Inject Methanol

7/27/2019 Inject Methanol

1/9

A SYSTEM FOR INJECTING METHANOL DOWNHOLE

DURING OFFSHORE TESTING TO PREVENT HYDRATE

FORMATION

D.I. WILKIE W.J. COOKE

this arti cle begins on the next page FF

PETROLEUM SOCIETY OF CIM ATM-08 THIS IS A PREPRINT - SUBJECT TO CORRECTION PAPER NO. 78 - 29 - 17 A SYSTEM FOR INJECTING METHANOL DOWNHOLE DURING OFFSHORE TESTING TO PREVENT HYDRATE FORMATION by D.I. Wilkie, Staff Drilling Engineer, Dome Petroleum Ltd./Canadian Marine Drilling Ltd., Calgary W.J. Cooke, Cooke Offshore Consultants Ltd. PUBLICATION RIGHTS RESERVE D THIS PAPER IS TO BE PRESENTED AT THE 29TH ANNUAL TECHNICAL MEETING OF THE PETROLEUM SOCIETY OF CIM IN CALGARY. JUNE 13 - 16, 1978. DISCUSSION OF THIS PAPER IS INVITED SUCH DISCUSSION MAY BE PRESENTED AT THE 29TH ANNUAL MEETING

AND WILL BE CONSIDERED FOR PUBLICATION IN CIM JOURNALS IF FILED IN WRITING WITH THE TECHNICAL PROGRAM CHAIRMAN PRIOR TO THE CONCLUSION OF THE MEETING. ABSTRACT Prior to the l977 offshore operations on Dome/Canmar's vessels in the Beaufort Sea, it was determined that hydrate formation could be encountered during gas well tests. Altho ugh systems had been developed for methanol injection in arctic land operations, offshore operations require d a system which would retain the advantages of a sub-sea test tree. A system was developed by Dome/Canmar, in association with Johnston Testers,which allowed the injection of methanol. up to 2,000' below the sub-sea test tree, and which would allow the unlatching of the sub-sea test tree, while maintaining the full pressure integrity of the test string. The systems also allowed closing of pipe rams during testing (to activate down hole pressure controlled test tools) and re-latching of the sub-sea test tree, if it was required to temporarily move off a well. The system was successfully used throughout Canmar's 1977 testing operations. INTRODUCTION Hydrates will form and accumulate in a natural ga!3 stream at predictablep r e ss u r es a nd t em p e ra t ur i @ s w h en t he f ol l ow i n g c o nd i t io n s a r e M E ! t ' ( 1, 2 ) 1 . T h e p r es s u re / t em p e ra t u re r e l at i o ns ' n ip o f th e ga s mu s t b e be l o w t h e d e w p o i nt ) f t i l e


2/9

PETROLEUM SOCIETY OF CIM PAPER NO. 7829

THIS IS A PREPRINT - SUBJECT TO CORRECTION

A SYSTEM FOR INJECTING METHANOL DOWNHOLE DURINGOFFSHORE TESTING TO PREVENT HYDRATE FORMATION

by0.1 Wilkie, StaU Dniling Engineer,

Dome Petroleum Ltd.lCanadlan Manne Dnlling Ltd . Calgary

W.J. Cooke, Cooke Oflshore Consultants ltd.

PUBLICATION RIGHTS RESERVED

THIS PAPER IS TO BE PRESENTED AT THE 29TH ANNUAL TECHNICAL MEETING OF THE PETROLEUM SOCIETY OF el M INCALGARY, JUNE 13 - 16, 1978. DISCUSSION OF THIS PAPER IS INVIT ED SUCH DISCUSSION MAY BE PRESENTED AT THE29TH ANNUAL MEETING AND WilL BE CONSIDERED FOR PUBLICATION IN el M JOURNALS IF FILED IN WRITING WITH THETECHNICAL PROGRAM CHAIRMAN PRIOR TO THE CONCLUSION OF THE MEETING.

ABSTRACT

E'r ior Co t he1977 o f f s h o r e o p e r a t i o n s onDome/Canmar's vesse ls in tn e aeauEort Sea, i t wasd E t e r u ~ n e dt h a t hydra te fOr@ation could beencountered during gas w e l l t es t s_ Althoughsystems had been developed for methanol i n j e c t i onin a r c t i c land o p e r a t i o n s , o f f s h o r ~opera t ionsrequi red a system which would re ta in th e advantages o f a sub-sea t e s t t ree_ A system was developed by Dome/Canmar , i n associa t ion withJohnston Testers , which allowed th e i n j ec t i onof methanol up to 2,000' below th e sub-sea t e s tt ree , and which would allow th e unla tch ing of th esub-sea t e s t t ree , whi le mainta in ing th e fu l lpressure i n t eg r i t y o f th e t e s t s t r i ng . Thesystem a lso al lowed c l o s i n g o f pipe rams duringtes t ing (t o act iva te down hole pressure c o n t r o l l e dt e s t too ls ) and r e - l a t ch i ng o f th e sub-sea t e s tt ree , i f i t was reqUired to t emporar i ly move o ff

a well_ The system was s u c c e s s f u l l y used t h r o u ~ h -o u t Canmar's 1977 t e s t i ng operations_

INTRODUCTION

Hydrates wi l l form an d accumulate in an a t u r a l gas stream a t predic table pressurES andtemp'eratures when th e fol lowing condi t ions aremet( l ,2) :

1. The pressure / temperature r e l a t i onsh ip 0th e gas must be below th e dew poin t of cwater vapour, an d l i qu id water must bepresent .

2_ The ga s muse be in motion an d subjec teda degree of turbulence .

J. A fore ign p a r t i c l e or a change in fLow pmust be p r e s e n t to i n t e r f e r e with clle gaflow pa t t e rn .

All th ree of these condi t ions can beexpected co occur during ga s w e l l t e s t i ng in thBeaufort Sea. Hydrates may form in th e upperpor t ion of th e tubing s t r i n g where th e ga s losei t s g rea t e s t amount o f h e a t an d may a c c u m u lfur ther upstream in th e flow head and manifOldsystem where the gas i s contro l led b e f o r ~i treaches the hea te r_ In each o f these loca t ions ,

hydra tes may r lug th e flm". path r e su l t i ng in at e rmina t ion of th e product ion t e s t an d .:J. l o s s 0effe '=. t ive r i 3 time due to th e need fo r d e ~ r ein g an d clear ing the system.

On th e o the r hand, hydra tes can be prevenfrom forming by mainta in ing r e l a t i v e l y high tema tures an d low pressures throughout the flow stro r by f u l l y i n h i ~ i t i n ~th e water phase in che g


3/9

to lower the hydrace tempera cure . High cemperacures ca n only be achieved a f t e r th e wel l ha dbeen placed OLI st.ream fo r a t.ime a t high r a t e s .However, dur ing th e i n i t i a l flow per iod , i n h i b it.ion i s the only method t ha t ca n r e l i ab ly preventth e formation of hydra tes .

ARCTIC OPERATOR EXPERIENCE OF HYDRATE FORMATIONDURING TESTING.

Sev e ra l o pe ra to r s were contac ted fo r f i e l devidence of hydra te formation on ga s wel l t e s t i ngin a r c t i c an d offShore areas .

On th e Nort.h Slope , downhole hydrateprevent ion procedures vary. One method involvesc i r c u l a t i n g hot. calcium c h lo r i de so lu t i on downconcent.r ic s t . r ings u n t i l a s u i t a b l e wellboret.emperature i s reached an d th e ga s i s maintainedou t s ide th e hydra te formation pressure / tempera turereg ion . Gas we l l F.T.H.P_' s in th is case are inth e range o f 2,800 to 3,000 p . s . i . g . which r equires a minimum ga s t empera ture o f approximately7S o F. to prevent hydra te formation, as shown inFigure 1. Another approach i s t.o use a d ie se lcushion l a ced with met.hanol t.o s ta r t . -up a wel l

t e s t while monitoring surface pressures an dt empera tures . Th e approach c a r r i e s a h ~ g hdegreeo f r i s k o f forming hyd ra t e s . However operat .orsare USing a method of p r ed i c t i ng the formationo f a plug by comparing a predic ted pressure an dtemperature t rend aga ins t th e ac tua l pressureand temperat.ure readings . I f a plug s t a r t s toform, th e wel l i s quickly shu t - in down-hole an da combination o f d i e s e l and methanol i s pumpedin to th e product.ion s t r i n g to mel t th e plug.

Both North Slope an d HacKenzie Del ta opera to r s have prepared t r a n s i e n t temperature programst ha t p r e d i c t ga s temperat.ure an d pressures in th es t r i n g a t var ious t ime i n t e r v a l s from s t a r t -upto a s s i s t them in t ak ing precaut ionary hydrateprevent ion measures. One o f th e main s e rv i ce st ha t these p't"ograms ca n prOVide in th e determina t ion o f t.he depth o f hot water c i r cu la t ion o rthe depth fo r i n j ec t ion o f i nh ib i to r ineo th es t r i n g so t.hat hydra te formation can be preventedduring a l l an t i c ipa t ed ga s wel l operat . ions.

Offshore opera tors in the North Se a and o f feh e East coase of Canada have experienced hydra teformation during o i l an d ga s t e s t s an d normalsub-sea t e s t - t r e e ope ra t i ons have been preventeddu e to f a i l u r e o f th e b a l l valves to c lose . Subsequent ly the sub-sea t e s e - t r e e s were modifiedto allow methanol i n j ec t ion a t th e sea f loor inth e viCini ty of tn e va lve s (3 ) .

HYDRATE FORMATION CONDITIONS,

Hydrate regimes in permafrost areas havebeen s tudied by Dome/Canmar an d s tare-upcond i t i ons in many ga s wel ls have been analysedto predict . t.he degree o f exposure to hyd ra t e s .Figures 2 - 5 show ac tua l ga s y e l l temperaturesduring s t a r t - u p an d th e eKposure t.o hydra teformation fo r four HacKenzie Del ta wells_

- 2 -

Not a l l test .s ha d methanol i n j ec t ed into th estream on a continuous bas i s an d some hydrat.eformation Y O S encountered.

Th e minimum r a t e o f methanol i n j ec t i onbe calcula ted using the Hammerschmidt equat ionfollows;

w

where W

10 0 dM

2335 + Hd

weight pe rcen t methanol in th e l i quphase .

d depress ion o f th e hydrate format.ionpoine .

M = molecular weight of methanol inject .

Table 1 shows ca l cu l a t i ons fo r th e amouto adequately depress the hydrate format.ion pin each o f th e four MacKenzie Del ta wel l t e s t s .The r eSu l t s i nd i ca t e th e i n j ec t ion rat.es arel e s s than 1 G.P.H.

Gas reservOirs in th e Beaufor t Sea coulposs ib ly be encountered where r e se rvo i rpressures may be abnormally high . Also i t ca nreasonably assumed from th e ag e and nature off o ~ t i o n st ha t th e ga s may be sa tu ra t ed withwater vapour and a l l th e ~ o n d i t i o n s[o r hydra tformation could be me t during a ga s wel l t e s t .

fNhen a wel l i s i n i t i a l l y placed on t e s tth e ga s i s expec ted to exper ience high hea t lofrom both high d i f f e r e n t i a l eemperatures arount.he wel l bore an d from pressure reduct ion int e s t s t r i n g . At t h i s time? th e pressure / temperat u r e r e l a t i onsh ip o f th e ga s w i l l most l i ke lywithin th e hydrat.e formation regime. Later inte s t , as flow r a t e s a re increased or as th ed i f f e r e n t i a l temperatures up th e wel l bore redu

th e ga s condi t ions move ou t s ide th e hydra teregime wit.hout fu r the r exposure to hydra teformation.

Gas temperature i s th e most c r i t i c a l fling condi t ion to analyse in determining whethep o t e n t i a l o f hydra te formation exis t . s . A reviwas therefore ca r r i ed out. o f th e expected int empera ture gradients in a wel l and includeddeterminat ion o f expec ted th icknesses of permaf r o s t sec t . ions which cause a la rge degree o fheat loss from th e wel l s t ream.

Data on permafros t was derived from actse ismic an d temperature i n t e rp re t a t ions providby s t a f f members of Dome/Canmar an d t ~ e 5 ~ e rMines and Resources Branch in Ottawa 4, . data indica ted that continuous permafrost th icnesses o f up to 2,000 fee t have been encountereon the Tuktoyaktuk Penninsula an d t ha t thinningoccurs in a seaward d i r ec t i on to th e nor th an dwest, as shown in Figure 6. The rea f t e r th epermafrost becomes discont inuous area l ly_ Theminimum t .emperature o f a continuous p e ~ a f rl aye r offshore could reach 2 ~ o F .which providesa very s i g n i f i c a n t cool ing e f f e c t on th e gas.


4/9


5/9


6/9


7/9


8/9


9/9

Documents

Inject Methanol