Array Induction Tools (AIT)

The Array Induction Imager Tools (AIT, Fig.2.27) accurately measure open borehole formation conductivity as a function of both well depth and radius into the formation at different borehole conditions and environments. Various tools cater to special operating environments, including slim wells and hostile, high-pressure, Fig. 2.26 Illustration of the conductivity and resistivity curves

2.3 Resistivity Logs 41

high-temperature (HPHT) environments. Wireline array induction tools use an array induction coil that operates at multiple frequencies. The distance between central coils of AIT tool ranges from several inches to dozens inches, for example, 6″, 9″, 12″, 16″, 21″, 27″, 39″, and 72″. The AIT tool has three operation frequencies, 25, 50, and 100 kHz; and it can provide three different vertical resolutions, 1′, 2′ and 4′, five different radial investigations with 10″, 20″, 30″, 60″, and 90″.

The Applications of AIT can be summarized as the following:

• Reservoir delineation

• Determination of true formation resistivity Rt

• Determination of Sw

Fig. 2.27 Coil layout for the high-resolution array induction tool

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• Hydrocarbon identification and imaging

• Determination of movable hydrocarbons

• Invasion profiling

• Thin-bed analysis

2.3.5 FMI (Full Borehole Microresistivity Image Logging)

Since the mid-1980 there has been an explosive development imaging technology, principally in terms of tools but also in terms of producing the image. The progress has been linked with availability of downhole digitization of signals and the pos-sibility of transmitting large data volumes in real time. Where the standard logs are sampled every 15 cm(6″), image logs may sampled every 0.25 cm(0.1″); where the standard logs have one measurement per depth point, image logs may have 250, the region of 200 kbits per second transmitted up the cable to achieve a collection rate of 60,000 samples per meter of borehole. Imaging technology is still evolving rapidly and is affecting the entire loggingfield. Table2.2lists typical image tools of some international oilfield service company.

Coils 1 are two identical 3-coil arrays equidistant either side of T. Coils 2–6 are 4-coil arrays. Not all bucking coils are shown. (From Beste et al.) (Right) Coil layout for the AIT-H tool. m are the main coils and b the bucking coils. In most cases the bucking coils are co-wound with the main coils of the next smallest array.

The spacings of the main receivers increase approximately exponentially from 6 in.

Coil spacings are given in inches.

In the mid-1980s, Schlumberger introduced theirfirst electrical imaging tool, the Formation MicroScanner (FMS), as an evolution of their SHDT dipmeter. Thefirst tools only provide as image of 20 % of an 8.5″ borehole, using just two pads. Since

Table 2.2 Imaging tools of different company

Company Symbol Name Description

Schlumberger FMI Fullbore Formation MicroImager (current tool 1996) Halliburton EMI Electrical MicroImager 6 independent pads

150(6 25)electrodes Western

Atlas

*STAR Simultaneous Acoustic and Resistivity (Borehole image)

6 independent pads 150(6 25)electrodes

2.3 Resistivity Logs 43

then there has been steady progress in borehole coverage. The present tool, the Fullbore MicroImager provide nearly 80 % coverage in an 8.5″ borehole of high quality images.

FMI consists of four arms, the adjacent two arms are orthogonal and each arm has two pads (Fig.2.28), the each pad has a hinged flap. Both pad and flap have arrays of 24 electrodes.

The FMI provides an electrical borehole image generated from up to 192 microresistivity measurements. Applications of FMI are listed below:

• Structural geology

• Structural dips, even in fractured and conglomeratic formations; detection and determination of faults.

• Sedimentary features

• Determination of sedimentary dips and paleocurrent directions

• Definition and characterization of sedimentary bodies and their boundaries

• Recognition of anisotropy, permeability barriers, and permeability paths

• Recognition and evaluation of thinly bedded reservoirs

Fig. 2.28 Schematic of the configuration of FMI tool

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• Rock texture

• Qualitative vertical grain-size profile; Determination of carbonate texture

• Detection and evaluation of secondary porosity

• Detection and evaluation of fracture systems

• Complement to coring and formation tester programs

• Depth matching and orientation for whole cores.

Figure2.29 shows that FMI provided image of different formation conditions, for (a), carbonate zone with facture on the wall of well, because of the mud invasion into the fracture, the resistivity decreased, and then a relative dark block corre-sponding to the fracture depth. (b) also shows features of fracture formation, nearly parallel to each other of the waves implying fracture occurring in the formation. The dark color in image (c) indicates shale or clay zone with resistivity. Nonuniform distributed dark color spots in the image (d) shows that a lot of caves and vugs existed on the sidewall of well.

2.3.6 ARI (Azimuthal Resistivity Imager)

The ARI azimuthal resistivity imager makes directional deep measurements around the borehole with a higher vertical resolution than previously possible from con-ventional laterolog tools. Using 12 azimuthal electrodes incorporated in a dual laterolog array (as shown in Fig.2.30), the ARI tool provides a dozen deep-oriented resistivity measurements while retaining the standard deep and shallow readings.

A very shallow auxiliary measurement provides data to fully correct the azimuthal resistivities for borehole effects.

The ARI tool is combinable with a wide variety of other tools and is particularly effective for thin-bed evaluations in combination with the IPL integrated porosity lithology tool string.

ARI images also complement images from the UBI ultrasonic borehole imager or the FMI full borehole formation microimager because of the tool’s sensitivity to features beyond the borehole wall and its lower sensitivity to shallow features.

Table 2.3 Properties of FMI

(c) (d)

(a) (b)

Fig. 2.29 FMI image for different formations. a FMI image of formation, b FMI image of fracture formation, c FMI image low resistivity shale zone, d FMI image of pors and caves

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Applications of ARI

Resistivity logs and images with vertical resolution of less than 1 ft Thin-bed analysis

Fracture identification and characterization Evaluation in deviated and horizontal wells Evaluation of heterogeneous reservoirs.