Drilling the 26 in. surface hole in the Tiwi Geothermal Field in the Philippines is characterized by low penetration rates and rough drilling Philippines is characterized by low penetration rates and rough drilling due to the presence of hard andesite boulders in the unconsolidated surface alluvium. This paper describes how drilling time and problems associated with rough drilling were reduced by using a downhole mud motor to power a stabilized 26 in. drilling assembly.


The Tiwi Geothermal Field is located on the flanks of the extinct Malinao Volcano approximately two hundred and fifty miles southeast of Manila on the Island of Luzon in the Republic of the Philippines. Approximately 80 wells have been drilled in the Tiwi Field since exploratory work was begun in 1972 by Philippine Geothermal, Inc., a wholly owned subsidiary of the Union Oil Company of California.

These geothermal wells range from 1000 ft. to 9000 ft. in depth, with the average well being about 5000 ft. deep. A typical casing program consists of 20 in. casing cemented in 26 in. hole at 300 ft., 13- program consists of 20 in. casing cemented in 26 in. hole at 300 ft., 13- 3/8 in. casing cemented in 17-½ in. hole at 1300 ft., and 9-5/8 in. casing cemented in 12-¼ in. hole at 3000 ft. An 8-½ in. hole is drilled through the producing reservoir and 7 in. perforated liner is set from the 9 5/8 in. casing shoe to the well perforated liner is set from the 9 5/8 in. casing shoe to the well TD as shown in Fig. 1.

The top 300 ft. of formation consists of unconsolidated surface volcanics and alluvium containing large andesite boulders up to 10 ft. in diameter and other volcanic debris from past eruptions. The extreme size of these boulders was proven during the excavation for the first power plant built in Tiwi. The underlying formation to 1300 ft. is altered to fresh andesite with streaks of clay. It is the upper sections of hole, especially the unconsolidated surface volcanics, where we found a down-hole mud motor extremely useful.

In the first wells, the 26 in. hole was completed by first drilling a 17- ½ in. pilot hole and opening it to a 26 in. hole. This method was very time consuming because of low penetration rates and very damaging to drilling equipment due to high torque encountered in drilling and opening unconsolidated formation. With the development of a 12 in. downhole mud motor, we are now able to drill a 26 in. hole using a stabilized bottom-hole assembly so that hole-opening and reaming are no longer required. By using an assembly consisting of 26 in. tungsten-carbide (TCI) bit, a 25-15/16 in. integral-blade nearbit stabilizer, a 12 in. mud motor, a 26 in. roller reamer and 10 in. drill collars, we have reduced rotating time in the 26 in. hole from 88 hours to 22 hours. Downtime due to rough drilling conditions has been reduced by almost 100 percent.

Original Drilling Procedure

In the Tiwi Geothermal Field, a 30 in. conductor is cemented in a hole dug with a backhoe to 15 ft. below ground level. A 26 in. hole is drilled to 300 ft. and 20 in. casing is cemented to consolidate sloughing hole, seal off surface water, and isolate the sometimes massive lost circulation zones associated with the unconsolidated surface rubble.

The extremely hard andesite boulder are sometimes encountered as soon as drilling out of the conductor pipe. Since only milled tooth bits were available in the 26 in. size at the beginning of the project, penetration rates would be as low as a few inches per hour while trying to drill through these boulders. In order to use available TCI bits, a 17-½ in. pilot hole was drilled to 300 ft. and then opened to 26 in. diameter. Even with the TCI bits, penetration rates were very low due to lack of adequate bit weight penetration rates were very low due to lack of adequate bit weight when drilling close to surface.

An additional problem was downtime due to the rough drilling conditions. Even with the use of multiple shock subs in the drill string, extreme vibration and bouncing caused by drilling the boulders would literally shake the drilling rig apart as nuts and bolts would fall from the derrick. The drill string not only suffered from the high impact forces due to the bouncing, but was also subject to many torsional failures as sloughing rocks would intermittently wedge the bottom-hole assembly. Rotary chains were constantly breaking and rotary tables suffered an unusually high rate of damage from the high torque. Under these conditions an average of 88 rotating hours was needed to drill and open the 26 in. hole.

First Use Of The Mud Motor

It was felt that a downhole mud motor with its high bit rpm would solve the problem of low penetration rates in the near surface section of the hole where there was inadequate weight on the bit to drill the hard andesite boulders. Using a 17-½ in. TCI bit on a 9-5/8 in. mud motor, the 17-½in. hole was drilled to 300 ft. with penetration rates increasing from 6 fph with the conventional assembly to 18 fph with the mud motor. The hole still needed to be opened to 26 in., slowing the combined penetration rate to 4 fph, an increase of 1 fph over the conventional rotating method. When a 26 in. milled tooth bit was used on the mud motor, the bit wore out so quickly it was not successful when drilling the andesite boulders.

In 1977, in cooperation with service companies, a 26 in. TCI bit was designed and manufactured for use on the downhole mud motor in drilling the very hard formations. The 26 in. TCI bit was used on a 9-5/8 in. mud motor to drill the 26 in. hole in Tiwi with exceptional results. Rotating time for the top 300 ft. of hole was reduced from an average of 88 hours to an average of 34 hours with the 9-5/8 in. mud motor on a 26 in. TCI bit.

Additional Problems Using The Downhole Mud Motor

Although using the downhole mud motor had improved the drilling of the surface 300 ft. significantly, there were still problems with the system. problems with the system.

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