A long, tight clearance liner was recently installed through a whipstock-milled window in a deepwater well. The length and weight of the liner, coupled with the tight clearances, pushed the limits of current technology. The liner was successfully run through a deep whipstock milled window and cemented at a depth below 24,000'. This paper describes the planning and execution involved in this critical well construction operation.
Introduction
Installing a long, tight clearance liner through a milled window is a critical operation that presents many technical and operational challenges. Recently in the U.S. Gulf of Mexico, ConocoPhillips Gulf Region Deepwater Exploration milled a window in 13–5/8" casing and subsequently ran 9551' of 11–3/4" liner through a 12–1/4" window to a depth of 24,382' MD. Planning, equipment selection and proper execution were the keys to success.
The first critical success factors were to construct a window with adequate clearance and minimal dogleg severity and then to provide a quality borehole below the window. A one-trip whipstock was selected to construct the window and provide rat hole below the window for drilling ahead. Various bottom hole assemblies were run to directionally drill and open the borehole to an interval depth of 24,382'. Prior to running the 11–3/4" liner, a borehole imaging survey was run to confirm suitable hole geometry and adequate clearance in the open hole interval.
Considerable effort was made to select the proper equipment to run and cement the 11–3/4" liner. Finite element analysis was applied to select casing connections that would provide maximum clearance while maintaining integrity to support high tensile loads combined with severe bending across the window. In addition, extra attention was given to selecting auxiliary casing equipment needed to ensure that the liner would be successfully run to bottom and cemented.
This paper illustrates the level of planning and detail required to successfully install a long, tight clearance liner. A case history of the planning and execution demonstrates the effort needed to successfully implement such a challenging operation.
Planning
Milling the Window
Method and Equipment Selection
After reviewing other sidetracking options including milling and casing recovery, a cased hole sidetrack utilizing a whipstock was selected as the most cost effective option with the lowest operational risk.
A technically advanced whipstock system capable of producing a clean, full-length, full gauge usable window followed by sufficient rat hole to accommodate a rotary steerable drilling assembly in one trip was desired. Two types of whipstock systems were considered for the cased hole sidetrack, 1) conventional and 2) extended gauge multi-ramp design. The conventional whipstock usually has a 1–1/2 to 3 degree single ramp and a cylindrical shaped mill head dressed with crushed carbide. Inherent problems with the conventional design are, 1) inconsistency in the shape and location of the window, 2) the mill progresses slowly at the center point of the casing, and 3) formation imposed mill limitations.
The extended gauge multi-ramp whipstock system features a specially designed whipstock face with multiple ramps, each with its own taper, and a milling tool with a conical shaped mill that can be dressed to accommodate different formation properties. This type of whipstock provides additional footage to the vertical face of the whipstock. The additional footage lengthens the usable portion of the window and reduces the dogleg through the window. By lengthening the vertical face of the whipstock, the departure angle of the milling tool is not sacrificed.
