1. Analysis of Heading in Artificially Lifted Wells
Feb-6, 2002 ASME Gas-Lift Workshop
Presented by: Efren Munoz
Edinburgh Petroleum Services Americas Inc
The idea of this paper is to share some experiences in actual prod-opt projects. Specifically wells exhibiting “heading”. In most fields these wells represent a very high percentage of the field and normally due to the difficulty to model them it is decided to work with stable wells first (show some results) and then work with the “problem” wells.

2. Introduction
Heading creates inefficiency in a continuous gas lift installation
This behavior can affect wells in the neighborhood when they share the flowline
This needs attention from the operations view point as it generates less production of fluids and more consumption of gas for injection
It has to be an important part of any production optimization study as “performance curves” assume perfectly “stable” production conditions
This operation/equipment section must be done prior to implement any recommendation from a production optimization study
First of all it is important to understand that heading happens and affects wells due to operating conditions when the equipment needs calibration and sometimes because the reservoir is depleting and the well can not keep the same productivity ie: continuous going to intermittent.

3. Heading Conditions
This behavior is very common and it is difficult to model by conventional tools
It is necessary to identify the cause in order to try and find the solution.

4. Modeling of gas-lift installations
Are the conventional “steady state” software tools able to diagnose this phenomenon?
Being a dynamic situation is the use of dynamic tools mandatory?
The conventional steady-state assumes perfectly stable prod conditions, which is very ideal.

5. Conventional Modeling Procedure
The following slides show a procedure using a “steady-state” tool with the ability to model “True Valve Performance”
This is an “iterative” methodology and the engineer has to have very good understanding of valve performance, expertise becomes critical at this point
The ability to model “true” or “real” gas-lift valves gives the engineers the power to simulate a more realistic situation. This is really a light in understanding non-ideal situations like heading for instance.

6. Conventional Steady State Analysis
CHP= 1120 PSIA
By conventional analysis this well would be injecting through the 2nd valve
With this ability (true valve modeling) it is possible not only to identify the actual operating valve but also how the others are behaving ie: open or close for a given moment. This well has 4 valves and is injecting through the 2nd, but it is necessary the status of the others too.
The Qgi measured on surface is MMscf/d

7. True Valve Performance Modeling
To this end the nodal theory is utilized, a “solution” node can be defined at any point within the system
In this case the solution node is the valve, an “operating point” can then be defined at each gas-lift valve’s depth giving the engineers a light about valve’s behavior under bottom-hole conditions ie: injection pressure and gas volume
This is just a brief introduction of AGVM (true valve performance modeling). It is based in the theory that in a nodal system the user can define a “solution” node where ever is needed.

8. What Do Valve Performance Curves Look Like ?
Data From Valve Performance Clearing House
Casing Pressure
Note valve does not close
at high Pcf
20/64ths Camco R-20
Ptro : 650 psig
Typical plot of gas passing through an orifice/valve. Y-axis shows gas volume and X-axis shows casing pressure, there is a throttling region and a critical flow region.
Throttling region

9. How True Valve Performance Data is Used?
The main use of valve performance curves in a design software is for a more correct sizing calculation to be performed.
Methodology consists of :
With the selected valve as the solution node a series of operating point calculations are performed over a range of gas injection rates.
This defines a production rate, flowing temperature, tubing and casing pressures at the selected valve for each gas injection rate.
An appropriate (selected) correlation for valve performance is then used to determine the gas flow rate through the valve for each combination of tubing and casing pressure and temperature calculated above.
Tubing pressure is plotted against gas injection rate for both the well and valve - intersection of the two curves implies compatibility between well and valve design

10. How True Valve Performance Data is Used?
Here AGVM (true valve performance) is shown, where the “inflow” is defined by the available CHP and the “outflow” is defined by the bhfp at the valve’s depth.

11. True Valve Modeling (1)
Using the true gas-valve modeling option, it can be seen that the first valve is also
open at CHP= 1120 PSIA
Through the first valve MMscf/D are passing from the annular to the tubing

12. True Valve Modeling (2)
According to the true valve modeling the second valve is open at CHP= 1120 PSIA
Through valve number two MMscf/D are being injected

13. True Valve Modeling (3)
Valves number three and four are also open at CHP= 1120 PSIA, but tubing pressure is higher than casing pressure, which activates the check-valve closing them

14. Injection Gas Balance (valves 1 and 2)
1st VALVE = MMSCF/D
2nd VALVE = MMSCF/D
TOTAL = MMSCF/D
CONCLUSION: If MMSCF/D are injected from surface and both valves allow an injection rate of MMSCF/D, then CHP can not be constant at 1120 PSIA, as a consequence CHP will decrease as this is equivalent to have a bigger port working

15. Sensitivity to Casing Head Pressure (1)
Running a sensitivity to CHP, it is possible to see that at 1090 PSIA valve Nbr-1 is closed, a very similar situation happens with valve Nbr-2

16. Sensitivity to Casing Head Pressure (2)
Running a CHP sensitivity it is observed that valve Nbr-2 closes when CHP goes to 1030 PSIA

17. Dynamic Gas-lift Simulation
This dynamic application uses all the equations used by a steady-state gas-lift software but adds the time variable to the system
Gas injection volume is a cumulative calculation in such a way that CHP will increase as more gas is being injected
The reservoir node honors all the input data already used for the nodal analysis
All the dynamic surface settings are translated into bottom-hole conditions using the selected vertical flow correlation, and…
True gas-lift valve data is utilized to simulate the real behavior of the valves under bottom-holes conditions

18. Dynamic Simulation of True Valves
1st and 2nd Valves open at CHP=1095 psia
Actual Qgi passing through 2nd valve
Valves 3 and 4 open at this CHP but check-valve closed

19. Dynamic Simulation of Wellhead Conditions
Simulating this phenomenon dynamically it is observed that both valves can be closed, but being a continuous injection design the MMscf/d injection rate will force the CHP to go up again
When CHP goes up again both valves are open and this cycle is repeated periodically
This phenomenon is known as Well Heading due to a lack of control on the injection rate, a re-calibration of the gas-lift valves is necessary to remedy this situation

20. Redesign for the Same Well
This redesign has been adjusted dynamically and valves positions verified in the steady state software
Looks like there is enough CHP to operate through the deepest mandrel (actually an orifice has been simulated), the shallower valve is closed already, remember that here the software assumes that the unloading process is already finished

21. Operating Valve Conditions on Bottom
An initial iteration would be to run a 10/64” orifice at the injection point, a much better control of the injection can be observed
Looks like there is still so much WHP and lowering it would help the well, however this has to be “iterative” with the filed simulation from where WHP will be defined

22. Testing a tentative solution – Dynamic Simulation
For one unloading valve and one 10/64” orifice at the injection point the well looks perfectly stable

23. Conclusions
Heading means inefficiency and needs especial attention to the gas-lift equipment in continuous injection installations
Gas-lift valves out of calibration can compromise the success of a production optimization effort
Wells exhibiting heading are difficult to simulate with conventional steady state applications, but they can be diagnosed properly provided that “true valve performance” is available and there is a good understanding of valve performance
Heading is a purely dynamic situation, a dynamic application for trouble-shooting and solution of the causes of this behavior will always give better results
Heading does not mean that a well can not be optimized, it means that the well needs an extra work to ensure the proper operation of the down-hole equipment

24. Conclusions (cont)
The steady-state model is still needed to feed the field model simulator
Training becomes critical as tools become more advanced (more complex to use)

25. Conclusions (cont)
Q: Are the conventional “steady state” software tools able to diagnose this phenomenon?
A: Yes, provided that they can model “true valve performance” a proper diagnose can be found
Q: Being a dynamic situation is the use of dynamic tools mandatory?
A: Yes, the only way to understand the cause and try to find and test a solution is to “see” the system working under more realistic operating conditions

26. Thank you!