Typical horizontal well design. Click on the photo to enlarge.
There are two general design options.
The first is shown above. In this case the tubing can be set just above the "build" section. The build section is the portion of the hole where the well is deviated from the vertical and the hole angle is "built" to the desired end value.
Note: A "horizontal well" may not be horizontal if it´s drilling into a formation which dips in the same direction as the well path, but that´s a minor point because the typical formation dip angles rarely exceed 5 degrees in the zones targeted by horizontal wells.
The second option is used less often. It involves drilling the build section with a very gentle curve, which allows the tubing to be run deep, sometimes into the liner itself. The gentle curve allows the pump to pass through the curve and reduces the rod wear problem. This is more applicable to wells using progressive cavity pumps (mentioned below).
Horizontal wells drilled into tight zones such as the Bakken will produce with a gas to oil ratio of about 1000 thousand cubic feet of gas per barrel of oil (1000 scf/bbl GOR). This makes the produced fluids quite "fizzy" and the wells will flow to the surface.
The well´s ability to flow to the surface is largely dictated by the water cut (adding water to the produced fluid stream increases its density and reduces the available pressure differential between the reservoir and the surface). Another factor is reservoir depletion. Fluid withdrawal will always reduce the well bottom hole pressure, this is more pronounced in tight zones. And this explains (in part) why the production rate drops so fast.
The artificial lift solution for tight wells completed in the Bakken will depend on water cut. The higher water cut wells will be easier to pump, but the 1000 GOR will lower pump efficiency (because it works in part as a compressor rather than a pump). I haven´t done the detailed calculations, but it seems to me gas lift may be a better option for wells drilled in a development which uses large multiwell pads and a central satellite station. Gas lift will allow the well to be lifted under highly variable conditions and will benefit from the increasing GOR, which is the expected behavior in a depleting reservoir of this type.
However, it seems the horizontal wells, such as drilled in North Dakota, have been drilled in a scattered fashion, the use of large multiwell pads is rare, and there´s a lack of flowline corridors. This leads me to think the better option may be a simple beam pump driven by rods. An alternative may be a progressive cavity pump (PCP). These pumps are very practical in high volume wells, and can handle some gas (but excess gas makes them overheat).
For those who wish to get into the subject in a little bit more detail, the following link takes you to an artificial lift page by Weatherford you may find useful. However, pump manufacturers don´t understand the full range of problems faced by well operators, therefore their answers are geared more to marketing their products rather than deliver the truly optimum solution.
No hay comentarios:
Publicar un comentario