Accessing previously untapped reserves by re-entering existing well bores can extend significantly the productive life of mature oil and gas fields. Yet, conventional sidetracking techniques pose limitations, both technical and economic, with a common drawback: time. A new casing exit technology is proving to be a valuable ally against time in a variety of re-entry applications. The polycrystalline diamond compact (PDC)-based window/formation mill was developed to mill one-trip casing exits through high alloy or chrome casings and difficult-to-mill/drill formations, and allow directional drilling to begin immediately after the casing exit. The new casing exit technology has been applied with impressive results in a variety of environments, from the deepwater Gulf of Mexico, to the North Sea, to the US Rocky Mountains and the Middle East.

Drawbacks of conventional casing exit techniques

Whipstocks and casing mills have been used for many years to exit existing well bores in preparation for sidetracking operations. Typically, a whipstock is set at a pre-determined depth with a window mill attached. The window mill then is unlatched from the whipstock and pushed ahead to mill the window. Historically, casing exits in hard and abrasive formations such as limestone/marble, sandstone, dolomite, anhydrite, and deep sandy shales and salts have required several trips because of premature wear on traditional tungsten carbide cutting elements. Most of the milling time was spent extending the bottom portion of the window and the rathole to a length that would accommodate the directional drilling bottomhole assembly (BHA), particularly when it was a rotary steerable assembly. Depending on window depth, each extra trip could consume 2 days of rig time and significantly increase well costs.
High-grade casing materials present similar challenges for casing exits. Chrome alloys have been widely used in highly corrosive downhole environments in fields that are now maturing. However, untapped reserves from these fields were considered unreachable with conventional techniques, which could not economically penetrate the high-grade casing.

A new solution

To overcome these challenges, Baker Oil Tools developed a window/formation mill with PDC cutters based on technology from Hughes Christensen's Genesis bit series. Called PathMAKER, the window/formation mill incorporates a special compound of the company's bit technology. The cutters are thermally stable and capable of milling alloy steel, stainless steel and formation. Cutter layouts are designed for maximum penetration rates and are arranged in a specially engineered spiral alignment that reduces vibration while enabling smoother, cleaner cuts. The blade design and hydraulic configuration provide large water courses that direct drilling fluid to efficiently clean and cool the cutter elements for increased endurance. This mill can complete a window and drill an extended rathole in thick, high-grade casing and difficult formations in a single trip.

Developed for North Sea Scott field

The casing exit technology was developed by Baker Oil Tools and Amerada Hess to enable the operator to stem declining production from its Scott field in the North Sea. Accessing untapped reserves economically would require through-tubing sidetracking in duplex 25% chrome casing that had been installed initially in many of the wells because of highly corrosive downhole environments. Successfully forming an exit profile in 25% chrome liners had eluded well teams to this point and was considered virtually impossible.

A team of 18 service company specialists worked with the operator's drilling engineer to overcome the challenge. The development program focused on existing milling technology, careful metallurgical analysis and new techniques to achieve a level of performance similar to that achieved with production machining. Repeatability testing for the new technology was conducted with both rotary and motor drilling systems. The systems then were used in three of Amerada Hess' Scott wells with outstanding success. For the operator, the option to use through tubing rotary drilling (TTRD) provided the potential to access smaller accumulations of reserves that otherwise would be sub-economic to pursue. The repeated success of the technique created opportunities for all operators with a current or future need to sidetrack wells in highly corrosive environments.

World's first UBD casing exit

PathMAKER casing exit technology was used to enable the world's first true coiled tubing-conveyed underbalanced (UBD) casing exit, in the Sajaa field in the United Arab Emirate of Sharjah. Field operator BP began a massive coiled tubing UBD campaign there in April 2003 to access additional reserves from the Thamama carbonate reservoir. To date, 21 multilateral wells have been completed with a total of 150,000 ft (45,750 m) of formation drilled. All drilling is performed with nitrogen gas. However, early attempts to mill the windows in a similar fashion proved unsuccessful due to the lack of cooling of the natural diamonds in the window mill. Windows were milled using water, which created an overbalanced condition that interrupted gas production and caused water loss to the formation.

A system using the mill and Navi-Drill Xtreme air driven motor was installed in well Sajaa 19 in June 2004. The window and formation rathole were completed in a single milling trip through a 7-in., 35 lb/ft liner at a depth of 11,424 ft (3,483 m) measured depth (MD) and a temperature of 300?F (150?C). A small amount of water was misted into the nitrogen flow to provide some lubrication for the motor bearings. The milling rate of progress was significantly greater than the previous windows milled in this project, and the milling assembly was conveyed on the drilling coiled tubing reel, which eliminated time consuming reel change-outs. Total casing exit time was reduced 60% over the previous average. Volume of water introduced into the well bore was reduced by 97% - to 175 bbl - compared to the previous average of 5,000 bbl. Reservoir damage was all but eliminated, and gas production continued uninterrupted. The system has since been used in two additional wells with operating time further reduced on each occasion. Window milling with nitrogen has enabled future development of the field by allowing high-rate production wells to be sidetracked.

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