Analysis of Hydrate Formation Temperature and Water Dew Point of Processed Crude oil and Gas using Unism Simulator

Flow assurance has been a topic of concern since the start of crude oil and gas production and transportation. The formation of Hydrates is an important issue likely to cause clogs in pipelines during production and transportation of oil and gas. Therefore, production and transportation of such fluids are simulated using software’s like Unism to know the possibility of hydrate occurrence so they can be avoided. This work is based on the simulation of processed well effluents from Rose Field to analyze the hydrate formation temperature and water dew point at different points of the process facility. At the crude oil line the hydrate formation temperature was 69.9565°C, while the water dew point was not defined because it’s a liquid phase. At the gas line the hydrate formation temperature was 4. 7975°C at 1803psia and water dew point was -42.7°C. These values are parameters necessary for hydrate formation prediction, hence, they were analyzed and recommendations made to manage effective flow assurance.


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Abstract-Flow assurance has been a topic of concern since the start of crude oil and gas production and transportation. The formation of Hydrates is an important issue likely to cause clogs in pipelines during production and transportation of oil and gas. Therefore, production and transportation of such fluids are simulated using software's like Unism to know the possibility of hydrate occurrence so they can be avoided. This work is based on the simulation of processed well effluents from Rose Field to analyze the hydrate formation temperature and water dew point at different points of the process facility. At the crude oil line the hydrate formation temperature was -69.9565°C, while the water dew point was not defined because it's a liquid phase. At the gas line the hydrate formation temperature was 4.°C at 1803psia and water dew point was -42.7°C. These values are parameters necessary for hydrate formation prediction, hence, they were analyzed and recommendations made to manage effective flow assurance.
Index Terms-Hydrate Formation Temperature, Water Dew Point, Flow Assurance, Unism Simulator.

I. INTRODUCTION
According to Young Bai et. al., flow assurance is a process analysis in engineering where designs are developed with guidelines of operation for the effective control of problems caused by deposited solids such as wax, asphaltenes and hydrates in subsea systems [7]. Also, erosion, corrosion and scale formation are considered as flow assurance challenges since they also hinder flow in some cases. However, they are dependent on the characteristics of the hydrocarbon fluid produced.
During production of oil and gas the engineer is tasked to ensure that the oil is efficiently transmitted from the reservoir to the end user without hitches, hence, flow assurance problems are foreseen during simulation and mechanisms to avert them are put in place.
Basic flow assurance challenges are: 1) Corrosion 2) Erosion 3) Scale formation 4) Asphaltene 5) Wax 6) Hydrates Hydrates: these are deposits formed when tiny nonpolar molecules (<9Å) mixes up with water at typical temperatures less than 100℉ folloed by a pressure typically above 180psia. The crystal formed in this process is known as hydrates.
For hydrates to form, the following should be present; 1) High Pressure 2) Low Temperature 3) Water 4) Gas composition Water Dew Point: This is the point on the temperature scale below which vapors of water in air or a body in air will not be able to wholly remain in vapor state.
The dew point of a body in air is dependent on the pressure and water vapor content. The water dew point at different points of transportation of oil and gas is a very important parameter to check when analyzing for hydrate formation.
Unism Simulator: This is a software used for process design and simulation. It performs similar process analysis as ASPEN HYSYS process simulator. It also develops dynamic models and steady state models for managing assets, designing plants, monitoring performance, business planning, and troubleshooting. This study used Unism as its simulator for flow assurance analysis on reservoir effluent processed and transported.

II. LITERATURE REVIEW
Mahmood explained that hydrate formation is dependent on time. Also, goes deeper to emphasize that the rate of hydrate formation depends on presence of crystal nucleation sites in the liquid phase, level of agitation, gas composition, etc. and concluded that in TEG system design water dew point should be considered strongly ahead of the hydrate formation temperature [5] Analysis of Hydrate Formation Temperature and Water Dew Point of Processed Crude oil and Gas using Unism Simulator Azubuike H. Amadi and Chukwuebuka E. Okafor Jerome Joel Rajnauth, in his analysis of the different components that could lead to the formation of hydrates, highlighted Carbon dioxide, Nitrogen and Hydrogen Sulphides as major impurities that affects hydrate formation in natural gas production [3].
Maurice Stewart et. al. used the Vapor-solid equilibrium constant to predict the formation of hydrates during his research. However, assumptions made was the composition of streams should be known [6].
The review of the Petroleum Engineers Guide to Oil field chemicals and fluids buttressed on the use of antifreeze agents to reduce hydrate formation temperatures. Some common Antifreeze agents are Glycols, Brines, Methanol, etc. However, with regards to brine, its corrosive nature makes it unsuitable [4]. J. A. Prajaka noted that hydrate formation clogs pipelines and occurs mostly at Dew Point Control Units when there is a rapid drop in temperature during separation of natural gas from heavy ends using Joule-Thompson effect. It was also shown in his work that hydrates form when water dew points are higher than the hydrate formation temperature. Prajaka also noted that water dew point is not influenced by the composition of natural gas but rather by the pressure [2].

III. METHODOLOGY
The name of this field used for this simulation is known as Rose field and contains the following fluid composition [1]. The meteorological and oceanographic data from the rose field has shown that the coldest month during the year is February at air temperature of 3.1℃ while the sea water can get as low as 2℃.  The work was made more tasking by the fact that the simulated effluent was made up of reservoir fluids from different reservoirs having different properties as shown in Table II.
The average of most of the properties across reservoirs in the Rose field was used during Simulation.
Ten wells were drilled and shared among the four sections of the Rose field. During simulation the streams which represented the wells where given an average property across all wells.    The Hydrate formation temperature of both the gas line and the oil line was generated.  As expected, the hydrate formation temperature for the processed crude came out in the negative (-69.95650 o C) due to the absence of water which is necessary for the formation of hydrate and no value for water due point because dew point is a gaseous phase property. However, at the associated gas process line, the hydrate formation temperature was 4.7975℃ while the water dew point was -42.7℃ at 1803psia. The low water dew point is as a result of effective dehydration at the TEG column during process. We are now faced with the challenge of making a decision to ascertain the if hydrate will form or not since varying system pressure could also affect the HTF.

V. CONCLUSION
From the above analysis, hydrate would not form in the processed crude oil line unless there is an unlikely change in transmission temperature leading to a drastic and unlikely reduction in the crude oil temperature to a value below the HFT (-69.9565℃ ) temperature at high pressure. While, for the gas line whose result was more technical, the low water dew point shows that there is very little or no water in the gas phase at 1803psia which implies that theoretically, at working condition lower than the HTF (4.7975℃) but higher than -42.7℃ the likelihood for hydrates formation is slim. Water is necessary in the formation of hydrates, therefore without water present it will be difficult for hydrate to form from gaseous to solid state.

VI. RECOMMENDATION
Irrespective of the fact that simulations have been carried out and results obtained, adequate monitoring of the system properties (pressure and temperature) should be carried out throughout the gas transmission to prevent the system from entering the hydrate phase. Also, adequate simulation should be carried out to determine the hydrate formation temperature at varying transmission pressures.
The case of the gas line which showed a low water dew point is theoretical, therefore in cases like this, the engineer should be careful in decision making, therefore, all necessary equipment and procedures such as pigs, shut down maintenance plan, inhibitors, etc. which could help in averting hydrates should always be available.