Prevention of surge wave instabilities in three phase gas condensate flowlines

Abstract

There is provided a process for the prevention or reduction of surge wave instabilities during the transport in a flowline of a three phase gas condensate comprising a gas phase, an aqueous phase and a condensate phase, characterized in that a dispersing agent is added to the three phase gas condensate which is able to disperse the aqueous phase in the condensate phase or the condensate phase in the aqueous phase, and a means for the prevention or reduction of surge wave instabilities during the transportation of a three phase gas condensate.

Claims

1. A process for the prevention or reduction of surge wave instabilities during the transport in a flowline of a three phase gas condensate comprising a gas phase, an aqueous phase and a condensate phase, said method comprising: adding a dispersing agent to the three phase gas condensate which is able to disperse the aqueous phase in the condensate phase or the condensate phase in the aqueous phase, wherein the addition of the dispersing agent prevents or reduces surge wave instabilities and enables the flowline to operate at a lower flow rate than when the same flowline is operated without addition of said dispersing agent; wherein the dispersing agent which is added to the three phase gas condensate is added at a dosing level of from 10 to 1,000 ppm.

2. The process according to claim 1, wherein the dispersing agent which is added to the three phase gas condensate is one or more dispersing agent selected from the group consisting of oil soluble surfactants and water soluble surfactants.

3. The process according to claim 1, wherein the dispersing agent which is added to the three phase gas condensate is an oil soluble surfactant.

4. The process according to claim 1, wherein the dispersing agent which is added to the three phase gas condensate is one or more dispersing agent selected from the group consisting of non-ionic oil soluble fatty acid monoesters of sorbitan and water soluble ethoxylated fatty acid monoesters of sorbitan.

5. The process according to claim 1, wherein the dispersing agent which is added to the three phase gas condensate is at least one Span surfactant selected from the group consisting of sorbitan stearate (Span 60), sorbitan tristearate (Span 65), sorbitan monooleate (Span 80) and sorbitan sesquioleate (Span 83).

6. The process according to claim 1, wherein the dispersing agent which is added to the three phase gas condensate is able to disperse the condensate phase in the aqueous phase.

7. The process according to claim 1, wherein the dispersing agent which is added to the three phase gas condensate is able to disperse the aqueous phase in the condensate phase.

8. The process according to claim 1, further comprising adding a processing chemical to a mixture produced after addition of the dispersing agent.

9. The process according to claim 1, further comprising adding a processing chemical to a mixture produced after addition of the dispersing agent, wherein said processing chemical is a demulsifying agent.

10. The process according to claim 1, further comprising adding a processing chemical to a mixture produced after addition of the dispersing agent, wherein said processing chemical is selected from the group consisting of acid catalyzed phenol-formaldehyde resin demulsifiers, base catalyzed phenol-formaldehyde resin demulsifiers, and epoxy resin demulsifiers.

11. The process according to claim 4, wherein the dispersing agent which is added to the three phase gas condensate is one or more dispersing agent selected from the group consisting of non-ionic oil soluble fatty acid monoesters of sorbitan (Span surfactants) and water soluble ethoxylated fatty acid monoesters of sorbitan (Tween surfactants).

12. The process according to claim 5, wherein the dispersing agent which is added to the three phase gas condensate is sorbitan monooleate (Span 80).

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The present invention will be described in the following in further detail with reference to the appended drawings, none of which should be construed as limiting the scope of the invention.

(2) FIG. 1 is a plot of surge volumes against frequency for a three phase condensate;

(3) FIG. 2 is a plot of pressure drop in a flowline as a function of production rate;

(4) FIG. 3 is a plot of liquid content in a flowline against production rate;

(5) FIG. 4 is a plot of hold up oscillation (%) against time due to three phase surge waves; and

(6) FIG. 5 is a plot of % aqueous phase recovered against time for a 35% emulsion shaken at 330 rpm for 15 hours, sowing the effect of varying the concentration of Span 80.

(7) As noted earlier, three phase surge wave instabilities are known to occur spontaneously in flowlines at otherwise stable conditions (particularly no changes in flowrate, pressure, composition or temperature) in a number of three phase flowlines, particularly gas condensate aqueous flowlines. This leads to a number of problems. First, these surge wave instabilities in three phase systems can cause liquid handling problems. Second, this entails a minimum flow rate for the flowline, leading to reduced total production. Third, the likelihood of hydrate formation is increased due to loss of hydrate inhibitor/water at the end of the flowline during surging. Finally, they result in restrictions in operating the flowlines.

(8) Based on our studies, experimental work and theoretical analysis, we expect that the addition of a dispersing agent to the liquid phase(s) to form a liquid dispersion removes the surge wave instabilities and increases the total production from the field because as a result it is possible to operate the flowlines at a lower flow rate. It is also possible to obtain improved regularity with stable flow. Furthermore, hydrate problems previously experienced are reduced. Finally, restrictions in operating the flowlines are reduced/eliminated.

(9) In a preferred aspect of the first embodiment of the invention, the dispersing agent which is added to the three phase gas condensate which is able to disperse the aqueous phase in the condensate phase or the condensate phase in the aqueous phase is any dispersing agent which is able to prevent slip between the aqueous phase and the condensate phase, i.e. one which forms an emulsion. Preferably, it is one or more dispersing agents selected from oil soluble surfactants for oil continuous flow and water soluble surfactants for oil continuous flow, preferably oil soluble surfactants. Further suitable dispersing agents will be immediately apparent to the person of ordinary skill in this field and these also fall within the scope of the dispersing agents suitable for use in the present invention.

(10) Suitable non-ionic oil soluble surfactants are those having a low HLB (hydrophilic-lipophilic balance), typically but not exclusively those having an HLB of 10 or lower. These promote the formation of water in oil emulsions. These include non-ionic oil soluble fatty acid monoesters of sorbitan (Span surfactants) such as Span 60, Span 65, Span 80 and Span 83. Span 80, for example, is sorbitan monooleate and has the following formula:

(11) ##STR00001##

(12) Other suitable oil soluble surfactants include polyoxyethylene alkylphenylethers, branched, such as polyoxyethylene (5) nonylphenylether, branched. Further non-ionic oil soluble surfactants will be readily apparent to the person of ordinary skill in this field and also fall within the scope of the non-ionic oil soluble surfactants suitable for use in the present invention.

(13) Suitable water soluble surfactants are those having a high HLB (i.e. typically but not exclusively those having an HLB of greater than 10). These promote the formation of oil in water emulsions. Suitable water soluble surfactants include water soluble ethoxylated fatty acid monoesters of sorbitan (Tween surfactants) such as Tween 20. Other suitable water soluble surfactants include poly(propylene glycol)-block-poly(ethylene glycol)-block-poly(propylene glycol) average M.sub.n=2,000 (PPG-PEG-PPG Pluronic 10R5). Further suitable water soluble surfactants will be readily apparent to the person of ordinary skill in this field and also fall within the scope of the water soluble surfactants suitable for use in the present invention.

(14) The amount of dispersing agent that needs to be added will vary depending upon the identity of the dispersing agent, the nature of the three phase gas condensate system, temperature, pressure and the like. Typically, the dispersing agent which is added to the three phase gas condensate is added at a dosing level of from 5 to 10,000 ppm (i.e. parts dispersing agent per million parts gas condensate), more preferably from 10 to 1,000 ppm, e.g. 10 ppm, 100 ppm, 500 ppm, 750 ppm and 1000 ppm.

(15) Further chemicals may be added to the mixture generated after addition of the dispersing agent to allow better processing. These include demulsifiers. Demulsifiers are added to separate the emulsion formed by the addition of a surfactant. This allows easier separation of the condensate and aqueous phases. Suitable demulsifiers include acid or base catalyzed phenol-formaldehyde resin demulsifiers and epoxy resin demulsifiers.

(16) The dispersing agent which is added to the three phase gas condensate is able to disperse the aqueous phase in the condensate phase, or it is able to disperse the condensate phase in the aqueous phase.

(17) In a further preferred aspect of the means according to the present invention for the prevention or reduction of surge wave instabilities during the transportation of a three phase gas condensate, a further injection means is provided which is in fluid communication with the flowline at a point downstream from the point of injection of the dispersing agent, the further injection means being suitable for the injection of a processing chemical into the mixture obtained after dispersion of the three phase gas condensate, e.g. a demulsifier.

(18) Although not wishing to be bound by theory, we believe that the following provides an explanation why surge waves are able to form in the three phase gas condensate flowlines but not in the two phase flowlines. An analysis of the governing equations for these types of surge flow has been done. The surges are long wave length mass waves. In order to simplify the equations the fast dynamics associated with the pressure waves compared to the dynamics of the mass waves were ignored. By assuming constant phase densities, the pressure wave velocity essentially goes to infinity, and the pressure wave dynamics can be decoupled from the mass wave dynamics. Furthermore, short wavelength gravity waves can be ignored and the equations simplified by invoking the long wavelength approximation. The mass equation for a two phase gas-liquid system can then be written:

(19) $\frac{h}{t}+\frac{}{x}\left(U_{\mathrm{SL}}\left(h\right)\right)=\frac{h}{t}+\frac{d{U}_{\mathrm{SL}}}{dh}\frac{h}{x}\frac{h}{t}+c_s\left(h\right).Math.\frac{h}{x}=0$

(20) This is a first order wave equation, describing the advection (transport) of holdup (h) with an amplitude dependent velocity (c.sub.s(h)). U.sub.SL is the superficial liquid velocity. By invoking a Neumann stability analysis of this equation it is readily seen that this equation is always stable. This indicates that it is not possible to have long surge wave instabilities (as seen in the Midgard field) in a two-phase system.

(21) Applying the same analysis to a three phase system (condensate, water and gas) we get the following mass equations for the liquids (h.sub.o is oil holdup, h.sub.w is water holdup):

(22) $\frac{}{t}\left(\begin{array}{c}{h}_o\\ h_w\end{array}\right)+\frac{}{x}\left(\begin{array}{c}{U}_{\mathrm{SO}}\\ U_{\mathrm{SW}}\end{array}\right)=\frac{}{t}\left(\begin{array}{c}{h}_o\\ h_w\end{array}\right)+\left(\begin{array}{cc}\frac{U_{\mathrm{SO}}}{h_o}& \frac{U_{\mathrm{SW}}}{h_w}\\ \frac{U_{\mathrm{SO}}}{h_o}& \frac{U_{\mathrm{SW}}}{h_w}\end{array}\right)\frac{}{x}\left(\begin{array}{c}{h}_o\\ h_w\end{array}\right)=0$

(23) For this equation system we have a 22 wave velocity matrix and a stability analysis readily shows that this system can have an unstable mode. This indicates that in a three phase system surge wave instabilities can develop.

(24) As demonstrated, the pure two phase surge waves are inherently stable, while in the three phase case it is possible for unstable waves to develop. We believe that this instability is the reason behind the long wavelength oscillations that has been observed on sgard B and similar gas condensate fields.

(25) Through the development of the present invention, we believe that by adding a dispersing agent to the liquid phase(s) to form a liquid dispersion it is possible to remove the surge wave instabilities and increase the total production from the field. As a consequence, it is possible to operate flowlines at lower flow rates, enabling increased production. For instance for the Mikkel-Midgard fields it is estimated that the total gas production will be increased by 2 GSm.sup.3 if the minimum flow rate of the flowlines can be reduced by 20%.

(26) Furthermore, it gives improved regularity with stable flow. Hydrate problems previously experienced are reduced. Furthermore, restrictions in operating the flowlines are reduced or eliminated.

(27) The present invention may be further understood by consideration of the following non-limiting examples.

EXAMPLES

Example 1

(28) In two and three phase experiments at Institute for Energy Technology (IFE), it has been demonstrated that surge wave instabilities occur in three phase flows but not in two phase flows, as predicted in our analysis of the governing equations above.

(29) In this example, the flow for a two phase mixture comprising an oil phase and a gas phase was measured. No instabilities were found to occur for this two phase flow. However, when water was added to the mixture to give a three phase mixture surge wave instabilities were found to occur (see FIG. 4). The instabilities occurred in the transition from low to high holdup at Usg=2.5 m/s for Usl=6 mm/s and a pipe inclination of 3.

Example 2

(30) Condensate from the Troll field and aqueous phase (60 wt % MEG in pure water) was transferred to a container. The mix had a 35% water-cut. Span 80 was added as an emulsifier in different concentrations. Concentration of emulsifier is given as ppm of the total amount of oil in the emulsion. The emulsification was carried out by shaking using an reciprocating shaker (HS 501 digital, IKA Labortechnik). The condensate/water-blend was shaken at 330 rpm for 2 or 15 hours. Negligible differences in emulsion behavior were observed upon a change in shaking time from 2 to 15 hours.

(31) mixing of the two phases was carried out by shaking as above. The stability towards gravity for emulsions (35% aqueous phase) shaken at 330 rpm for 15 hours is illustrated in FIG. 5. The Span-80 concentration was varied from 25 to 250 ppm. As soon as the agitation stopped, the destabilization process started. The kinetics of destabilization and the final level of resolved aqueous phase were determined by the Span-80 concentration. By tuning the concentration of added emulsifier, it was found that it was possible to produce a system which gives a stable w/o-emulsion during agitation, and a destabilization without agitation. These types of data have also been obtained for emulsions mixed by means of gentle manual shaking.

(32) It has been demonstrated that a mixture of water and MEG can be emulsified into a condensate by using Span-80. The emulsification can be achieved independent of the magnitude of energy input through tuning of the Span-80 concentration.

(33) It has been demonstrated that Span 80 is highly efficient in forming a dispersion of aqueous phase in Troll condensate. By using Span-80 as the emulsifier it is possible to produce an emulsion stable during agitation and unstable after agitation. This emulsification can be achieved independent of the magnitude of energy input through tuning of the Span-80 concentration. It has also been demonstrated that with increasing concentration of Span 80 the emulsion can be stable for a long time even at rest (for instance in a separator). This indicates that for this surfactant one may need a demulsifier to improve the separation.

(34) Thus, it is possible to conclude that through the use of a suitable dispersing agent such as the w/o emulsifier Span-80 it is possible to transform a two-phase flow system to a stable single phase liquid flow. As it is demonstrated in Example 1 that surge wave instabilities occur in three-phase flows but not two phase flows, this transformation of the two liquid phases to a single stable phase will enable the removal of the surge wave instabilities and increase the total production from the field.