GERMANY'S DEEP DRILLING PROJECT ADVANCES TECHNOLOGY IN CORING

Germany's Kontinentales Tiefbohrprogramm (KTB) deep drilling project at Windischeschenbach, Bavaria, has demonstrated a major advance in coring technology.

The government funded research hole (OGJ, Nov. 11, 1991, p. 49) reached 6,672.4 m (21,885 ft) with a diameter of 12 1/4 in. July 22, said Claus Chur, head of drilling for the project. Hole inclination was only 0.08.

Seven core runs were made during the drilling, Chur reported. Four of the runs employed Eastman Teleco's large diameter diamond coring system (LDCS). The system drilled a 9 1/4 in. core in the 12 1/4 in. borehole that cut mainly through amphibolite.

In all, 10.7 m (35.1 ft) of large diameter core were retrieved from a 13.7 m stretch for a yield of 79%.

The LDCS's major components are:

A diamond impregnated 12 1/4 in. core bit with lip width of 38.5 mm (1.5 in.) with an integral stabilizer of 12 3/16 in. With such a thin lip, a smaller volume of rock is destructively drilled, yielding a larger core.
An outer core barrel of 11 3/4 in. OD and with a 12 3/16 in. stabilizer.
A 6 m (18 ft) long, 238 mm (9 3/8 in.) ID inner core barrel with the capability of holding two core catchers.
A jam indicator and antijam device. If the inner core barrel begins to jam, a hydraulic signal is sent to the surface. This pressure increase warns the driller and helps eliminate jamming of the core and core barrel.
A core barrel head with a ball sub. The sub is between the coring assembly and the positive displacement motor, which provides a high cutting speed for the diamond impregnated core bit.

During tripping in the hole, drilled debris can enter and accumulate in the inner core barrel. When the bit reaches the well bottom, the inner core barrel is flushed clean with a flow rate of less than 1,600 l./min (425 gpm).

An increase in the flushing rate then releases the ball from the ball sub. Conventional systems, without a positive displacement motor, often use a ball dropped from the surface.

The ball seats on the port above the inner barrel and diverts the fluid flow to the annulus between the inner and outer core barrels and on around the bit. The flushing fluid then does not wash out the core.