Process Description
Production Manifold
The Production Manifold collects fluids produced by the three wells. The Production Manifold normally operates at 612 PSIG and 87.9 DEG F with only Well #1 in service at its design production rate. The combined fluids are piped to the Line Heater F200.
Detailed Process Description: Well #1
Natural Pressure Gas Well
Well #1 is a conventional gas well that produces natural gas along with small amounts of entrained hydrocarbon liquids and water from an underground reservoir. The reservoir operates at a downhole pressure of 1000 PSIG and 664 PSIG at the wellhead. Gas flows up the well tubing to the wellhead at the surface, through surface shutdown valve SDV-001, through the choke valve HV-001 and to the Production Manifold where it combines with any flow from Wells #2 and #3. In case of an emergency the well is isolated by SDV-001 which is closed by activation of interlock I-01.
Detailed Process Description: Well #2
Gas Well with Natural Mechanical Liquid Lift
Well #2 is a conventional gas well that mainly produces natural gas along with small amounts of entrained hydrocarbon liquids and water from an underground reservoir like Well #1. Although the reservoir conditions are the same as Well #1, the velocity of fluid in Well #2 cannot adequately remove all the produced liquid and water which, in turn, builds up in the well tube over time. As the liquids fill the tube they act to plug the well and gas flow begins to drop off. To counteract this, Well #2 is outfitted with a Plunger Lift P-002 to periodically lift accumulated liquids out of the well tube using the natural pressure of the reservoir.
Gas and liquids from Well #2 flow out of MOV-002 through surface shutdown valve SDV-002, through the choke valve HV-002 and to the Production Manifold where it combines with any flow from Wells #1 and #3. In case of an emergency the well is isolated by SDV-002 which is closed by activation of interlock I-02.
Well #2 Plunger Lift P-002
The Plunger Lift system consists of the following equipment:
- The Plunger Lift P-002 which is essentially a tall bucket that fits in the well tube and is outfitted with a specially engineered valve on the bottom which closes when the plunger falls to the bottom of the well and opens when the plunger ascends to the wellhead.
- The plunger Lubricator/catcher assembly SP-002 which sits on top of the wellhead. This assembly opens the valve on the bottom of the plunger lift when it arrives and holds the plunger lift in the assembly until the plunger lift controller releases it so it can fall to the bottom of the well.
- The lubricator outlet motor-operated valve MOV-002 which is opened and closed by the plunger lift controller XIC-002.
Well #2 P-002 Plunger Cycle
Lifting of liquids out of the well tube is accomplished using the natural pressure of gas in the reservoir in the following way:
- The valve on the Plunger Lift P-002 is opened in the Lubricator SP-002.
- Under stable flow and pressure conditions, controller XIC-002 releases P-002 from SP-002 so it can fall down the well by gravity. As it falls, produced gas and any accumulated liquids in the well tube will pass through the open valve.
- At the bottom of the well, a spring-loaded assembly closes the valve as the Plunger Lift falls onto it.
- After an adjustable time from release of the Plunger Lift, controller XIC-002 closes MOV-002 to block in the well at the surface. This causes the pressure at the bottom of the outside of well pipe to increase as gas from the reservoir and surrounding formation continues to flow in. The pressures in the well tube will equalize without any surface flow.
- After a fixed time, MOV-002 is reopened and a surge of gas leaves the top of the well causing the wellhead pressure to drop. A nozzle fitted in the bottom of the well tube and below the Plunger Lift spring-loaded assembly directs accumulated gas outside the well tube to the base of P-002 resulting in a large differential pressure between the bottom of P-002 and the wellhead. This differential pressure is normally large enough to lift any liquid sitting above P-002 to the surface.
- P-002 rises to the surface and pushes any accumulated liquid out of the well pipe and is captured in the Lubricator SP-002. Its arrival is detected on XI-002.
This cycle is repeated until controller XIC-002 is turned off.
Detailed Process Description: Well #3
Well #3 is a mature oil well that has lost its pressure due to previous production. In order to lift the crude oil from the reservoir to the surface, a Sucker Rod Pump P-003 is used.
Liquids from Well #3 flow through surface shutdown valve SDV-003, through the choke valve HV-003 and to the Production Manifold where it combines with any flow from Wells #1 and #2. In case of an emergency the well is isolated by SDV-003 which is closed by activation of interlock I-03.
The produced liquid from Well #3 contains a relatively small amount of dissolved light gases. It also contains a significant fraction of water.
Well #3 Sucker Rod Pump P-003
Sucker Rod Pump P-003 consists of the following components:
- A downhole plunger assembly that consists of a container (plunger) that fits into the well tubing and contains a series of check valves that effects filling of the plunger with liquid on its down-stroke and lifting of liquid on its up-stroke.
- A long rod string that is inserted through a packing assembly in the wellhead and attaches to the downhole plunger.
- A motorized surface assembly consisting of cranks, shafts, arms and counterweights to smoothly and cyclically move the rod string up and down in the well. The most prominent component of the surface assembly is referred to as the “horse head” and is attached to the rod string.
A variable speed motor is provided and its speed is operator-adjustable so that production flow and surface pressures may be optimized.
Detailed Process Description: Line Heater F-200
The Line Heater warms up the flow of produced liquids, gases and water from the Production Manifold before reaching the Three Phase Separator. The Line Heater is a cylindrical vessel through which tubing passes through a warm glycol/water solution that fills the vessel. The warm glycol solution is heated using combusted gas taken from the Three Phase Separator.
There are two tubing passes through the Line Heater. The high pressure pass heats the flow from the Production Manifold at supply line pressure. The warmed fluid is then directed to a letdown valve HV-201 in order to depressure the fluid from inlet pressure to the outlet pressure which operates near the Three Phase Separator’s pressure. Whenever gas and lighter liquid hydrocarbon condensates are depressured, the temperature of the fluid will decrease. Depending on the composition of the mixed fluids and the pressure ratio across HV-201, the temperature decrease may be sufficient enough to result in ice and hydrates formation which can plug up the downstream line to the Three Phase Separator resulting in decreased production rates from the Tank Battery. Warming the fluid prior to flashing ensures that potential plugging of HV-201 from ice and hydrates is minimized.
After depressurization and cooling in HV-201, the production flow is warmed some more in the low pressure pass of F-200. The warmed production fluid is then sent to the Three Phase Separator D-500.
A combustor assembly is mounted on the side and operates like a typical household gas furnace. Fuel gas is taken from the Three Phase Separator and depressured by a series of two pressure regulators for the main gas flow. A third pressure regulator provides a small flow of gas for the light-off pilot burner.
A temperature control valve TV-203 allows adjustment of the gas flow rate to the combustor assembly. TV-203 is calibrated so that at zero output of TIC-203 a minimum flow of gas is obtained that provides a stable flame in the combustor assembly. A gas shutoff valve SDV-301 is situated between the two pressure SPM-3050 Manual Operator Doc Revision 0 DSS-100 Version 7.1 Page 8 regulators and will close off gas to the combustor assembly whenever F-200 is turned off.
Line Heater F-200 Glycol Tank
In addition to the tubes handling produced flows from the wells, the Line Heater also contains tubes for warming the glycol solution using a small flow of combusted gas from the Three Phase Separator.
The hot combusted gas flows from the combustor assembly through warming tubes that pass through the glycol solution within F-200 and exhaust to a flue gas vent pipe after giving up most of the heat from combustion of the gas to the glycol solution. The tubing arrangement of F-200 results in natural circulation of the glycol solution as it is heated. This method of using a circulating fluid to transfer heat from a high temperature heat source to a process stream is known as indirect heating. This approach is much safer and easier to engineer and build than a direct-fired heater for a service that only requires modest heat input to the process stream.
Line Heater F-200 Interlock Circuit
F-200 is protected by an interlock circuit I-04 that stops fuel gas in case of a trip of the Three Phase Separator, a high glycol solution temperature or a loss of flame in the combustor assembly. The loss of flame is automatically and momentarily bypassed immediately after turning on the Line Heater.
Detailed Process Description: Three Phase Separator D-500
Gas, condensates, water and crude oil from the Line Heater are brought into one end of the Three Phase Separator, D-500 and enter the upper section of the vessel which contains a baffle to minimize the disturbances from slugs of oil and water entering with the feed. Oil, water and gas in the feed flow under this baffle into the main separation compartment of the Three Phase Separator.
Light gases such as methane and nitrogen along with flashed light hydrocarbons from feed condensate separate into the vapor space at the top of the vessel, while immiscible water and oil separate by gravity and form two liquid layers in the separation compartment. The separation compartment is bounded by another baffle extending from the bottom of the vessel to halfway between the top and bottom of the vessel. Separated water and oil collect on the feed side of this baffle. The water/oil interface level is controlled at halfway up the separation baffle (which, therefore, is one quarter of the height of the vessel) by LIC-501 which adjusts the takeoff flow of water from the bottom of the separation compartment. The water contains small amounts of oil and is sent to the Water Tank T-2001 before it is taken offsite to remove the oil before disposal.
Separated crude oil and unflashed light condensate floats on the water phase and accumulates and spills over the separation baffle into the collection compartment of the Three Phase Separator. The level of collected oil in this compartment is controlled by LIC-500 which adjusts the flow of oil taken from this compartment to the Oil/Condensate Tank T-2000.
NOTE
LIC-501 indicates 100% when the water/oil interface level in the separation compartment is at the height of the separation baffle. If the interface level goes higher than 100%, water will flow over the separation baffle and mix with oil on the collection side. In this situation, water will be sent to the Oil/Condensate Tank and will settle at its bottom.
D-500 Gas Separation
Gas that is flashed and desorbed from the feed is collected in the vapor space of the Three Phase Separator and flows to the opposite end. The residence time that allows water to separate from the oil also allows entrained gas in the oil to desorb from the oil.
To make sure there are as few droplets of oil and water leaving with the gas, a demisting pad extends from the top of the vessel to the middle of the vessel over the separation compartment. This ensures the vapor must pass through the demisting pad as the level of oil in the separation compartment will be slightly higher than the liquid separation baffle. The demisting pad is made of crinkled wire mesh screen (CWMS). Any entrained droplets of liquid will impact the wires and the droplets will coalesce and drain into the oil phase of the separation compartment.
D-500 Pressure Control
The pressure in the Three Phase Separator normally floats on the gas collection pipeline pressure. HIC-501 can be used to open and close HV-501 in order to isolate the Three Phase Separator from the pipeline. A check valve is installed in the line to prevent back flow of pipeline gas to D-500. In the event of over pressurization of D-500, controller PIC-501 automatically vents gas through a separate line to the flare boom located safely away from the equipment. A small line takes gas from D500 to the Line Heater to be used for warming the feed from the Production Manifold.
A pressure safety valve set at 500 PSIG protects D-500 from over pressurization in case PIC-501 fails to safely regulate its pressure.
Detailed Process Description: Storage Tanks T-2000 & T-2001
Crude oil/hydrocarbon condensate from the Three Phase Separator is collected in the Oil/Condensate Tank T-2000. The working capacity of the tank is 2,000 BBL which roughly provides 2 weeks storage capacity when producing crude oil from Well #3 (5.0 BPH oil production). Oil/condensate from T-2000 can be transferred by gravity to tanker truck using HIC-600. The maximum unloading rate of T-2000 is 350 BPH. A volume meter, FQI-600, shows the total volume unloaded. The meter automatically resets at its high range (9,999 BBL).
Produced water from D-500 is collected in the Water Tank T-2001. The tank is identical to T-2000 and roughly provides 6 weeks storage capacity when producing crude oil from Well #3 (2.0 BPH water production). Water from T-2001 can be transferred by gravity to tanker truck using HIC-601. The maximum unloading rate of T-2001 is 350 BPH. A volume meter, FQI-601, shows the total volume unloaded. The meter automatically resets at its high range (9,999 BBL).
Both tanks are fitted with a near-atmospheric pressure safety valve which routes any accumulated gases to the flare system. A tie-line between the upper sides of the two tanks allows sharing of the other tank’s unused volume in case one tank gets overfilled. This tie-line also allows any hydrocarbons that separate from water in T2001 to spill over to T-2000.
A small vacuum breaker admits air into the top of each tank to avoid a vacuum lock when transferring liquid to a tanker truck.
Instrumentation
The pressure of the Production Manifold is indicated by PI-004. The temperature of the Production Manifold is indicated by TI-004.
The pressure of the wellhead upstream of choke HV-001 is indicated by PI-001.
The position of the choke is adjusted using HIC-001.
Switch HS-001 is used to activate and reset interlock I-01 for Well #1. Interlock I-01 is also activated when Separator interlock I-05 is activated. When I-01 is activated, surface shutdown valve SDV-001 on the wellhead is closed and the choke hand controller HIC-001 is placed in manual mode with an output of 0% to close the choke HV-001. I-01 will lock these actions until it is reset using HS-001.
The pressure of the wellhead upstream of the Plunger Lift surface equipment is indicated by PI-002A. The pressure at the outlet of the surface equipment is indicated by PI-002B.
The position of the choke HV-002 is adjusted using HIC-002A.
P-002 Plunger Lift
The arrival of the Plunger Lift at the Lubricator SP-002 is indicated by switch XI-002. The position of switch XIC-002 indicates the status of MOV-002 per the Plunger Lift sequencing control logic. Switch HS-002B is used to put the sequencing control logic in the RUN or STOP state. In the RUN state, the sequencer operates as described in the process description for Plunger Lift P-002 above. The output of HIC-002B is used to adjust the time the sequencer waits before closing MOV-002. At 0% output the wait time is 300 seconds; at 100% the time is 1800 seconds. The time is linearly adjusted between these two settings for any other output of HIC-002B. In the STOP state, no sequencing occurs and MOV-002 is opened.
Switch HS-002A is used to activate and reset interlock I-02 for Well #2. Interlock I-02 is also activated when Separator interlock I-05 is activated. When I-02 is activated, surface shutdown valve SDV-002 on the outlet of the Plunger Lift surface equipment is closed, choke hand controller HIC-002A is placed in manual mode with an output of 0% to close the choke HV-002, and the Plunger Lift sequencing logic switch HS-002B is placed in the STOP state. I-02 will lock these actions until it is reset using HS-002A.
The pressure of the wellhead is indicated by PI-003.
The position of the choke HV-003 is adjusted using HIC-003A.
P-003 Sucker Rod Pump
The motor for P-003 is switched on and off using switch HS-003B. The speed of the motor is adjusted using the output of hand controller HIC-003B. At 100% output the motor operates at full speed; at 0% output the motor operates at 50% speed.
Switch HS-003A is used to activate and reset interlock I-03 for Well #3. Interlock I-03 is also activated when Separator interlock I-05 is activated. When I-03 is activated, surface shutdown valve SDV-003 on the outlet of the wellhead is closed, choke hand controller HIC-003A is placed in manual mode with an output of 0% to close the choke HV-003, and the motor of the Sucker Rod Pump is stopped by placing HS-003B in the STOP state. I-03 will lock these actions until it is reset using HS-003A.
The inlet pressure of the production fluid is indicated by PI-201. Its temperature is indicated by TI-201.
The outlet pressure of the production fluid is indicated by PI-202. Its temperature is indicated by TI-202.
The position of the letdown valve HV-201 is adjusted using HIC-201.
The temperature of the glycol solution inside the vessel is controlled by TIC-203. Control valve TV-203 is calibrated so that it is sufficiently open to provide a minimum flow of gas to the burners of the combustor assembly at 0% output of TIC-203 while having a stable flame.
THH-203 is an independent measurement of the glycol solution for the protective interlock I-04. TI-204 is another independent indication of the glycol solution for monitoring purposes.
PI-401 indicates the pressure of the fuel gas supply to the pilot and main fuel pressure regulators. The setpoint of self-regulating pressure control valve PCV-401 is 25.0 PSIG. PI-301 indicates the pressure of the main fuel gas downstream of self-regulating valve PCV-301 whose setpoint is 5.0 PSIG.
BLL-301 indicates the flame status detected in the combustor assembly.
Switch HS-200 is used to activate and reset interlock I-04 for the Line Heater. Interlock I-04 is also activated when Separator interlock I-05 is activated. THH-203 will activate I-04 when the temperature exceeds 200 DEG F. A loss of flame as detected by BLL-301 will also activate I-04.
When I-04 is activated, fuel gas shutdown valve SDV-301 on the fuel gas supply line is closed. The valve remains closed until I-04 is reset.
Interlock I-04 also contains logic to start combustion of fuel gas when the interlock is reset by changing HS-200 to the ON state. In order to do this the trip signal from BSL-301 is bypassed for 15 seconds when HS-200 is switched to the ON state. If BSL-301 does not indicate a flame after 15 seconds, I-04 will activate. A local control board handles lighting of the pilot flame.
Whenever starting the Line Heater care should be made to place TIC-203 into manual with 0% output to avoid large surges of fuel gas when the flame lights off.
The oil level of the Three Phase Separator D-500 is controlled by LIC-500 which regulates the control valve on the line to the Oil/Condensate Storage Tank. The oil flow is indicated on FI-500.
The water/oil interface level of D-500 is controlled by LIC-501. The produced water flow is indicated on FI-501.
The pressure of the gas produced in D-500 is indicated by PHH-500 which serves as a trip to interlock I-05. The produced gas flow from D-500 to the pipeline is indicated on FI-503. Hand controller HIC-501 is used to adjust the opening of HV-501 on the produced gas line. The sales gas pipeline pressure is indicated on PI-502.
PIC-501 is an emergency vent to flare that will open if the pressure of D-500 rises above 250 PSIG. It is designed to open before the pressure safety valve PSV-500 opens (setpoint = 500 PSIG). It can also be used to depressure D-500 at shutdown. If the pipeline pressure operates near or above the setpoint of PIC-501, the setpoint needs to be increased to prevent unnecessary venting of gas to flare. The maximum setpoint should not exceed the trip setting of PHH-500 (see below).
Switch HS-500 is used to activate and reset interlock I-05 for the Three Phase Separator. LHH-500 indicates the top level of liquid over the separation compartment of D-500 and will activate I-05 when the level exceeds 95%. Unlike the other two level control instruments, LHH-500 spans most of the height of D-500. PHH-500 will activate I-05 when it exceeds 350 PSIG.
Interlock I-05 does not directly control any process equipment. Instead, it activates the four other interlocks (I-01 through I-03 for the wells; I-04 for the Line Heater). I-05’s control signal is maintained until it is reset with HS-500. Therefore, the other interlocks will remain activated until I-05 is reset and each interlock is individually reset.
Oil/Condensate Tank T-2000
The level of liquid in T-2000 is indicated on LI-600.
The output of hand controller HIC-600 adjusts the position of gravity unloading valve HV-600. FQI-600 indicates the volume of oil/condensates delivered to the delivery truck. FQI-600 automatically resets to zero indication when it reaches its upper range.
Water Tank T-2001
The level of liquid in T-2001 is indicated on LI-601.
The output of hand controller HIC-601 adjusts the position of gravity unloading valve HV-601. FQI-601 indicates the volume of oil/condensates delivered to the delivery truck. FQI-601 automatically resets to zero indication when it reaches its upper range.