Planning for Successful Window Milling Operation

Innovative materials technology advances sidetracking capabilities and offers a cost-effective approach to creating multiple laterals from the same mother bore. The key component to this sidetracking system is a mill designed with polycrystalline diamond (PCD) inserts. Current technology dictates that sidetrack milling be performed with mills dressed with crushed or pre-formed tungsten carbide that is manually applied. Regardless of technology, the success of the sidetrack is dependent upon on the skill and experience of the welder to properly dress the mill. The PCD mill eliminates the dressing process and the related performance uncertainties. This paper discusses how PCD cutters, which are commonly used in drilling, were modified and applied to a casing-sidetracking mnill. The mill design capitalizes on their ability to effectively and swiftly cut a window in the casing and drill a rat hole in formations with compressive strengths of up to 40,000 psi. The benefits of such a mill are:using the same cutting element for both steel and hard formations,substantial cost savings when constructing multiple laterals from the same mother bore,increased consistency in mill manufacture,reliable milling performance andimproved efficiency in the sidetracking operation. This paper profiles the development and testing from concept to a field-proven tool. The paper details:laboratory-milling tests that identify the best material for cutting steels,Oklahoma field tests in 9–5/8 in. casing to verify the feasibility of PCD casing milling and formation drilling,West Texas and Colombia field runs in operators' 7 and 9–5/8 in. casing to mill a window and drill hard formations. Additionally, the paper will elaborate on the future potential of integrating this technology to a directional drilling assembly to drill laterals with the same equipment and preferably in the same trip for short laterals. Introduction The individual performance of mills used to cut a casing window and sidetracking a well can differ widely. This is due to varying downhole conditions, operating parameters and applications.1 Properly managed quality and process controls, instituted for the manufacturing of milling tools, are essential for consistent performance. Current technology dictates that casing sidetracking mills are dressed with crushed tungsten carbide and/or pre-formed tungsten carbide. It is this cutter dressing that is the cutting structure responsible for removing steel and formation. The dressing must be able to withstand the diverse grades and weights of casing and the varying hardness and abrasiveness of the rat hole formations. The dressing process requirements for successful manual application of the crushed and pre-formed tungsten carbide are very demanding. The quality of the process is dependent upon the skill of the applicator or welder. The consistency of the application is paramount to the success of the mill. Sidetracking mills must adhere to critical design criteria. For this reason, elimination of the often difficult and usually inconsistent dressing process and potential performance uncertainties is preferred. Sidetracking Synergy In light of the irregularities that have been associated with conventional sidetracking mills, the concept of using PCD inserts in place of conventional cutting structures for casing sidetracking was developed. Utilizing PCD inserts as the cutting structure could potentially increase consistency in product manufacturing and performance. The cylindrical PCD cutting elements would be precisely placed in the cutting structure of the PCD Mills. The PCD Mill could deliver a much more reliable milling performance and overall improved efficiency in the sidetracking operation, because it would no longer have to rely on the labor-intensive process of correctly or specifically applying crushed and/or pre-formed tungsten carbide.

PCD Technology Advances Sidetracking Capabilities

Childers¹,

Miller²

2000

Milling Variable Window Openings for Sidetracking

Desai¹,

Dewey²

2000

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.

Planning and Execution of a Long, Tight Clearance Liner Through a Whipstock Milled Window

Dooley

Faul

Courville

et al. 2004

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.