Microresistivity logs

Microresistivity devices measure the resistivity of the flushed zone and delineate permeable beds by detecting the presence of mudcake.

Basics

When invasion is moderate to deep, knowledge of resistivity of the invaded zone (R_xo) is required to derive the resistivity of the uninvaded zone (R_t) from the deep-resistivity measurement. To evaluate a formation with logs, the R_xo/R_t ratio is required for some saturation-estimation methods. In clean formations, a value of the formation resistivity factor F can be computed from R_xo and R_mf if S_xo is known or can be estimated.

Tools designed to measure R_xo have a very shallow depth of investigation, because the flushed zone may extend only a few inches beyond the borehole wall. To avoid the effect of the borehole, a sidewall-pad tool is used. The pad, carrying an array of closely spaced electrodes, is pressed against the formation to minimize the short-circuiting effect of the mud. Currents from the electrodes on the pad must pass through any mudcake to reach the flushed zone.

Microresistivity readings are affected by mudcake; the effect depends on the mudcake resistivity and thickness (h_mc). Mudcakes are usually anisotropic, with the resistivity parallel to the borehole wall lower than the resistivity across the mudcake. This increases the mudcake effect on microresistivity readings to make the effective, or electrical, mudcake thickness greater than the physical thickness indicated by the caliper.

Microresistivity measurements have evolved from the first microlog, through the obsolete microlaterolog and proximity-log devices, to the current MicroSFL and Platform Express MCFL microresistivity measurements.

Microlog

The microlog is still used qualitatively for its ability to detect permeable intervals with a fine vertical resolution, but not for the evaluation of R_xo. The measurement comprises two short-spaced devices with different depths of investigation, providing resistivity measurements of small volumes of mudcake and formation adjacent to the borehole. The presence of a mudcake, identified by a "positive" separation of the two curves, indicates an invaded and, therefore, permeable formation. As a qualitative log, the microlog is usually presented on a linear grid.

Principles

The flexible oil-filled microlog pad is pressed against the borehole wall by arms and springs. The face of the pad has three small in-line electrodes spaced 1 in. [2.5 cm] apart. The electrodes record a 1×1-in. "microinverse" log and a 2-in. "micronormal" log simultaneously.

In an invaded permeable zone, R_mc is usually significantly lower than R_xo. The 2-in. micronormal device has a greater depth of investigation than the microinverse. It is, therefore, less influenced by the mudcake and reads a higher resistivity when mudcake is present. In impermeable formations, the two curves read approximately the same resistivity or have a small negative separation, and the resistivities are usually much greater than in permeable formations.

Positive separation occurs in a permeable zone. Although the microlog curves identify permeable formations, quantitative inferences of permeability are not possible.

Under favorable circumstances, R_xo values can be derived from microlog measurements using charts provided by the service companies. R_mc values for this purpose can be measured directly or estimated from other charts, and h mc is obtained from comparing the caliper curve to bit size. The limitations of the method are as follows:

1. The ratio R_xo/R_mc must be less than approximately 15 (porosity more than 15%).
2. The value of h_mc must be no greater than 0.5 in. [1.3 cm].
3. Depth of invasion must be greater than 4 in. [10 cm]; otherwise, the microlog readings are affected by R_t.

MicroSFL log

The MicroSFL (MSFL) device is a pad-mounted spherically focused logging sensor with two distinct advantages over the microlaterolog and proximity tools it replaced. Unlike these earlier R_xo devices, the MSFL tool is combinable with other logging tools, which eliminates the necessity of a separate logging run to obtain R_xo information. The MSFL log also performs better in shallow invaded zones in the presence of mudcake.

Fig. 1 shows the electrode arrangement (right) and current patterns (left) of the MSFL tool. The surveying current flows outward from a central electrode, A0. Bucking currents, passing between electrodes A0 and A1, flow in the mudcake and the formation. The measuring current, I₀, is confined to a path directly into the formation, where it quickly spreads and returns to a remote electrode. By forcing the measure current to flow initially directly into the formation, the effect of mudcake resistivity on the tool response is minimized, yet the tool still has a very shallow depth of investigation.

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  Fig. 1 – Pad layout of the MicroSFL tool.

Synthetic microlog curves (microinverse and micronormal) can be computed from MSFL parameters, because the measure current sees mostly flushed zone and the bucking current sees primarily mudcake.

Environmental corrections

Although the influence of mudcake on the readings is relatively small, MSFL measurements must be corrected for thickness. Mudcake thickness is normally deduced from a comparison of the actual borehole size, as measured with the caliper, to the known bit size.

MCFL log

The Schlumberger MCFL microresistivity measurements made with Platform Express tool strings are different from previous measurements in several respects:

1. Electrodes are mounted on a rigid, mostly metal pad that is not deformed by the borehole wall, allowing a more consistent standoff measurement.
2. Survey currents are independently focused in planes parallel and perpendicular to the tool axis, reducing the sensitivity to borehole geometry.
3. Three measurements are made with different depths of investigation, which allows a more reliable resolution of mudcake and formation properties with independent response equations.
4. The microresistivity sensors are interlaced with the density sensors, so both measurements sample the same volume of formation at the same time.

A vertical resolution of the raw measurements better than 1 in. [2.5 cm] is achieved, and R_xo, R_mc, and h_mc are solved simultaneously. Two curves can be displayed in a microlog-like presentation. When the two curves are superimposed, they both read R_xo. Any separation indicates pad standoff from the formation, which is usually caused by mudcake and indicates a permeable formation.

Because R_xo, R_mc, and h mc are obtained directly from the Platform Express microresistivity measurements by inversion processing, no mudcake thickness corrections are required. The values of R_xo can be used directly with medium and deep-resistivity measurements (or array-resistivity measurements) to derive R_t.

Nomenclature

R	=	resistivity (ohm•m)
Rann	=	resistivity of the annulus
Rh	=	resistivity in the horizontal direction (ohm•m)
Rm	=	resistivity of the mud column (ohm•m)
Rmc	=	resistivity of the mudcake
Rmf	=	resistivity of the mud filtrate
Rxo	=	resistivity of the invaded zone
Rt	=	resistivity of the uninvaded formation
Rv	=	resistivity in the vertical direction (ohm•m)
Rw	=	resistivity of the formation connate water (ohm•m)
Rwa	=	apparent water resistivity from deep resistivity and porosity
Sxo	=	water saturation of the invaded zone

Noteworthy papers in OnePetro

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External links

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See also

Resistivity and spontaneous (SP) logging

Electrode resistivity devices

Formation resistivity determination

Spontaneous (SP) log

Well log interpretation

PEH:Resistivity_and_SP_Logging