Lateral Re-Entry : Case Study in Pearson Field

Last year, Oryx Energy Company re-entered a vertical well in the Pearson field, Texas, USA, completed in Austin Chalk. Horizontal drilling in Austin Chalk using mud commonly encounters almost total lost circulation. To reduce mud losses, formation damage and costs, water is often used as drilling fluid. This decreases bottomhole hydrostatic pressure to less than formation pressure - underbalanced drilling. To combat annular pressure from formation flow during drilling, conventional rigs use a rotating stripping head or rotating BOPs to seal the annulus. The wells are killed each time a trip is made.

By using a CTU (coilded tubing unit) , which has its annulus sealed throughout drilling by the stripper, Oryx was able to run in and out of hole without killing the well. This imporved safety and avoided the expense of potential damaging effects to the formation of pumping brines to kill the well prior to tripping.

To prepare the well, Oryx used a conventional service rig to remove the existing completion hardware, set a whipstock and sidetrack out of 4,5-in. casing at a true vertical depth of 5300 ft (1615 m). Drilling was the continued using 2-in. coiled tubing, downhole mud motors, wireline steering tools, a mechanical downhole orienting tool and 37/8-in bits. An average buildup rate of 15o/100 ft was achieved and a horizontal section drilled for 1458 ft (444 m). The main bottomhole assembly (BHA) components were :

• Drillstring -Oryx employed a reel comprising 10,050 ft (3060 m) of 2-in. outside diameter coiled tubing with 5/16-in. mono conductor cable installed inside the tubing. 
• Orientation tool - Because coiled tubing cannot be rotated from surface to alter drilling direction, a downhole method of changing tool face orientation is needed. To achieve this, Oryx deployed a mechanical tool that converts tubing reciprocation into rotation-compression rotated the tool face to the right, extension to the left. Once adjusted, the tool face was locked in place using a minimum 250-psi differential pressure across the tool.
• Directional survey tool - The survey tool inside a nonmagnetic collar relayed directional information to surface via the wireline. 
• Directional BHA - Two assemblies were used, depending on the build rates required - a double bend assembly consisting of a conventional 2 7/8-in. bent housing mud motor coupled to single bent sub, or a steerable assembly comprising a single bend motor.
• Bit - Thermally stable diamond bits were used to drill the curve and build sections and polycrystalline diamond compact (PDC) bits to drill the lateral section. 

Oryx motive for drilling this well was to prove that coiled tubing could be used to drill a lateral well in a controlled manner. This was achieved - the final wellbore trajectory came within a 50-ft vertical window along the horizontal section.