Abstract
Drilling the 26 in. surface hole in the Tiwi Geothermal Field in the 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.
Introduction
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., 13–3/8 in. casing cemented in 17–1/2 in. hole at 1300 ft., and 9–5/8 in. casing cemented in 12–1/4 in. hole at 3000 ft. An 8–1/2 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 TD as shown in Fig. 1.
Schematic diagram showing casing detail and surface formations drilled in Tiwi Geothermal field
Schematic diagram showing casing detail and surface formations drilled in Tiwi Geothermal field
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 17–1/2 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 boulders 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 17–1/2 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 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 17–1/2 in. TCI bit on a 9–5/8 in. mud motor, the 17–1/2 in. 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.
When the downhole mud motor was drilling through the softer portion of the alluvium agglomerate, penetration rates would reach as high as 80 fph generating 4.9 cu. ft. of cuttings per minute. This overloaded the surface mud equipment as well as created a problem of fill and sticking on connections.
In order to handle this amount of cuttings, the shale shakers on the drilling rig were increased from two to four shakers. Since the mud viscosity was held high to support the large number of cuttings and to prevent fill and sticking on connections, a large amount of the high viscosity mud was being lost as it was carried over the shakers with the cuttings. Screen size on the shakers had to be increased from a 40 mesh to 20 mesh to prevent screen clogging and excess mud carry-over. As the screen size was increased, a large percentage of fines was allowed in the system and additional desanders and desilters had to be run to keep the solids content of the mud at a level that would not cause excessive wear to the mud pumps, drill string, and downhole motor. In some cases, where penetration rates became very high and the cuttings volume could not be handled efficiently, the penetration rate was purposely lowered to solve the problem.
A second problem encountered with the 26 in. bit on the 9–5/8 in. mud motor was ledges in the hole. As the 350 rpm bit encountered smaller but hard boulders in the agglomerate, it tended to take the path of least resistance and drill off to the side of the boulders. When the 26 in. bit was past the boulder and again into softer formation it would swing back to vertical since the 9–5/8 in. mud motor and 10 in. drill collars contained no stabilization. This left a hole that was vertical at 300 ft. but contained sufficient ledges that the 20 in. casing could not be run to bottom. The hole then had to be reamed with a stiff reaming assembly consisting of a 17–1/2 in. × 26 in. hole opener on 10 in. drill collars, with 26 in. roller reamers at 30 and 60 ft. This operation usually took an average of 10 hours and again subjected and rig components and drill string to high torsional and impact shocks while removing the ledges in the 26 in. hole.
In an attempt to solve the ledge problem, a 26 in. OD roller reamer was placed in the bottom-hole assembly on top of the mud motor and the drill string was routed at 30 to 50 rpm. As the reamer was 30 ft. off bottom it did not help the crooked hole problem and tended to hang up on the ledges, keeping string weight off the bit and slowing penetration. The reamer would also knock loose rubble which would fall down and stall the mud motor, requiring the string to be pulled free before continued drilling.
Introduction of the 12 in. Mud Motor
The 9–5/8 in. mud motor, which was designed for 12–1/4 in. to 17–1/2 in. hole, was too underpowered to be using the 26 in. bit. The bit caused stalling of the mud motor even at low bit weight and when rubble fell down on top of the bit. In 1979, during the latter runs with the 9–5/8 in. motor and large bit. industry was able to provide a 12 in. downhole mud motor that tripled the amount of torque available at the bit. With this tool available, we were able to start improving the bottom-hole assembly so as to eliminate the problem of ledges. A 25–15/16 in. integral-blade nearbit stabilizer was added to the bottom-hole assembly beneath the mud motor. The stabilizer was embedded with tungsten-carbide slugs to resist the high rate of abrasion from the andesite boulders. The stabilizer was kept as short as possible to minimize weight below the mud motor. Combined weight of the stabilizer and 26 in. bit is over 3,300 lbs. The 12 in. mud motor had no problems rotating this load with drilling weights up to 30,000 lbs. on bit. but off-bottom reaming and circulating were kept at a minimum to reduce load on the thrust bearings of the mud motor.
A clamp-on 25–1/2 in. OD stabilizer was added to the body of the 12 in. mud motor. This stabilizer slips on the body of the mud motor and is attached by tightening the end caps on the stabilizer. It is placed just above midpoint on the 33 ft. long mud motor to further stabilize the system. This stabilizer was built slightly undergauge to reduce the torque that could tend to loosen the locking mechanism. The 26 in. roller reamer on top of the mud motor completed the stabilized bottom-hole assembly.
Present Operation
Fig. 2 shows the bottom-hole assembly that is currently used to drill the 26 in. hole. This assembly includes the 26 in. TCI bit, 25–15/16 in. OD integral-blade stabilizer with tungsten-carbide slugs on the blades, the 12 in. mud motor with the 25–1/2 in. OD clamp-on stabilizer located about midpoint, a 26 in. OD roller reamer, a 10 in. shock sub, two 10 in. steel drill collars, and four 8 in. steel drill collars.
Surface equipment includes four conventional type shale shakers with 20 mesh screens, a four 8 in. cone desander and a fourteen 4 in. cone desilter, an 800 bbl active surface mud system, and mud pumps capable of delivering 800 gallons per minute to the mud motor. Special tools for the 12 in. mud motor include 12 in. slips and tong jaws and a break-out unit for maintenance of the mud motor and installation and removal of the clamp-on stabilizer.
The drilling mud used is a lightweight bentonite-water mud with weight kept at 63–65 pcf. viscosity at 45–55 sec.qt, PV at 16. gel strength at 7/12. water loss at 20+ cc/30 min., and solid content 3–5 percent. Pump rates are 800 gpm with annular velocities from 25 to 35 fpm in the 26 in. hole. No jets are run in the 26 in. bit.
Weight on bit is usually 25,000 to 30,000 lbs. in the 26 in. hole, with the 12 in. mud motor turning the bit at 125 rpm and the drill string rotating at 50 rpm. a total bit rpm of 175.
By using the stabilized drilling assembly on the 12 in. mud motor, average rotating time to drill the 26 in. hole has been reduced to 22 hours and penetration rate increased to 12 fph.
Results of using the Downhole Motor
Table No. 1 shows the overall reduction in rotating time for drilling the top 300 ft. of 26 in. surface hole. This table follows the reduction in rotating time from the original method of drilling 17–1/2 in. hole and opening it to 26 in. hole to the present stabilized bottom-hole assembly using the 12 in. mud motor. A total of 66 hours is saved in drilling the 26 in. hole, while penetration rates have increased by 300 percent. After compensating for the additional costs associated with using the mud motor assembly, this results in a savings of $16.000 per well at current drilling prices. This does not include the intangible costs savings from reduced rig downtime and equipment maintenance because of rough drilling conditions.
SUMMARY OP ROTATING TIME AND PENETRATION RATES FOR DIFFERENT BOTTOM-HOLE ASSEMBLIES IN 26 IN. HOLE
Type of Bottom-Hole Assembly Used . | Drilling Time and Rate . | Hole Opening Time and Rate . | Reaming Time and Rate . | Combined Time and Rate . | ||||
---|---|---|---|---|---|---|---|---|
(Hours) . | (Ft/Hr) . | (Hours) . | (Ft/Hr) . | (Hours) . | (Ft/Hr) . | (Hours) . | (Ft/Hr) . | |
Drill 17–1/2 in. hole and open to 26 in. hole | 47 | 5.5 | 41 | 6.3 | - | - | 88 | 2.9 |
Drill 17–1/2 in. hole with 9–5/8 in. mud motor and open to 26 in. hole | 14 | 18.2 | 55 | 4.7 | - | - | 69 | 3.8 |
26 in. TCI bit on 9–5/8 in. mud motor | 23 | 11.1 | - | - | 11 | 23.9 | 34 | 7.7 |
26 in. TCI bit on 12 in. mud motor with stabilized assembly | 19 | 13.8 | - | - | 3 | 94 | 22 | 11.8 |
Type of Bottom-Hole Assembly Used . | Drilling Time and Rate . | Hole Opening Time and Rate . | Reaming Time and Rate . | Combined Time and Rate . | ||||
---|---|---|---|---|---|---|---|---|
(Hours) . | (Ft/Hr) . | (Hours) . | (Ft/Hr) . | (Hours) . | (Ft/Hr) . | (Hours) . | (Ft/Hr) . | |
Drill 17–1/2 in. hole and open to 26 in. hole | 47 | 5.5 | 41 | 6.3 | - | - | 88 | 2.9 |
Drill 17–1/2 in. hole with 9–5/8 in. mud motor and open to 26 in. hole | 14 | 18.2 | 55 | 4.7 | - | - | 69 | 3.8 |
26 in. TCI bit on 9–5/8 in. mud motor | 23 | 11.1 | - | - | 11 | 23.9 | 34 | 7.7 |
26 in. TCI bit on 12 in. mud motor with stabilized assembly | 19 | 13.8 | - | - | 3 | 94 | 22 | 11.8 |
Conclusions
The following conclusions were reached from our experience using a downhole mud motor to drill unconsolidated surface alluvium.
The use of a downhole mud motor will effectively increase penetration rates in hard near-surface drilling where there is little weight available to the bit.
Fishing jobs and equipment downtime due to high torque and shock loading caused by rough surface drilling can be reduced by almost 100 percent.
Milled tooth bits are not effective on a mud motor in hard formations due to short bit life caused by tooth wear. TCI bits used on a mud motor in unconsolidated formations containing hard boulders need additional gauge protection.
A drilling assembly using a mud motor in an unconsolidated formation containing boulders must be stabilized to prevent ledges that will result in additional reaming time before running casing.
Surface mud systems must be matched to mud motor volume requirements and penetration rates for maximum effectiveness.
Although equipment costs are increased using a mud motor, overall well costs are reduced because of increased penetration rates and project on-stream time is reduced since more wells are drilled per year.
Acknowledgements
The authors wish to acknowledge the assistance and cooperation of the staff of SII. LOR and the Drilling Department of Philippine Geothermal, Inc., for without their contributions this program would not have been possible. The authors also wish to thank the management of the Union Oil Company of California for permission to publish this paper.