Experimental Investigation and Modeling of the Effects of Rising Gas Bubbles in a Closed Pipe
- Saeed Al-Darmaki (General Holding Corporation) | Gioia Falcone (Texas A&M University) | Colin Hale (Imperial College) | Geoffrey Hewitt (Imperial College)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- September 2008
- Document Type
- Journal Paper
- 354 - 365
- 2008. Society of Petroleum Engineers
- 5.1.1 Exploration, Development, Structural Geology, 5.3.2 Multiphase Flow, 4.1.2 Separation and Treating, 5.6.4 Drillstem/Well Testing, 1.6 Drilling Operations, 5.8.8 Gas-condensate reservoirs, 4.1.5 Processing Equipment, 5.2.1 Phase Behavior and PVT Measurements
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Transient multiphase flow in the wellbore causes problems in well-test interpretation when the well is shut in at surface and the pressure is measured downhole. Pressure-buildup data recorded during a test can be dominated by transient wellbore effects (i.e., phase change, flow reversal, and re-entry of the denser phase into the producing zone), making it difficult to distinguish between true reservoir features and transient wellbore artifacts (Gringarten et al. 2000).
This paper is a follow-up to paper SPE 96587 (Ali et al. 2005), which presented experimental results of phase redistribution effects on pressure-buildup data. Though the results of the experiments were revealing, they are complex because they reflect the real well situation. To obtain results in which the phase redistribution in the well is studied independently of the interaction with the reservoir, a further set of experiments was carried out. In these experiments, the tube (simulating the well) was isolated at both the top and the bottom at the same time. The pressure distribution was measured during the transient following shut-in and for the steady-state final condition, in which there was a liquid-filled zone at the bottom of the test section and a gas-filled zone at the top. A substantial number of tests were conducted in the bubbly-flow region and could therefore be analyzed by a simple 1D model for bubbly flow. The results of the comparison between the model and the experimental data are presented in this paper.
In the first study (Ali et al. 2005), experiments were carried out to investigate the effects of wellbore phase redistribution (WPR) and phase re-injection on pressure-buildup data. Single-phase- and two-phase-flow tests were conducted with air and water in the long-tube system (LOTUS) at Imperial College. The LOTUS test layout, as described in paper SPE 96587 (Alii et al. 2005), was designed to simulate a reservoir connected, by a resistance, to the base of a flowing well. The "reservoir" was recreated by a pressurized vessel, while the "well" was simulated by a 10.8-m -long, 32-mm-internal-diameter vertical pipe (i.e., the main LOTUS test section). The well was flowed at controlled rates to mimic those encountered in gas/condensate reservoirs. After steady-state conditions had been attained, the well was shut in at the top of the rig (i.e., at the surface) and the associated transient phenomena were monitored through distributed measurements of pressure, temperature, liquid holdup, and wall shear stress. Pressure-buildup data were interpreted with established well-test-analysis techniques.
These initial experiments provided a qualitative and quantitative understanding of the effects of gas rates, liquid rates, and rising gas bubbles on WPR and phase re-injection. Gas flow rate was found to have a higher effect than water flow rate on WPR. This was most probably because of annular flow being the predominant flow regime for the experiments. Phase re-injection was simulated successfully. The lower the reservoir pressure, the higher the liquid re-injection, an analog to low-permeability reservoirs. For a closed system, WPR took place. Rising gas caused an increase in bottomhole pressure.
The focus of the second study, presented here, was to investigate WPR independently of the interactions with the reservoir. The LOTUS tube was isolated at both the top and the bottom at the same time. The test section was again the LOTUS 10.8-m-long, 32-mm-internal-diameter vertical tube. A two-phase flow was set up with known air- and water-flow rates. The pressure distribution and void fraction were measured for the steady-state flow, and the flow subsequently was shut down by closing valves at the top and bottom of the test section simultaneously.
Although the experiments covered a wide range of conditions, a substantial number of tests were conducted in the bubbly-flow regime. A simple, 1D model for bubbly flow was developed and implemented for comparison with the experimental data.
Earlier efforts toward understanding the physics of gas-bubble migration in wells were carried out by Hasan and Kabir (1994; 1993) and Xiao et al. (1996). Aremu (2005) provided an overview of bubbly-flow models applied to the problem of gas kicks while drilling. A detailed review of previously published work on research into transient wellbore phenomena is presented by Falcone (2006). In recent years, much work has been carried out on the phenomena occurring in bubbly flows with a wide range of local measurements, and increasingly, many use computational methods to represent the detailed motions and interfacial deformations of the bubbles.
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