fracture apertures from electrical borehole scans

7/26/2019 Fracture apertures from electrical borehole scans

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GEOPHYSICS, VOL. 55, NO JULY 1990 ;P. 821-833, 15FIGS.

Fracture apertures from electrical borehole scans

S

M

Luthi and P ouh itet

ABSTRACT

Three-dimensional finite-element modelingwas per

formed to investigate the response to fractures of the

Formation MicroScanner Mark of Schlumberger ,

which records high-resolution electrical scans of the

borehole wall. is found that the equation

W

c A R ~ R o b

describes, over two orders of magnitude of resistivity

contrasts between borehole mud and the formation,

the relationship between fracture width W in mm ,

formation resistivity R

xo

mud resistivity R

m

and the

additional current flow caused by the presence ofthe

fracture. is the additional current which can be

injected into the formation divided by the voltage,

integrated along a line perpendicular across the frac

ture trace. Coefficient c and exponent b are obtained

numerically from forward modeling. Tool standoffs of

up to 2.5 mm and fracture dips in the range from 0 to

40 were found to have an insignificant effect on the

above relation.

INTRODUCTION

Fluid flow rates through fractures with smooth surfaces

are proportional to the cube of the aperture, but decrease

with increasing roughness such as found on natural fracture

surfaces Brown,

1987;

Jones et al., 1988 . The estimation of

fracture apertures in wellbores penetrating fractured reser

voirs is, therefore, of paramount importance for assessing

reservoir productivity. Reflections of the Stoneley wave

measured by an array sonic tool Hornby et al., 1989 have

recently been proposed as an in-situ measurement of frac

ture aperture. The technique presented in this paper ad

dresses the same problem, albeit with an entirely different

downhole geophysical measurement principle.

The Formation MicroScanner is a wireline device produc-

A three-step approach to detect, trace,and quantify

fractures is used. Potential fractures in Formation

MicroScanner images are detected as locations where

conductivity exceeds the local matrix conductivity by

a statistically significant amount. Integration over a

circular area is performed around these locations to

gather all excessive currents; this integral is then

geometrically reduced to approximate the line integral

Line sharpening and neighborhood connectivity

tests are done to trace the fractures, and apertures are

computed for all fracture locations.

Results from a well into basement in Moodus Con

necticut show that the method successfully traces

fractures seen on Formation MicroScanner images.

The resulting fracture apertures range from 10urn to 1

mm. For the wider fractures there is acceptable agree

ment with apertures obtained from Stoneley wave

reflection measurements. This unique and novel tech

nique for characterizing fractures in wellbores has a

very low detection threshold of around 10 urn and

resolves fractures as little as I em apart. Furthermore,

it provides azimuthal orientation of the fractures.

ing electrical scans of the borehole wall Ekstrom et al.,

1987 .

The scans are achieved by arrays of small electrodes

mounted on pads held at a known potential with respect to a

return electrode in the upper part of the tool Figure

1 .

Currents emitted from these electrodes are recorded at a

high sampling rate typically 0.1 inches, or 2.5 mm , and are

used to produce conductance images of the part of the

borehole wall covered by the pads while traveling upward.

These images can be oriented with respect to geographic

north through continuous downhole measurement of the

sonde orientation by a triaxial fluxgatemagnetometer. Thus,

dip and azimuth of fractures and bedding planes can be

measured if the electrical images are displayed in an azi

muth-depth plot Plumb and Luthi,

1986;

Pezard and Luthi,

1988;

Luthi, 1990 .

Manuscript received by the Editor August 22, 1989;revised manuscript received December 8, 1989.

*Schlumberger-Doll Research, Old Quarry Road, Ridgefield, CT

06877 4108.

*Etudes et Productions Schlumberger, rue de la Cavee, Clamart, France.

e 1990Society of Exploration Geophysicists. All rights reserved.

82


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822 Luthi and souhalte

Open fractures are among the most prominent features

seen on electrical images because of the large conductivity

contrasts between the fluid in the fracture-typically as

sumed to be the drillingmud nd the surrounding rock. In

many boreholes drilled with water-based mud this contrast

may be several orders of magnitude. Examples of open

fractures on electrical images are documented by Plumb and

Luthi

1986 ,

Ekstrom et al. 1987), and Pezard and Luthi

1988 .

They show up as conductive streaks exhibiting a

large variety of morphologies ranging from short, irregular

shapes to planar. Fractures typically affect several adjacent

samples because the electrode diameter is approximately

twice the sampling distance, accounting for some vertical

and horizontal overlap, and also because the electrical flow

lines are severely distorted in the vicinity of the fracture. It

is, therefore, of interest to find a relationship between the

Button

Trajectory

I

I

I

I

I

I

I

I

I

Fracture

MODELING OF ELECTRICAL FRACTURE RESPONSE

Technique

electrical signal produced by the fracture and fracture pa

rameters such as aperture, dip angle, resistivity of the fluidin

the fracture, resistivity of the rock, and the distance from the

tool to the borehole wall tool standotl). We address this

problem through forward modeling of the electrical field

using a three-dimensional 3-D) finite-element modeling

code. To invert electrical borehole scans for fracture param

eters, we then present a statistical method to identify and

trace fractures on Formation MicroScanner images and a

technique to compute fracture apertures for each sample

located on the fracture trace.

The finite-element method has been used successfully by

Chang and Anderson

1984

to model electromagnetic bore

hole devices such as the induction tool. In our approach,

which is closely related to the technique of Chang and

Anderson

1984 ,

the current emitted by a single Formation

MicroScanner button in front of a fracture Figure 2) is

simulated using the finite-element method which solves

Laplace s differential equation for the electrical field over an

adaptive three-dimensional grid in and around the borehole.

Grid node spacing is very close in the vicinity of the

electrode button, covering at least 20 nodes in the sensitive

area along a line across the fracture and increases progres

sively away from this area with a minimum of 10more nodes

in each direction. The total number of grid points is about

70000; the progressive variation in element size away from

the sensitive area avoids discontinuities which may be

detrimental to the computational accuracy. The fracture is

modeled as a thin-sheet element with a uniform resistivity

equal to the mud resistivity. Current densities are computed

on the nodes covering the tool pad, and button currents are

obtained by multiplying current densities with their corre

sponding area. All computations assume planar, parallel

fractures of infinite extent. The dip of the fracture, i.e., the

4

R

SONDE

TELEMETRY

HYDRAULICS

INCLINOMETER

INSULATING SUB

PREAMPlIFICATION

CARTRIDGE

AMPLIFICATION

CARTRIDGE

INSULATING

SLEEVE

FLEX JOINT

FIG. 1. Sketch of the Formation MicroScanner tool config

uration discussed in this paper after Ekstrom et aI., 1987 .

Two of the four pads are equipped with the array of imaging

electrodes shown on the left. A newer tool design has fewer

electrodes on all four arms.

Borehole

FIG. 2. The modeled situation of an electrode button crossing

a fracture on the borehole wall.


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Fracture Apertures from Electrical Scans

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fracture apertures from electrical borehole scans

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