Well Testing by Design: Transient Modelling for Predicting Behaviour in Extreme Wells

Abstract

Abstract Offshore rig rates are at an all time high and wells are becoming bigger and longer, in deeper waters and in more complex reservoirs. Well testing in this environment has become more challenging, where well clean-up and flow assurance issues such as slugging and hydrates can significantly extend the planned duration of well tests. The ability to predict and being prepared to deal with such problems by appropriate design of well test equipment can reduce operational risk, minimise safety hazards and environmental impact and potentially save millions of dollars in rig-time. Traditional well flow software only models steady-state flow. Predicting the transient behaviour of wells, from the unloading of completion fluids until steady state flow conditions are reached, requires specialised software. Dynamic flow simulation software is a proven tool which has been applied for years by facilities engineers for pipeline and slug-catcher design, but its application for well testing is a new practice. Key outputs from dynamic well simulation include slug sizes and frequency, fluid composition and pressure-temperature trends at any time and at any point in the well. Such information enables optimum design so all parameters are within the equipment's allowable operating envelope at any time of the well test operation. This paper describes how dynamic simulation, using the software package OLGA, was applied to a big-bore gas well with 9 5/8" production tubing. The dynamic simulation study: provided better understanding of well unloading behaviour at different flowrates (by using different choke sizes).assessed the effect of an emergency shut-down (ESD) during a well clean-up operation.defined the minimum flowrate required to cleanup the well (for sizing well testing equipment). Results from the dynamic simulation indicated that a standard well test package may be adequate for cleaning up this big-bore gas well with 9 5/8" production tubing, though the equipment would be operated at or near its limits and would take quite some time for clean-up. A significantly faster clean-up could be achieved with a high rate well test package at additional cost.