Milling Variable Window Openings for Sidetracking

Abstract

Historically, the profile of the window milled with a conventional whipstock system resembles an inverted tear drop shape. The resulting full gage window opening is relatively cramped for subsequent drilling assemblies and liner completions. For example, in 7" casing the total window opening is usually 10 feet long out of which only about 2 feet of the opening has full diameter access for drilling and completion tools. This paper presents a unique whipstock design and milling tool design, the combined performance of which produces a full gage window opening of up to 85% of the total window length and thereby provides an abundant clearance for drilling assemblies and liner completions. Test conducted on 7"-29 lbs./ft. casing resulted in a total window opening of 9.8 feet (118") out of which 7.5 feet (90") was a full gage 6.00" opening. Test on 9–5/8"-47 lbs./ft. casing produced 16.83 feet (202") total window length of which 14.50 feet (174") of the opening measured full gage width of 8.50". The whipstock can easily be modified to produce the desired length of full gage opening. Optionally, the window can be milled without the need for drilling a rat hole. The benefits of a longer full gage window aretrouble free entries and re-entries through the window for drilling and completion of lateral,ideal for short radius departure,allows sidetracking in hard formation without drilling rat hole,compensates for any mismatch in depth tally calculations, andeliminates problems associated with skewed window (longer full gage window is vertically straight). The paper details the test set up, test results, photograph and profile of actual milled windows and summarizes field run results. Introduction The conventional whipstock systems, currently in use, require single or multiple trips downhole to mill an appropriate window for sidetracking the well. In all cases, the whipstock includes a 1–1/2 to 3 degree tapered ramp, the length of which varies from 8 ft. to 18 ft. depending upon the casing size and manufacturer. The face of the ramp incorporates a concave profile to facilitate the guiding of the milling tool. In multiple trip operations, a starter mill is used to make the initial cut out followed by a window/watermelon mill combination milling tool which usually finishes the window. In a single trip operation only one milling tool is used to complete the window. In either case, the profile of the window looks like an inverted tear drop as shown in (Fig. 1). This shape is produced by milling the window using a continuous, single angle whipface. The full gage opening required for the subsequent drilling assemblies is usually less than 25% of the total widow length. This relatively cramped opening becomes more hazardous due to the jagged edges of the window resulting from the rough downhole milling operation. The BHA and completion equipment, particularly the packers may get tangled and prematurely damaged. In many cases, it may require additional reaming trips with a watermelon mill to elongate and smooth out the window profile. The above described systems have been in use for many years. The design of the whip stock and the milling tools were derived from the earlier versions of remedial tools. Relatively shallow 1–1/2 to 3 degree tapered ramps on the whipstock face were designed to generate gradual milling torque on the casing and to minimize the damage to the whipstock. Different shape mills were designed and run to elongate and dress the window to reduce the damage to the drilling and completion components. However, there were no serious attempts made to completely alter the window profile to eliminate potential damage to other components while passing through the window until now.

A Mechanical Analysis of BHA Used in Casing-Window-Milling for Sidetracking

“…The polishing milling bit is used to make smooth the window barbless. The biggest advantage of this kind of muti-bits BHA is that a smooth window can be successfully milled out by one trip and the subsequent drilling string can go through the window [1,2].…”