Frontier Geothermal Drilling Operations Succeed at 500 C BHST
Authors: Saito S. , Sakuma S. , Uchida T. 
Published: December 2007

Abstract
The Japanese government funded geothermal exploration well WD-1A reached 3,729 m at TD, where the BHST is more than 500 C. A trajectory correction run was carried out with a PDM and MWD tool where the formation temperature is greater than 350 C. A TDS was used to cool the BHA while running each drill pipe stand in the hole. A borehole dynamic temperature experiment and drill bit tests were carried out in this well.

Introduction
The NEDO exploration well WD-1A was drilled to delineate deep-seated geothermal resources in the Kakkonda geothermal area, located approximately 500 km north of Tokyo (Fig. 1, Kato et al., 1994). The well was planned to be drilled to a 4,000 m depth using rotary methods. Eleven trajectory correction runs were needed to penetrate to the assigned target area. The well reached 3,729 m at total depth (TD) in July, 1995, but did not encounter steam production zones. But the formation temperature, measured by thermal indication materials, was about 500 C at 159 hours recovery time after pumping ceased. At 2,600 m depth, where the formation temperature is greater than 350 C, the last trajectory correction run was successfully carried out with a positive displacement motor (PDM) and a measurement while drilling (MWD) tool. A mud cooling system was used to cool returned mud. And a top drive system (TDS) was used to cool the bottom hole assembly (BHA) while running each stand of drill pipe into the hole for this operation. At 2,650 m depth, a two-and-one-half day borehole dynamic temperature experiment was conducted. This verified the borehole temperature data, both with and without pumping mud, and the cooling effect by continuous pumping with TDS while running the BHA.
The O-ring seal and diaphragm conditions of three-cone bits were inspected after use. From this study, it was verified that a few bit seals had survived even where the formation temperature is over 400 C.

Geology and Formation Temperatures
The Kakkonda Geothermal Field is one of the highest temperature geothermal areas in the world. More than 70 geothermal wells, ranging in depth from 1,000 to 3,000 m, have been drilled and geothermal generation has been conducted since 1978 in this area.
The over-laying tertiary formation, which has a thickness of about 2,200 to 2,500 m, consists mainly of dacitic pyroclastic rocks, tuffaceous sandstone and black shale. The pre-tertiary formation is highly metamorphosed and is of a few hundred meters thickness, confirmed thus far by drilling. The neogranitic pluton is thought to be one of the heat-source rocks in this area, that has intruded into tertiary and pre-tertiary formations (Fig. 1). Most of the rocks are very abrasive. Also the geological structure is characterized as a fold structure. Therefore, bit walk is often encountered and well trajectory control is very difficult in this area (Saito, 1993a).
In general, formation temperatures in this area reach 200 C at a few hundred meter depths, 300 C at 1,500 m depths and over 350 C at around 2,000 m depths.

Purpose for Drilling WD-1 Well
The purpose of this project is to delineate deep-seated geothermal resources by drilling a 4,000 m well and includes a comprehensive supporting research program (Sasada et al., 1993, Muraoka et, al., 1995, Yasukawa et al., 1995, Sawaki et al., 1995, Uchida et al., 1996). Several research studies have been involved in this project.
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