API_Intro to Oil and Gas Production

8/13/2019 API_Intro to Oil and Gas Production

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CHAPTER 7SEPARATION TREATMENT AND STORAGEWell fluids ar e often a complex mixture of

l i q u i d hydrocarbons, gas, and some impurities. I ti s necessary t o remove th e gas and some impuritiesfrom th e liquid hydrocarbons before they a r estored. transported. and s o l d . Liquid hydrocarbonsand objectionable impurities must also be remo v edfrom natural g as before th e gas goes t o a s a l e sl i n e . Impurities t h a t might be found i n some wellstreams are hyd ro gen s u l f i d e , carbon dioxide, f r e ewater. water vapor, mercaptans. nitrogen, helium.and s o l i d s . Nearly a l l of th e impurities cause var-ious types of operating problems.The separation of natural g a s . l i q u i d hydrocar-bons, and impurities i s accomplished by various

field-processing methods, depending upon t h e com-position o f th e well stream and th e desired endproduct. These met hod s include time, chemicals.gravity, heat, mechanical or e l e c t r i c a l processes,and combinations of t h e s e .

SeparatorsSeparation of well-stream g as from f r e e liquids

i s th e most common and simplest form of f i e l dprocessing. The e qui pment most widely used f o rt h i s type of processing i s referred t o a s a separa-t o r . The separation of natural gas from liquidsand or impurities i n a separator combines gravity.time. mechanical processes, and occasionallychemicals.The s i z e of th e separator i s dependent upon r a t eof flow of th e natural gas and; or l i q u i d s going i nth e v e s s e l . The operating pressure of t h e v e s s e l i sdependent upon t h e pressure of the gas s a l e s l i n e ,th e flowing pressure of th e w e l l , and t h e operatingpressure desired by th e l e a s e operator.Separators are b u i l t i n various designs, such a sv e r t i c a l , horizontal and spherical. The internals ofth e v e s s e l , t o aid i n th e mechanical separation of

th e gas and l i q u i d s , are of a special design.depending upon th e manufacturer. Alt hou g h mostseparators a r e tnvo-phase i n design F i g . 1 ) . sepa-rating th e gas and l i q u i d s , they can be b u i l t three-phase t o separate natural g a s , l i q u i d hydrocarbons,and f r e e water F i g . 2 ) .Under certain conditions i t i s desirable t o us emore than one stage of separation t o obtain morerecovery of f l u i d s .

Alt hou g h natural gas leaving th e separatorlonger contains f r e e l i q u i d s , th e gas may be satrated with water vapors S ee DehydrationNatural Gas ). Liquid hydrocarbons leaving tseparator do no t contain f r e e g a s ; h o w e v e r , thmay contain water, basic sediment, and othimpurities S ee Oil Treating ).

Natural g as contains substantial amountswater v a p o r when produced from a g as wellseparated from liquid hydrocarbons. When gand o i l are separated a t low pressure below 1p s i g ) , t h e g as i s normally gathered i n a low presure gathering system. The g as goes t o a gas plaw h e r e th e g as i s compressed and additional l i q uhydrocarbons, water, and other impurities aremoved. I n th e hig h pressure separation of oand gas above 100 p s i g ) , the gas may be metereand sold t o th e pipeline company i n t h e f i e l ds e n t t o a g as plant for further processing. The gi s then sold t o a gas transmission company.

P

DR IN

SECONDARYV S NSECTON

F i g . I - V e r t i c a l two-phase s e p a r a t o r .

GAS OUTSET


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3 0 Introduction t o O i l Gas Production

PRESSUREGAUGE -

INLET

PR/MARYSPARAr/OSECT ON

F i g . 2 - Horizontal three-phase s e p a r a t o r .

OIL OUTLETL- CONTROL VALVEWATER OUTLET

Water v apor i n g as a t h ig h p re ss ure can causeserious operating problems. T hes e p rob lem s canbest be i l l u s t r a t e d by a typical example:Gas i s separated a t 1 0 0 0 psig and 90 F . At t h i s

pressure and temperature, g as a t water saturationcontains 46 pounds of water v a p o r pe r millioncubic f e e t of g a s . I f th e g as i s cooled t o 70 F . i tw i l l contain only 25 pounds of water vapor permillion cubic f e e t of g a s . This means 2 1 pounds ofwater v a p o r have condensed t o f r e e water. Freewater has a tendency t o c au se c or ro sio n andreduce flow r a t e s i n the gas transmission l i n e . Freewater also promotes th e formation of g as hydrates.Gas hydrates are i c e - l i k e crystalline compoundsformed i n natural gas i n th e presence of f r e e waterunder conditions of high pressure and turbulence.

F i g . 3 - Glycol Dehydrator.

cErroE A E a G ^ ^ E ^SERGE

These hydrates form a t various temperatures. oftenwell a b o v e th e f reezing temperature of water.When hydrates f o r m i n a gas-gathering or distribu-tion l i n e . t o t a l or p a r t i a l blockage of the pipelinemay r e s u l t .

Dehydration of Natural GasT h e r e are several methods t o prevent hydrates

from forming i n a gas l i n e . Some of th e mostcommonly used met hod s are a s follows:

1 . Heating th e gas stream so that the tempera-ture of th e g as w i l l no t drop t o th e l e v e l a tw h i c h hydrates form.

Addition of an anti-freeze agent such a smethanol or glycol t o th e g as s tr ea m.

3 . Removal of water v a p o r by us e of a glycoldehydrator F i g . 3 ) . The most common formof glycol dehydration consists of a verticalpressure vessel called a glycol absorbertower) that allows th e glycol t o flow down-w a r d a s th e g as flows u pward . The mixing ofth e glycol and g as occurs a s th e g as bubblesthrough bubble caps of a t r a y . The pressurev e s s e l usually has four t o eight t r a y s . Theglycol absorbs th e water v apor from th e g a s .As th e glycol becomes saturated with water.th e glycol and water a re c ir cu la te d t h r o u g ha reboiler where th e mixture i s heated t o32 5 F . t o 35 0 F . , boiling of f th e watervapor. The glycol i s then recycled through th eglycol absorber tower.


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Separation. Treatment an d Storage 3 1

4 . Dehydration using s o l i d d es ic ca nt s d ry in gagents) such a s alumina, s i l i c a - g e l , s i l i c o n -alumina beads. and molecular-sieve. Gas flowsthrough t h e desiccant bed w h e r e water i sadsorbed. On a time cycle b a s i s , t h e g asstream i s switched through another bed andth e f i r s t bed i s heated t o remove t h e water.For continuous operation there must be a tl e a s t tw o beds.

5 . Dehydration by expansion refrigeration.w h i c h can be accomplished i f there i s a s u f -ficient pressure drop between well-flowingp r e s s u r e and separator p r e s s u r e . This i s a c -complished by u s e o f h e a t exchangers andexpansion o f t h e g a s .

Most dehydrated gas that goes t o th e s a l e s l i n econtains no more than seven pounds o f waterv a p o r per million cubic f e e t of g a s .

Objectionable amounts of ot he r im pur it ie s i n t h enatural g as stream such a s hyd ro gen s u l f i d e andcarbon dioxide a r e removed by v a r i o u s p r o c e s s e s .T h e met hods may b e broadly c a t e g o r i z e d a s t h o s edepending on chemical reaction, physical solution,and absorption.

Gas transmission companies s e t s p e c i f i c a t i o n s onhow f r e e of impurities th e gas must h e b ef ore i t i spurchased.

O i l TreatingWhen crude o i l i s produced. various amounts of

a s water and other impurities are mixed withth e o i l Some of t h i s mixture comes a s f r e e o i lsome a s f r e e water. and some a s a homogeneousmixture known a s an emulsion. The g a s , wateran d o t h e r i m p u r i t i e s known a s b a s i c sediment andw a t e r ) must b e removed b e f o r e s e l l i n g t h e o i l . Thisseparation process i s c a l l e d o i l t r e a t i n g .

T re at in g s ys te ms are important parts of l e a s eequipment. Experience i n a particular f i e l d or areai s valuable i n determining th e best e qui pment forth e application.

I n selecting a treating system, a number of f a c -t o r s should be considered t o determine t h e mostd e s i r a b l e method o f t r e a t i n g t h e crude o i l t o p i p e -l i n e requirements. Some of these factors a r e :

1 . Tightness of emulsion. Specific gravity of th e o i l and produced

water.3 Corrosiveness of th e o i l g a s . an d p rodu ced

water.

4 . S ca ling t en de nc ie s o f th e produced water.5 . Quantity of f l u i d t o be treated and percent of

water i n t h e f l u i d .6 . A v a i l a b i l i t y o f s a l e s l i n e f o r t h e g a s .7 . Desirable operating pressure f o r t h e equip-

ment.8 Paraffin-forming tendencies o f th e crude o i lO i l - f i e l d emulsions are usually of th e water-in-

o i l type; however, a f ew of th e emulsions a r e o i l -in-water type and are c a l l e d reverse emulsions.Emulsions a r e complex and e ach sh ou ld b e c ons id -ered individually.

I n order t o break a crude o i l emulsion andobtain clean o i l , i t i s necessary t o displace th eemulsifier and i t s f i l m . This brings about th e coa-lescence o f droplets of water and furnishes ameans and time p eri od of undisturbed s e t t l i n g ofth e coalesced water drops. There are severalmethods used i n c on ju nc ti on w it h o ne a no th er t otreat an o i l emulsion.

Heater-Treaters

Treaters equipped w i th e le ct ro de s a r e normallyhorizontal i n design. They are referred t o a s Elec-trostatic Coalescers or Chem-electric treaters F i g . 5 ) . I n some applications these treaters are t h emost desirable because they w i l l t r e a t a t a lowertemperature than a conventional heater-treater,saving f u e l and o i l g r a v i t y .

A heater-treater F i g . 4 ) i s normally used i ntreating o i l emulsions. The h eater-treater usesthermal. gravity, mechanical. and sometimes chem-i c a l and r or e l e c t r i c a l methods t o break emulsions.

Free Water Knockouts FWKOs)When there i s sufficient f r e e water production

on a l e a s e , a FWKO F i g . 6 ) i s often i n s t a l l e d t oseparate f r e e g as and f r e e water from f r e e o i l ande mu ls io n. T hi s v e s s e l can be either horizontal orv e r t i c a l i n design. The s i z e i s dependent upon t h edesired retention time and th e volume of water pe rday t o be handled.The methods used t o f a c i l i t a t e separation when

FWKOs are used are time, gravity, mechanical.a nd s om et im es chemical.

Heater-treaters can be v e r t i c a l or horizontal i ndesign. The s i z e i s dependent upon th e volume ofo i l and water t o be handled.


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32 I n t r oduction t o O i l Gas ProductionWhen heat must be used t o break an emulsion,much fuel gas can be saved b y using t h e FWKO.

Heating unnecessary water i s no t only u s e l e s s , buti t takes more than twice a s many BTUs t o heatwater t o a given temperature a s i t does t o heat o i l .This can be v ery c o s t l y .

Gun B a r r e l

s e t t l e toward t h e bottom o f a tank and o i l w i l l r i s et o t h e top due t o t h e w a t e r s having a higher s p e -c i f i c g r a v i t y than t h e o i l . Heat and chemicals maybe used t o shorten t h e time required f o r s e t t l i n gand t o improve t h e separation of t h e tw o l i q u i d s .Th e s e t t l i n g v e s s e l i s known a s a g un b a r r e l o rwash tank F i g . 7 ) .T h e g un b a r r e l comes i n various d e s i g n s , h o w -e v e r . i t u s u a l l y h a s s u f f i c i e n t h e i g h t t o allow t h ec l e a n o i l t o g r a v i t y flow i n t o t h e stock t k T hn s eIn some cases an oil-water emulsion i s no t very water i s drawn o f f through th e water l e g , w h i c hstable and, i f s u f f i c i e n t time i s allowed, water w i l l also regulates th e oil-water interface l e v e l .

StackeadStack

Stack Brace

Ga s Equalizing L i n e

Outside Siphon

Condensing Head

Centrifugal Gas Scrubber

Thermostat ConnectionThermometer-_n

f-1Stack breeching .__6^-r.

G as Val ve

F i e l d Piping

G as S cr ub b er DripTrapF i g . 4 - Flow diagram.for a v e r t i c a l h e a t e r - t r e a t e r .


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WATER ,AS

F i g . 5 - E l e c t r o s t a t i c c o a l e s c e r .

INLET

WATFF

Separation. Treatment and Storage

AWOA I L EMULSION

OUTSIDE SHRLLOI

GAS

OIL S.R000

J fATEROIL NTLME TC SF4E )E3t

GAS L

ACCESS MANWA

F i g . 6 - Free water knockout w i t h o il s pl i t o p t i o n ) .

AS OUTLE-OIL SPLITTER ORiEN. SHO`NN

3 3


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GASEMULSION

OILWATER

JUG HEATEROPTIONAL)

GAS EQUALIZER

F i g . 7-Schematic flow diagram ofgun b a r r e l wash tank) i n s t a l l a t i o n .

Storage TanksOil that i s f r e e of impurities t o th e extent that i t

w i l l meet pipeline specifications i s referred t o a sclean o i l or pipeline o i l . I t i s o i l from a separator,f r e e water knockout, heater-treater, or g un b a r r e l ,depending upon th e type of treating necessary t oobtain th e clean o i l . The pipeline o i l goes from th etreating f a c i l i t i e s t o th e s to ra g e tanks , known a sstock tanks.

The number and s i z e of stock tanks dependupon th e volume of o i l produced each day.method of s e l l i n g th e o i l t o th e pipeline, and howfrequently and a t what r a t e o i l i s taken by th epipeline company.

Introduction t o O i l Gas Production

The separation, t r e a t i n g , and storage f a c i l i t i e sar e commonly referred to as a tank battery F i g . 8 ) .The t w o basic types of stock tanks a r e bolted

s t e e l an d w elded s t e e l . Bolted s t e e l stock tanks arenormally 5 0 0 barrels or larger and ar e assembledon location. Welded s t e e l stock t an ks r an g e i n s i z efrom 90 barrels t o several t h o us a nd b a r r e l s .Welded tanks up t o 4 0 0 barrels i n capacity a nd i nsome cases 5 0 0 b arr el s) a re s hop -w eld ed a nd aretransported a s a complete u n i t to th e tank batterys i t e . Larger tanks a r e welded on location. Weldedtanks can be internally coated t o protect themfrom corrosion. Bolted tanks o f f e r th e option ofinternal lining or g a lv a ni zed c on st ru ct ion f o r pro-tection against corrosion.


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Separation. Treatment an d StorageVapor R e co v er y S ys te m

When o i l i s treated under pressure and thengoes t o a stock tank a t near atmospheric pressure.some liquid hydrocarbons f l a s h t o g a s . Some f a c -t o r s that determine t h e volume of f l a s h gas a r e :

I Type o f l i q u i d hydrocarbons.2 . Treating p r e s s u r e .3 . Treating temperature.4 . Volume o f l i q u i d hydrocarbons.5 . Temperature o f l i q u i d hydrocarbons e n t e r i n g

t a n k .6 . Diameter o f t a n k .7 . How liquids ente r t he tank.8 . How long l i q u i d hydrocarbons s t a y i n tank

before going t o p i p e l i n e .

For many years th e flash g as or vapors werevented t o t h e atmosphere. I t i s no longer a ques-

NET OILCOMPUTER

NET OILCOMPUTER

F i g . 8 - General l e a s e s e r v i c e i n s t a l l a t i o n .

3 5

tion of economics t o j u s t i f y v apor recovery sincegovernment agencies are i n s i s t i n g on vapor r c -coverv t o reduce a i r pollution. Many improve-ments i n production practices and e qui pmentdesign i n th e past f ew years h a v e made recovery oflow-pressure hydrocarbon vapors p r a c t i c a l . botheconomically and ecologically.

A v a p o r recovery unit F i g . 9 ) consists of a con-t r o l p i l o t mounted on a tank f o r control of com-pressors, a scrubber t o keep l i q u i d hydrocarbonsout of th e compressor. a compressor. and controlpanel. The e l e c t r i c motor-driven compressor w i l ls t a r t by a signal from th e control p i l o t a t approx-imately one ounce of g as p re ss ur e. I t w i l l shut o f fa t approximately 1 / 4 ounce g as pressure. I t i snecessary t o keep a positive pressure i n th e tank t okeep ou t a i r and prevent evaporation of th e crudeo i l . Air contamination of th e g as can create explo-s i v e mixtures and accelerate corrosion of equip-ment. Stock tanks are normally designed t o holdliquid hydrocarbons with a maximum of fourounces of positive gas pressure.


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3 6

Handling Produced WaterIntroduction t o O i l Gas Production

Most o i l and g a s w e l l s produce some w a t e r .Some of th e produced waters are f r e s h whileo t h e r s a r e s l i g h t , medium o r high i n s a l t c h l o -r i d e s ) c o n t e n t . I n many i n s t a n c e s d i s p o s a l o f t h eproduced waters p r e s e n t s an operational problemt h a t must b e s o l v e d t o meet l o c a l , s t a t e an d f e d e r a lenvironmental requirements.

T h e method of d i s p o s a l o f t h e produced watersdepends on many f a c t o r s such a s volume o f w a t e r ,t y p e of w a t e r , l o c a t i o n o f o i l o r g a s f i e l d , t y p e o fr e s e r v o i r from which water i s produced. govern-ment r e g u l a t i o n s , e t c The most acceptable methods

VENT LINE BACKPRESSURE VALVE

SUCTION LINE

CRUDE OIL 0 w ELECTRICSTOCK TANK I - TR LON OU PANELL4 0 0 4 N C UNLOADING

VALVE

0

GAS SALESMETER RUN

CONDENSATEDUMP RETURN

o f disposing o f produced waters a r e a ) i n j e c t i o ni n t o underground s a l t water bearing formations, b ) i n j e c t i o n i n t o o i l bearing underground r e s e r -v o i r s from which t h e o i l an d water i s producedan d c ) d i s p o s a l o f c a r e f u l l y t r e a t e d water i n t o t h eocean from o f f s h o r e production platforms.

Depending on t h e q u a l i t y o f t h e waters t o b ei n j e c t e d and t h e permeability of t h e formation, i tmay be necessary t o t r e a t t h e waters t o remove a smany s o l i d s an d o i l p a r t i c l e s a s p o s s i b l e .A typical water disposal system consists of a

treating v e s s e l t o remove s o l i d s and o i l p a r t i c l e s ,an accumulation o r storage tank, pump with primemover, controls, and a water disposal w e l l .

CONTROL PILOT

ELECTRIC DRIVENROTARY COMPRESSOR

F i g . 9 - Typical stock tank vapor recovery s v s t e n m .


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API_Intro to Oil and Gas Production

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