UNLOADING SEQUENCE
UNLOADING SEQUENCE
UNLOADING SEQUENCE
FIGURE 3-1 FIGURE 3-1
The fluid level in the casing and the tubing is at surface. No gas is being injected into the casing and no fluid is being produced. All the gas lift valves are open. The The fluid level in the casing and the tubing is at surface. No gas is being injected into the casing and no fluid is being produced. All the gas lift valves are open. The pressure to open the valves is provided by the weight of the fluid in the casing and tubing.
pressure to open the valves is provided by the weight of the fluid in the casing and tubing.
Note that the fluid level in the tubing and casing will be determined by the shut in bottom hole pressure (SIBHP) and the hydrostatic head or weight of the column of Note that the fluid level in the tubing and casing will be determined by the shut in bottom hole pressure (SIBHP) and the hydrostatic head or weight of the column of
fluid which is in turn determined by the density. Water has a
fluid which is in turn determined by the density. Water has a greater density than oil and thus the fluid level of a column of water will be lower than that of greater density than oil and thus the fluid level of a column of water will be lower than that of oil.oil.
INJ
INJ ECTION ECTION GASGAS CHOKE CLOSED CHOKE CLOSED
TO SEPARATOR/STOCK TANK TO SEPARATOR/STOCK TANK
TOP VALVE OPEN TOP VALVE OPEN
SECOND VALVE
1000 50005000
C
6000 70007000
FIGURE 3-2 FIGURE 3-2
Gas injection into the casing has begun. Fluid is
Gas injection into the casing has begun. Fluid is U-tubed through all the open gas lift valves. No U-tubed through all the open gas lift valves. No formation fluids are being produced because the pressure in theformation fluids are being produced because the pressure in the wellbore at perforation depth is greater than the rese
wellbore at perforation depth is greater than the reservoir pressure i.e. no rvoir pressure i.e. no drawdown. All fluid produced is from the casing drawdown. All fluid produced is from the casing and the tubing. All fluid unloaded fromand the tubing. All fluid unloaded from the casing
the casing passes through the open gas lift valves. Because of this, it is important that the well be unloaded at a reasonable rate to prevent damage to the gas liftpasses through the open gas lift valves. Because of this, it is important that the well be unloaded at a reasonable rate to prevent damage to the gas lift valves.
valves.
INJ
INJ ECTION ECTION GASGAS CHOKE OPEN CHOKE OPEN
TO SEPARATOR/STOCK TANK TO SEPARATOR/STOCK TANK
TOP VALVE OPEN TOP VALVE OPEN
SECOND VALVE
FIGURE 3-3 FIGURE 3-3
The fluid level has been unloaded to the top gas lift valve. This aerates the fluid above the top gas lift valve, decreasing the fluid density. This reduces the pressure in The fluid level has been unloaded to the top gas lift valve. This aerates the fluid above the top gas lift valve, decreasing the fluid density. This reduces the pressure in the tubing at the top gas lift valve, and also reduces pressure in the tubing at all valves below the top valve. This pressure reduction allows casing fluid below the top the tubing at the top gas lift valve, and also reduces pressure in the tubing at all valves below the top valve. This pressure reduction allows casing fluid below the top gas lift valve to be U-tubed further down the well and unloaded through valves 2, 3 and 4.
gas lift valve to be U-tubed further down the well and unloaded through valves 2, 3 and 4.
If this reduction in pressure is sufficient to give some drawdown at the perforations then the well will start to produce formation fluid.
If this reduction in pressure is sufficient to give some drawdown at the perforations then the well will start to produce formation fluid.
INJ
INJ ECTIOECTION GASN GAS CHOKE OPEN CHOKE OPEN
TO SEPARATOR/STOCK TANK TO SEPARATOR/STOCK TANK
TOP VALVE OPEN TOP VALVE OPEN
SECOND VALVE
FIGURE 3-4 FIGURE 3-4
The fluid level in the annulus has now been unloaded to just above valve number two. This has been posssible due to the increasing volume of gas passing through The fluid level in the annulus has now been unloaded to just above valve number two. This has been posssible due to the increasing volume of gas passing through number one reducing the pressure in the tubing at valve two thus enabling the U-tubing process to continue.
number one reducing the pressure in the tubing at valve two thus enabling the U-tubing process to continue.
INJ
INJ ECTION ECTION GASGAS CHOKE OPEN CHOKE OPEN
TO SEPARATOR/STOCK TANK TO SEPARATOR/STOCK TANK
TOP VALVE OPEN TOP VALVE OPEN
SECOND VALVE
1000 50005000
DRAWDOWN DRAWDOWN
6000
6000 70007000
FBHP FBHP
FIGURE 3-5 FIGURE 3-5
The fluid level in the casing has been lowered to a point below the second gas lift valve. The top two gas lift valves are open and gas being injected through both The fluid level in the casing has been lowered to a point below the second gas lift valve. The top two gas lift valves are open and gas being injected through both valves. All valves below also remain open and continue to pass casing fluid.
valves. All valves below also remain open and continue to pass casing fluid.
The tubing has now been unloaded sufficiently to reduce the flowing bottom hole pressure (FBHP) below that of the shut in bottom hole pressure (SIBHP). This gives The tubing has now been unloaded sufficiently to reduce the flowing bottom hole pressure (FBHP) below that of the shut in bottom hole pressure (SIBHP). This gives a differential pressure from the reservoir to the wellbore producing a flow of formation fluid. This pressure differential is called the drawdown
a differential pressure from the reservoir to the wellbore producing a flow of formation fluid. This pressure differential is called the drawdown
INJ
INJ ECTIOECTION GASN GAS CHOKE OPEN CHOKE OPEN
TO SEPARATOR/STOCK TANK TO SEPARATOR/STOCK TANK
TOP VALVE OPEN TOP VALVE OPEN
SECOND VALVE
1000 50005000
DRAWDOWN DRAWDOWN
6000
6000 70007000
FBHP
FBHP SIBHPSIBHP
FIGURE 3-6 FIGURE 3-6
The top gas lift valve is now closed, and all the gas is being injected through the second valve. When casing pressure operated valves are used a slight reduction in the The top gas lift valve is now closed, and all the gas is being injected through the second valve. When casing pressure operated valves are used a slight reduction in the casing pressure causes the top valve to close. With fluid operated and proportional response valves, a reduction in the tubing pressure at valve depth causes the top casing pressure causes the top valve to close. With fluid operated and proportional response valves, a reduction in the tubing pressure at valve depth causes the top valve to close. Unloading the well continues with valves 2, 3 and 4 open and casing fluid being removed through valves 3 and 4.
valve to close. Unloading the well continues with valves 2, 3 and 4 open and casing fluid being removed through valves 3 and 4.
INJ
INJ ECTIOECTION GASN GAS CHOKE OPEN CHOKE OPEN
TO SEPARATOR/STOCK TANK TO SEPARATOR/STOCK TANK
TOP VALVE CLOSED TOP VALVE CLOSED
SECOND VALVE
1000 50005000
DRAWDOWN DRAWDOWN
6000
6000 70007000
FBHP
FBHP SIBHPSIBHP
FIGURE 3-7 FIGURE 3-7
The No. 3 valve has now been uncovered. Valves 2 and 3 are both open and passing gas. The bottom valve below the fluid level is also open.
The No. 3 valve has now been uncovered. Valves 2 and 3 are both open and passing gas. The bottom valve below the fluid level is also open.
Note that the deeper the point of injection the lower the FBHP and thus the greater the drawdown on the well. As well productivity is directly related to the drawdown Note that the deeper the point of injection the lower the FBHP and thus the greater the drawdown on the well. As well productivity is directly related to the drawdown
then the deeper the injection the greater the production rate.
then the deeper the injection the greater the production rate.
INJ
INJ ECTION ECTION GASGAS CHOKE OPEN CHOKE OPEN
TO SEPARATOR/STOCK TANK TO SEPARATOR/STOCK TANK
TOP VALVE CLOSED TOP VALVE CLOSED
SECOND VALVE
1000 50005000
DRAWDOWN DRAWDOWN
6000
6000 70007000
FBHP
FBHP SIBHPSIBHP
FIGURE 3-8 FIGURE 3-8
The No. 2 valve is now closed. All gas is being injected through valve No 3. Valve No 2 is closed by a reduction in casing pressure for casing operated valves or a The No. 2 valve is now closed. All gas is being injected through valve No 3. Valve No 2 is closed by a reduction in casing pressure for casing operated valves or a reduction in tubing pressure for fluid operated and proportional response valves. Valve No 3 is the operating valve in this example. This is because the ability of the reduction in tubing pressure for fluid operated and proportional response valves. Valve No 3 is the operating valve in this example. This is because the ability of the reservoir to produce fluid matches the ability of the tubing to remove fluids (Inflow/Outflow Performance). The operating valve can either be an orifice valve or can be a reservoir to produce fluid matches the ability of the tubing to remove fluids (Inflow/Outflow Performance). The operating valve can either be an orifice valve or can be a gas lift valve. The valve in mandrel No 4 will remain submerged unless operating conditions or reservoir conditions change.
gas lift valve. The valve in mandrel No 4 will remain submerged unless operating conditions or reservoir conditions change.
INJ
INJ ECTION ECTION GASGAS CHOKE OPEN CHOKE OPEN
TO SEPARATOR/STOCK TANK TO SEPARATOR/STOCK TANK
TOP VALVE CLOSED TOP VALVE CLOSED
SECOND VALVE
1000 50005000
DRAWDOWN DRAWDOWN
6000
6000 70007000
FBHP
FBHP SIBHPSIBHP