Directional Drilling Equipment and Techniques for Deep, Hot Granite Wells
Authors: Brittenham T.L. , Neudecker J.W. , Rowley J.C. , Williams R.E. 
Published: December 2007

Summary
Conventional directional drilling technology has been extended and modified to drill the first well of a subsurface geothermal energy extraction system at the Fenton Hill, NM, hot dry rock (HDR) experimental site: Ambitious borehole geometries, extremely hard and abrasive granite rock, and high formation temperatures combined to provide a challenging environment for directional drilling tools and instrumentation.

Introduction
Completing the first of a two-wellbore HDR system has resulted in the definition of operational limitations of many conventional directional drilling tools, types of instrumentation, and techniques. The successful completion of the first wellbore, Energy Extraction Well 2 (EE-2), to a measured depth of 15,300 ft (4.7 km) in granite reservoir rock with a bottomhole temperature of 600°F (315°C) required the development of a new high-temperature downhole drilling motor and modification of existing wireline-conveyed steering tool systems. Conventional rotary-driven directional assemblies were modified successfully to accommodate the very hard and abrasive crystalline rocks encountered during drilling of a nearly 8,500-ft (2.6-km) directional hole to a final inclination of 35° from vertical at a controlled azimuthal orientation.

Drilling Objectives
The HDR geothermal resource is derived from a subsurface region that exhibits a relatively high geothermal gradient. At the Fenton Hill site, granitic basement rock is encountered at a depth of 2,400 ft (730 m) and exhibits a static geothermal temperature of 600°F (315°C) at a true vertical depth (TVD) of 14,500 ft (4.42 km). Hydraulic fractures in the granitic rock are vertical and preferentially oriented in a northwesterly direction. The rock matrix is slightly porous (&lt;1%) but has very low permeability (from 1.0 to 0.0 µd).
The method of heat extraction experiments currently under way at the Fenton Hill site requires that two boreholes - one injection well and one production well - be drilled to a depth exhibiting an economically attractive reservoir temperature. To enhance reservoir production objectives, the two wells will be inclined 35° from the vertical through the reservoir region at an azimuthal direction normal to the preferred fracture orientation. The wells will be drilled vertically coplanar with a constant separation of 1,200 ft (370 m) between the underlying injection well and the overlying producer. Fig. 1 illustrates the already described geometry of the Well EE-2/Well EE-3 extraction system in the 11,000- to 14,500-ft (3.35- to 4.42-km) reservoir region. The sequentially formed interconnecting fracture system will be inflated hydraulically, and water will be circulated at a total flow rate of approximately 1,500 U.S. gal/min (95 dm3/s).

Drilling Problems
The implications for drilling-related problems1 during the construction of such a system are significant. The flow capacity requirements of the system require a minimum production drilled-hole diameter of 8 3/4 in. (22.2 cm) and a minimum intermediate drilled-hole diameter of 12 1/4 in. (31.1 cm).
The extremely hard and abrasive rock requires that all roller cone bits have a tungsten carbide cutting structure. All drilling tools, bottomhole assembly components, and the drill string are subjected to severe abrasive wear that limits useful life.