the Induction tool is preferred. When this ratio is approximately equal to 2.5, it may be necessary to use both the Induction and Laterolog for proper evaluation.

Microresistivity devices are used to find resistivities in the flushed zones near the borehole. In addition to many other uses, this data can be used to correct invasion effects for Induction and Laterolog tools.

Resistivity Logs - Resistance, Resistivity, Conductivity,

Resistivity (or electrical logs) logging is basically the concept where an ohm­meter circuit measures the impedance to the flow of electrical current though a material, The resistance (r) will depend upon the length (L), the cross sectional area (A), and the resistivity (R) of the sample. Stated mathematically r = R L/A. Formation resistivity is a basic property of material, just as the color, smell, density, and hardness are. Resistivity, can then be shown as R = r A/L.

The resistivity devices currently used by Century at this time are indicated below:

Resistivity                           

Rxo = Resistivity of the flushed zone

Rt = Resistivity of uninvaded zone

Rm = Resistivity of Mud

Guard Resistivity

The Guard Resistivity logs are another type of resistivity device. A small measure electrode, with large guarding electrodes above and below the measure electrode, focus the current path into the formation (Figure 6). The guard electrodes and measure electrode are all maintained at the same electrical potential. The tool circuitry compares the measure electrode current with that of the guard electrodes current which is adjusted to maintain this equal potential.

 Guard Resistivity System

In this method, the signal of the measure current is maintained at a precise magnitude. The voltage at the measure electrode varies in response to the changing lithology as described below:

Em = KiRg

Where,

Em = signal voltage at the measurement electrode.

Ki = the tool coefficient as determined by geometrical factors and excitation current Im in Amps/Meter

Rg = the apparent formation resistivity in ohm­meters measured by the guard system

The system therefore measures Em, which is in a direct proportion to the measured parameter Rg. The guard resistivity has some distinct advantages over other resistivity devices. Number 1, is the focusing of the beam which allows for thinner beds to be recognized and evaluated. Number 2, bed boundaries are sharper and more defined, but in some cases, the thickness of the formation using the guard log results in a smaller thickness than the actual formation.

Our resistivity tools base on this concept. Deep Guard Resistivity and Short Guard Resistivity placed in different tool. Deep Guard Resistivity placed in combination with CNT 9073 tool, Short Guard Resistivity placed in combination with CDT 9039 tool.

DEEP GUARD RESISTIVITY (DG)

CNT 9073 with Deep Guard Resistivity

Specifications
 

Offset Parameter

Operation Parameter

CDT 9039 with Short Guard Resistivity

Specifications
 

Offset Parameter

Operation Parameter

SPONTANEOUS POTENTIAL LOG (SP)

The SP records naturally occurring electrical potentials in the borehole. The SP is usually run in combination with other tools in an open-hole where the salinity of the drilling fluid differs from that of water in the formation.

Measurements of the small “millivolt” potential between the electrode on the sonde and a surface electrode generally show two useful characteristics – a straight line opposite impermeable strata when the salinity of the formation water is greater that the salinity of the drilling fluid. These variations give qualitative information concerning permeability, but not quantitative information since there is no definite correlation between the amplitude of the curve and the degree of porosity or permeability of the formation.

The principle uses of the SP curve are:

·         To determine a value for formation water resistivity (Rw).

·         To correlate formations from well to well or log to log.

·         To locate bed boundaries.

·         To indicate permeable beds.