Development of a Hydrate Inhibition Monitoring System


Evaluation with Produced Water Solutions Aqueous fluids in oil and gas pipelines usually contain multiple


components of salts, such as NaCl, CaCl2, KCl and so on, other contaminants, such as oil droplets and chemical additives, such as corrosion inhibitors (CIs) and scale inhibitors (SIs). To investigate the effect of these factors on the performance of the prototype C-V device, two real produced water samples were used. These samples contained unknown concentrations of MeOH, a CI, multi-component salts and some condensate. The MeOH and salt concentrations were measured using the C-V device. The measurement results showed that the concentrations of MeOH and salt in the first produced water sample (PW-1) were 2.9 and 22.2 mass%, respectively and 3 and 23 mass% in the second produced water sample (PW-2). The reliability of the C-V device measurements was checked in two ways. For PW-1, the hydrate phase boundary was determined by the C-V prototype for a typical natural gas (88.3mol% methane, 5.4mol% ethane, 1.5mol% propane, 0.2mol% isobutene, 0.3mol% normal butane, 0.1mol% isopentane, 0.09mol% normal pentane, 2.39mol% nitrogen and 1.72mol% carbon dioxide) and then compared with that determined by the flame photometric detector (FPD) method for the same natural gas. For PW-2, one hydrate dissociation point was experimentally measured at 7.4°C and 139.4 bar, which was compared with the hydrate phase boundary determined by the C-V device. Figure 3 shows that the deviation between the C-V device (blue line) and the FPD (red line) is less than 0.5°C for pressures lower than 200 bar and that the measured dissociation point (purple points) is very close to the C-V results (green points). The required dosage of KHIs is relatively low by comparison to THIs such as MeOH and MEG, typically, 1–3 mass% in the aqueous phase. Therefore, it is particularly important to examine the possible influence of other chemical additives and multi-component salts on the accuracy of the C-V device for KHI systems. Produced water taken from a pipeline was used to examine the possible influences. It contained 4.97 mass%


of multi-component salts including NaCl (3.11 mass%), CaCl2 (0.16 mass%), KCl (0.34 mass%), MgCl2 (0.33 mass%) and others (1.03 mass%). Two aqueous solutions were made from a KHI (Luvicap


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EG) and the produced water. One contained 1.2 mass% PVCap (3 mass% Luvicap EG) and the other 1.1 mass% PVCap (2.75 mass% Luvicap EG), 500ppm of a CI and 550ppm of a SI. Table 2 shows that the presence of multi-component salts and the CI and SI did not have any measurable effect on the performance of the C-V device as long as NaCl was the dominant component.


Evaluation in Field Conditions The prototype C-V device was also evaluated in field laboratories by leading oil companies, including Total in Pau, France; Statoil in Trondheim, Norway; Dolphin Energy (Total) in Qatar; Petronas in Nouakchott, Mauritania; NIGC (National Iranian Gas Company) in South Pars Gas Complex (SPGC) fields; and Champion Technology, Aberdeen, UK. The evaluation results from Dolphin Energy were reported by Lavallie et al.28


and demonstrated


that the C-V device is a simple and reliable device for monitoring KHI concentration compared with conventional chemical analysis methods. Recently, the C-V technique has been patented29


been commercialised with a tradename HydraCHEK® by Hydrafact Ltd. Conclusions


A novel hydrate inhibition monitoring system has been developed based on measurements of acoustic velocity and electrical conductivity of downstream aqueous samples. It determines concentrations of salt and inhibitor (both THIs and KHIs) and the hydrate phase boundary (hence, the hydrate safety margin) given that the gas or oil composition and the temperature and pressure in a pipeline are known. Results of the lab and field evaluations demonstrated that technique can be used for monitoring the hydrate safety margin and optimising inhibitor injection rates, thus improving the reliability and economics of oil and gas production and reducing their impacts on the environment. n


Acknowledgments


This work was financially supported by BP, Chevron, Petronas, Statoil, TOTAL, NIGC and the Scottish Enterprise SMART programme, which is gratefully acknowledged.


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23. Yang J, Tohidi B, Characterization of inhibition mechanisms of kinetic hydrate inhibitors using ultrasonic test technique, Chem Eng Sci, 2011;66(3):278–83.


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28. Lavallie O, Ansari AA, O’Neil S, et al., Successful field application of an inhibitor concentration detection system in optimising the kinetic hydrate inhibitor (KHI) injection rates and reducing the risks associated with hydrate blockage, Presented at: International Petroleum Technology Conference, 7–9 December, 2009, Doha, Qatar, 2009:13765-MS.


29. Tohidi B, Yang J, Chapoy A, Hydrate monitoring system, WO2010/004289, 2010.


EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 1 and the prototype has


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