A New Transient Model to Simulate and Optimize Liquid Unloading with Coiled Tubing Conveyed Gas Lift

Abstract

Nowadays, unloading gas wells with coiled tubing is a common application to the field. However, it still lacks of adequate understanding of dynamic behavior of the unloading process. This paper investigates the process of liquid unloading by gas lift with coiled tubing under transient condition. This unloading process can be divided into three stages: liquid rising in tubing, liquid slug production, and liquid production by entrainment. In each stage, the mass and linear momentum conversation equations are applied as governing equations. The components of each stage includes coiled tubing, coiled tubing-tubing annulus, liquid slug, gas bubble, and liquid film. Empirical correlations have been used including surface gas injection choke, check valve, friction factor, the relationship between the gas bubble and the liquid slug velocity, inflow performance relationship, and blackoil fluid properties. From the above, the dynamic model coupling real-time change of inflow performance relationship. Using the LU factorization and Euler's method to solve the proposed dynamic model in time domain. Among all these variables, the most important ones include gas injection rates, pressures at various locations, length of the liquid slug and gas bubble, and various velocities. By the simulation efforts, the mechanism of liquid unloading process is revealed. Gas lift is commonly constrainted by gas availability. This is pioneer study on liquid unloading with coiled tubing is critical in design phase to optimize the usage of injected gas as well as choose appropriate pump and cost-saving for electricity expense.