PETROLEUM & GAS PROCESSING TECHNOLOGY (PTT 365) SEPARATION OF PRODUCED FLUID

Transcription

1 PETROLEUM & GAS PROCESSING TECHNOLOGY (PTT 365) SEPARATION OF PRODUCED FLUID Miss Nur Izzati Bte Iberahim

2 Introduction Well effluents flowing from producing wells come out in two phases: vapor and liquid under a relatively high pressure. Fluid pressure should be lowered and its velocity should be reduced in order to separate the oil and obtain it in a stable form and usually done by admitting the well fluid into a gas oil separator plant (GOSP) The primary objective is to allow most of the gas to free itself from these valuable hydrocarbons, hence increasing the recovery of crude oil.

3 Introduction Oil-field separators can be classified into two types based on the number of phases to separate: - Two-phase separators : to separate gas from oil in oil fields, or gas from water for gas fields. - Three-phase separators : to separate the gas from the liquid phase and water from oil.

4 TWO-PHASE GAS-OIL SEPARATION

5 The Separation Problem High-pressure crude oils containing large amount of dissolved gas flow from the well head into the flow line The gas, lighter than oil, fills the upper part of the vessel. Crude oils with a high gas oil ratio (GOR) must go through two or more stages of separation. In a series of repeated runs, the piston is raised to allow for a decrease in pressure, keeping the temperature constant. The volume of the gases separated in each run is measured and reported until reaching atmospheric conditions

6 Theory of Gas-Oil Separation Well effluent hydrocarbon mixtures contain essentially three main groups of hydrocarbon - Light group, which CH4 (methane) and C2H6 (ethane) - Intermediate group consists of two subgroups: the propane/butane (C3H8/C4H10) group and the pentane/hexane (C5H12/C6H14) group. - Heavy group, which is the bulk of crude oil and is identified as C7H16.

7 Objective of gas oil separation process - Separate the C1 and C2 light gases from oil - Maximize the recovery of heavy components of the intermediate group in crude oil - Save the heavy group components in liquid product Methods for the mechanics of separation (minimize the loss and maximize liquid recovery) - Differential or enhanced separation - Flash or equilibrium separation In differential separation, light gases (light group) are gradually and almost completely separated from oil in a series of stages, as the total pressure on the well-effluent mixture is reduced.

8 For flash separation, gases liberated from the oil are kept in intimate contact with the liquid phase. Then thermodynamic equilibrium is established between the two phases and separation takes place at the required pressure. Differential Separation - most of the light gases take place at the earlier high-pressure stages, the opportunity of loosing heavy components with the light gases in low pressure stages is greatly minimized. Flash separation is inferior to differential separation because of greater losses in heavy hydrocarbons that are carried away with light gases due to equilibrium conditions

9 Methods of Separation Modification to the basic flash separation technique: - Conventional methods - Modified methods - Replacing the conventional methods by a stabilizer and a vapor recompression unit Conventional method is a multistage flash separation system and is recommended for comparatively high-pressure fluids. Number of stages in a multistage conventional separation process is a function of: - The API gravity of the oil - The gas-oil ratio (GOR) - The flowing pressure

10 High-API-gravity oils with high GOR flowing under high pressure would require the greatest number of stages (from three to four). First modified method of separation implies adding several stages of gas compression to recompress the separated gas from each flash stage. Liquids from inter-stage vessels between the compressors can be collected and processed as liquid natural gas (LNG) stock.

11 Second modified method of separation use of crude stabilizer columns. These columns have top feed trays with no rectifying section and no condenser, but are provided with interstage reboilers and feed preheaters. Crude stabilization systems are advantageous because they occupy less space than conventional GOSPs.

12 Gas-Oil Separation Equipment Characteristics of the conventional separator: - It causes a decrease in the flow velocity, permitting separation of gas and liquid by gravity. - It always operates at a temperature above the hydrate point of the flowing gas. Hydrate gas - ice-like solids that form when free water and natural gas combine at high pressure and low temperature

13 Functional Components of a Gas-Oil Separator Gas oil separators consist of four functional sections: - Section A: Initial bulk separation of oil and gas takes place in this section. - Section B: Gravity settling and separation is accomplished in this section of the separator. - Section C: Known as the mist extraction section, it is capable of removing the very fine oil droplets which did not settle in the gravity settling section from the gas stream. - Section D: This is known as the liquid sump or liquid collection section.

14 Two step mechanism of separation gas from oil: To separate oil from gas - Density difference or gravity differential is responsible for this separation. At the separator s operating condition of high pressure, this difference in density between oil and gas becomes small (gas law). This could be a sufficient driving force for the liquid particles to separate and settle down. To remove gas from oil - To recover and collect any non solution gas that may be entrained or locked in the oil. (settling, agitation, and applying heat and chemicals)

15 Commercial Types of Gas-Oil Separator Most common types of separator are horizontal, vertical, and spherical.

16 Comparison

17 Test Separator Main Equipment Use to separate and measure at the same time the well fluids. The oil produced is measured by a flow meter (normally a turbine meter) at the separator s liquid outlet and the cumulative oil production is measured in the receiving tanks. An orifice meter at the separator s gas outlet measures the produced gas.

18 Low-Temperature Separators Low-temperature separators (LTSs) are used to effectively remove light condensable hydrocarbons from a high-pressure gas stream (gas condensate feed). Liquid (condensate) separation is made possible by cooling the gas stream before separation. Temperature reduction is obtained by what is known as the Joule Thomson effect of expanding the well fluid as it flows through the pressure-reducing choke or valve into the separator.

19 Modern GOSPs Safe and environmentally acceptable handling of crude oils. If the effect of corrosion due to high salt content in the crude is recognized, then modern desalting equipment could be included as a third function in the GOSP design. The water produced with the crude is a brine solution containing salts (mainly sodium chloride) in varying concentrations. Functions of a modern GOSP: - Separate the hydrocarbon gases from crude oil - Remove water from crude oil - Reduce the salt content to the acceptable level

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21 Controllers and Internal Components of Gas- Oil Separators Liquid Level Controller Use to maintain the liquid level inside the separator at a fixed height. Consists of a float that. Exists at the liquid gas interface and sends a signal to an automatic diaphragm motor valve on the oil outlet. Pressure Control Valve The pressure control valve (PCV) is an automatic backpressure valve that exists on the gas stream outlet. The valve is set at a prescribed pressure

22 Pressure Relief Valve The pressure relief valve (PRV) is a safety device that will automatically open to vent the separator if the pressure inside the separator exceeded the design safe limit. Mist Extractor The function of the mist extractor is to remove the very fine liquid droplets from the gas before it exits the separator. Type of mist extractor: - Wire-Mesh Mist Extractor (made of finely woven stainless-steel wire wrapped into a tightly packed cylinder of about 6 in. thickness) - Vane Mist Extractor (consist of a series of closely spaced parallel, corrugated plates) - Centrifugal Mist Extractor ( use centrifugal force to separate the liquid droplets from the gas)

23 Wave Breakers Consist of vertical baffles installed perpendicular to the flow direction in order to avoid unsteady fluctuations in the liquid level and would negatively affect the performance of the liquid level controller. De-foaming Plates The foam, having a density between that of the liquid and gas, will disrupt the operation of the level controller. Foaming problems may be effectively alleviated by the installation of defoaming plates within the separator. In some situations, special chemicals known as foam depressants may be added to the fluid mixture to solve foaming problems

24 Vortex Breaker Normally installed on the liquid outlet to prevent formation of a vortex when the liquid outlet valve is open. The formation of a vortex at the liquid outlet may result in withdrawal and entrainment of gas with the exiting liquid (gas blow-by). Sand Jets and Drains Sand will settle and accumulate at the bottom of the separator. This takes up separator volume and disrupts the efficiency of separation. Produced water is injected though the jets to fluidize the accumulated sand, which is then removed through the drains.

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26 Optimum Pressure for Gas-Oil Separators Proper operating pressure has to be selected and its value has to be between the two extreme cases in order to maximize the oil yield.

27 Effect of the operating pressure on the recovery of stock tank-oil: - High-pressure operation: This will diminish the opportunity of light hydrocarbons in the feed to vaporize and separate - Low-pressure operation: large quantities of light hydrocarbons will separate from the gas oil separator, carrying along with them heavier hydrocarbons, causing a loss in the recovered oil Total pressure on the system is increased, the mole fraction in the gas phase has to decrease. Tendency of vaporization diminishes as the pressure inside the gas oil separator increases.

28 Selection and Performance of Gas-Oil Separators Operating temperature: A higher temperature will cause more evaporation of the hydrocarbons, diminishing the recovery of the liquid portion Operating pressure: A higher pressure will allow more hydrocarbons to condense, increasing liquid recovery. However, after reaching a certain peak, a higher pressure causes liquid to decrease. Number of stages: Increasing the number of stages in general will increase the efficiency of separation, resulting in a higher yield of the stable stock tank oil. Composition of the well streams has to be considered in evaluating the performance of a gas oil separator.

29 Number of tests are commercially carried out to evaluate the efficiency of operation: - Evaluation of particle size: The method requires determining the size of liquid particles entrained by the gas stream. The efficiency of gas oil separator is thus evaluated based on this size. - Determination of the quantity of liquid carryover: The method requires determining the volume of liquid entrained or carried over by the gas stream. - Stain test (handkerchief): consists of holding and exposing a white cloth in the gas stream leaving the separator.

30 THREE-PHASE GAS-OIL SEPARATION

31 Introduction Water produced with the oil exists partly as free water and partly as water in-oil emulsion. Free water produced with the oil is defined as the water that will settle and separate from the oil by gravity To separate emulsified water, heat treatment, chemical treatment, electrostatic treatment or combination of these treatments would be necessary in addition to gravity settling Free-water knockout method used to separate free water, oil and gas The vessel is called three-phase separator when there is a large volume of gas to be separated from the liquid (oil and water).

32 Horizontal Three Phase Separators Liquid collection of three-phase separator handles two immiscible liquids (oil and water). Produced fluid stream coming either from producing well or free water knockout Enters the separator and hit the inlet diverter Initial bulk separation of the gas and liquid due to change in momentum and different in fluid densities. The gas flows horizontally through the gravity settling section. Entrained liquid droplets down to a certain minimum size are separated by gravity Gas then flow to the mist extractor Bulk of liquid separated at inlet diverter, flows downward through water layer (water washing)

33 Known as the bucket and weir design-eliminates the need for an interface controller. The oil and emulsion flow over the oil weir into the oil bucket. The water flows through the space below the oil bucket

34 Vertical Three Phase Separators The produced fluid stream enters separator and hits the inlet diverter and bulk separation of the gas from liquid. Gas flows upward through the gravity settling section. Gas flows through the mist extractor (small liquid droplets are removed) Gas leaves the separator at the top and liquid flows downward through the down-comer. Liquid comes out of the spreader, the oil rises to the oil pad Oil flows over a weir into an oil chamber and out of the separator.

35 Liquid-liquid interface controller will function effectively if there is an appreciable difference between the densities of the two liquids. Water-oil emulsion and water emulsion interface will be present in the separator instead of a water-oil interface. Chemical known as demulsifying agents are injected into the fluid stream and help in breaking the emulsion. Or by using heat addition to the liquid within the separator.