hope this helps
quattro44 - 19 Sep'07 - 09:57 - 56938 of 56942
I shall attempt to summarize the above points as a person who worked for Halliburton doing frac jobs for many years.
Hydraulic Fracturing is the process of creating a fracture in a porous medium by injecting a fluid under pressure great enough to cause failure in that medium.
Proppant (spherical and rounded Sand or synthetic sand of average size about 0.50mm) is mixed with the fluid at surface so that it follows the fluid into the fractures and on pressure release remains trapped in the fractures allowing permeable flow back into the well bore greater than before fracturing.
Fracturing is an extremely complex procedure with many variables, but the essential items are that the fluid MUST be kept moving in order to maintain proppant mobility, and the proppant concentration must not be allowed to increase above optimum. The result of either the fluid slowing or the concentration getting too high will cause proppant drop-out or 'screening out'.
Screening out can happen anywhere in the well bore or in the tubing or in the formation. It can even happen at surface in the pumps or the mixing blenders. Once proppant becomes immobile it is effectively a solid and can only be washed out slowly.
There are many reasons why a planned fracturing job can stop early :
-the design was incorrect and the formation was not able to fracture as expected and the fractures did not materialize. One always starts a fracturing job with pure fluid to initiate the fractures so the proppant has somewhere to go once introduced. If the fractures are only small - sufficient to take fluid but not much proppant - then you will get early screenout.
- Too high a concentration of proppant added at some point.
- A sudden increase in pressure forcing a slowdown in fluid pumping velocity allowing proppant to drop out and screen off the fractures. (This is what happens towards the end of a fracture job as standard)
- a sudden down hole drop in pressure from a cement bond failure, or a screen failure or a massive fracture being created which allows the proppant to 'fall' rather than be pumped or carried thus causing screen out.
- Poor perforating of the casing which allows fluid through but very little proppant.
Etc etc and hence why they do not immediately know.
Fracturing is such a common thing these days, especially in Texas, with immense experience that an error in design is extremely unlikely. Screen outs are also very common and mean nothing but just that - a screen out. They will clean it out and try again or test the well as is to attain essential information.
It is always possible that the fracture already created, although it only took 6% of the design proppant, could have overcome the lack of permeability to the well bore and the well will flow satisfactorily for these initial testing purposes. Early indications of gas in the returns is obviously encouraging.
Nothing to worry about imho dyor etc.
q44