Characterising Gas-lift Instabilities with OLGA2000 ASME/API/ISO Fall 2003 Gas-Lift Workshop 21-22 October 2003, Kuala Lumpur Bin Hu Ph.D Candidate Department of Petroleum Engineering and Applied Geophysics Norwegian University of Science and Technology E-mail: hubin@ipt.ntnu.no Telephone: +47 7359 4975 Fax: +47 7394 4472

If gas injection is not critical... Casing heading may happen To thoroughly eliminate casing heading, make the gas injection critical

Replace the orifice with a venturi Is the well unconditionally stable if gas injection is critical? Replace the orifice with a venturi

OLGA2000 simulation settings Vertical air/water two-phase flow No slug tracking Iso-thermal Initiated from steady-state Boundary conditions Static IPR Psep is constant Constant gas source is given near the bottom of the well

Density wave instability can occur!

OLGA simulation results PR is 90bara and air injection rate is about 18000Sm3/D

OLGA simulation results PR is 90bara and air injection rate is about 40000Sm3/D

OLGA simulation results PR is 90bara and air injection rate is about 54000Sm3/D

Summary Increasing reservoir pressure and gas injection rate increases stability. Increasing well depth, tubing diameter, PI and system pressure decreases stability Instability occurs only when

Production loss due to density wave instability

Apply feedback control to the well Variation of choke opening after controller is started

Variation of Pwf and wellhead production rate

Manual control at the same choke opening

Conclusions Density wave instability can occur in deep depleted gas-lift wells. The instability not only causes operating problem, but also reduces production. Active feedback control is an effective method for both stabilising and avoiding production loss. OLGA is at least capable of qualitatively capturing the instability dynamics.