Detailed gas lift modeling tool improves reservoir output

Optimizing production from gas-lifted wells and fields is a complex task. Most existing modeling tools approach the problem by simulating gas lift in terms of a steady state solution, in the process making sweeping assumptions about the performance of the valves and the completion.

New Windows-based software from Edinburgh Petroleum Services (EPS) called Dynalift offers the user improved understanding of the various interacting processes through building a comprehensive model of the well, reservoir, completion and operating conditions. Dynalift is thought to be the first commercially available dynamic gas simulation software, and has already been evaluated by several North Sea players as well as attracting interest in the Far East and Australia.

The software simulates the individual steps towards unloading a gas lifted well by calculating the detailed physical parameters in time steps as unloading advances. This means that gas lift designs can be validated and checked against varying reservoir conditions problematic wells with heading, multi-pointing and valve cutting can be diagnosed gas lift valves and designs can be evaluated to optimize workovers.

The upshot, claims Edinburgh, UK-based EPS, is that gas lift wells can be modeled in much greater detail than was previously feasible, which in turn leads to improved performance and lower downtime.

Detailed completion modeling allows the user to enter data for annular casing strings and variable tubing strings. Then pressure drop calculations are performed for the injection gas and produced fluids, with accurate temperature calculations applied to these fluids as they rise through the completion.

Generic valve performance curves are also provided. These and manufacturers' performance curves can be selected for each mandrel installed in the completion. Simulation can also be halted, or valve failure can be simulated, to evaluate the impact on system performance. "If you inject gas at too high a rate, valves can be damaged," says EPS spokesperson John Cumming. This process warns you, by calculating flow rates of gas through each valve, whether the valve is being eroded and how much gas is traveling through. These calculations have not been done before."

Performance modeling

A correctly designed gas lift completion takes into account reservoir performance. Early on in the gas lift unloading process, the well injects into the reservoir until hydrostatic pressure has been reduced so as to allow well inflow. When reservoir flow does begin, first fluid produced will be that injected into the reservoir. Only when all this fluid has been produced will the lift process benefit from the gas in the reservoir's produced oil. This chain of events is modeled by Dynalift.

Later in the gas lift simulation, well flowing pressure is determined partly reservoir productivity. When the well is lifting off bottom, the amount of produced fluids will be governed as much by inflow performance as by the gas lift design.

As for the system's controls, settings are provided for both injection and production chokes, and the unloading method is selectable. Different scenarios can also be simulated without having to stop and start up the main simulation in progress. Dynalift, which draws on Decker Technologies' experience in valve performance, forms part of EPS' FloSystem suite of production optimization software.