OTC 8235

Formation of Hydrates in Shut-down Pipelines in Offshore Conditions.

Y.F. Makogon, SPE, Texas A&M University

Copyright 1996, society of Petroleum Engineers, mc

Thm paper was wepared for p4esentabon at the Offshore technology Conferemcs held mHcuston, Texas, 6-9 May 19S6

TPIS pqmr was seiecied for prasentabon by the OTC Program Commdtee followng rewew ofinfommtion con!aimd in q n abstract submitted by the autfwr(s) Ccntents of the paper, aspresented, have not been rewewwd by the Offshwe Tectmc40gy Ccmference and are subjectICIccfredion by the author(s) The material, as presented, does not naessanly reflect anypos!ticmof the Offslwwe Technology Conference or Ils ofhcars Parmlswon to copy IS restrctedto an abstracl of not more than XIO wc+ds Illustrakms may nol ba cop!ed The abstr~tshdd contain conep!cuous acknowledgment of where and by whom the paper waspmseflted

AbstractOperation of oil and gas pipelines in deep-sea is sharplycomplicated by the formation of gas hydrates. The experienceindicates that large gas hydrate plugs in gas and oil pipelinesform most actively during the period of unforeseen long shut-ins. In static conditions three types of hydrate crystals form:surface-contact films and massive hydrates which form bysorption of gas and water molecules on the surfaces ofgrowing crystals; bulk diffusional whisker-like which formboth in the volume of gas, and in the bulk of liquid waterthrough sorption of molecules on the growing crystal surfaceand by tunnel sorption of molecules at the base of the crystal;gel-like soft crystals which form in the bulk of liquid water ata deficiency of dissolved gas in water. Under the P-Tconditions of hydrate formation there may be a simultaneousformation of crystals and decomposition of other crystals.Equilibrium content of gas dissolved in water which is incontact with a solid hydrate surface is lower than in waterwhich is in contact with free gas.

IntroductionGas hydrate is a metastable mineral. Its formation, stableexistence and decomposition depend on pressure, temperature,and the compositions of water and gas.

Gas hydrates are inclusion compounds, solid solutions, inwhich water is the solvent. Molecules of water tied togetherby hydrogen bonds form a space tilling lattice. Mobilemolecules of gases, or volatile liquids are absorbed, orincorporated in the cavities of the water lattice.

Gas hydrates are widespread in nature. They easily form intechnological systems of production, transportation andprocessing of gases, Natural gas hydrates are a tremendoussource of energy. Resources of hydrocarbons accumulated onearth in a hydrate state are estimated as 1.5*10t6 m. Several

countries have already started to implement intensively theirprograms for development of gas hydrate deposits.

An intensive growth of oil and gas production isaccompanied by putting the fields in sub-Arctic regions anddeep offshore into operation. At that the thermodynamiccharacteristic of the technological systems of hydrocarbonproduction and transportation almost always corresponds tothe conditions of hydrate formation. There is a knowledgeabout numerous cases of large hydrate plugs formation inwells and gas pipelines. Analysis of the facts about theformation of large hydrate plugs in wells and oil or gaspipelines reveals that the most dangerous periods of time arethe unplanned long shut-ins.

Expenses for the prevention of hydrate formation are 10-15%.of the production cost. According to Savidge (I), on anaverage gas or gas-condensate field annual expenses forhydrate prevention are 5 to 15 million US $. Expenses forremoval of a complete hydrate plug in a well or a gas or oilpipeline offshore are usually several millions of US dollars. Atthat the shut-in losses for up to several months must be takeninto account. In a number of cases the formation of a hydrateplug in a pipeline halts the production of oil and gas on thewhole field for the complete period of hydrates removal.

Unfortunately, there is no clear understanding about theconditions of hydrate plugs formation both in wells and inpipelines. It should be emphasized that the kinetics of hydrateformation and decomposition remains to be one of the mostcomplex and least studied one in the problem of gas hydrates.

This work presents the results of experimental investigationperformed in the gas hydrate laboratory of the PetroleumEngineering Department, and the Offshore TechnologyResearch Center at the Texas A&M University. The presentedresults of studying the kinetics and morphology of hydratecrystals do not pretend to reveal all the aspects of hydrate plugformation. However, they will aid a more properunderstanding of the hydrate formation process and designingof the more effective means of preventing hydrates.

As a result of this work a dependence of the gas volubilityin water on temperature and pressure was determined athydrate stability conditions. Three types of hydrate crystalswere found: growing on interface, bulk-divisional, and softgel-like. All three types of crystals may form simultaneouslyat equal P-T conditions. The property of hydrate crystals todissolve in water (2) at the hydrate stability conditions was

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2 FORMATION OF HYDRATES IN SHUT-DOWN PIPELINES IN OFFSHORE CONDITIONSQTc 8235

confirmed. The direction of hydrate forrners diffusion duringhydrate formation and many forms and rates of crystal growthwere revealed. Only several results obtained during the studyof hydrate formation processes are presented due to thebrevity of the paper volume.Experimental TechniqueA high-pressure cell of an original design was used. This cellallowed to conduct a complex investigation of both formationand decomposition of hydrate in static and in dynamicconditions (Fig. 1). The cell volume was variable from 820 to1020 cm, operating pressure - up to 200 kg/cm2. The cell hadfive round windows with diameter of 84.6 mm for visualobservation, a system of temperature control in gaseous and inliquid phases with an accuracy of 0.05 C, and a pressurecontrol system with an accuracy of 0.1 A. The cell wasequipped with a special mechanism which allowed todetermine the dynamics of phase correlation of componentsduring hydrate formation without disturbing of the P-T andkinetic conditions.

The obtained results were obtained for methane Ultra HighPurity CGA Ouher 350 (LB-170) obtained from MathesonGas Production Inc., and a double-distilled water.

The cell pressure could be kept constant, or preset-varyingwith supplying gas into the gas volume, or by bubbling gasthrough a layer of water. Pressure could also be controlled byan injection of a fresh water, or of water pre-saturated withgas through a ALCOTT-760 HPLC 390 micro pump with awater flowrate of 0.01 -1.0 cm3/min. Water could be suppliedinto the cell in a bulk liquid or in a microdispersed state with apressure drop at the throttle of up to 200 kg/cm2.

The reactor cell was equipped with a remote magneticstirrer with a speed of 0.5 - 10 rev./see, and a mechanicalsystem providing the creation of a free gas-water interface at arate equal to the rate of a hydrate film formation.

The cell allows to study the morphology of crystals, toconduct direct measurements of thickness of the formedhydrate film, and to determine various properties of hydrates.

Types of Hydrate CqstalsThe earlier studies of the kinetics and morphology of gashydrates (2- 10) have indicated that the start of hydrateformation always occurs at the free gas-water interface (with agaseous or a liquefied gas). At that microcrystals of criticalsize rCrare formed. The critical radius of such crystals dependson many factors and is determined as (3, 8, 11)

2rsT,> 20V ~rcr= Q(T,> -To)= ~=

...................

2dfh

:[$Q.nt)+Dv;+;+(1)

Magnitude of the surface energy o is a function of pressure,temperature, composition, and the state of interface. Fig. 2presents the dependence of the surface tension at the methane-water interface on pressure and temperature,

Growth of microcrystals which formed at the methane-water interface is accompanied by the formation of a completehydrate film on the whole surface of a tlee gas-water interface(Fig. 3). Thickness of the forming film depends on pressure,degree of subcooling, composition of the forming hydrate.With pressure the required initial degree of subcoolingdecreases, and with the decrease of the degree of subcoolingthe rate of nuclei formation and the rate of the hydrate filmformation decrease. For example, the radial rate of hydratefilm growth on a methane-water interface at a pressure of 84kglcmz and a temperature of 6.6 C is 3 cm/min. For acomparison, the growth rate of needle-like crystals in similarconditions may reach 36 to 70 cm/min., and the rowth rate of

Fwhiskery crystals may range from 0,1 to 1.7*1O crdmin. Therate of hydrate formation at a free gas-water interface isdetermined mostly by the rate of heat removal.

After the hydrate formation on the interface, the interracialprocess becomes a bulk diffusion-dependent process. At thatthe rate of hydrate accumulation is determined mostly by thediffusion of water through a hydrate film (3,4) because of thedifference between the diffusion coefficients of water and gasmolecules. Three types of hydrate crystals may formsimultaneously at the same external P-T conditions:a) massive - on the surface of the formed hydrate crystals bysorption of water and gas molecules (Fig. 4);b) soft gel-like crystals;c) whiskery (thread-like) crystals.Whiskery crystals grow both in the gas, and in the aqueousphases. Whiskery crystals grow as linear or twisted mono-raysor colonies, with or without branches. The whiskery crystalsusually develop in the points of dislocations on the surface ofthe formed hydrate in the gas phase. At that the crystal growthis caused by the axial dislocation(s) with a finite screwcomponent which creates a non-curing growth step at thecrystal top (Fig. 5) or at the crystal base. Crystal formationproceeds by a sorptional addition to the growing step (12).The rate of crystal growth by addition of molecules to agrowing surface can be very high, but the time of formation isshort (shooting crystals) and is determined by the water vaporcontent in the gas phase, difference between the chemicalpotentials of water molecules dissolved in gas and in a hydratelattice. When an equality of these chemical potentials isreached, the process of crystal formation stops. The lineardimension of such crystals varies broadly from fractions of amillimeter to several centimeters. During a consequentdiffhsive influx of water molecules to the gas phase theformation of new whiskery or massive hydrate crystals mayresume (Fig. 6).

Whiskery hydrates may form simultaneously from atunneling influx and a diffisive influx of hydrate forming

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molecules to the base of growing crystals (Figs. 7). By solvingthe equation of diffusion

Cffv(x,f) =D ~2~(xJ) ~(xt)+/v. ,.,., . ........ .. .. .. ...(2).

dt (3X2 rwe can determine the rate of whiskery hydrates formation.Such crystals have a high initial growth rate with a consequentdecrease in the rate.

Usually, whiskery crystals form afier the formation of ahydrate film on the gas-water interface in the points of contactof the hydrate film with sharp angles of a hydrophilic surface -in the points where capillary forces are the most expressed. Inthese points water is at the pressure which is lower than thevapor pressure by the amount of the capillary pressure, andfavorable conditions for whiskery hydrates formation arecreated. Shift of the phase transition pressure APs due to theeffect of capillary pressure Pc can be written as

./w, =%(dP ,!, ,,- f,- 1 1 10 12 16 36 u GTIME. b

Fig. 8 Variation of the iinear growth rate of whisketymethane hydrates with time.

10

v, cmlg

1

0 5 15 20Tempe~ture, C

Fig. 9 Dependence of the solubiiity of methane inwater on pressure and temperature with a freeinterface.

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8 FORMATION OF HYDRATES IN SHUT-DOWN PIPELINES IN OFFSHORE CONDITIONSA-rm O-*C

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