Gas well deliquification : critical rate analysis and artificial lift design & review workflow

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Date

2020-03-31

Authors

Tayyab, Imran

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Abstract

Liquid loading is an inevitable phenomenon for most gas wells. Liquid loading occurs when fluids accumulate in the wellbore instead of producing to the surface. This causes additional hydrostatic pressure that lowers formation drawdown and reduces production. The process of de-watering gas wells is commonly known as gas well deliquification. This involves quantifying if liquid production is the source of un-optimization in the well and selecting an appropriate artificial lift system to offload the well. This study presents a gas well deliquification workflow, that quantifies the extent of liquid loading through production analysis, critical unloading rate, and nodal analysis. Once liquid loading is confirmed, a design & review workflow is suggested that compares different artificial lifts to select the most effective choice. Production analysis includes; (a) evaluating decline trend of rate and estimated bottom hole pressure, (b) Nodal analysis, to create a calibrated baseline model that is used as a reference during artificial lift design, (c) VLP Stability, and Flow-point analysis to qualitatively understand unstable flow in the wellbore. A new critical rate calculation workflow is developed to quantitatively confirm liquid loading. This workflow utilizes published critical gas rate correlations and wellhead pressure as a weighing criterion to estimate a weighted average critical rate. A separate data-driven model, where machine learning is used to estimate critical rate for a target well given its well parameters is also formulated. Both workflows are shown to better predict critical gas rate than most published models. If liquid loading is confirmed, applicable lift systems are designed, and their production impact is gauged through nodal analysis. With a direct comparison of all applicable systems, most suitable system is selected that maximizes incremental production. Design & Review workflow is applied to a field in Lower Indus Basin, Pakistan. Several wells are evaluated to check if liquid loading is a problem and artificial lift can improve production. S-field is the largest field in this dataset, where 10 wells are evaluated. Among the technologies suggested for these wells are Gas lift, Coiled Tubing Gas Lift, and Plunger Assisted Gas lifts. Beam lift and velocity strings are found to be less effective in the specific case of S-field. Several artificial lift selection workflows are published that focus on selecting lifts for oil wells. Most only focus on the lift selection and do not include any production analysis to ascertain if liquid loading is the cause of low production. Further, many critical rate correlations are published however most are applicable for specific ranges of well parameters. This study attempts to provide a thorough gas well deliquification workflow. It includes production analysis to quantify root cause, new critical rate calculation that is universally applicable on most wells, and artificial lift selection process, specific to gas well deliquification, to select the most suitable lift system

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