SNS Salting Study

April 2018

Impact of Salt Deposition on Production Losses

© OGA 2018

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Contents

FBHP1 Executive summaryBHP2 Background

3 Production losses due to salt formation

4 Operator salt management strategies

5 Salt prediction, monitoring and surveillance

6 Salt mitigation

7 EBN and TNO led initiatives

8 Recommendations for further work

Gas fields in the Southern North Sea (SNS) arecurrently experiencing production losses due to saltdeposition. Halite precipitation occurs in the SNS duea number of reasons, including high salinity information water, methanol overdosing, excessivedrawdown, pressure depletion and changes inreservoir conditions.

In 2017, the Oil and Gas Authority (OGA) carried outa study to quantify the impact of salt deposition onproduction losses, with participation from sevenSouthern North Sea operators who have either directexperience of salting and salt management, oranticipate salt formation and deposition at a futurepoint in the lifecycle of their facilities.

The study evaluated the extent of self-scaling saltdeposition across the SNS, the effectiveness ofcurrent treatment options and methods of mitigation.The study also established a view on productionlosses in the SNS due to salting.

The study identified that at least one fifth of allproducing fields in the SNS are likely to be affectedby salt deposition with a diverse range of saltmitigation measures adopted, but with limitedconsistency of approach across operators.

Based on the OGA’s 2016 UKCS StewardshipSurvey, SNS production efficiency (PE) is estimatedat 64%. This is the lowest across the UKCS. TheOGA estimates daily production loss totalling 130mmscf/d attributable to salting, equivalent to 20% ofthe SNS’s total production losses.

1. Executive summary

Figure 1: Illustration of salt build-up and blockages in topsides gas flowlines and processing equipment. Images courtesy of Perenco UK.

SNS Salting Study 4

1 in 5At least

SNS producing fields are likely to be affected

by salt deposition

PE in the SNS is the

in the UKCS, with

of production losses attributable to salting

worst

20%

2. Background

Production wells in gas reservoirs can experience rapidperformance decline as recovery progresses. In manycases, this behaviour may be attributed to haliteprecipitation in the near well bore area around theperforated pay zone, but it can also deposit within thewellbore, tubing or within the process plant.

Salt deposition is a problem encountered in a number ofUKCS SNS gas fields as well as similar reservoirsacross the North Sea. Salt deposition gives rise to areduction in well deliverability and flow rates and inextreme cases blockages.

The OGA study evaluated the extent of self-scaling saltdeposition in UKCS SNS gas fields, including predictivetechniques, monitoring and surveillance of the wellborefor salt deposition, options for mitigation and removaland possible further study.

Salt in this context refers to sodium chloride (althoughother salts such as calcium chloride may be present).Elsewhere, in HPHT reservoirs, other self-scaling salts,for example zinc sulphide, may form but such scaleswere not investigated in the OGA study.

Salt deposition is relatively common and is believed tobe a problem for at least 20% of the SNS gas fields inthe UKCS. Despite the prevalence of salting, there aresignificant gaps in the industry’s ability to predict thepropensity of salting and potential for formation damage.Technology for the removal of salt in UKCS SNS wellsrelies mainly on intermittent water washing, althoughcapillary strings are being used in some wells. Thereappears to be useful opportunities to encourage thesharing of industry experience, knowledge and bestpractice in dealing with salt deposition and also topromote or sponsor research into salt deposition.

Salt is predominantly sodium chloride (NaCl) and itsformation is a self-scaling phenomenon, in contrast totypical scale formation which usually occurs whendifferent waters mix. Its formation is more commonly aproblem in gas wells.

There are two main mechanisms of salt deposition inhydrocarbon producing wells:

1. The first is due to gas stripping fresh water fromthe produced formation water at reducedpressures which increases brine salinity. In thiscase, salt deposition is normally experienced ingas wells with a low formation water to gas(WGR) ratio (at the same magnitude as watercontent in the reservoir gas, e.g. < 5 stb/MMscf).Salt drop-out most likely occurs in the formation,near the wellbore and at the wellbore.

Wells affected by salting can experience a ‘saw-tooth shape’ in their production profile and adramatic reduction in the well productivity causedby salt induced formation damage and formationof the bridge plugs in the completion. Watersamples collected from topsides can be of lowsalinity (e.g. < 10,000 mg/l total dissolved salts)due to dilution by the condensed water from theproduced gas, and sometimes only condensedwater.

2. The second mechanism of salt deposition is dueto halite solubility reduction at a reducedtemperature and pressure. For this condition toapply, salt deposition is normally experienced ingas wells with a high WGR (e.g. WGR > 20stb/MMscf). Water samples collected fromtopsides will show high salinity or will besaturated with halite. Salt drop-out in thisscenario is expected in the upper completion andsurface facilities.

Salt deposition usually occurs in mature, depletedgas fields, though can also occur in the early life ofsome wells. The solubility of water in natural gasincreases at lower pressures and gas can becomeunder-saturated with respect to water after pressuredrops, as occurs when gas flows into a well. Suchsalting is observed in gas fields across the UK andDutch sectors of the SNS.

5 SNS Salting Study

The OGA has estimated the impact of salting onrecoverable reserves using a representative sampleof wells known to be affected by salting.

Well histories (including decline curves and P/Zplots), have been obtained from operators whichhighlight the key events in the lifecycle of a well.These have been used to compare recoverablereserves both pre- and post- salt deposition and inturn provide an estimate of potential productionlosses.

Using the above approach across the currentlyaffected SNS well stock, the OGA has estimatedsome 96 bcf of losses due to salting over a two yearperiod, equivalent to 130 mmscf/d or 20% of theSNS’s production losses in 2016.

From the study undertaken by the OGA, there isevidence that some SNS operators mayunderestimate the impact of their production lossesattributable to salting, with the majority estimating theimpact of salting to be less than a 10% contributor totheir losses.

Production efficiency returns to the OGA (ref: OGApublication UKCS Production Efficiency in 2016),currently provide limited insight into the extent oflosses due to salting, beyond the bespoke analysisundertaken by the OGA as part of the 2017 study.

There is an opportunity to therefore improve theapproach taken to production loss measurement inthe UKCS by aligning with that of EBN in the Dutchsector. This could provide a more comprehensivedata set and insight into the magnitude of saltinglosses across the SNS and therefore the level ofeffort required to mitigate such losses.

3. Production losses due to salt formation

Figure 2: Well histories both pre- and post- salt deposition havebeen used to provide an estimate of potential production losses.

SNS Salting Study 6

The OGA study involved a series of structuredinterviews with a selected number of SNS operatorswith known salt management issues and challenges.The following table seeks to summarise some of thefindings from those interviews, including the level ofconsistency and approach to the management of salt.It is evident from the table that there are a diversenumber of approaches to the management of saltacross the SNS, along with a common request forsupport and the establishment of a forum for thedissemination of best practice on the subject.

Ope

rato

r

Salt

Man

agem

ent

Stra

tegy

Salt

Pred

ictio

n An

alys

is

Monitoring Surveillance Mitigation

Dai

ly P

VT

PW s

alin

ity

chec

k

Inve

stig

ate

prod

uctiv

ity

step

cha

nge

Wel

l in

terv

entio

n /

Wire

line

surv

ey

Man

ual d

ilute

m

etha

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ash

Man

ual f

resh

w

ater

was

h

Rem

ote

was

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clud

ing

foam

ing

skid

sW

irelin

e/

Coi

led

Tubi

ng

inte

rven

tion

Con

tinuo

usfr

esh

wat

er

inje

ctio

n

A ● ● ● ●B ● ● ● ●C ● ● ●D ● ● ●E ● ● ● ● ● ●F ● ● ● ● ●G ● ● ● ● ● ● ● ●

4. Operator salt management strategies

Figure 3: Approach to salt management by SNSoperators included within the OGA study

7 SNS Salting Study

Daily Monitoring of

WHP, Temperature

and Flowrates 50%

Wireline Sampling

& HUD run20%

Wireline Camera Surveys

10%

Monitor Topside Pressure

drop10%

Monitor Produced

water salinity

10%

Drawing on the insights from the OGA study, producedwater salinity is the leading cause of salt precipitation,with deposition occurring as pressure and temperaturereduces in the well bore, as shown in Figure 4.

Building on this, more than half of the wells reviewed inthe study affected by salting in the UKCS wereassociated with Permian age fields, followed closely byCarboniferous age fields. Carboniferous fields howevertypically have higher salt concentrations in formationwater compared to Permian and Triassic fields.

The OGA study identified limited evidence of predictiveanalysis or techniques to identify the onset of saltingand furthermore halite deposition envelopes. This isdespite the availability of proprietary software tools.Similarly, very few operators were found to have agood understanding of formation water composition,nor did they carry out regular sampling and analysis ofproduced water fluids in support of their predictiveanalysis.

Well performance (rate, temperature and pressure)was found to be the primary means by which operatorsdiagnose potential salting issues downhole and atsurface, as shown in Figure 5. Thereafter, wirelinecamera surveys and sampling surveys were found tobe undertaken on an infrequent basis and consistentwith the intrusive nature of such activities.

There is a clear opportunity for SNS operators toimprove the management of gas fields with salt issuesby using established techniques and software topredict the onset of salt deposition. Improvements inthe regularity and frequency of produced watersampling for salinity is another key measure andtypically impacted by the more limited access to “notnormally attended installations” (NPAIs), which arecommonplace in the SNS.

Low WGR22%

Methanol overdosin

g22%

Excessive drawdown

11%

Water production

45%

5. Salt prediction, monitoring and surveillance

Figure 5: Monitoring andsurveillance techniques as identifedby the 2017 OGA salting study

Figure 4: Causes of salt formation asidentifed by the 2017 OGA salting study

SNS Salting Study 8

Though water washing is the most common method ofsalt removal, there are a range of approaches adopted byoperators, from bullhead batch washing through tocontinuous down hole capillary washing, as shown inFigure 6). A summary is provided below:

• Manual Washes. Treatments requiring manualintervention can often be delayed due to theavailability of fresh water for subsequent treatment ofthe wellbore but also the ability to access SNS NPAIs

• Remote Water Wash Skid Installation. Wells whichhave been proven to have significant salt problemshave had specialist water wash skids installed toallow regular washes to be conducted without theneed for access to NPAIs

• Trickle washes. Pumping water downhole whilst awell is shut-in but with the sub-surface safety valve(SSSV) open. Due to the prolonged period duringwhich the tubing is exposed to a flow of water, this isconsidered to have the greatest effect when there issalt deposition at multiple points within the tubing

• Batch and fall wash. The wash is performed with thewell shut in and the SSSV closed. Once a givenvolume of water has been pumped, the SSSV isopened and the water dropped as a slug. Thistechnique is believed to be best utilised when there isa “salt bridge” formed in the lower completion

• Continuous downhole capillary wash. Small borecapillaries are installed as part of the completion andfacilitate continuous water wash of the well bore.Installation of a permanent capillary must ideally beplanned along with the completion of the well

• Gargling. Gargling is the process of opening the wellon a limited choke whilst performing a water washsoak. This is intended to encourage the dissolving ofa potential salt bridge

• Use of Potassium Chloride (KCl). A 3% KClsolution is used in some wells as the wash water, inorder to limit clay swelling (particularly whereSmectite rich clays are present) and associatedformation damage

Consistent with common practice, the OGA studyidentified that water washing accounted for almosthalf of the salt removal interventions carried out in theSNS, with a success rate of approximately 70%reported by operators.

This was followed by temporarily stopping theinjection of methanol (demonstrated to be 100%effective in Permian wells), followed by dilutemethanol washing (found to be 100% effective inCarboniferous wells).

Wireline interventions, including the use of bailers andgauge cutters were noted to have limited applicationbut used to recover salt samples and remove saltplugs within the wellbore. The approach clearlybecomes prohibitive for subsea wells or dry wells withlimited remaining economic life.

The 2017 OGA study identified little evidence ofconsistency and best practice amongst the SNSoperators, with a number calling for the establishmentof a forum for the dissemination of lessons learnedand best practice in the UKCS.

FBHP Control

6%

Temporarily Stop

Methanol Injection

19%

Dilute Methanol

Wash12%

Water Wash44%

Gauge cutter6%

Awaiting Treatment

13%

6. Salt mitigation

Figure 6: Salt mitigation approaches andtechniques as dentifed by the 2017 OGAsalting study

9 SNS Salting Study

Both EBN and TNO place significant emphasis on salt /halite management within the Dutch sector. Similar tothe UKCS, EBN observes significant production lossesassociated with salting amongst its productionoperators. Regional work by EBN also suggests thatUKCS production losses due to salting areunderestimated; application of the same EBN criteriacould be applied to UK SNS gas fields to provide afurther estimate of the likely number of fields affectedand losses at stake.

In recognition of the importance of salt management,both EBN and TNO lead on a number of initiatives.

• SNS Gas Well Salt Precipitation Forum: EBNhosts an annual Salt Precipitation Forum whichconsists of a series of presentations from operatorsbut also contractors, vendors and academicspeakers to provide a balanced view of topics.Feedback from the Forum suggests that there is anongoing role for the OGA, either to present UKCSinsights and findings or furthermore to promotecollaboration between the UKCS and Dutch Sectoron salt management initiatives and data gathering.

• Innovation Programme in Upstream Gas. ThisTNO led programme of work has already initiated anumber of projects with industry and academia toaddress salting issues. This multi-year programme ispart funded by industry but also the Dutchgovernment. Partners include EBN, Engie E&P,Total, Oranje Nassau Energie and Wintershall.

The programme includes the development of a modelthat predicts salt precipitation behaviour in the near wellbore / perforation region as well as the development ofremediation strategies, such as wash water frequencyand volumes. The programme also includes the realtime monitoring of salt precipitation, and thedevelopment and trialling of salt crystal growthinhibitors.

The OGA has established relationships with both EBNand TNO and therefore has an opportunity to promotesynergies between the UKCS and the Dutch sector onsalt / halite management issues.

Figure 7: Effects of novel crystal growthinhibitors on halite deposition in porousmedia and control without inhibitor (bottom).Images courtesy of TNO.

0 100 200 300 400 500 600 700 800 900 1000

1.5

2

2.5

3

3.5

4x 10

5

Time (days)

Qg (

m3/d

ay)

Gas Production vs Time; optimized

0 100 200 300 400 500 600 700 800 900 10001.8

2

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

3.8

4

Time (days)

Qg (

m3/d

ay)

498 washes

158 washes

Initial approach

Optimised approach

7. EBN and TNO led initiatives

Figure 8: Use of predictive modelling tooptimise the frequency and volume ofwash water in gas wells. Images courtesyof TNO.

SNS Salting Study 10

The OGA will maintain an active role in the monitoring ofsalting and halite management issues across the SNS,particularly whilst this continues to represent asignificant proportion of the SNS’s production losses.

Based on the feedback received from the variousparticipants in the 2017 OGA study, as well as theOGA’s broader considerations on the subject, thefollowing actions and recommendations are beingprogressed:

1. The EEEGR Southern North Sea RejuvenationSpecial Interest Group will seek to engage relevantSNS operators in a collaborative effort on saltmanagement and furthermore establish a SaltingWork Group

2. The EEEGR Salting Work Group will seek todevelop industry best practice, from inhibition andprediction through to mitigation and intervention aspart of its terms of reference

3. The EEEGR Salting Work Group will seek to identifypotential JIPs and / or deployment of technologies insupport of salt management, including access toexisting programmes of work currently beingprogressed by TNO

4. The OGA will raise the profile of production lossesdue to salting and salt management practices withSNS operators both for new developments as wellas existing developments through the OGA’s AssetStewardship process

5. The OGA will identify any additional datarequirements from operators in support of improvedsalt management and seek to access such datathrough the Asset Stewardship process and / or theannual Stewardship Survey

6. The OGA will seek to promote cross bordercollaboration with TNO and EBN; including datasharing opportunities where possible, sharing of bestpractice and lessons learned

8. Recommendations for further work

11 SNS Salting Study

Glossary

FBHPEBN Energie Beheer NederlandBHPEEEGR East of England Energy Group

FBHP Flowing Bottom Hole Pressure

HUD Hold up Depth

HPHT High Pressure & High Temperature

JIP Joint Industry Project

NPAI Not Permanently Attended Installation

OGA Oil & Gas Authority

PVT Pressure, Volume and Temperature

PW Produced Water

SNS Southern North Sea

TNO The Netherlands Organisation for applied scientific research

UK United Kingdom

UKCS United Kingdom Continental Shelf

WGR Water Gas Ratio

WHP Well Head Pressure

SNS Salting Study 12

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