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Locally Sourced, Ecofriendly Hydrate Inhibitor Effective in Simulated Offshore Environment

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Gas hydrates pose a serious flow-assurance problem in offshore environments where accessibility is restricted. The complete paper investigates gas-hydrate inhibition in a simulated offshore environment using a plant extract (PE) as a local inhibitor. The work aims to identify an effective biodegradable gas-hydrate inhibitor from locally sourced materials and ascertain its effectiveness compared with the conventional hydrate inhibitor monoethylene glycol (MEG). Experiments were conducted using a mini-flow loop, and involved mitigating hydrate formation using varying weight percentages of the inhibitor (1, 2, and 3 wt%) and evaluating their effect on hydrate inhibition in the mini-flow loop. Sensitivity charts for pressure, temperature, and time for both the PE and MEG were made. In 1 and 2 wt% of the PE, better inhibitory capacity than MEG was demonstrated, while 3 wt% of the PE and MEG had a close match. Because of its ecofriendly and biodegradable nature, the PE is therefore recommended for field trial.

Introduction

Kinetic hydrate inhibitors (KHIs) are ­water-soluble polymeric compounds that prevent or delay hydrate formation. Antiagglomerants (AAs) function as surfactants and prevent hydrates from sticking together and clumping. The hydrate still forms, but the crystals do not plug and can be transported through pipelines. Unlike thermodynamic hydrate inhibitors (THIs) such as methanol or glycols, low-dosage hydrate inhibitors (LDHIs) do not stop hydrate from forming completely because they do not cause a ­hydrate-curve shift. Once hydrate forms, it cannot be eliminated by LDHIs because the operating conditions cannot be changed (lowering pressures or raising temperatures); thus, a well is still at risk when LDHIs are used. THIs must still be available on site, especially when shutting in or starting a well.

The local inhibitor is a water-­soluble PE containing flavonoids, tannins, alkaloids, and saponins. Flavonoids act against inflammation and are anticoagulants. They are polyphenolic compounds found in vegetables, fruits, and beverages, acting as powerful oxidants that protect against reactive oxygen. Tannins are a heterogeneous group of high-molecular-weight polyphenolic compounds with the capacity to form reversible and irreversible complexes. They are also found in fruits, legumes, and grasses. Polyphenolics are a class of chemical compounds consisting of a hydroxyl group bonded directly to an aromatic hydrocarbon group. They are otherwise called polyhydroxylphenols and are phytochemicals. They are a structural class of mainly natural products. Condensed tannins are the most-abundant polyphenols and are found in virtually all families of plants. Alkaloids are natural products that contain heterocyclic nitrogen atoms and are basic in character. They are synthesized naturally by a large number of organisms including animals, plants, bacteria, and fungi. Saponins are a group of secondary metabolites found widely distributed in plant life. They form a stable foam in aqueous solutions such as soap. Chemically, saponins as a group include compounds that are glycosylated steroids, hypernoids, and steroid alkaloids. Saponins also prevent agglomeration and are surface-acting agents with bubbles and foams acting as barriers that provide good stability.

Materials

The materials used include compressed natural gas (CNG) of a specific gravity of 0.5 and a makeup of mostly methane. A minihydrate mini-flow loop is used. The stainless-steel loop is approximately 39.4 in. long, of 0.5-in. internal diameter encased in a skid-mounted, ArmaFlex-insulated 4-in. polyvinylchloride (PVC) pipe. A control panel houses the three switches that control the process, with a refrigerating unit used to simulate the offshore environment. The setup contains three pumps, five pressure gauges, three temperature gauges with two differential pressure gauges, including a manual pump, and seven valves. An inhibitor/mixing vessel, a CNG bottle that can withstand pressure up to 100 bar, and a visual flowmeter are present in the setup as well.

Experimental Procedures

The control panel is connected to the power source; then, the system is flushed with ordinary water to ensure that the loop is free of debris or rust. To achieve this, water is poured into the inhibitor vessel and Pump 3 is switched on from the control panel. Water is drawn into the loop until a pressure of 25 psia is attained. The valve is closed and Pump 3 switched off. The water is then vented out through one of three other valves. The process is repeated three or four times.

After this process, approximately 2660 ml of water is measured into the inhibitor vessel, water is drawn into the loop to approximately 25 psia, and then Valve 4 is closed. The CNG tank is turned on with Valve 1, then an orifice and Valve 6 are opened to build the pressure up to 150 psia, after which the valves and orifice are closed. Pump 2 is turned on to draw water into the refrigerator and to allow water to circulate in the PVC pipe in order to lower the fluid temperature to hydrate-formation temperature. Pump 1 is turned on after Pump 2 (that is, after circulation is achieved through Pump 2) and is set at 150 V or 250 ft3/hr to cause agitation in the loop. Ice blocks are added to the refrigerator (approximately 0.7 m of the 0.5-in. pipe is a spiral loop immersed in cold water in the refrigerator) to facilitate the cooling process. This spiral portion aids in retention time for increments of the fluid under test in the coldest part of the mini-flow loop where hydrate is most likely to form. The different readings for temperature, differential pressure, and pressures are noted at intervals of 5 minutes throughout the duration of the experiment, which is 120 minutes in total.

For inhibition experiments, the same procedure is followed, but instead of water only, an inhibitor is added into the inhibitor vessel. The inhibitor is added on the basis of the weight percent in the water phase to be used for the experiment (i.e., 1, 2, or 3 wt% of inhibitor in relation to the water used) and then the experiment is commenced per usual.

Results and Discussions

In the uninhibited experiment, the differential pressure dropped and then increased gradually until the end of the experiment. The temperature reduced from 30 to 27°C in 50 minutes, after which it began to rise to approximately 29.5°C for another 35 minutes. This is an indication that Type 1 or S1 hydrates have begun forming. Pressure declined from 150 to 80 psia in 60 minutes, then declined steadily to 36 psia at the end of the experiment, indicating the formation of hydrate in this system.

When 1 wt% of local inhibitor was used, the pressure reduced from 150 to 132 psia in 2 hours. The temperature reduced from 28 to 9.5°C in the first 85 minutes and then to 9°C for the rest of the experiment. The differential pressure oscillated around 0.3 bar. This shows good inhibitory characteristics. Also, for 2 wt% of the PE, pressure reduced from 150 to 105 psia, and temperature reduced from 30.5 to 6.6°C in 60 minutes and was constant for another 25 minutes before it decreased again to 6°C and remained constant until the end of the experiment. Differential pressure was relatively stable. No temperature increase was recorded, meaning that hydrate was inhibited. For 3-wt% inhibitor addition, pressure reduced from 150 to 120 psia over a duration of 120 minutes, which shows very good inhibitory capacity. Temperature reduced from 31 to 22°C in 15 minutes and to 16°C in another 10 minutes, and then decreased to 10°C and then 7°C until the end of the experiment. Differential pressure was relatively steady. Hydrate was inhibited.

Comparison of the PE and MEG is shown in Fig. 1. For the various weight percentages compared, 1 wt% PE had better inhibitory capacity than MEG. At 2 wt%, both inhibitors were close but were outperformed by PE at 1 wt%. At 3 wt%, both were closely matched.

Fig. 1—Plot of pressure and time for uninhibited, 1, 2, and 3 wt% of PE and MEG.

 

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 193439, “Gas-Hydrate Inhibition in a Simulated Offshore Environment Using a Local Inhibitor,” by E.V. Urunwo, SPE, S.S. Ikiensikimama, J.A. Ajienka, SPE, O. Akaranta, M.O. Onyekonwu, SPE, T.O. Odutola, SPE, and E.O. Okon, University of Port Harcourt, prepared for the 2018 Nigeria Annual International Conference and Exhibition, Lagos, 6–8 August. The paper has not been peer reviewed.

Locally Sourced, Ecofriendly Hydrate Inhibitor Effective in Simulated Offshore Environment

01 September 2019

Volume: 71 | Issue: 9

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