L. R. Aalund
Managing Editor-Technology
The third phase in the world's most sophisticated deep drilling project ended this summer with the borehole at a depth of 3,003 m or 9,852 ft.
The scientific probe into the lithosphere near the German/Czechoslovakian border in Bavaria is on schedule and now well into the fourth phase that will carry it to a depth of 6,000 m (i 9,684 ft). At mid-September, the hole depth was 4,250 m. The final objective is a depth of 10-12 km or some 33,000-40,000 ft.
The operations reflect the intense planning that has gone into the project, designated the German Continental Deep Drilling Program, or KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland).
Although all the drilling has been and will be in crystalline rock, it is certain that many of the new procedures, hardware, and measuring systems developed and being used will have practical application in conventional oil and gas drilling.
The massive research rig near the small town of Windischeschenbach does have enough of the systems and equipment found on today's advanced rigs to make an experienced drilling engineer or rig crew feel at home on the drill floor. But the automation and power available and the redundancy and sophistication of the systems make this resemblance only superficial.
BACKGROUND
The KTB project, which is funded by the German Ministry of Research and Technology at a level of DM 497 million ($298 million), began in 1984 and will continue through 1994 (OGJ, Feb. 27, 1989, p. 64).
The objective of the program is the basic research of the physical and chemical conditions and processes in the deeper earth crust with the goal of understanding the structural development, the dynamics, and the evolution of the intracontinental crustal regions.
After a study of 40 locations, a scientific committee selected the site near Windischeschenbach which is in a region where the Moldanubic continental tectonic plate has been thrust over the Saxothuringic plate. These conditions are similar to those found in other areas of the world and will give the research findings broader implications.
The KTB project has a complex organizational structure with the federal research ministry at top connected to various levels of oversight boards and advisory committees. The Geological Survey of the German federal state of Lower Saxony (Niedersachsen) manages the effort in which German universities and industry are participating. KTB project management reports to the Geological Survey and a scientific oversight council. Reporting to KTB project management are four directorates, two for geological sciences, one for project administration, and another for technology.
It is this latter directorate that is responsible for day-today planning and drilling the hole in a manner that suits the needs of the scientists. It is also responsible for environmental control at the site.
The staff and management of the scientific directorates come from German universities. They collect the extensive data and test samples from the operation, run analyses, if necessary, and analyze data and results. Much of the work is done in a modern field laboratory at the site with the latest in chemical and geophysical analytical equipment. For example, one group of scientists continuously measures by spectrometry nine gases at the outflow of the shale shaker. A sample trap has been devised to obtain the samples uncontaminated by air.
Such facilities and activities make the point that this is much more than a drill site. It is actually a geophysical research facility with an ongoing drilling operation. Drilling and completion operations also provide opportunities for testing new equipment and procedures and are another form of research.
In an oil industry analogy, the KTB technology department would be the operator of a drilling project.
The contractor doing the drilling is UTB Ultratief Bohrgesellschaft mbh, Bad Bentheim, Germany, a venture of three German firms active in contract drilling and rig design and construction. UTB also designed and constructed the drilling rig with input from the three partners. They and their stake in UTB are Deutsche Tiefbohr AG, Deutag (50%), Deutsche Schachtbau und Tiefbohrgesellschaft mbh, DST (35%), and ITAG Tiefbohr Gmbh & Co. KG (15%).
DST drilled the 4,000 m (13,120 ft) pilot hole at the Windischeschenbach site. This hole, which was completed in 1989, was cored for over 90% of its depth. These cores now are neatly cataloged and filed in row upon row of floor-to-ceiling shelves in a special core room. As a result, coring in the ultra deep hole will begin below 4,000 M. Then 1,500 m of coring is planned down to the 10 km mark. The pilot hole obtained a maximum of geoscientific information at lower costs than could be obtained with the heavier rig. It reduced core and logging runs in the large diameter upper section that is necessary in the primary hole. Also, the pilot hole served as a test of drilling and logging tools and yielded valuable information on hole stability and the most important temperature profile.
THE HOLE
Borehole temperature will determine the final depth of the ultradeep well. The temperature in the pilot hole reached 118 C. (244 F.) at 4,000 m. This was some 30% higher than expected. It therefore appears that the critical temperature of 300 C. (572 F.) will be reached at 10,000 m. Temperatures above 570 F. can damage electronic and other logging systems and cause other major problems to crop up. Temperature has therefore put the initial target depth in doubt.
If the temperature is lower than expected, drilling will continue toward 12,000 m or nearly 40,000 ft. The decision to go for the deeper depth will be made when the hole reaches 8,000 m (26,240 ft).
The most famous superdeep hole to date is the Soviet Kola SG-3 hole west of Murmansk. The hole, which was spudded in 1970, reached 12,066 m (39,586 ft) in 1984 when drilling was suspended.
A factor in setting the bit size/casing size program shown in Fig. 1 was borehole stability, on which the pilot hole provided valuable information.
Breakouts caused by structural stress over time can be a major problem. They are oval in shape and can be from 20 to 40 m in vertical length, posing hole caliper problems. As a result, open hole sections are kept to a maximum of 3,000 m.
The desire to use conventional bits and casing was also a factor.
Another was of course bit size at total depth. The KTB planners decided that an 8 1/2 in. size would be the best compromise after weighing several factors, including bit life expectations and drilling and logging tool diameters.
The result of all these considerations was the bit size/casing size program shown in Fig. 1. The 17 1/2 in. bit was used to spud the ultradeep well on Oct. 6, 1990, to allow logging operations and permit use of the vertical drilling systems. The hole was then opened with hole openers.
However, to reach the 10,000 or 12,000 m mark with this bit/casing program, the employment of two other concepts-vertical and slim clearance drilling-are necessary.
These two techniques go hand in hand. Because of the stiffness of large size casing, the hole must be drilled very straight. But because the planners are not sure the electronics of a self-steering vertical drilling system (VDS) will operate properly at the high temperatures expected in the lower sections, they allowed larger clearances in this part of the hole. To retain the same amount of casing strings while using standard oil and gas drilling clearances throughout the hole would have required starting the hole with a 35 in. bit.
Table 1 shows the clearances in the KTB hole while employing slim line drilling.
The combination of slim hole drilling and a self-steering vertical drilling system worked well during the third phase, which was completed with the cementing of 3,000.5 m of 16 in. and subsequent necessary work (OGJ, July 8, p. 28).
While drilling the 3,003 m of the this phase, twenty-three 17 1/2 in. bits were used, mostly type HUG EP1084. The average bit life of all bits was 127 m and the rate of penetration was 1.4 m/hr.
Inclination of this section of the hole averaged only 0.5. The outside diameter of the Hydril connectors on the 16-in. casing is 16 1/4 in. while the hole size is 17 1/2 in., leaving a clearance of only 5/8 in.
KTB reports that the cementing operation required 232.5 cu m of cement, including 20.5 cu m of HEC spacer and 26 cu m of heavy tail end pill. Table 2 shows the difference in hole volume when employing slim line drilling in contrast to conventional drilling.
VERTICAL DRILLING
The development of a downhole, closed loop drilling tool that automatically compensates for any deviation from the vertical has had a high priority during drilling of the ultradeep hole.
KTB now believes that a design, called the VDS-3 (vertical drilling system) from Eastman Christensen GmbH is the best answer. It was employed in Phases 2 and 3 and is being used successfully in Phase 4.
The latest design has an "internal" steering system in contrast to "external" steering. In the latter, the steering pistons actuated to control the inclination worked against the borehole.
The VDS Type 3 has an internal steering system where pistons correct the inclination by working from an outer, nonrotating shell against the inner rotating shaft.
The rotating shaft connects to the bit on the bottom and the universal joint of the downhole motor on top of the shell. The shell houses the hydraulics, sensorics, electronics, and batteries.
The VDS-3 has steering and centralizing pistons with paired cylinders. Both cylinders are supplied with pressurized mud from an inlet bore to the inside of the shaft. The centralizing pistons are powered constantly, holding the shaft in a controlled central position within the housing. When a deviation from the vertical occurs, the electronics cause one or two of the control valves to close and activate the corresponding steering pistons. These will exceed the forces generated by the centralizing pistons and push the shaft in the proper direction. Downhole electronics translate the inclination and temperature data for transmission to the surface by mud pulse technology.
As a result of using this and a previous externally steered drilling system, inclination averaged only 0.5 over the first 3,000 m. This was achieved at some costs though because problems with the earlier vertical drilling systems caused excessive trips out of the hole.
The latest design called VDS-4 has an external steering system. In this type the steering piston actuated to control inclination works on the expandable stabilizer ribs against the borehole.
According to KTB, the tendency to deviate from the vertical is more severe in crystalline rock than in sediments. The verticality achieved is therefore a major achievement, particularly when compared with ultradeep Russian and Swedish wells that experienced inclinations of over 20. KTB believes that the principles and components employed in the VDS system will eventually be useful in steering systems for horizontal and deviated boreholes.
The VDS-3 system is being successfully employed in the 14 3/4 in. fourth phase of the ultradeep well. In was used four times in drilling 247.2 m in the 609 m drilled between June 18 and July 24. It held the inclination between 0 and 0.5 during these runs.
Nine conventional 14 3/4 in. bits were used. The types and JADC codes were:
1 GS88/5-4-5
2 GM88/6-2-5
1 S62A/6-2-5
3 ATM33CG/6-1-7
1 X44/6-1-5
1 X77/7-3-5
Average life was 66.55 m with the maximum being 102.9 m. Rate of penetration was 1.25 m/hr.
RIG, OPERATIONS
It would perhaps be more accurate to call the KTB drilling rig a drilling facility (See cover photo). Anchored in a heavy duty foundation that covers the drill site and powered by electricity from the regional system, it will never be moved as a single unit to another locations.
Although the modification of an existing rig was considered, the consortium of drilling contractors charged with developing the rig decided with the KTB engineers that a new on-purpose unit would be the best choice.
The result is officially designated UTB-1 GH 3000 EG, the world's largest onshore rig. It has installed capacity of 12,900 hp and a total weight of about 2,500 metric tons (including all rig components). It meets environmental standards of the highest order with a quiet, fully electric drive, sound proofing, and elaborate systems for controlling and treating all effluents. Also, an elaborate system continually monitors all important drilling parameters.
Following are details on various components and operations.
DERRICK AND SUBSTRUCTURE
The derrick and substructure are of permanent construction. The total height of the rig is 83.175 m (272.9 ft). The maximum hook load is 8,000 kiloNewtons. There is racking capacity for 12,000 m of drill pipe and collars.
The derrick accommodates an automated pipehandling system and pipe conveyor. The drill floor is 13 m x 13 m and 11.75 m above ground level. There is a driller control room on the drill floor (Fig. 2).
Although there is some question as to whether a blowout preventer is needed in such hardrock drilling, the rig does have one.
DRAWWORKS
Given the fact that more than 800 round trips are expected during the drilling of the ultra deep hole and that the time for a 1,000 m round trip will be 2 hr, the drawworks have several notable features. They are mounted on the floor of the substructure to reduce vibrations, and are gear driven with a maximum input of 2,200 kw. The cable reel is some 2 m in length and is driven by a four-speed gearbox. Remotely controlled multi-disk clutches allow fast gear selection under load.
PIPE
A KTB study showed that with conventional drilling stands of 27 m (88.5 ft) that roundtrips during the course of the 4-5 year project with the 10,000 m hole would require some 450 days. KTB, therefore, decided to use triple stands of 40 m (131 ft), made up of 13.3 m singles. Actually, this is a standard API pipe (Range 3), however, and not believed to be used by anyone else. With this length and a hook retractor system, KTB expects to cut the roundtripping time by a third.
When tripping, the hook raises the drill pipe upward to a point where it is gripped by the pipehandler. At that time the hook is retracted out of the center of the well and is moving down as the connection is being broken by an iron roughneck. Normally, the hook would have to wait until the stand is set aside before moving down. This is now a simultaneous action, coordinated by a computer. The savings in time justifies the investment in the pipehandler. The 53 m long pipehandler is stationary and rotates with a star-shaped finger board (Fig. 3). This is a new development. In addition, an independent pipe conveyor allows pick up and lay down of drill pipe for inspection without interruption of drilling. The drill pipe has experienced two types of problems, a shearing of thread metal and corrosion.
The connections of 5 1/2 in. full hole drill pipe suffered the thread damage probably because of the higher weight of the stands and the geometry of the thread. To solve the problem, the operators are remachining thread to NC 56. Thirty stands have already been remachined and eventually the entire drilling string will be converted. KTB believes a case can be made for conventional applications of this thread.
The other drill pipe problem is pitting corrosion. This is believed to be a function of the specially developed drilling fluid required for the drilling program. Basically, it meets the conventional requirements put on a drilling fluid, but is stable, particularly at high temperatures, and does not influence or disturb geoscientific analyses or detection. The formulation chosen is a water-based mud containing a synthetic clay mineral designated Dehydril HT. Because of its high cation exchange capacity value, it imparts high viscosity with far less material addition requirements in comparison to traditional natural clays such as bentonite.
KTB believes that when this mud encounters an unconformity on the drill pipe it forms a small bubble that remains immobile. This become a sort of venting cell that doesn't allow air in but helps transport iron ions out, causing the pitting corrosion.
The operators have therefore added an organic polymer called Hostadrill to the mud in an effort to hinder the coagulation that promotes the venting cells. The compound also hinders water loss, cutting it from about 30 API to 15.
The scientists don't have a problem with the Hostadrill but it is likely a corrosion inhibitor will have to be eventually added.
And when the borehole temperature reaches temperatures greater than 200 C., probably around 6,000 m, the operation will have to be ready to shift to an oil-based mud.
This depth will therefore be the start of a major transition. It will end Phase 4 and probably mark for the first time the entry into the Saxothuringic plate.
CREW/STAFF
Not surprising, most of the supervisors, drilling crew, and service company personnel and technicians come from the oil industry. In total, 120 persons work permanently at the Windischeschenbach site. Of this number, 55 supervisors and crew are directly involved in the drilling; 45 as UTB employees, 10 from service companies. The remainder are support personnel or with the universities or KTB in the laboratories.
Claus Chur, a former drilling engineer for BEB Erdoel Erdgas Gmbh, Hannover, is head of operations for KTB at the site. He has been there since the pilot hole was started in 1986 and is responsible for the drilling operation and all related work such as testing.
As in industry, Chur has a tool pusher who works with his counterpart at UTB in carrying out the drilling program. The work goes forward in three shifts, each with a driller, assistant driller, and three roughnecks.
Three roustabouts work in the daytime.
Also employed are a pipe inspector, electronic and service company technicians, and drilling and other specialists.
BIBLIOGRAPHY
Rischmueller. Prof., Dr. H, "Ultra Deep Drilling in Lithosphere Enhances R&D in Drilling Technology as Demonstrated in German Deep Drilling Program (KTB)," presentation to DGMK Annual Meeting, Sept. 21, 1990, Muenster.
Sperber, A., "The Casing Concept for the KTB Ultradeep Well," Fifth International Symposium on the Observation of the Continental Crust through Drilling, Sept. 10-11, 1990, Regensburg, Germany.
Oppelt, J., Chur, C., Feld, D., and Juergens, R., "New Concepts for Verical Drilling of Boreholes," 1991 SPE/IADC Drilling Conference, Amsterdam, Mar. 11-14.
Wohlgemuth, L., and Chur, C., "The KTB Drilling Rig," Fifth International Symposium on the Observation of the Continental Crust through Drilling, Sept. 10-11, 1990, Regensburg, Germany.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
