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8/6/2019 ion en Ingles

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New Applications for Resistivity Tools

With advances in logging technology, the leading oilfield technology

companies have developed an impressive array of tools for a broad range

application. In this section, we will discuss a variety of high resolution

resistivity tools.

Resistivity Imaging Tools

Resistivity imaging tools were introduced during the mid-1980s, as an

outgrowth of dipmeter technology. These tools utilize four to six independent

arms, each with articulating pads containing multiple electrodes. This

combination of multiple pads and numerous electrodes results in vastly-

improved vertical resolution -to the tune of mere fractions of an inch.

A typical tool emits an electrical "survey" current into the formation, while

another current focuses and maintains a high-resolution measurement. The

currents measured by each electrode vary according to formation conductivity,

which reflects changes in fluid properties, permeability, porosity, rock

composition, and grain texture. These variations are processed and converted

into synthetic color or gray-scale images, which are interpreted according to

the following convention:

Light Colors -reflect low micro-conductivity zones, (low porosity, low

permeability and high resistivity)

Dark Colors -reflect high micro-conductivity zones, (high porosity,

high permeability and low resistivity)

Resistivity Imaging Applications

Borehole imagers use a fixed-contrast presentation for gross correlations, and

a dynamic averaging display to enhance local features.

The fixed, or absolute contrast allows the viewer to correlate colorvalues between different zones of interest within the well, or between

images from different wells.

The dynamic averaging display is applied to local events, to allow the

viewer to distinguish features on a smaller scale, such as oil-filled

pores, or tight sands.


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When integrated with a traditional suite of logs, the images produced by a

resistivity imaging tool enable the analyst to differentiate laminated reservoirs

from low-permeability shaly sands. The tool produces quantitative, high-

resolution micro-resistivity measurements that aid in estimating hydrocarbon

saturation and reserves in thin-bedded reservoirs, thus improving the net pay

estimation of laminated reservoirs.

Resistivity Imaging Services

Each of the three leading oilfield technology companies offers their own

unique version of the resistivity imaging tool. And because each company has

its own impressive design, we will feature a photo of each. Examples include:

Halliburton Electrical Micro Imaging (EMITM

)Tool

This tool has six independent arms, with an articulating pad on each arm

(Figure 1: EMITM

tool, courtesy of Halliburton Energy Services).

Figure 1

Each pad contains 25 sensors, with a resolution of 0.2 inches. The central


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Using the high-resolution resistivity measurement results in improved

saturation calculations and more realistic net pay estimations.

Schlumberger Formation MicroImager (FMI TM) Tool

In addition to a 24-button microelectrical array pad on each of four arms (192

buttons total), the FMITM

mounts an extendable pad below each arm, to

increase pad coverage to about 80% of an 8-inch borehole. (Figure 3:FMITM

Tool; courtesy of Schlumberger Oilfield Services) Resolution is 0.2 inch

(5mm),

Figure 3

and the tool is rated to 350r F, and 20,000 psi.

Hybrid Resistivity Imaging Devices

In this section, we discuss two rather unique imaging devices, each of which

features specialized capabilities and operating modes.


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Baker Atlas Simultaneous Acoustic/Resistivity (STARTM

) Tool

Rather than taking only resistivity measurements, this tool simultaneously

acquires high-resolution images of borehole features that have resistivity

contrast or acoustic impedance. This combination of acoustic and resistivity

measurements partially compensates for any shortcomings inherent in eitherof the individual measurements. The six-arms on the tool use a powered

standoff to improve pad contact with the borehole, thus providing resistivity

coverage of 60% of an 8-inch hole, and 100% acoustic coverage. (Figure 4:

STARTM

Tool; courtesy of Baker Atlas.)

Figure 4

The tool is rated to 350r F.

Schlumberger Azimuthal Resistivity Imager (ARITM

) Tool

Instead of relying on pad contact, this tool uses an array of 12 azimuthalelectrodes, spaced 30 degrees apart. (Figure 5:Conceptual drawing of ARI

TM

tool; courtesy of Schlumberger Oilfield Services).


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Figure 5

This dual laterolog array is able to measure deep resistivity, but with a vertical

resolution of only eight inches. This makes for a laterolog reading that is

similar to the laterolog deep curve, but with a vertical resolution thatapproaches that of the MSFL curve. As an imaging tool, the ARI

TMis less

sensitive to borehole rugosity than the FMI electrical imaging tool, and can

also provide coarse structural dip measurements. The tool was developed for

evaluation of heterogeneous reservoirs, thin-bed analysis, and fracture

identification. It is rated to 350rF, and 20,000 psi.

Consult your logging representative for more information on the resistivity

imaging services that their company can provide.

Digital Array Induction Logs -

Digital array induction tools use multiple receivers and multiple logging

frequencies which provide capabilities that are not available with conventional

induction tools.


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this situation, high-resolution data near the borehole are added to

the deeper measurements so that all curves are presented with the

same matched vertical resolution, of 1, 2, or 4 feet.

Schlumberger Array Induction Imager Tool (AITTM

)

This tool uses 8 induction-coil arrays operating at multiple frequencies to

generate five resistivity curves. The log curves have median depths of

investigation of 10, 20, 30, 60, and 90 inches, and vertical resolution options

of 1 foot, 2 feet, and 4 feet. When the logs are radially deconvolved to produce

a detailed radial description of formation conductivity, the conductivity

description can be presented as a color-coded image or as discrete log curves.

Halliburton High Resolution Induction (HRITM

) Tool

tool features five radii of investigation (90, 60, 50, 40, 30, and 24 inches).

Their log also displays a resistivity map to indicate formation resistivity as a

function of depth and radial distance from the HRI tool.

Consult your logging representative for more information on the array

induction services that their company can provide.

3D Multicomponent Resistivity Tool

Conventional induction logging tools use transmitter and receiver coils that

are aligned with the long axis of the tool. In wells drilled perpendicular to

bedding, these tools measure formation conductivity parallel to bedding.

When a reservoir is composed of thinly bedded, highly conductive shales and

hydrocarbon-bearing sands that are below the vertical resolution of the tool,

the result is measurements experience the problematic low-contrast, low-

resistivity pay effect (Mollison, 2001).

Baker Atlas 3D Explorer Induction Logging Service (3DEXTM

)

Baker Atlas has developed a resistivity tool unique to the industry, which isdesigned to overcome the limitations of conventional induction tools in thin

bedded, low-resistivity shaly-sand formations. The Baker Atlas 3D Explorer

Induction Logging Service (3DEXTM

) provides both vertical and horizontal

resistivity measurements independently of borehole deviation or formation

dip.


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The 3DEX features three transmitter-receiver coil arrays, which are mounted

orthogonally in the X, Y, and Z planes relative to the tool axis. These coil

arrays provide 3-D coverage in their resistivity measurements:

two coils (XX and YY) measure resistivity in transverse directions

(parallel to the tool body),

a third coil (ZZ) measures resistivity in the direction of conventional

resistivity tools (perpendicular to the tool body)

in addition, there are cross component measurements (XY and XZ).

These arrays induce currents that flow, for the most part, across laminated

sand-shale sequences, and are far more sensitive to hydrocarbon-bearing sand

resistivity, as shown in Figure 6: Basic principle of operation: Laminated

sand/shale intervals are surveyed by three orthogonal coil arrays.

Figure 6


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Inversion processing of XX-YY-ZZ measurements obtained through the tools

orthogonal coil configuration are used to determine vertical and horizontal

resistivity Rv and Rh. The 3DEX horizontal resistivity is always determined

parallel to the bedding plane, consequently, the vertical resistivity is always

measuredperpendicularto the bedding plane. Thus, regardless of changes in

borehole deviation or apparent strike and dip, the 3DEX measurements of Rv

and Rh remain properly oriented to the formation bedding.

Where there is a difference in values between Rv and Rh, the formation is said

to be electrically anisotropic.

Electrical Anisotropy Effect

Conventional induction logging tools are limited to measurements in one

dimension because their sensors are aligned along the length of the tool (its Z-axis). Such measurements are satisfactory only when formations are at least as

thick as the tools vertical resolution, which is generally several feet.

In the presence of small apparent formation dips, the conventional induction

tools induce currents that mainly flow in the highly conductive beds (typically

shales) of hydrocarbon bearing sections. Thus, when pay zones occur within

thinly bedded sand-shale sequences, the conventional horizontal induction

measurement is dominated by the lowestresistivity, usually found in the shale

layers. As a result of this induced current flow pattern, the horizontal

resistivity (Rh) is relatively insensitive to the higher resistivity of thehydrocarbon-bearing sands. In this manner, relatively small volumes of

conductive shale can significantly reduce the apparent resistivity, thereby

reducing the accuracy of computed hydrocarbon saturations for the sand

layers.

Vertical resistivity, however, is dominated by the highest resistivity

component. In a hydrocarbon reservoir, Rv measurements provide more

information on the resistive sand components, thus yielding more accurate

fluid saturations in the sand layers. The 3DEX tool capitalizes on this

principal, with coil arrays aligned to resolve vertical resistivity.

In a thinly laminated sand-shale sequence, effective horizontal and vertical

resistivities are derived through parallel and series resistor models. The

corresponding formulae are:


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(Equation 1)

where

Rh = horizontal resistivityRsh = shale resistivity,

Rsd = sand resistivity

Vsh = shale volume, and

Vsd = sand volume

such that

Vsh + Vsd= 1

and

(Equation 2)

where

Rv = horizontal resistivity

Rsh = shale resistivity,

Rsd = sand resistivity

Vsh = shale volume, and

Vsd= sand volume

These equations are key to understanding the important differences between

horizontal and vertical resistivity. Equation 1 helps to explain how horizontal

resistivity is affected by shale or by sand:

horizontal resistivity (Rh) is strongly dependent on shale resistivity (usually

low) and shale volume

horizontal resistivity exhibits poor sensitivity to sand resistivity.

Conventional induction tools, with their coils aligned along the length of the

tool, are only able to measure perpendicular to formation bedding, and thusare only sensitive to horizontal resistivity.

Equation 2 demonstrates that vertical resistivity averages the contributions

from both sand and shale, and thereby provides a much better indicator of

thin hydrocarbon-bearing sands.


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The 3DEX tool capitalizes on vertical and horizontal conductivity

measurements to determine the laminar shale volume and laminar sand

conductivity. A Thomas-Stieber-Juhasz evaluation technique is applied to

determine the volume of dispersed shale along with the total and effective

porosities of the laminar sand fraction. By removing laminar shale

conductivity and porosity effects, the laminated shaly sand problem is reduced

to a single dispersed shaly sand model to which the Waxman-Smits equation

can be applied. (For additional details, see the Petrophysical Evaluation

described below.)

Log Example

In this example from Mollison, et al. (2000), the 3DEX tool was used to

evaluate a shaly-sand interval containing three distinct zones, each of which

exhibit different ranges of electrical anisotropy and shale content. Theresulting log is shown in Figure 7.

Figure 7


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The upper sand, from x100 to x145 feet, exhibits a fining-upward

sequence of moderately shaly sand. The data show significant electricalanisotropy (Track 1), as demonstrated by the separation of Rv and Rh (Track

2).

The middle sand, from x145 to x169 feet, is a gas producing zone with

low shale content. This interval exhibits little anisotropy, as would be

expected in a massive, high-porosity sand.

The lower sand, from x169 to x220 feet, is characterized by higher shale

content and higher electrical anisotropy than the upper sand. Conventional,deep-induction resistivity data, HDIL, shown in track 2, would not be able to

effectively identify this interval as a potentially productive sand-shale

sequence. However, the Rv and Rh data improve evaluation accuracy of the

lower sand and properly identify this as a finely laminated sand interval.

Petrophysical Evaluation of the Log

Directional resistivity measurements from the 3D Explorer tool can be used to

compute both the volume of laminar shale and the resistivity of the sand

fraction of a laminated formation without reference to other measurements orshale indicators. The 3DEX petrophysical evaluation model removes the

laminar shale conductivity effects by utilizing electrical anisotropy

measurements Rv and Rh.

In sand-shale sequences, Rv and Rh measurements provide a close link to the

petrophysical model through the direct computation of laminar shale. This

laminar shale volume may be compared to Thomas-Stieber style volumetric

laminar shale calculations, thus yielding a validation of the both petrophysical

models.

Petrophysical analysis of the above log reveals that the shales are

predominantly laminar, with minor amounts of dispersed shale (Track 4). In

the upper sand interval (100 to 145), the calculated laminar-sand resistivity,

Rsd, is 3 to 5 ;-m higher than that indicated by either the deep induction of the

HDIL tool or the horizontal resistivity Rh of the 3D Explorer (Track 2). Water

saturation from the laminar sand analysis is 10% to 15% lower than that


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obtained by standard saturation analysis (Track 3), indicating commercial

hydrocarbon production rates are probable from this interval.

This comparison of laminar shale volumes may also provide valuable

geological information. For example, the presence of anisotropic resistivity

allows important additional interpretation as to the geometry of the layers, i.e., parallel bedding. The lack of resistivity anisotropy would point to a lack of

parallel bedding, such as disturbed, folded or slumped bedding. Such intervals

often tend toward low producibility.

The lower sand interval, from x169 to x220 feet, is the most interesting in this

well. Total shale volume in this interval is 60% to 70%. The separation

between the Rh and Rv curve, together with the resulting anisotropy ratio,

indicate that the formation is almost entirely laminar and thin-bedded, with an

average net-to-gross ratio of 35%. Water saturation through the laminar sandis calculated at 40% to 55%, which agrees well with water saturation values

obtained in the upper sand interval. The net result is a possible 18 feet of

additional pay that might not have been identified by standard resistivity tools

and traditional water saturation analysis methodology.

The 3D Explorer can also provide supplemental measurements for the High

Definition Induction Log (HDIL). It can be run on the same toolstring and and

logged simultaneously, at the same logging speed required by the HDIL tool.

Data processing at the wellsite is provided to expedite the decision-making

process (e.g. testing and completion).

Nuclear Magnetic Resonance Tools

When microporosity, conductive mineralogy, or altered framework grains are

the cause of low-resistivity pay problems, then perhaps an alternative

approach to logging would help the formation evaluation program. In this

case, Nuclear Magnetic Resonance logging, which does not depend on rock

conductivity, can be used to accurately determine hydrocarbon saturation and

distinguish between free water and bound water in the reservoir. (In fact, the

esimation of bulk volume irreducible water, orBVI, is one of the earliest andmost widely used applications of NMR logging.)

The conventional neutron, bulk-density, and acoustic-travel-time porosity-

logging tools are influenced by components of the reservoir rock. Because

reservoir rocks typically have more rock framework than fluid-filled space,


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these conventional tools tend to be much more sensitive to matrix materials

than to pore fluids.

Conventional resistivity-logging tools, while being extremely sensitive to the

fluid-filled space, are traditionally used to estimate the amount of water

present in reservoir rocks, but cannot be regarded as truefluid-loggingdevices. These tools are strongly influenced by the presence of conductive

minerals and, for the responses of these tools to be properly interpreted, a

detailed knowledge of the properties of both the formation and the water in the

pore space is required.

NMR logging tools use a permanent magnet to produce a magnetic field that

excites formation materials. An antenna transmits an oscillating magnetic field

in precisely timed bursts of radio-frequency energy into the formation.

Between these pulses, the antenna is used to listen for the decaying echosignal from hydrogen protons that are in resonance with the field from the

permanent magnet. Since this magnetic resonant frequency depends on the

local strength of the magnetic field, the measurement zone of the tool is a

function of the magnetic field generated, and the radio frequency used.

NMR measurements respond primarily to hydrogen protons in the pore spaces

of the formation, thus providing a measure of water or hydrocarbons in the

rock. Unlike conventional porosity measurements (such as the compensated

neutron tool), this measure of NMR porosity does not include hydrogen bound

in the matrix of the rock, thus providing porosity values that are not influencedby lithology. (Figure 8:MRIL porosity model, Courtesy of Baker Atlas) With

only fluids visible to the NMR tool, it does not need to be calibrated to

formation lithology.


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This response characteristic makes NMR logging tools fundamentally

different from conventional logging tools.

Unique Formation Measurements

NMR tools can provide three types of information, each of which make these

tools unique among logging devices:

information about the quantities of fluids in the rock,

information about the properties of these fluids, and

information about the sizes of the pores that contain these fluids.

Specifically, NMR tools are used determine total porosity, effective porosity,

capillary bound water volume, free water volume, hydrocarbon volume, and

permeability. The basic physics behind NMR interpretation is common to all

such data; however, each of the current NMR logging service companies -

Baker Atlas, Halliburton, and Schlumberger have their own proprietary

interpretation methods. In addition, there are now several companies thatspecialize in the interpretation of NMR data, including NuTech and NMR+.

Schlumberger Combinable Magnetic Resonance Tool (CMRTM

)

The Schlumberger Combinable Magnetic Resonance tool uses a directional

antenna sandwiched between a pair of bar magnets to focus the CMR


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measurement on a 6-in. [15-cm] zone inside the formationthe same rock

volume scanned by other essential logging measurements. As shown in

Figure 9 (CMR tool), it is a compact skid-mounted tool that was designed to

be combinable with many other standard logging tools. The CMR tool is run

in an eccentered configuration.

Figure 9

The vertical resolution of the CMR measurement makes it sensitive to rapid

porosity variations, as often seen in laminated shale and sand sequences. The

sensitive region of the tool is shown in red in Figure 10 (Cross-section of the

CMR tool). This region is approximately 0.5 x 0.5 by 6 long, and is located

about 1.1 inches inside the formation.


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Figure 10

Baker Atlas Magnetic Resonance Imaging Log (MRIL) Service

The Magnetic Resonance Imaging Log run by Baker Atlas provides the

capability to run in combination with other openhole logging instruments

(Figure 11: Schematic of combined tool configuration; Courtesy of Baker

Atlas).


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Figure 11

The tool is run in a centralized configuration to ensure that the sensitive

volume does not include the borehole fluid, and is unaffected by borehole

rugosity. The MRIL measurements can investigate the formation at diameters

of up to 18 inches. This tool can be operated simultaneously at different

frequencies to increase the sensed volume, improve the signal-to-noise ratio,

and allow multiple NMR measurements to be obtained at one time.

HalliburtonMagnetic Resonance Imaging Log (MRIL) Tool

The MRIL-Prime tool was introduced in 1998. Like other MRI tools, this MRI

probe can be tuned to be sensitive to a specific frequency, thereby allowing

the MRI to image narrow slices of the rock formation. Figure 12 (Cylinders of

investigation: Courtesy of Halliburton Energy Services) illustrates the

measurement concept behind the MRIL-Prime tool.


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Figure 12

The diameter and thickness of each thin cylindrical region are selected by

simply specifying the central frequency and bandwidth to which the MRIL

transmitter and receiver are tuned. The diameter of the cylinder is temperature

dependent, but typically ranges from approximately 14 to 16 inches.

Consult your local service company representative for more information on

NMR logging tools.

Log Example

In the first log Figure 13, we see a classic example of a low resistivity zone,

which does not show any potential for future completion.


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Figure 13

This example, provided by Halliburton Energy Services, shows MRI and

resistivity data obtained in a Low Resistivity Pay zone. (Figure 14:MRIL Log

presentation, Courtesy of Halliburton Energy Services)


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Figure 14

Log Description -

Following is a list of curves presented in each track of the log.

Track 1: MRIL porosity derived from T2 bins, along with

Caliper, Gamma Ray, and SP measurements from conventional

logs.

Track 2: MRIL permeability, derived from MRIL

Porosity, Bound Water, and Free Fluid measurements, along with

Deep and Shallow Resistivity from conventional logs.

Track 3: T2 distribution from partially polarized activation

with TE of 0.6 ms (left side of track 3), which is typicallyindicative of clay bound water, and the T2 distribution from fully

polarized activation of a TE of 1.2 ms (right side of track),

usually indicative of capillary bound water and free fluids.

Track 4: The difference between two T2 distributions,

each taken with a TE of 1.2 ms at different polarization times,


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yields hydrocarbon signals within the free fluids. Relative

position indicates hydrocarbon type and viscosity value.

Track 5: Time Domain Analysis of calculated volumes of

oil, gas, and free water in the pore space, which provides a

complete description for the fluids in the invaded zone.

Track 6: MRIL-Resistivity display of MRIAN (MRI

Analysis) model to calculate volume of hydrocarbon and free

water in the pore space, which provides a complete description of

fluids in the uninvaded zone.

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