Prediction of Slug-to-Annular Flow Pattern Transition (STA) for Reducing the Risk of Gas-Lift Instabilities and Effective Gas/Liquid Transport From Low-Pressure Reservoirs
- Peter R. Toma (P.R. Toma Consulting Ltd.) | Edinson Vargas (U. of Alberta) | Ergun Kuru (U. of Alberta)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- August 2007
- Document Type
- Journal Paper
- 339 - 346
- 2007. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 1.6.9 Coring, Fishing, 4.1.4 Gas Processing, 3.1.6 Gas Lift, 5.3.2 Multiphase Flow, 3.1.2 Electric Submersible Pumps, 3.1.8 Gas Well Deliquification, 1.8 Formation Damage, 4.1.2 Separation and Treating, 4.2.4 Risers, 3.1 Artificial Lift Systems, 7.2.2 Risk Management Systems, 5.4 Enhanced Recovery, 1.7.7 Cuttings Transport, 2.2.2 Perforating, 1.6 Drilling Operations, 5.2.1 Phase Behavior and PVT Measurements, 1.7.1 Underbalanced Drilling, 5.4.1 Waterflooding, 5.4.2 Gas Injection Methods, 1.6.3 Drilling Optimisation
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Flow-pattern instabilities have frequently been observed in both conventional gas-lifting and unloading operations of water and oil in low-pressure gas and coalbed reservoirs. This paper identifies the slug-to-annular flow-pattern transition (STA) during upward gas/liquid transportation as a potential cause of flow instability in these operations. It is recommended that the slug-flow pattern be used mainly to minimize the pressure drop and gas compression work associated with gas-lifting large volumes of oil and water. Conversely, the annular flow pattern should be used during the unloading operation to produce gas with relatively small amounts of water and condensate. In both procedures, a considerable decrease in tubing pressure from the perforation levels to the wellhead levels results in a major increase in superficial gas velocity. It is believed that this occurrence may well induce the unstable flow that occurs during STA.
Understanding and accurate prediction of flow pattern transitions, such as those that occur in STA, is imperative for the development of suitable production operations and effective gas-lifting and unloading strategies. New and efficient artificial lifting strategies are required to transport the liquid out of the depleted gas or coalbed reservoir level to the surface. This paper presents field data and laboratory measurements supporting the hypothesis that STA significantly contributes to flow instabilities and should therefore be avoided in upward gas/liquid transportation operations. Laboratory high-speed measurements of flow-pressure components under a broad range of gas-injection rates including STA have also been included in the paper to illustrate the onset of large STA-related flow-pressure oscillations. The latter body of data provides important insights into gas deliquification mechanisms and identifies potential solutions for improved gas-lifting and unloading procedures. A comparison of laboratory data with existing STA models was performed first. Selected models were then numerically tested in field situations. Effective field strategies for avoiding STA occurrence in marginal and new (offshore) field applications (i.e., through the use of a slug or annular flow pattern regimen from the bottomhole to wellhead levels) are discussed in this paper.
Simultaneous production of gas, liquid hydrocarbons, and water through vertical wells is a common occurrence in both land and offshore production operations. However, the current depletion of conventional oil and gas reservoirs, coupled with unrelenting global demand for these resources, requires a re-examination of conventional production strategies. In particular, procedures for liquid and gas production in extremely low-pressure reservoirs, as well as methods for offshore transportation of large fluid volumes, must be re-evaluated.
Because of the combined effects of low reservoir pressure and bottomhole accumulation of water, many gas wells are now dormant. Deep coalbed reservoirs, considered to be a valuable source of gas, are experiencing similar liquid-water unloading problems. Knowledge of mechanistic modeling of gas/liquid flow systems is essential for the development of suitable production strategies for both depleted and mega offshore gas/oil reservoirs (Oliemans 1994; Taitel et al. 1980; Turner et al. 1969).
Slug-to-annular (STA) flow pattern transition, including the intermediate churn condition, is considered to be a potential source of flow-pressure instability and is given special attention in this paper. Whether churn is a stand-alone flow pattern or a transition stage between slugs and developed annular flowis currently under debate (Taitel et al. 1980; Pickering et al. 2001; Mao and Dukler 1993). Highly turbulent gas flow makes it difficult to visually assess the occurrence and evolution of STA and, as a result, the definition of churn remains in dispute.
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