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May 12, 2008
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Home / Issue Archive / 2006 / December #12 / Scientific Projects Give Drilling Practice a New Dimension

№ 12 (December 2006)

Scientific Projects Give Drilling Practice a New Dimension

The successful development of vocational training for industry specialists and the effective work of scientific and research organizations in the former Soviet Union have created a comprehensive engineering knowledge base for drilling technology developing.

By Yakov Gelfgat, Mikhail Gelfgat

The industry saw the successful development and application of various new technologies, such as the hydraulic and electric downhole motors, directional and clustered well drilling, underbalanced drilling, deep wells construction etc. These developments accumulated during several decades, and hundreds of inventions represented a significant asset of drilling science. Unfortunately, some of that knowledge has never found its way into the industry despite its great potential.

Construction of superdeep wells for scientific purposes was one of the remarkable projects that significantly affected drilling industry in the FSU and provided unique operational experience.

The continental scientific drilling program realized in the FSU provided the unique chance to develop advanced drilling techniques.

The overall scientific and technical program for investigation of the deep crust structure by the superdeep borehole drilling for scientific purposes was developed in the USSR in 1965.

This program was in many senses a unique long-term large scale field experiment (Fig. 1). The only way to finance such a program was a centralized economical system and budget financing applied by the USSR Ministry of Geology and supported by the State Planning Committee (Gosplan) and the State Committee on Science and Technology (GKNT). The GKNT was actually the organization with responsibility for coordinating and managing many projects within the general program targets, including site survey, equipment manufacturing, drilling technologies development, and operations itself. Thanks to these programs, the research institutes, production plants, and drilling enterprises received proper financing to execute projects.

The world record of borehole depth was set during this program implementation - 12,262 m was reached by Kola SG-3 superdeep borehole. The 9 5/8-in. casing string was run to the depth of 8,770 m in the same borehole. The 80 percent of drillstring was composed of 147-mm diameter aluminum drill pipes. Three different aluminum alloys were used for pipes manufacturing: high temperature AK4T1 for the bottomhole section, D16T for the middle section, and high strength 1953T1 for the upper section of the drillstring. The rig capacity amounted to 4,000 kN.

Several other superdeep boreholes (5-9 km depth) were constructed in crystalline and sedimentary rocks.

The Krivoy Rog SG-8 drilling was of the greatest interest from both technological and engineering viewpoints. The borehole was spud in September 1984 by a standard Uralmash-4E rig with hook-load capacity 2,000 kN. After running the 720 mm surface casing down to 70 m, the drilling for conductor was carried out with the rotary-turbine drill RTB-640, utilizing two 195-mm turbines and 295-mm diameter drill bits. A 508 mm conductor was set up at the depth of 848 m.

The 215.9 mm diameter borehole with continuous coring operations has been drilled further. The borehole had a pronounced tendency to deviate due to penetrating extremely hard rock layers with big dip angle. The naturally curved borehole was drilled down to 3,460 m, where the inclination exceeded 30°. The technology of drilling without pulling out the drill pipes (DWPP) was tested in this borehole including retractable drill bits and reamers.

The new borehole was sidetracked at the depth of 950 m and drilled to the depth of 2,808 m with the use of the RTB-480M, equipped with 195-mm turbine and 215.9-mm drill bits. In March 1989, the 426 mm diameter casing string was set up. Borehole was continued with the RTB-394 down to the depth of 3,241 followed by 240-mm turbine with 295-mm drill bit.

The most serious problem in SG-8 borehole was tectonically unstable zones. The penetration of complicated intervals was significantly time consuming in terms of cleaning cavernous zones and its temporary cementing. The additional efforts required to prevent the borehole deviation. The inclination angle in the main borehole was kept within the range of 4-6°.

Due to financial problems, the Krivoy Rog SG-8 drilling was terminated in 1993 at the depth of 5,432 m. Overall 85 percent of the studied geological section was cored with 64 percent average core recovery.

A scientific program for investigation of the deep crust structure by the superdeep borehole drilling was of great benefit to geological science; besides, the following techniques were successfully and extensively field-tested:

• the use of high strength and high temperature aluminum drill pipes;

• the use of reduction gear turbodrills for hot wells (up to 250 C);

• coring techniques for hard abrasive rocks;

• DWPP using retractable drill bits and retrievable down- hole motors;

• vertical drilling technology for hard unstable formations.

One of two technologies, which Aquatic Company has been working on during the last 15 years - aluminum drill pipes - has been well presented by OGE. Another one was DWPP, or casing drilling.

From 1948 through 1952, several experimental intervals in some wells were drilled near Krasnokamsk and in the Saratov region using the casing drilling technique. The 8 5/8-in. (219 mm) casing string and a pilot expandable underreamer have been used. About 1,000 m were drilled in three 295-mm wells, proving the operating capability of the method. During the following years, engineers designed, fabricated, and tested various modifications of the retractable two-cone and three-cone bits. From 1964 through 1975, the new method was used to drill 30 wells with a total penetration of 50,000 m at the depth up to 3,000 m. Wells with diameters of 295 mm and 220 mm were drilled using 8 5/8-in. and 6 5/8-in. (168 mm) special drillpipes.

In 1975-1979, the DWPP was tested in a satellite borehole «Sputnik» near the Kola SG-3 borehole. From 1985 through 1992, the method was successfully used to drill a section from 3,000 m to 5,450 m in the Krivoy Rog SG-8 with continuous coring. Coring was carried out in an advancing pilot borehole of 132-mm diameter by retrievable BHA, which was pulled out by the drilling mud reverse circulation without drillpipe tripping. The proper BHA and drilling parameters were optimized for the task to only obtain informative (self-descriptive) core material. In the course of works, the core disintegration by axial and lateral vibrations of the BHA was minimized. That technique allowed receiving sound core recovery and quality of sampling. Table 1 shows details of coring in several pilot holes drilled with reduction gear turbodrill.

After the completion of coring in the pilot borehole, which naturally tended to deviate, the main borehole was reamed using vertical drilling technique. That technology allowed improving core recovery with practically no additional time consumption as the reaming (new borehole drilling) was carried out with the full borehole size bits, which significantly surpassed core bits of the same size in durability and performance.

Since 1991, the method has been used to drill offshore scientific and stratigraphic boreholes in the water depth up to 1,800 m. During the work at Voring Plateau, Norwegian Sea in 1993 six boreholes was drilled and cored using retrievable BHAs (Fig. 2). Retractable drill bit applied to ream pilot borehole upon completion of coring without drillstring tripping. The water depth at the drilling sites ranged from 1,445 m to 1,502 m when the penetration ranged from 41 m to 231.5 m.

In late 1990s Tesco Corp., Canada has been developing casing drilling techno logy, which was in fact very similar to one applied in the FSU back in 1950s. Certainly Tesco development has been based on the up-to-date achievement in rig design, modern drill bit technology and casing pipes running system. Casing drilling has occupied certain niche in well construction techniques and other companies are providing different options of casing drilling technology applicable in different field conditions.

Since 2004 casing drilling became again subject of development in Russia and all previous experience was immediately on demand.

The Prospects of Scientific Drilling

Unfortunately, the term «scientific drilling» has practically disappeared in Russia. Russia is not participated both in the International Ocean Drilling Program (IODP) and International Continental Drilling Program (ICDP).

Meanwhile the first offshore scientific drilling program «Mohole» has started in 1957, almost same time as continental scientific drilling program in the FSU. This program enabled very important technologies for oil and gas offshore activity such as drilling platform dynamic positioning, re-entry operations at the water depth 3000 m and others. Coring operations at the seabed of Gulf of Mexico proved possible hydrocarbon reserves. It was no question shall USA industry and government support continuous scientific drilling activities or not. IODP today involves three drilling platforms: Japanese drillship «Chikyu» equipped with riser, USA riserless drillship «JOIDES Resolution» and one of the European drillships on the project by project basis. IODP is discussing now superdeep well drilling from the ocean floor with water depth up to 4,000 m and borehole depth 6,000 m and more. Such ambitious projects are certainly boosting up drilling technology innovations, but Russian drilling engineering community still is not involved in this work.

Again, the R&D activities in the FSU could be used as a pattern for modern joint arrangements by both industry and governmental agencies.

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