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temporary wireline deployment, says

Tad Bostick, Weatherfords vice president

for business development in intelligent

completion technologies in Houston. But

permanent in-well seismic sensing needs

a different approach, one where the

equipment in the well can survive the

demands of the downhole conditionswhile continuing to perform reliably for

field lives of up to 20 years.

Among the conventional borehole

seismic techniques he refers to, vertical

seismic profiling (VSP) has been most

practised to date. This involves hanging

an array of geophone sensors

temporarily in the wellbore on a wireline,

while moving a seismic source across the

surface (see figure right) . The sound

waves generated by the source penetrate

the earth and are reflected by rock and

fluid interfaces, the reflected signals

being picked up by the geophones. As the

surface source is moved, a different set of

reflections is received, helping to build upa seismic image of the subsurface in the

vicinity of the well. Compared with

normal seismic surveys which have both

source and sensors at the surface, the

result is a more detailed image of the

reservoir in this region the distance

that can be seen from the wellbore is

roughly determined geometrically by the

depth of the well and the sensor

locations.

A variation on this theme is to locate

the seismic source in a nearby wellbore

rather than on the surface, both of these

methods being classed as active

monitoring because of the use of a

seismic source.Additional information can be gathered

using conventional downhole instrumen-

tation by passive monitoring, where no

seismic source is used. The downhole

sensors instead detect the natural

seismicity of the reservoir the sounds

emitted by rock formations as they are

compacted and crack. Making use of such

microseismic signals has been around

for a while, one notable case being in the

Ekofisk field offshore Norway where the

seabed was discovered to be sinking as

gas was extracted.

More recently it has been demonstrated

that microseismics can detect fluid

movement as well, although at presentthis is still a relatively qualitative

measurement, says Bostick.

But while such seismic data can be

gathered using short-term wireline-

deployed geophones, the readings can

only be taken periodically and require

well intervention. Permanent geophone

arrays have also been deployed in shallow

wells; however, current technology does

not allow complex instruments and their

associated electronics to withstand high

temperatures and pressures encountered

downhole for long periods of time. In

contrast, optical sensors are significantly

more resilient, and being based on optical

fibres, have no moving parts or

electronics in the downhole environment,

allowing them to be installed perma-

nently to collect seismic data on demand.

We want to show that an optical

sensor-based seismic array capable of

both active and passive seismic

monitoring can be deployed, and thatthese sensors will track fluid movement,

explains Bostick. Optical sensors

deployed in wells are already capable of

detecting pressure, temperature, distri-

buted temperature, flow and even phase

fraction, all now available on a

commercial basis. Seismic detection,

although still in the pre-commercial

stage, is the next step for optical sensing

it will mean much more than just reading

parameters at the wellbore itself, such as

temperature and pressure, and will

provide a valuable view out into the

reservoir.

Among the handful of companies

offering downhole optical sensingsystems, Weatherford is a recognised

leader with some 30 well installations to

date to its credit, many of which include

multiple optical sensing systems. In 2001,

the company acquired the oilfield optical

sensing division of CiDRA, which itself

had been in partnership with Norways

Optoplan, an early pioneer in applying

optical sensing technology to downhole

operations, having been conducting field

trials since the early 1990s. The

Norwegian connection is prominent in

the current Izaute field trials, as the

project has its roots in the countrys

Demo 2000 technology development

programme. TotalFinaElf, Statoil, Norsk

Hydro and BP are supporting the Izaute

project.

Izaute was selected for the

demonstration trials for several reasons.

Installation of the system onshore in a

shallow well is relatively straightforward,

and much is known about thecharacteristics of the storage reservoir;

conventional bore hole and surface

seismic data exists, as do well logs, and

seasonal movement of gas is reasonably

predictable. But a key to the project is the

fact that the seismic image of the

interface between gas and water is one of

the most distinctive, helping to monitor

the migration of gas in the reservoir.

The Weatherford optical sensing

system was installed in well 102 at Izaute

in November last year. Described by the

company as an all optical multi-station,

multi-component permanent in-well

seismic imaging and monitoring array,

the system is claimed to be the first of itskind in the world, and is capable of both

time-lapse VSP and microseismic data

gathering.

The optical sensor array was installed

in the well, attached to the outside of 4in

diameter production tubing inside 95/8in

casing the operation took two days in

all, using mobile cranes to run the

tubing.

The single optical cable, about 6mm in

diameter and with three fibres inside,

links the sensor arrays together and

relays data to the surface wellhead. From

here, the information is transferred by

optical cable on the surface to the seismic

downhole monitoring

w w w. o f f s h o r e - e n g i n e e r. c o m O F F S H O R E E N G I N E E R | a p r i l 2 0 0 3 | 27

Vertical seismic profiling is used to build up an image of the subsurface near to the wellbore.

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recording devices nearby (see figure

right).

Six optical sensor stations are

integrated into the cable, at locations

several hundred metres down into the

well. The sensor locations were

determined from the results of a

conventional VSP made in November2001. Five of the sensor stations are

suspended above the reservoir, spaced

over a few metres, to collect VSP seismic

imaging data. The sixth sensor station is

located a few hundred metres lower than

the VSP array, nearer to the reservoir, for

microseismic monitoring.

The sensors are coupled to the casing to

allow them to detect seismic signals in

the reservoir using a purpose-designed

device; this also substantially decouples

the sensors from the tubing to prevent

them from picking up tubing or cable-

borne noise.

The optical sensors in the cable include

Bragg grating technology. The Bragggratings are used to assist in determining

strain in the optical fibre by reflecting

light back along the fibre at specific

wavelengths the strain detected in the

fibre can be translated into the parameter

being measured (see panel on page 22). In

this case, the parameter is acceleration,

the minute movement of the earth caused

by the excitation of seismic waves

travelling through it. Weatherfords

optically-based sensors are configured to

detect acceleration in three mutually

perpendicular directions the three

components of multi-component 3C

seismic.

The breakthrough here is being able todetect very small accelerations, down to

micro-Gs and even nano-Gs, in three

directions simultaneously, which means

we can tell where a seismic signal is

coming from, says Bostick. And also in

being able to do this in a compact device

around 25mm in diameter which can stay

downhole at pressures up to 1000bar and

temperatures up to 175C. There are no

electronics that can match this

combination of performance while

surviving the downhole conditions.

At this level of sensitivity, high

frequency events such as the cracking of

rock can be detected, enabling

microseismic signals to be heard andlocated, allowing an image of the

reservoir near the well to be determined.

Extraneous events, such as the passing of

trucks, can be identified and eliminated

from the survey by high tech software

that employs very sophisticated

algorithms to analyse and interpret the

results data is only stored by the system

when there is an event of relevance.

The results of the field trial to date are

very encouraging, says Weatherford. The

first snapshot VSP taken in November

last year using the optical system, when

the reservoir was full of gas, compares

well with the quality of the previous

conventional VSP taken in November

2001, and shows greater detail of the

subsurface than conventional surface

surveys can generate. The plan now is to

take another snapshot optical VSP this

spring when the reservoir is at an empty

condition, for comparison. Continuous

microseismic measurements are also

being analysed to demonstrate that the

reservoirs seismic response can be

correlated with gas migration. When

combined together the results of the

different surveys will help to optimise

gas storage at Izaute.

With the cost of various types ofoptical sensors currently available from

Weatherford already being competitive

compared with conventional electronic-

based equivalents, looking ahead, Bostick

can see that optical permanent in-well

seismic sensing will follow suit as the

technology becomes more widely

deployed.

Offshore wells stand to gain

significantly from in-well seismic, he

believes. Here, the higher frequencies of

conventional surface seismic surveys are

knocked out as they pass through the

soft seabed, a condition also experienced

in desert locations, leaving only the

longer range view being generated by

lower frequencies, around 30-50Hz.

Wellbore seismic enables higher

frequency sounds typically 80-100Hz to

reach the downhole sensors, proving the

detail of a localised view.

This is also of particular interest in

deep wells in reservoirs lying below salt

formations such as those encountered in

the Gulf of Mexico. Seismic signals are

scattered by salt formations, making

imaging of deep reservoirs difficult in-

well surveys can deliver much clearer

images. Obtaining good images below gas

formations is also problematic whenusing conventional seismic techniques. In

these circumstances, optical in-well

seismic surveys could reveal much

needed reservoir data, for example,

detecting the movement of water

injection fronts, indicating how fast a

well should be produced.

Further trials of the Weatherford in-

well optical seismic sensing system are

planned for this year, in the North Sea,

Gulf of Mexico and Europe.

Our goal is to make intelligent wells a

viable completion option to enhance

production optimisation and reservoir

recovery for as many field developments

downhole monitoring

28 | O F F S H O R E E N G I N E E R | a p r i l 2 0 0 3 w w w. o f f s h o r e - e n g i n e e r. c o m

Power Telephone

Wellhead

Splitter

Opticalsurfacecable

Opticaldownhole

cable

Fibre opticinterrogation unit

Seismicrecorder

Optical

interstationcable

3-C Opticalseismic

accelerometers

Portacabin

Layout of the optical

sensing system at

Izaute.

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as possible, Graham Makin, Weather-

fords marketing director for intelligent

completions, tells OE. The adoption of

simple, low cost, easily understood

hydraulically actuated flow controls is

one half of this strategy. The other half is

to displace unreliable electronic

permanent monitoring systems withoptical sensing systems. The adoption of

a suite of all -optical , permanently

installed in-well sensors gives a step

change in reliability, and also opens up

the possibility of a vastly enhanced

sensing capability in terms of the types of

data that can be retrieved, adds Makin.

In the downhole arena, detecting

seismic signals by listening with lightlooks to have a bright future.

downhole monitoring

30 | O F F S H O R E E N G I N E E R | a p r i l 2 0 0 3 w w w. o f f s h o r e - e n g i n e e r. c o m

Bragg grating

Optical fibres are thin flexible

strands of silicon glass, having

an inner core and an optical

cladding, together typically

measuring around 125 microns in

diameter, encased in a buffer

coating.

Pulses of light can be transmitted

down the core of a fibre. They

bounce off the inside of the optical

cladding, an effect known as total

internal reflection. In this way,optical fibres are able to convey

large volumes of digital data, such

as audio and video signals for

telecommunications, and are now

being used increasingly in downhole

sensing as they can withstand high

temperatures and pressures.

Bragg gratings are incorporated

into the latest optical sensors. A

Bragg grating is photo-imprinted

onto a small section of the core of

the optical fibre using ultraviolet

light, changing the property of the

fibre at that location. This causes

the fibre to reflect a very specific

wavelength of light back along itslength but allows all other light to

pass through.

Some types of sensors employ

Bragg gratings as the sensor itself.

When a strain is applied to the fibre

location containing the grating, for

example by changing the

surrounding pressure or

temperature, a different wavelength

is reflected back, the shift in

wavelength being related to the

strain placed on the Bragg grating

the greater the change in the sensed

parameter, the greater the

wavelength shift. Measuring this

shift in reflected wavelength permitsthe temperature or pressure to be

calculated.

In the case of Weatherfords

seismic sensors, the Bragg gratings

are isolated from direct strain and are

used as mirrors that define a sensing

region, consisting of a length of fibre

exposed to strain, known as an

interferometer. It is the straining of

the length of fibre between the Bragg

gratings that acts as the sensor in

this particular case. Depending on the

length of fibre, interferometric

sensors are capable of detecting

much smaller strain changes (thanBragg grating sensors). The

configuration of Weatherfords

sensors can detect strain caused by

accelerations coming from any of

three directions, caused by the

earths structure being excited by

seismic sound waves.

The light which passes on beyond

the grating can be used to interact

with other sensors, tuned to different

wavelengths. This feature means an

optical fibre can be multiplexed,

simultaneously carrying multiple data

channels in a single fibre.

Strain

Inputspectrum

Reflectedspectrum

Transmittedspectrum

TransmittedlightReflectedcomponent

Pattern copied toUV-sensitive core

UV grating(alternating seriesof light and dark

regions)

IntersectingUV laserbeams

Bragg gratings in optical fibres can be used to

detect strain by changes in reflected

wavelength. Weatherfords sensors detect

acceleration caused by seismic sound waves.

UV laser beams

Transmittedlight

Photo-inscribedgrating

Reflectedcomponent

Inputspectrum

Reflectedspectrum

Transmittedspectrum

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