Chen. I think you need to be a lot more specific and provide more accurate details of what your problem actually is.
You say you experienced a restriction passing through a window with a 4.75” RSS.
Did the mills come back within acceptable gauge size?
Do you know how long your milled window actually is?
Are you hanging up and if so is it at the top of the whip or the bottom of the window or are you just losing some weight but can pass through?
What is the hole angle and what is the whipstock orientation in relation to high side?
What size bit are you running on the RSS?
Was the widow fully qualified before pulling the milling BHA out of the well, could you run the milling BHA through the window with no rotation and no pumps?
Did you generate a rathole and if so how long?
You say you are experiencing a restriction but does this mean you can still pass through?
The more information you supply the better the responses will be.Regards
Why not look at undergauge hole size through window. If your drift is 6.000 then drill a 5.875 hole thru 6.00 window. Whenever possible (and practical) drilling slightly undergauge hole thru a window is the preferred option.
Not enough facts to discern the reason but these days guys are not so much cutting a window but "can openning", milling with too much weight and /or not holding penetrations rates back to "hold" the mills on the ramp particularly once past "centre point" where the chances of jumping the ramp increase.
If the Fastrack Mill completed the window but failed to complete the rathole due to the compressive strength of the formation and provided that the Follow Mill on the Fastrack Tri-Mill has passed the bottom of the window i.e. the window is essentially complete, then it is recommended that a bit (IADC Code 117 or IADC Code 447 or similar) is run instead of another Mill.
If there is a substantial amount of cement to be drilled out - following a remedial cement job, it is recommended that a Tri-Cone Bit (IADC Code 117 or IADC Code 447 or similar) is run instead of a Mill. The Bit will perform better whilst drilling out the cement.
Was the whip set in vertical or horisontal well, and what was the orientation of the whip ? Another things to consider is the length of the rathole,as some RSS BHA`s require longer rathole than others.(I have no info on what type of the RSS BHA in your application).We run RSS assemblies( all suppliers) across our whips on a daily basis without any issues, so unless you ran a special BHA, this should not be the reason why you experienced problems.
Always take care at all times when passing through a window with any bottom hole assembly.
It has been observed that very occasionally a drilling assembly requires to be ”œwalked” past the Top of the Whip to allow the drilling assembly to pass through the window.