Frontier Geothermal Drilling Operations Succeed at 500°C BHST
- Saito Seiji (Japan Metals & Chemicals Co. Ltd.) | Sumio Sakuma (Japan Metals & Chemicals Co. Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2000
- Document Type
- Journal Paper
- 152 - 161
- 2000. Society of Petroleum Engineers
- 5.9.2 Geothermal Resources, 5.1.1 Exploration, Development, Structural Geology, 1.12.1 Measurement While Drilling, 1.11.4 Solids Control, 2.2.2 Perforating, 5.6.1 Open hole/cased hole log analysis, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.6.2 Technical Limit Drilling, 1.6.1 Drilling Operation Management, 1.6.6 Directional Drilling, 3 Production and Well Operations, 1.6 Drilling Operations, 1.9.4 Survey Tools, 1.10 Drilling Equipment, 1.14 Casing and Cementing, 4.2.3 Materials and Corrosion, 1.5 Drill Bits, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.11 Drilling Fluids and Materials
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The Japanese government-funded geothermal exploration Well WD-1A reached 3729 m at total depth, where the bottomhole static temperature is more than 500°C. A trajectory correction run was carried out with a positive displacement motor and measurement while drilling tool where the formation temperature is greater than 350°C. The combination of large surface mud containers and sufficient mud cooling equipment were used to cool return mud from the well. A top drive system was used to cool the bottomhole assembly while running each drillpipe stand in the hole. A borehole dynamic temperature experiment and drill bit tests were carried out in this well.
The New Energy & Industrial Technology Development Organization (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).1 The well was planned to be drilled to a 4000 m depth using rotary methods. Eleven trajectory correction runs were needed to penetrate to the assigned target area. The well reached 3729 m at total depth (TD) in July 1995, but did not encounter steam production zones. The formation temperature, measured by thermal indication materials, was about 500°C at 159 hours recovery time after pumping ceased. At 2600 m depth, where the formation temperature is greater than 350°C, the last trajectory correction run was carried out successfully with a positive displacement motor (PDM) and a measurement while drilling (MWD) tool. A mud cooling system was used to cool returned mud. A top drive system (TDS) was used to cool the bottomhole assembly (BHA) while running each stand of drillpipe into the hole for this operation. At 2650 m depth, a 2 1/2-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 the 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 1000 to 3000 m, have been drilled. Geothermal generation has been conducted in this area since 1978.
The overlaying tertiary formation, which has a thickness of about 2200 to 2500 m, consists mainly of dacitic pyroclastic rocks, tuffaceous sandstone, and black shale. The pretertiary formation is highly metamorphosed and is a few hundred meters thick, as confirmed by drilling. The neo-granitic pluton is thought to be one of the heat-source rocks in this area that has intruded into tertiary and pretertiary 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.2
In general, formation temperatures in this area reach 200°C at a few hundred meter depths, 300°C at 1500 m depths, and over 350°C at around 2000 m depths.
Purpose for Drilling WD-1 Well
The purpose of this project is to delineate deep-seated geothermal resources by drilling a 4000 m well. It includes a comprehensive supporting research.3-7 Several research studies have been involved in this project.
As for drilling technologies, an evaluation of existing drilling technologies at the high-temperature downhole condition has been planned. This plan includes a PDM, MWD, drill bits, a drill mud cooling system, and multiple-stage cementing tools. Also, mud coolers and a TDS, which enables continuous pumping of mud and cooling of the BHA while running drillpipe stands in the hole, were planned for evaluation. In addition, casing corrosion tests included in production tests at 3000 and 4000 m and casing damage evaluations are planned.
WD-1 was planned with a target depth of 4000 m in the neo-granite where formation temperature was expected to be 400°C. The drilling strategy was devised to drill with five holes of different diameter to safely reach the planned depth. Many lost circulation zones were expected in the shallower depths where previously drilled shallow wells are producing geothermal fluids at depths ranging from 1000 to 1500 m. A 3000 m class rig was employed from spud-in until the setting and cementing of the 13 3/8-in. casing (Fig. 2). All drilling operations were suspended for six months, until the next fiscal year. The rig was then changed to a 5000 m class rig and operations began again in January 1995. The expected productive reservoir was not encountered at about 3000 m, therefore, the well was deepened to 3729 m 6 July 1995 with 8 1/2-in. bits. At 2860 m depth, the well entered the quaternary granite formation, and a 900 m section of the same rock was drilled. Below 3451 m, the drilling mud in the hole deteriorated because of a temperature increase while round trips to change bits were made. CO2 gas was ejected when the bottom part of the mud was returned to the surface. Lime was added to the drilling mud to control the CO2 gas. After a new bit was run to 3642 m and mud was circulated at that depth, high H2S gas-content mud returned to the surface. The H2S gas was thought to be derived from the formation and continuously flowing into the well, even if at a very low rate. A very thin mud system (mud density was less than 1.1 g/cm3) had been used for drilling to prevent mud gelation. It was necessary to raise mud density to control the H2S gas ejection, but the borehole temperature was thought to be too high to continue drilling with higher density mud. The drilling operation terminated at 3729 m because of safety concerns. A closed injection system, which was able to dump H2S gas content fluids into an injection well without venting H2S gas to the atmosphere, was installed to accomplish borehole surveys. Temperature surveys and many wireline logs were performed by various methods. Also, chemistry samples were taken from the TD by the reverse circulation method. The well was plugged again in August 1995 at 2400 m. This was done in preparation for a planned sidetracking operation and re-drill toward a steam production zone at 3000 m depth, to start in October 1996.
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