Basics of Gas Lift Recent Techniques

Gas lift is commonly used to increase oil production rates by injecting compressed gas into the tubing through an orifice or gas lift valve (GLV) that communicates production tubing with annulus. However, some challenges might exist during lifting wells.

This article briefly summarizes some novel techniques that were recently introduced to solve some of those challenges connected with gas-lift injection. Section I refers to "Liquid Assisted Gas Lift Unloading LAGL" while Section II refers to "Light oil Gas Lift Assisted injection for Heavy oil producing wells".

SECTION I

Problem Statement:

In conventional gal-lift unloading, multiple gas lift valves exist that are used to unload the well only, in addition to reducing unloading surface injection pressure, and are not used during production. However, one challenge of gas-lift producing wells could be the possibility of valves malfunctioning, leak or failure, which could require loss of production to replace those valves. Instead, another approach could be replacing gas-lift valves with dummy valves just after the unloading process, yet this has the disadvantage of well intervention high costs as well as large production downtime. Another challenge of gas-lift unloading system is the requirements for high injection pressure on surface. Liquid Assisted Gas Lift Unloading is an alternative technique that could be used to solve such challenges, however it is worth mentioning that LAGL method also generates extra cost for liquid pumping. Hence, it would be needed to make a decision based on economic study for both scenarios. In addition, attention should be paid to the downward multiphase flow in the annulus. The effectiveness of LAGL method depends on void fractions in annulus flow as it controls the mixture density.

Concept of LAGL Unloading

Coutinho et al. (2018) introduced an alternative technique to unload wells using a mixture of gas-liquid injection and demonstrated that liquid-assisted-gas-lift technique could allow the transport of gas to a deeper injection point while reducing injection pressure to a value lower than the required pressure for single-phase gas injection. The main idea is to use only less gas-lift valves in the well, to mitigate GLVs challenges, as liquid-gas mixture would have a higher density resulting in a reduced surface injection pressure to reach a single deep gas-lift valve. Briefly, gas/liquid mixture is injected until it starts to flow upward the tubing. Next, the mixture injection GLR slowly increases till only gas is injected into the annulus, which indicates the end of LAGL unloading and the start of conventional GL unloading. They validated the technique in a 2,788 ft-deep field test well with injection mixture of water and gas with constant flow rates. The experiment depicted that liquid injection rate should be carefully monitored to optimize this process and that simulation models would be required. The experiment also showed that the required surface injection rate using LAGL technique could be 75% less than that of a single-phase gas injection procedure.

System Analysis and Optimization

Coutinho (2018) described and analyzed two-phase downward flow in the annulus, flow through gas lift valves orifice, and upward flow inside the tubing. They demonstrated that bubbly and intermittent flow regimes are preferred to optimize and minimize surface injection rate for LAGL unloading process. They used commercial simulation software (OLGA), which showed the potential of this method to reduce surface injection rate by 50% than the pressure required for single-point injection during unloading. For two-phase flow through GLV orifices, they conducted an experiment, which showed that pressure ratio across the orifice increases with GLR due to the increased density and velocity of the mixture resulting in higher pressure drop. Coutinho et al. (2020) observed bubbly, intermittent, and annular flow in the annulus downward two-phase flow. Annular flow was observed for a wide range of superficial gas and liquid velocities while intermittent and bubbly flow are limited to narrower conditions, however, they are preferred for LAGL unloading as they represent higher mixture density and hydrostatic pressure. For optimization, they used OLGA software to simulate 1) downward two-phase flow in annulus, 2) vertical tubing flowline, and 3) orifice gas-lift valve.

SECTION II

Zhong et al. (2018) conducted an experiment on using gal-lift assisted light oil injection for heavy-oil production wells, which can’t be easily lifted by gas-lift injection only, and validated results on a test-well in Tuha oilfield, Xinjiang, China. Validation showed a threefold increase in oil production while using nitrogen as source for gas injection. They showed that gas-lift assists in mixing heavy oil with light oil leading to improved lifting efficiency. They developed and solved a model for coupling pressure and temperature where they used Mukherjee and Brill model for simulating downward two-phase flow pressure profile in the annulus. They introduced a design for heavy oil gas lift assisted with light oil using an iterative method where bottomhole was set to be the nodal analysis solution point. The experiment revealed that using gas lift only in that case, through four different production rates, could not lift heavy oil. In addition, they showed that diluting heavy oil with light oil could assist with low production rates. However, using both gas lift and much lower diluting light oil could significantly enhance production rates.

REFERENCES:

1. Coutinho, R.P., Williams, W.C., Waltrich, P.J., Mehdizadeh, P., Scott, S., Xu, J. and Mabry, W., 2018. The Case for Liquid-Assisted Gas Lift Unloading. SPE Production & Operations, 33(01), pp.73-84.
2. P Coutinho, R., 2018, September. System Analysis of Liquid-Assisted Gas-Lift Unloading. In SPE Annual Technical Conference and Exhibition. OnePetro.
3. Coutinho, R.P., Tornisiello, L. and Waltrich, P.J., 2020. Experimental investigation of vertical downward two-phase flow in annulus. Journal of Energy Resources Technology, 142(7).
4. Zhong, H.Q., Zhu, S., Zeng, W.G., Wang, X.L. and Zhang, F., 2018. Research on heavy oil gas lift assisted with light oil injected from the annulus. Journal of Petroleum Exploration and Production Technology, 8, pp.1465-1471.