Short Radius Drilling System
Authors: Pustanyk Randal H. 
Published: December 2007

Industry trends to re-enter existing well bores has created high demands to improve the technology associated with short radius drilling systems. A system and methodology has been developed which allows for the drilling of short (40-90 ft) radius curves with lateral sections potentially in excess of 1500 feet.

The system employs a combination of articulated motors and measurement while drilling survey tools that enable low rpm drillstring rotation within the safe limits of drillstring design.

Introduction

The economic environment of the oil and gas industry has placed demands on operator and service sector companies to develop strategies and technology that reduces the cost associated with drilling reserves.

Despite extensive exploration efforts, most basins on a global basin are approaching maturity. To extend reservoir life many operators are turning to programs which employ the drilling of horizontal re-entries of existing wells, of these a significant number are of the short radius type.

Other applications have also driven up the demand for short radius drilling; Drilling thin producing reservoirs below unstable formations which must be cased, and surface location constraints where target location requires the use of short radius curves.

Where operators were once content with shorter horizontal sections afforded with the initial technology (100-300 meters) demands have now required the ability to drill further sections (450+ meters). Several issues have previously restricted the ability to complete these objectives; primarily excessive well bore drag in the horizontal associated with sliding (essentially static) friction, and erratic profiles. Improvements in the drilling systems offered have overcome these obstacles and allow longer horizontal sections to be drilled at faster rates than before.

Definition of Short Radius

For reference in this paper short radius wells will be deemed to have radii ranging from approximately 12m to 30m. Wells with radii above 30m typically do not require specialized (articulated) equipment to drill. Slightly modified motors and MWD systems have proven themselves adequate. Wells with Radii significantly under 12m typically employ alternative (rotary) technologies as described in reference 1. Curves in the 1m range can be accomplished with jetting technology and are often referred to as "Ultra Short".

Previous Technologies

Previous short radius systems have utilized articulated build and lateral drilling motors combined with flexible wire line steering tools and methods outlined in reference 2.

To drill the build section an articulated build motor is employed. It is configured with a large bend located near bit and articulations located above the bend, and between power section stages. The lowermost articulation typically would. be located 1.25 to 1.5m from the bit. Above the motor a non magnetic tubing joint is run with a universal bottom orienting sub (UBHO).

If kicking off in casing or from vertical immediately below casing, a gyro is generally utilized to orient and survey with. As the vibration of the down hole motor while drilling is detrimental to the gyro, it is generally pulled up the well bore and locked in place to prevent damage. As a result the first 7 to 10m of well must be drilled essentially with a gyro single shot method.

On deeper wells the reactive torque in the BHA will often cause a poor kick off direction to be established.