Vrije Universiteit Brussel

Effect of hydrogen gas on FBG-based optical fiber sensors for downhole pressure andtemperature monitoringHuang, Ji-Ying; Van Roosbroeck, Jan; Vlekken, Johan; Kinet, Damien; Martinez, A.B.;Geernaert, Thomas; Berghmans, Francis; Van Hoe, Bram; Lindner, Eric; Caucheteur,ChristophePublished in:Optics Express

DOI:10.1364/OE.27.005487

Publication date:2019

Link to publication

Citation for published version (APA):Huang, J-Y., Van Roosbroeck, J., Vlekken, J., Kinet, D., Martinez, A. B., Geernaert, T., ... Caucheteur, C.(2019). Effect of hydrogen gas on FBG-based optical fiber sensors for downhole pressure and temperaturemonitoring. Optics Express, 27(4), 5487-5501. https://doi.org/10.1364/OE.27.005487

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Download date: 05. Jul. 2020

Effect of hydrogen gas on FBG-based optical fiber sensors for downhole pressure and temperature monitoring

JI-YING HUANG,1,2,* JAN VAN ROOSBROECK,2 JOHAN VLEKKEN,2 DAMIEN

KINET,1 ANTONIO BUENO MARTINEZ,1 THOMAS GEERNAERT,3,4 FRANCIS BERGHMANS,3,4 BRAM VAN HOE,2 ERIC LINDNER,2 AND CHRISTOPHE CAUCHETEUR

1 1Electromagnetism and Telecommunication Department, Faculté Polytechnique, Université de Mons, Boulevard Dolez 31, B-7000 Mons, Belgium 2FBGS International NV, Bell Telephonelaan 2H, B-2440 Geel, Belgium 3Vrije Universiteit Brussel, Department of Applied Physics and Photonics (TONA), Brussels Photonics (B-PHOT), Pleinlaan 2, B-1050 Brussels, Belgium 4Flanders Make, Oude Diestersebaan 133, B-3920 Lommel, Belgium *ji-ying.huang@umons.ac.be

Abstract: The influence of hydrogen gas on Fiber Bragg Grating (FBG)-based optical fiber sensors has been validated experimentally. More in particular, the focus was on FBGs written in the so-called Butterfly Micro Structured Fiber that targets simultaneous pressure and temperature monitoring with a minimum in cross-sensitivity to be used in, for example, downhole applications for the oil and gas market. The hydrogen-induced pressure and temperature errors from this type of sensor have been quantified as a function of the partial hydrogen pressure. The induced errors can be related to the diffusion of the hydrogen into the microstructure and to refractive index changes due to the presence of the hydrogen in the micro holes and penetration of it into the fiberglass. Furthermore, we have also shown that the hydrogen-induced errors scale with the partial hydrogen pressure.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction

Fiber optic sensing has demonstrated its benefits for sensing of different critical parameters in downhole applications [1–5]. With respect to downhole temperature monitoring, Distributed Temperature Sensing (DTS) is a well-known intensity-based detection method within this industry [3,6,7]. DTS allows picturing the well temperature as a function of depth down to several kilometers with meter sized resolution. The technique relies on the encoding of temperature information contained in the Raman back scattered signal (Stokes and Anti-Stokes components). Another method relies on the use of Fiber Bragg Gratings (FBG) [4,8,9]. This technology yields quasi-distributed sensing: FBGs with different Bragg wavelengths can be inscribed along the same sensing fiber but the number of FBGs that can be multiplexed is rather limited when compared to DTS. Each Bragg resonance is intrinsically sensitive to temperature and strain and thus by eliminating strain effects, they can be equally employed to capture temperature profiles down an oil well.

A common characteristic for downhole environments is the presence of hydrogen [10]. Intensity-based sensing systems, like DTS, are affected by hydrogen as it diffuses into the optical fiber and thereby induces photo-darkening which increases the optical attenuation in the sensing fiber [5,11]. Researchers have proposed different mitigation techniques, such as reducing the penetration rate of hydrogen by means of a hermetic fiber coating [12] or by using self-calibrating methods involving either dual wavelength scanning or exploiting two sensing fibers [13–15]. For FBG wavelength-based sensing systems, the attenuation itself is

Vol. 27, No. 4 | 18 Feb 2019 | OPTICS EXPRESS 5487

#353338 https://doi.org/10.1364/OE.27.005487 Journal © 2019 Received 3 Dec 2018; revised 4 Feb 2019; accepted 5 Feb 2019; published 13 Feb 2019

less of an issu[16]. This drisilica and the the temperatu

More recehave also beeThe effect of in an addition[25,27,28]. Thbar of hydrogshift of ~100

Recently, More particulfibers for simsuch specialtyreported in thpressure monibased sensorsdiffusion attrithe disappearareason for theload the sensoinduced errorhydrogen indupressure. Bashydrogen on quantified wenvironment. applications, e

2. Experime

2.1 Gas load

To create a commonly usinscription [32

ue but the presft of the Braggformation of G

ure [17–22]. ently, FBG-basen considered the presence onal wavelengthe induced wagen pressure at°C, considerinwe have also

larly, the usagemultaneous meay sensors whenhis document.itoring with thes. The modificibuted to the mance of pressu

e apparent presors with hydrogrs for each seuced effects ha

sed on the expMS-FBG sen

ith respect toWith that, th

especially to th

ental set-up

ding facility

Fig. 1. Auto

hydrogen-rich ed in view of i2,33]. The auto

sence of hydrog wavelength sGe-OH or Si-O

sed sensors fabfor sensing cri

of hydrogen onh shift due to

avelength shift t 80 °C for a wg a typical temreported on t

e of FBGs writtasurement of pn exposed to a We specificae proposed MScation of the r

main cause on ture difference dsure error. In sgen. Section 3 nsor when beave been studi

periments whicnsor is estimao partial hydrhe product adhe influence of

oclave facility for

environment,increasing the oclave consists

ogen results in stems from the

OH depending o

bricated in Micritical paramete

n such sensors ho the diffusion

was found to week, which co

mperature sensitthe usage of Mten in highly-bpressure and tehydrogen-rich

ally evaluate tS-FBG sensor trefractive indexthe apparent temdue to hydrogesection 2, we incovers the exp

eing loaded wiied when usingch were discusated. More imrogen pressuredaptability of f the hydrogen

simulating a hydr

, we use a hyphotosensitivits of a stainless

an additional e change of theon the hydroge

ro-Structured oers in downhohas also been s

n of hydrogen be nearly 1 nmorresponds to tivity of aroun

MS-FBGs for birefringent Buemperature [29

h environment wthe performantogether with ox in the fiber mperature shif

en diffusion apntroduce the auperimental dataith pure hydrog a gas mixturessed in each se

mportantly, the e and workin

f the MS-FBGgas can be eva

rogen-rich environ

ydrogen loadinty of (standard

s steel cylinder

Bragg wavelene refractive inden pressure lev

optical fiber (Mole applicationsstudied [25,26]in the micro

m after exposua fictitious tem

nd 10 pm/°C. downhole app

utterfly micro-s9–31]. The bewas studied an

nce of temperaother conventioglass due to h

ft. For pressureppears to be theutoclave facilita and the analyogen. In sectioe with partial hection, the infl

induced errong temperaturG sensor in daluated.

nment.

ng autoclave d) fibers prior tr with an inner

ngth shift dex of the vel and on

MS-FBG) s [23,24]. ] to result structure

ure to 200 mperature

plications. structured havior of

nd will be ature and onal FBG hydrogen e sensing, e primary ty used to ysis of the on 4, the hydrogen

fluence of or can be re in the downhole

which is to grating cavity of

Vol. 27, No. 4 | 18 Feb 2019 | OPTICS EXPRESS 5488

20 cm in height and 13 cm diameter. It is equipped with four epoxy-based fiber feed-throughs with FC-connectors, which allow for online monitoring of samples during gas loading. The temperature of the autoclave can be controlled by an external oven. The epoxy-based fiber feed-throughs limit the maximum operating pressure and temperature to 80 bar and 80 °C. The temperature and pressure in the autoclave are continuously monitored with reference gauges. Figure 1 shows a schematic arrangement of the autoclave set-up.

2.2 FBG-based optical fiber sensors

We experimented with three types of FBG-based sensors. The first is an FBG in a Polyimide coated pure silica fiber written with a femtosecond (fs) laser by means of a phase mask. This sample will further be referred to as the Femto Second Grating or ‘FSG’. The second is a Draw Tower Grating or ‘DTG’ with Ormocer coating written with a UV laser using a Talbot interferometer. The sample was provided by FBGS International. The third is an FBG in the Butterfly Micro-Structured fiber (MS-FBG) written again with a fs-laser and phase mask. A typical reflection spectrum of the Butterfly MS-FBG sensor features two Bragg resonances due to the birefringent nature of this fiber. One peak corresponds to light polarized along the fast axis, whilst the other to light polarized along the slow axis. More information about the typical reflection spectrum of the Butterfly MS-FBG sensor and the cross section image of the Butterfly MSF can be found in [29–31]. The unique ability of this sensor is that pressure changes appear as changes in the peak separation (i.e. the wavelength difference between the fast and slow axes) while temperature can be monitored by tracking the change of the individual Bragg wavelengths. Since temperature appears as common mode for both Bragg wavelengths, it becomes cancelled in the peak separation and in this way, most of the pressure-temperature cross-sensitivity can be eliminated [29–31]. We used two such MS-FBG sensors. The first has its fiber end sealed by collapsing the micro-holes with an electrical arc of a fusion splicer in order to be sensitive to environmental pressure. Since this sealing process is done under atmospheric pressure, sealed MS-FBG sensors will sense the differential pressure i.e. the difference in pressure with respect to the atmospheric pressure that is present in the micro-holes. The other sample has its fiber end untreated and so with open micro-holes. This sample is not sensitive to pressure changes because the pressure inside the micro holes will always be the same as the pressure outside. The Bragg resonances of each FBG sensor are tracked using a Micron Optics SM125-500 interrogator combined with an HP 11896A polarization scrambler. All sensors are spliced to a FC/APC pigtail with a lead-in cable spooled on a metal rim. The sensing region of the MS-FBG sensors was kept straight by taping it before and after the FBG to the inner wall of the autoclave.

3. Experiments with sensors in a single gas environment

3.1 Nitrogen loading

The aim of this first experiment was primarily to verify whether both the autoclave and the sealed MS-FBG sensor are properly sealed. We examine this by loading nitrogen gas into the autoclave since this is not explosive. The experiment was carried out in the following stages: (1) pressurization of the autoclave with nitrogen from atmospheric pressure to 20 bar and then to 80 bar sequentially, (2) heating up the autoclave to a temperature of 80 °C with a holding time of a few hours, (3) leave the autoclave to cool down to room temperature (RT) and (4) release the pressure in the autoclave down to atmospheric pressure. Notice that the heating step also causes the pressure to increase since the autoclave should be regarded as a closed volume.

Figure 2(a) shows the evolution of the Bragg resonances of the open MS-FBG sensor during nitrogen loading, together with the reference pressure and temperature readings. The recorded wavelength shifts for both the slow and fast axes have been normalized for clarity. The figure also indicates the different stages (1) to (4) explained above. A few transients in

Vol. 27, No. 4 | 18 Feb 2019 | OPTICS EXPRESS 5489

the wavelength shifts for both Bragg resonances can be observed during stage (1), which actually correspond to the moments when the pressure is increased in the autoclave. The sudden change in pressure in the closed volume of the autoclave induces a sudden change in temperature, which quickly dissipates again over time. Since the individual wavelengths are sensitive to temperature changes, this explains the transients. Apart from that, both Bragg resonances shift first to longer (shorter) wavelength as the nitrogen pressure increases (decreases) during the warming up (cooling down) period and we can see a good correspondence between the wavelength shift and the pressure reading. However, the magnitude of the shift for the fast axis Bragg resonance is larger than that for the slow axis. The wavelength correspondence to pressure and the change in peak separation can both be related to the change in refractive index that occurs due to the presence of the pressurized nitrogen gas in the micro holes. As the pressure increases, the refractive index will change accordingly. This also causes the Bragg wavelength to increase since the light from both modes overlaps partly with the air holes. However, the light guided along the fast-axis has more overlap with the micro-holes and therefore it experiences a larger change in effective refractive index (and wavelength) compared to the slow-axis and hence there is a net change in peak separation: a pressure increase corresponds to a drop in peak separation and vice versa, see Fig. 3. The peak separation decreases with 42 pm with a pressure increase of 80 bar and returns to its initial value when the autoclave pressure has been released. As stated before, this initial loading test with nitrogen was primarily intended as a sealing check of the autoclave and sealed senor. But it also nicely illustrates the effect of the gas in the micro-holes on the refractive index and wavelengths. This effect will also be present during the hydrogen loading tests, see next section.

Similarly, Fig. 2(b) shows the evolution of the Bragg resonances of the sealed MS-FBG sensor. Both Bragg resonances respond to pressure in the way it is understood for sealed samples of this fiber type [29,30]: the slow axis Bragg resonance features a positive and larger pressure sensitivity, whilst the fast axis Bragg resonance has a negative and lower pressure sensitivity during stages (1) and (2). The sealed MS-FBG sensor operates as an actual pressure sensor as expected. The sensing functionality of the sealed MS-FBG becomes obvious when considering the peak separation as shown in Fig. 3, which follows the reference pressure reading. The influence of temperature changes on the pressure measurements is negligible since both Bragg resonances experience the temperature variation as a common mode effect that is cancelled out by taking the difference. We obtain a pressure sensitivity of the sealed MS-FBG sensor of 2.8 pm/bar (a total peak separation change of 229 pm over a pressure difference of 81.7 bar). This proves that the sealed sensor is indeed properly sealed.

The FSG and DTG samples behave similarly to standard FBG based sensors during pressure cycling. At stage (1), a typical pressure sensitivity of around −0.2 pm/bar was observed. Afterward, the wavelength shifts are mainly dominated by the change in temperature. The net wavelength changes are around + 0.746 nm and + 0.651 nm for the DTG and FSG sensor respectively.

The findings of the nitrogen loading tests can be summarized as follows. The sealed MS-FBG acts as a pressure sensor as the change of hydrostatic pressure can be encoded into the change of peak separation and a positive pressure sensitivity in peak separation was observed. On the other hand, the open MS-FBG sensor is acting as a refractive index sensor since the pressure changes will predominantly cause the refractive index of the nitrogen in the micro holes to change and this in turns induces wavelength shifts in the light guided in the fast and slow axes. Due to the difference in overlap, the effect is larger in the fast axis and this causes a net negative change in peak separation.

Vol. 27, No. 4 | 18 Feb 2019 | OPTICS EXPRESS 5490

Fig. 2combi

Fig. 3combi

3.2 Hydroge

After the inihydrogen loadsince it is knconducted as monitoring thautoclave up tin step), (3) heating of thcompletely sta

3.2.1 Diffusin

Figure 4 showfor the open Mboth Bragg atmospheric p

2. Normalized wavined nitrogen pres

3. Bragg resonancined nitrogen pres

n loading

tial verificatioding on the dinown to diffusfollows: (1) lo

he pressure stto 80 °C and wcooling down

he autoclave babilized (diffus

ng in

ws the evolutioMS-FBG sensoresonances shpressure to 80

velength shifts of tsure and temperatu

ce peak separationsure and temperatu

on step with nfferent FBG sese into solids oading with hytability using wait until the se to room temback to 80 °Csing out step).

on of the indivor during stagehift to longer

bar hydrogen

the (a) open and (ure cycle.

n of the open andure cycle.

nitrogen gas, ensors. Hydroglike e.g. silic

ydrogen gas upthe reference

ensor readings mperature, (4) r

C and wait a

vidual Bragg rees (1) to (4) as

wavelengths gas at room t

(b) sealed MS-FB

d sealed MS-FBG

we also evalugen is expecte

ca. The hydrogp to 80 bar at r

pressure gauhave complete

release of the again until all

esonances ands described abo

when the prtemperature. H

BG sensor during a

G sensors during a

uated the infled to behave dgen loading teroom temperatuuge, (2) heatinely stabilized (hydrogen gas

sensor readin

d of the peak sove. It can be ressure increa

However, it can

a

a

luence of differently ests were ure while

ng of the (diffusing s and (5) ngs have

separation seen that ses from n be seen

Vol. 27, No. 4 | 18 Feb 2019 | OPTICS EXPRESS 5491

that the wavewavelengths micro-holes, observe againresulting in a any sign of leit is safe to in20 bar in the −13 pm. Thisincrease in theffect should holes. The mothe fiber glascore will alsseparation sin

Note that mainly requirFBG sensor iAfter cooling gas present inopen MS-FBG

Fig. 4during

Similarly, separation forFBG reacts toeach other, rseparation sliconstant. ThisMSF. The ditemperature towith the additemperature cpeak separatiodiffuses throusealed MS-FB

elength shifts ato stabilize. Twhich is not i

n that the shift net decrease o

eakage during tncrease the tem

closed autocla change in peae micro holes be present thatost plausible es at the level oo increase the

nce the light frothe long waiti

red for diffusios of the order odown (stage 3

n the glass andG sensor return

4. (a) Absolute wavg the hydrogen loa

Fig. 5 shows r the sealed Mo the 80 bar oresulting in anightly decreases indicates thatiffusion rate iso 80° yields anitional pressurclearly also speon almost dowugh the glass, tBG sensor is pl

are not instantaThis can be lininstantaneous of the fast ax

of the peak septhe first 24 hou

mperature up toave volume. Tak separation issince this shout is more impo

explanation is tof the core dure refractive inom the slow axing time of almon in the sealeof tens of minu

3) and release othe micro-hol

ns to its initial v

velength shifts andading experiment c

the evolution MS-FBG sensorof hydrogen gan increase of es along stage t the hydrogen s low becausen additional inre increase of eeds up the dif

wn to the initialthe differentiallaced in a hydr

aneous but it tanked to the difbecause the his Bragg resonparation, as shurs at room temo 80 °C, stage (The peak separs opposite to wuld result into aortant than the that this is the ring the initial ndex but its exis now has themost 150 houred MS-FBG seutes, in line wiof the hydrogenles diffuses outvalue.

d (b) change in pecorresponding to s

of the individur during stagesas with both B0.21 nm in p(1), while the

is already starte we are still ncrease in peak

20 bar at theffusion rate anl value at atmol pressure will rogen rich env

akes a certain ffusion of the

holes are initianance is largerhown in Fig. 4(mperature durin(2). It results inration then incwhat is expectea further decrerefractive inderesult from thheating. Hydr

effect will be e most overlap rs at 80°C and ensor. The diffith the time con gas pressure t again and the

eak separation of tstages (1) to (4).

ual Bragg resos (1) to (4). InBragg resonancpeak separatioe reference prting to diffuse at room temp

k separation of e beginning ofnd it leads to aospheric pressueventually com

vironment for a

amount of timhydrogen gas

ally filled withr than that of s(b). We cannong stage (1). Tn a pressure in

creases from −ed in view of aease. Thereforeex increase in the hydrogen perogen entering

opposite on with the glass 100 bar in sta

fusion in the oonstant reporte

(stage 4), the he peak separati

the open MS-FBG

onances and ofnitially, the seaces moving aw

on. Note that ressure reading

into the air hoperature. Incref 51 pm, whichf stage (2). Tha quick reductiure. Since the hmpletely vanisa sufficiently lo

me for the s into the h air. We slow axis, ot observe Therefore, ncrease of 21 pm to

a pressure e, another the micro enetrating the fiber the peak core.

age (2) is open MS-d in [28]. hydrogen ion of the

G

f the peak aled MS-way from the peak

g remains oles of the asing the

h matches he higher ion of the hydrogen

sh when a ong time.

Vol. 27, No. 4 | 18 Feb 2019 | OPTICS EXPRESS 5492

On the other hlonger wavelebehavior is simresonance is lat the end of the sealed MSFBG sensor isthe open MS-behavior in th

As we reledifferential prit largely excnegative (fromFig. 5(b). Dustarts to diffus

Fig. 5FBG s

Figure 6 sstages (1) to (pressure withslight increaseroom temperbeginning of and the hydrwavelength shNote that the loading. We diffusion in senecessary neithydrogen pre0.594 nm at rshifts are com

hand, the indivengths when tmilar to that olarger than thastage (2) as sh

S-FBG sensor s now filled wi-FBG. Both Mhe same hydrogease the pressuressure since noeeds the pressm + 0.21 nm iuring stage (4)se out of the m

5. Change of (a) nosensor during the h

shows the cha(4) of the hydr

h a minor and ne of 10-20 pmature and becstage (2), Bragrogen diffusionhift stabilizes awavelength shwill estimate ection 3.3. Simther for the FSssure, the remoom temperatu

ming purely from

vidual Bragg rethe hydrogen gf the open MS

at of the slow ahown in Fig. 5reaches a valuith hydrogen a

MS-FBG sensorgen rich enviroure in stage (3)ow the 80 bar osure at the outsinitial separatio, the peak sep

microstructure a

ormalized Bragg rhydrogen loading

ange in Bragg rogen loading negative pressu

m during the fircause the sensgg resonances n rate increasat + 1.248 nm ahift stems fromthe error in t

milar to the opeSG nor for the

maining hydrogure for the FSGm the hydroge

esonances of thgradually diffu

S-FBG sensor. axis. Both curv5(a). After almue of −12 pm. Tat 100 bar and 8rs (open and s

onment. ), the sealed Mof hydrogen is side. This is reon to −0.21 nmparation graduaat room temper

resonances and (b)experiment corres

wavelength foexperiment. Bure sensitivity rst 24 hours as sor gratings armove to longe

se. After arounand + 1.308 nm

m both the temtemperature anen MS-FBG seDTG sensor. A

gen induced wG and DTG senn diffusion into

he sealed MS-Fuses into the mThe waveleng

ves eventually most 150 hours,

The microstruc80 °C and hen

sealed) eventua

MS-FBG sensor trapped insideeflected in the

m after externaally increases rature.

) peak separation sponding to stages

or the FSG anBoth sensors sim

during stage ( the hydrogen re protected b

er wavelength nd 20 – 24 hm for the FSG

mperature differnd in pressureensor, the 150 hAfter cooling d

wavelength shifnsors, respectivo the silica.

FBG sensor botmicrostructure,gth shift for the

join almost co, the peak sepacture of the se

nce becomes idally feature an

r experiences ae the microstrue peak separatial pressure releagain as the h

of the sealed MS-(1) to (4).

nd DTG sensomply react to h(1). They also diffusion rate

by the coatingas both the temhours in stageand DTG, resprence and the he induced by hhour waiting tidown and releaft is + 0.567 nvely. These wa

th shift to , and this e fast axis ompletely aration of aled MS-

dentical to identical

a negative ucture and ion being ease), see hydrogen

-

ors during hydrogen feature a is low at

g. At the mperature e (2), the pectively. hydrogen hydrogen ime is not ase of the nm and + avelength

Vol. 27, No. 4 | 18 Feb 2019 | OPTICS EXPRESS 5493

Fig. 6hydro

3.2.2 Diffusin

In stage (5), tprocess. FiguThe hydrogenas the autoclanecessary fortemperature cexpected that effect.

Fig. 7sensor

Similarly, the out-diffuscomplementarout of the micin a decrease almost 150 hosealed MS-FB

6. Change of Braogen loading exper

ng Out

the autoclave ture 7(a) shows n contained in tave pressure wr the open Mchange, as show

both Bragg re

7. (a) Absolute wavr during hydrogen

Fig. 8 shows sion step. Botry way to the rcrostructure leaof the individu

ours, the peak sBG sensor has b

agg resonance forriment, correspond

temperature wathe Bragg wa

the air holes hawas released (

MS-FBG sensorwn in Fig. 7(besonances expe

velength shifts and out-diffusion.

the wavelengtth the Bragg wresults reporteading to a decrual Bragg resonseparation reacbeen restored t

r (a) FSG in pureding to stages (1) to

as increased toavelength evoluas already diffustage 4). Therr: both Bragg). The peak seerience the tem

d (b) change of pe

th evolutions fwavelengths ad in Fig. 5. Threase of the tranances and an ches zero, indicto atmospheric

re silica fiber ando (4).

o 80 °C to speutions for the used out of therefore, this oug resonances separation remaimperature chan

ak separation for t

for the sealed Mand the peak he trapped hydapped hydrogen

increase of thecating that the

c pressure.

d (b) DTG in the

eed up the out-open MS-FBG

e microstructurut-diffusion stasimply responins stable, indinge as a comm

the open MS-FBG

MS-FBG sensseparation evo

drogen gas nown pressure and e peak separatiinternal pressu

e

-diffusion G sensor. re as soon age is not nd to the icating as

mon mode

G

or during olve in a

w diffuses resulting

ion. After ure of the

Vol. 27, No. 4 | 18 Feb 2019 | OPTICS EXPRESS 5494

Figure 9 sthe out-diffusdiffuses out o

Fig. 8during

Fig. 9hydro

3.3 Hydroge

For the sealedlinear calibratby the pressurtemperature s

where λB1,0 atemperature apressure and pm/°C, c = 2.equation can

shows the chansion. The hydrof the fiber.

8. Change of (a) Bg the hydrogen out

9. Change of Braogen out-diffusion.

n-induced me

d MS-FBG sention model. It re sensitivity (aensitivity (b an

and λB2,0 indiand pressure. temperature ca.725 pm/bar an

be inverted

nge of the Bragogen induced w

Bragg resonances at-diffusion.

agg resonances for

easurement e

nsors, the chaninvolves the c

a and c) and thnd d), as given

a b

c d

Δ

cate the fast The sensitivit

alibrations. Thnd d = 9.4801 and doing so

gg resonances wavelength sh

and (b) peak separ

r (a) FSG in pur

error

nge of the Bracontribution ofhat of the tempby Eq. (1) [31

1

2

B

B

p

T

λ λλ λ

−Δ = −Δ

and slow axity coefficientshey were found

pm/°C. Since o yields the c

for the FSG anhifts gradually

ration of the sealed

re silica fiber and

agg resonancesf the pressure perature change

].

1,0

2,0

B

B

λλ

is Bragg resos could be obd to be a = −0the coefficien

calibration for

nd DTG sensovanish as the h

d MS-FBG sensor

d (b) DTG during

s can be descrichange (Δp) m

e (ΔT) multipli

onances at a btained from d0.1 pm/bar, b

nts are not idenrmulas to con

ors during hydrogen

r

g

ibed by a multiplied ied by the

(1)

reference dedicated = 9.4705

ntical, the nvert the

Vol. 27, No. 4 | 18 Feb 2019 | OPTICS EXPRESS 5495

wavelengths into pressure and temperature data, see Eq. (2). These equations will be used to calculate the pressure and temperature errors by comparing the calculated values with the reference measurements.

1 1,0 2 2,0

2 2,0 1 1,0

1[ ( ) ( )]

1[ ( ) ( )]

B B B B

B B B B

p d bad bc

T a cad bc

λ λ λ λ

λ λ λ λ

Δ = − − − −Δ = − − − −

(2)

For the open MS-FBG sensor, the above equations and coefficients are still used to calculate the corresponding temperature and pressure. The wavelength changes of the open MS-FBG sensor are mainly originating from the refractive index modifications when hydrogen is present in the air holes and in the fiber glass. The open MS-FBG sensor can quickly reveal the response of both Bragg wavelengths when hydrogen diffuses through the open channels. Doing so yields the data presented in Fig. 10(a). The temperature error for the open MS-FBG sensor at 80 °C and 100 bar of hydrogen is around 74 °C, which comes purely from the refractive index changes from the hydrogen in the MS-FBG. Similarly, the refractive index induced drop in peak separation leads to an apparent pressure drop of −4.6 bar, causing the overall pressure error to increase from 100 to 104.6 bar.

For the sealed MS-FBG sensor, the influence on temperature monitoring is rather limited in the beginning when the hydrogen diffusion rate is low, as shown in Fig. 10(b). The temperature difference however starts to grow when the hydrogen starts to diffuse into the air holes and through the glass, yielding eventually to a total induced temperature error of around 75 °C. We thus find good agreement between the two MS-FBG sensors at the end of the hydrogen diffusing in step. In terms of pressure monitoring, the pressure reading in the sealed MS-FBG sensor gradually drops to zero as the hydrogen diffuses into the air holes. One could expect that the pressure difference becomes zero from the moment when the hydrogen pressure in the air holes is the same as in the autoclave. But due to the difference in effective refractive index induced by the hydrogen in the air holes and glass, the calculated pressure for the sealed MS-FBG sensor will be approximately 4 bar lower. Again, good agreement between open and sealed MS-FBG sensors could be found.

When the hydrogen is removed from the autoclave, the sealed sample initially gives a similar temperature difference as just before the hydrogen removal. The pressure error becomes negative because the pressure of the trapped hydrogen is larger than the pressure in the autoclave and hence the net pressure difference is negative. During the out-diffusion, both the temperature as well as the pressure error gradually restore to zero.

For the conventional FBG sensors, the response of the temperature changes can be approximated to the response of the wavelength variations with a linear relationship. The used linear coefficients are 10.25 pm/°C and 11.6 pm/°C for the FSG and for the DTG sensor, respectively. Figure 11 shows the calculated temperature during the hydrogen loading test for the FSG and DTG. Due to the diffusion of hydrogen, the wavelength shifts will be translated into apparent temperature shifts. Eventually, the temperature differences are around 58 °C and 49 °C for the FSG and DTG sensor, respectively. These apparent temperature shifts will vanish again once all hydrogen has diffused out from the fiber.

In these initial experiments, the pressure and temperature errors have been quantified under a pure hydrogen gas environment. In the next step, we will study the influence on the same set of FBG sensors in case of hydrogen-nitrogen gas mixtures.

Vol. 27, No. 4 | 18 Feb 2019 | OPTICS EXPRESS 5496

Fig. 1during

Fig. 1hydro

4. Experime

For gas mixtusum of the inare concernehydrogen preexperimentedMS-FBG senused the FSG

4.1 Nitrogen

We follow a two gases. (1)to 80 °C and temperature aheat the autocdown to room80 °C again a

10. Calculated temg the hydrogen loa

1. Calculated temogen loading tests.

ents with sen

ures, Dalton’s ndividual partiaed, hydrogen ssure and shou

d with sensors sors, one opensince the beha

-hydrogen loa

procedure that) first we load 2wait until the d

and then we inclave again to

m temperature and we wait for

mperature and presading tests.

mperature and press

nsors in a ga

law of partial al pressures of induced measuld not be relaexposed to a n

n and one sealavior of the DT

ading

t is similar as 20 bar of hydrodiffusion is co

ncrease the tota80 °C and waand then we rer the out-diffus

ssure for the (a) op

sure from the (a) F

as mixture

pressures stateeach gas [34]

surement erroated to the ovenitrogen-hydroled. As for theTG was found t

described in thogen at room t

omplete, (3) weal pressure up it sufficiently elease the pression to comple

pen and (b) sealed

FSG and (b) DTG

es that the ove. Hence, as far

ors should onerall gas press

ogen gas mixtue conventional to be very simi

he preceding stemperature, (2e let the autoclto 80 bar by along, (5) we lessure, (6) we hete and (7) fina

d MS-FBG sensor

G sensor during the

erall pressure er as FBG base

nly result fromsure. To verifyure. We used a

FBG sensors,ilar.

sections, but n2) we heat the alave cool downadding nitrogeet the autoclavheat the autoclaally we let the a

r

e

equals the d sensors m partial y this, we again two , we only

now using autoclave n to room n, (4) we

ve to cool ave up to autoclave

Vol. 27, No. 4 | 18 Feb 2019 | OPTICS EXPRESS 5497

to cool dowtemperature a

4.2 Nitrogen

4.2.1 Diffusin

The results prsealing conditcycle and is ntemperatures and the diffupressure errorthe induced rscheduled popreserved in loaded into thpressure differefractive indrelease of the

Fig. 1MS-F

The resultpressure errorstage (2). Fro−1.4 bar but adiffusion is nautoclave to hydrogen andtemperature isthe continuatiapproximatelytotal of 27.3 from the senswith 20 bar of

wn to room teand pressure err

-hydrogen ind

ng in

resented in thition of the autonot presented for the open M

usion of hydrogr is 27.3 bar (26refractive inde

ower maintenathe autoclave

he autoclave, reerence increaseex in the air hpressurized ga

12. Calculated temBG sensor during

ts for the sealers are gradual

om the open saat the end of stanot yet fully cocreate the mix

d therefore sees increased to 8ion from the pry 30 bar belowbar coming frositivity coefficif hydrogen com

emperature agrors like it was

duced measu

is section correoclave was chein the plots be

MS-FBG samplgen into the f6 bar actual prex changes). Tance. Nevertheafter the main

esulting into a e from −1.4 b

holes. These errases from the o

mperature and presthe different loadi

ed MS-FBG sely increasing

ample, we wouage (2) it has r

ompleted. Durixture. The seales only around80°C. Initially,revious step. Itw the referenceom the open sients (a, b, c ampared to a sim

gain. We wills done in sectio

urement error

espond to the ecked in the firelow. Figure 1le. Due to the

fiber glass, theessure of hydroThe gap betweless, the loantenance. Nexttemperature er

bar to −11 barrors stay const

open MS-FBG

ssure as a comparing steps with the

ensor are preseas hydrogen i

uld expect the reached only aring stage (3), 6led MS-FBG sd 59 bar (80 −, we can still set gradually leve pressure. Thisensor but thisand d), that aremilar sensor fill

l immediatelyon 3.3.

r

aforementionest 84 hours bef12 shows the cpresence of hy

e temperature ogen at 80 °C

ween stages (3aded hydrogent, an additionarror increase frr due to the ctant for the nexsensor.

rison to the referehydrogen-nitrogen

ented in Fig. 13is diffusing intfinal apparent

round −0.9 bar60 bar of nitrosensor already−20 - 0.9 bar)ee some remainels off to a conis is slightly lo

s difference is e slightly diffeled with 1 bar

y look to the

ed stages (1) -fore starting thcalculated presydrogen in the error is 20 °Cand 1.4 bar ext

3) and (4) is n gas is still al 60 bar of nirom 20°C to 48change of the xt 144 hours b

ences for the openn gas mixture.

3. The temperto the air holet pressure readr. This indicateogen is injectedy has 20 bar o). During stagening diffusion,nstant reading,ower than the most likely or

erent for a senof air.

induced

- (4). The he loading ssure and air holes

C and the tra due to due to a properly

itrogen is 8°C and a effective

before the

n

ature and es during

ding to be es that the d into the

of trapped e (4), the , which is , which is expected

riginating nsor filled

Vol. 27, No. 4 | 18 Feb 2019 | OPTICS EXPRESS 5498

Fig. 1MS-F

So to sumthe induced tshown in Fig.the induced teTherefore, thethe partial hyd

The apparduring stage (temperature combination wTherefore, als

4.2.2 Diffusin

This section calculated temdiffused out oof tens of midiffuse out tonegative pressthe gas mixtupressure diffeat 80 °C. In th

5. Summary

In this documvalidated exptemperature ainterest were (MS-FBG). Bhydrogen andkept in an auevaluation, thcompared wit

3. Calculated temBG sensor during

mmarize: in an etemperature an. 10(b). And inemperature ande hydrogen-inddrogen pressurrent temperatur(2) and aroundwith 5°C comwith the applieso here no extra

ng out

reports the remperature and of the open air nutes. For theo the autoclavsure differenceure has been reerence for the ohe end, both ca

y and conclu

ment, the effecperimentally. Tand pressure m

the femtosecoBoth open as wd later on also utoclave wherhe measured th the readings

mperature and presthe different loadi

environment wnd pressure errn case the partid pressure erroduced error in re and the errorre shift for the d 20 °C duringmes primarilyed pressure (8a temperature e

esults of stagpressure returholes. The out sealed MS-FB

ve after the gae around the coeleased. The amopen MS-FBG alculated readin

usion

ct of hydrogenThe focus wa

monitoring, whond written FB

well as sealed sato a hydrogen

re both pressupressure and from the refer

sure as a compariing steps with the

with a partial hyror can reach ial hydrogen pors also reducetemperature an

rs scale almost FSG sensor in

g stage (4), resy from the p0 bar during serror could be

es (5) - (7). rn to their norm-diffusion procBG, the trappeas mixture in

ore region is felmount of negawhen its air ho

ngs match with

on different Fas primarily oere hydrogen i

BG sensors in amples of this

n-nitrogen mixture and tempe

temperature rence sensors s

ison to the referenhydrogen-nitrogen

ydrogen pressu104 bar and

pressure is redue to 27.3 bar and pressure selinearly with t

n the gas mixtuspectively. Thipressure sensistage 4 insteadobserved by ad

For the openmal status whcess for the FSed hydrogen inthe autoclave

lt by the sealedative pressure oles are filled wh the reference

FBG based opon FBG sensois known to bethe Butterfly MS-FBGs we

ture. In both cerature could

from the FBo that the meas

nces for the sealedn gas.

ure of 100 bar 75 °C, respect

uced to 26 bar and 20 °C, respensing is only rthe partial presure test is arouis reduction in tivity of the

d of 20 bar in dding the nitro

n MS-FBG sehen the gas miSG sensor is of n the air holese has been reld MS-FBG senalso corresponwith 26 bar of hgauges again.

ptical fiber senors used for de present. Of pMicro-Structu

ere exposed fircases, the sampbe controlled.

BG-based sensurement error

d

at 80 °C, tively, as at 80 °C,

pectively. related to ssure. und 25 °C

apparent FSG in stage 2).

ogen.

ensor, the xture has

f the order s starts to leased. A nsor when nds to the hydrogen

nsors was downhole particular ured fiber st to pure ples were . For the sors was rs coming

Vol. 27, No. 4 | 18 Feb 2019 | OPTICS EXPRESS 5499

from the hydrogen could be quantified. From the pure hydrogen loading tests, it was found that the responses from the sealed samples evolved towards those of the open samples and eventually, their response was found to be identical. This indicated that the hydrogen diffuses through the glass into the microstructure for the sealed samples. Because the diffusion time is different for the closed compared to the open samples, we could clearly separate the two main mechanism behind the hydrogen induced errors: (1) the first being the disappearance of the (partial) hydrogen pressure from the pressure reading due to the diffusion of the hydrogen through the glass into the microstructure and (2) the second being a change of the individual Bragg wavelengths and also of the peak separation due to a change in the refractive index originating from the hydrogen entering the micro structure and penetrating the fiber glass. The changes in the individual wavelengths are causing apparent temperature shifts whereas a change in the peak separation causes an apparent pressure shift.

The tests with the hydrogen-nitrogen mixture was done to confirm that the driving parameter behind this process is the partial hydrogen pressure. This could be confirmed and it indicates basically that the size of the hydrogen induced errors scale almost linearly with the partial hydrogen pressure. Although the pressure and temperature errors shown in this work may seem relatively large, it should be kept in mind that these are for relatively large partial hydrogen pressures (80 bar and 20 bar). In practice however, the hydrogen level in a downhole environment is typically a few order of magnitude smaller. Therefore, the hydrogen induced errors will scale accordingly and therefore can be expected to be much smaller. The adaptability of the MS-FBG sensor for temperature and pressure monitoring in downhole applications is feasible if the hydrogen level is of the order of the present work. Some mitigation techniques would be required for MS-FBG sensor if the hydrogen induced error is above the required measurement accuracy.

Funding

European Space Agency GSTP6.2; the Belgian Science Policy. Fonds de la Recherche Scientifique - FNRS (F.R.S.-FNRS).

Acknowledgments

The authors would like to acknowledge financial support from Methusalem and Hercules foundations.

References

1. J. Eck, U. Ewherido, J. Mohammed, R. Ogunlowo, J. Ford, L. Fry, S. Hiron, L. Osugo, S. Simonian, T.Oyewole, and T. Veneyrso, “Downhole monitoring: The story so far,” Oilfield Rev., 11(3), 18–29 (1999).

2. T. K. Kragas, B. F. Turnbull, and M. J. Francis, “Permanent fiber optic monitoring at Northstar: pressure/temperature system and data overview,” SPE Western Regional/AAPG Pacific Section Joint Meeting (2002), Society of Petroleum Engineers.

3. P. Ranjan and G. Mcolpin, “Fiber optic sensing: Turning the lights on downhole,” (Halliburton, USA, November 2013), http://www.halliburton.com/public/pinnacle/contents/Papers_and_Articles/web/hart-energy-fiber-optics.pdf

4. S. Ma, J. Guo, Y. Guo, J. Cao, and B. Zhang, “On-line monitoring system for downhole temperature and pressure,” Opt. Eng. 53(8), 87102 (2014).

5. C. G. Blount, K. E. Friehauf, B. E. Smith, and D. P. Smith, “Improving fiber-optic system reliability in permanent installations,” SPE Annual Technical Conf., SPE-174928-MS, 1–13 (September 2015).

6. I. Majid, K. Wei, G. Drenzek, C. H. Wang, and A. Soufiane, “Novel optical fiber design for hydrogen-rich harshenvironments,” Proc. SPIE 6004, 60040H (2005).

7. G. Brown, “Downhole temperatures from optical fiber,” Oilfield Rev. (20) 4 (2009). 8. A. D. Kersey, “Optical fiber sensors for permanent downwell monitoring applications in the oil and gas

industry,” IEICE Trans. Electron. 83(3), 400–404 (2000).9. X. Qiao, Z. Shao, W. Bao, and Q. Rong, “Fiber Bragg grating sensors for the oil industry,” Sensors (Basel)

17(3), 429 (2017).10. D. Brondel, R. Edwards, A. Hayman, D. Hill, S. Mehta, and T. Semerad, “Corrosion in the oil industry,” Oilfield

Rev. 6(2), 4–18 (1994).11. J. P. Lemaire, “Reliability of optical fibers exposed to hydrogen: prediction of long-term loss increases,” Opt.

Eng. 30(6), 780–789 (1991).

Vol. 27, No. 4 | 18 Feb 2019 | OPTICS EXPRESS 5500

12. G. Williams, G. Brown, W. Hawthorne, A. Hartog, and P. Waite, “Distributed temperature sensing (DTS) to characterize the performance of producing oil wells,” Proc. SPIE 4202, 39–54 (2000).

13. W. Hill, J. Kübler, and M. Fromme, “Single-mode distributed temperature sensing using OFDR,” Proc. SPIE 7653, Fourth European Workshop on Optical Fibre Sensors, 7653, 1–4 (9 September 2010).

14. M. Jaaskelainen, ““Fiber optic distributed sensing applications in defense, security, and energy,” Proc. SPIE 7316,” Fiber Optic Sensors and Applications VI 7316, 1–9 (2009).

15. K. Suh and C. Lee, “Auto-correction method for differential attenuation in a fiber-optic distributed-temperaturesensor,” Opt. Lett. 33(16), 1845–1847 (2008).

16. B. Malo, J. Albert, K. O. Hill, F. Bilodeau, and D. C. Johnson, “Effective index drift from molecular hydrogendiffusion in hydrogen-loaded optical fibres and its effect on Bragg grating fabrication,” Electron. Lett. 30(5),442–444 (1994).

17. Y. Liu, K. Zheng, Y. Chen, T. Chen, and S. Jian, “Comparative study on effects of uniform UV pre�exposureand post�exposure on fiber Bragg gratings written on hydrogen�loaded fiber,” Microw. Opt. Technol. Lett. 46(4), 350–353 (2005).

18. L. B. Fu, G. Tan, W. J. Xu, H. L. An, X. M. Cui, X. Z. Lin, and H. D. Liu, “Ultraviolet-initiated reactions ofH(2) with germanosilicate fibers and H(2) concentration dependence of the Bragg wavelength of a fiber grating,” Opt. Lett. 25(8), 527–529 (2000).

19. Y. Masuda, M. Nakamura, C. Komatsu, K. Fujita, M. Yamauchi, M. Kimura, Y. Mizutani, S. Kimura, Y. Suzaki, T. Yokouchi, K. Nakagawa, and S. Ejima, “Wavelength evolution of fiber Bragg gratings fabricated from hydrogen-loaded optical fiber during annealing,” J. Lightwave Technol. 22(3), 934–941 (2004).

20. P. L. Swart and A. A. Chtcherbakov, “Study of Hydrogen Diffusion in Boron/Germanium Codoped OpticalFiber,” J. Lightwave Technol. 20(11), 1933–1941 (2002).

21. P. L. Swart, A. A. Chtcherbakov, W. L. Joubert, and M. G. Shlyagin, “Study of the pressure dependence of hydrogen diffusion in optical fiber by an interferometric technique,” Opt. Commun. 217(1–6), 189–196 (2003).

22. C. Martelli, A. Mendez, A. L. C. Triques, A. M. B. Braga, J. Canning, K. Cook, R. Llerena, and V. Takahashi, “Impact of hydrogen-induced effects on optical fiber Bragg gratings,” Proc. SPIE 7753, 775385 (2011).

23. P. St. J. Russell, “Photonics-crystal fibers,” J. Lightwave Technol. 24(12), 4729–4749 (2006).24. C. Jewart, K. P. Chen, B. McMillen, M. M. Bails, S. P. Levitan, J. Canning, and I. V. Avdeev, “Sensitivity

enhancement of fiber Bragg gratings to transverse stress by using microstructural fibers,” Opt. Lett. 31(15),2260–2262 (2006).

25. D. Grobnic, S. J. Mihailov, R. B. Walker, G. Cuglietta, and C. W. Smelser, “Hydrogen detection in high pressure gas mixtures using a twin hole fibre Bragg grating,” Proc. SPIE 7753, 77537D (2011).

26. Y. Wang, H. Bartelt, M. Becker, S. Brueckner, J. Bergmann, J. Kobelke, and M. Rothhardt, “Fiber Bragg grating inscription in pure-silica and Ge-doped photonic crystal fibers,” Appl. Opt. 48(11), 1963–1968 (2009).

27. J. Villatoro, M. P. Kreuzer, R. Jha, V. P. Minkovich, V. Finazzi, G. Badenes, and V. Pruneri, “Photonic crystal fiber interferometer for chemical vapor detection with high sensitivity,” Opt. Express 17(3), 1447–1453 (2009).

28. T. Geernaert, M. Becker, P. Mergo, T. Nasilowski, J. Wójcik, W. Urbanczyk, M. Rothhardt, C. Chojetzki, H. Bartelt, H. Terryn, F. Berghmans, and H. Thienpont, “Bragg grating inscription in GeO2-doped microstructured optical fibers,” J. Lightwave Technol. 28(10), 1459–1467 (2010).

29. T. Martynkien, G. Statkiewicz-Barabach, J. Olszewski, J. Wojcik, P. Mergo, T. Geernaert, C. Sonnenfeld, A. Anuszkiewicz, M. K. Szczurowski, K. Tarnowski, M. Makara, K. Skorupski, J. Klimek, K. Poturaj, W. Urbanczyk, T. Nasilowski, F. Berghmans, and H. Thienpont, “Highly birefringent microstructured fibers withenhanced sensitivity to hydrostatic pressure,” Opt. Express 18(14), 15113–15121 (2010).

30. T. Geernaert, F. Berghmans, and H. Thienpont, “Butterfly microstructured fiber complements FBG-based sensing,” Laser Focus World 51(7), 43–47 (2015).

31. J. Y. Huang, J. Van Roosbroeck, J. Vlekken, A. B. Martinez, T. Geernaert, F. Berghmans, B. Van Hoe, E. Lindner, and C. Caucheteur, “FBGs written in specialty fiber for high pressure/high temperature measurement,”Opt. Express 25(15), 17936–17947 (2017).

32. P. J. Lemaire, R. M. Atkins, V. Mizrahi, and W. A. Reed, “High pressure H2 loading as a technique for achieving ultrahigh UV photosensitivity and thermal sensitivity in GeO2 doped optical fibers,” Electron. Lett. 29(13), 1191–1193 (1993).

33. K. O. Hill and G. Meltz, “Fiber Bragg grating technology fundamentals and overview,” J. Lightwave Technol. 15(8), 1263–1276 (1997).

34. M. Silberberg, Chemistry: the molecular nature of matter and change, 5th ed. (McGraw-Hill, 2009).

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