Natural Downhole Separation and Its Impact on Production Performance
- H.A. Asheim (Norwegian U. of Science and Technology)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 1999
- Document Type
- Journal Paper
- 315 - 320
- 1999. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 1.10 Drilling Equipment, 2.2.2 Perforating, 4.1.2 Separation and Treating, 5.3.2 Multiphase Flow, 5.6.8 Well Performance Monitoring, Inflow Performance, 5.2.1 Phase Behavior and PVT Measurements, 4.6 Natural Gas
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This article deals with phenomena associated with gravity-induced separation along the completed interval of inclined wells. The model developed computes liquid accumulation and production performance. From the results it is shown that both the overall inflow performance and the gas-liquid ratio observed at the surface are affected by the downhole separation.
It is shown that downhole separation may be constrained by either backseepage to the reservoir or countercurrent flooding along the wellbore. The liquid backflow rate limit due to flooding has been derived explicitly, providing a design parameter for downhole separation devices.
With multiphase flow along the completed wellbore, there will be some tendency of accumulation of heavier and more viscous fluids. If the heavier fluid accumulates in an upwardly inclined wellbore, its weight will increase the bottom well pressure. This reduces the inflow. If the bottom well pressure exceeds the surrounding reservoir pressure, some of the accumulated fluid may seep back to the reservoir, thus reducing the liquid fraction produced.
It is commonly recognized that a pressure drop along a completed wellbore may affect local inflow and thereby also overall inflow performance. Work on this topic has mostly considered a pressure drop caused by single phase flow.1 However, most wells produce more than one fluid, which usually implies a larger pressure drop than is predicted by single phase flow models.
Experimental work on two-phase flow along horizontal pipes with local inflow was undertaken by Ihara and co-workers.2,3 They found that the pressure drop along horizontal, completed wellbores can be predicted by standard two-phase models, although an improved model was also proposed. Gonza´lez-Guevara and co-worker presented an inflow simulation model.4 They found that inflow along the wellbore is strongly affected by the incline of the wellbore. These works have simplified the modeling by neglecting the backseepage of liquids from the wellbore to the reservoir and the corresponding backflow of liquids along the wellbore. However, when backseepage occurs, it can have a profound effect on the downhole separation and inflow performance, as will be shown.
The current work includes the backseepage and backflow of liquids along the wellbore with special emphasis on the associated phenomenon of countercurrent flooding, which may limit the downhole separation. The model may be used to predict well performance and to develop a completion design that minimizes the detrimental effects of liquid accumulation. The mechanisms quantified may also be utilized to promote downhole separation of unwanted liquids.
Wellbore Performance Prediction
Fig. 1 illustrates the inflow, liquid accumulation, and backseepage along an inclined wellbore. For a constant production rate over a period of time, the downhole conditions should approach steady state. This implies a steady-state distribution of inflow and liquid holdup along the completed interval. In an upwardly inclined wellbore, the accumulated liquid will provide backpressure, which increases towards the bottom of the well.
In Fig. 1, the location where the wellbore pressure reaches the reservoir pressure is denoted by xL This will be called the liquid level. Inflow of gas and liquid occurs above this liquid level. Below the liquid level, the wellbore pressure exceeds the reservoir pressure. Thus, there will be no inflow, and liquid that flows back along the wellbore will accumulate there. Backseepage may occur below the liquid level and is governed by pressure and injectivity.
For natural backseepage, driven by gravity, the liquid level will vary with the production rate. For backseepage driven by downhole injection devices, the location and operation of such devices will determine the location of the liquid level.
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