Don Pearce
Union Pacific Resources Co.
HoustonMichael Johnson, Bo Godfrey
Drilex Systems Inc.
Houston
Bent-housing steerable downhole motors helped maintain course for a deep, hot, horizontal well in the Austin chalk.
The Navasota Unit No. 1 was planned as a B zone, single downdip lateral, Austin chalk horizontal well with a maximum departure from vertical of 3,767 ft and a planned total depth (TD) of 17,342 ft measured depth (MD)/14,172 ft TVD. The Austin chalk was found significantly deeper in this well than planned, which resulted in an actual TD of 17,899 ft MD/14,993 ft TVD, the deepest (TVD) horizontal well in the Austin chalk to date. The well was spudded on Aug. 6, 1994, and took 52 days to reach TD. The static bottom hole temperature was almost 350 F.
Early horizontal Austin chalk wells were shallow (6,000-8,000 ft TVD) with 2,000-3,000 ft lateral sections. These wells had low temperatures (140-180 F.). Later wells commonly had 4,000-6,000 ft lateral reach sections, and these wells were hotter.
Drilling multilateral sections became somewhat common in the Austin chalk. The ash boundary within the Austin chalk prompted the drilling of multilateral sections stacked above each other. It is believed the ash beds form a seal; therefore, single horizontal wells drilled below or above the ash layer may leave producible reserves in the ground.
As the chalk play moved east, the lease sizes became larger, the TVD thickness of the chalk increased, and the ash bed presence diminished. These factors led to opposed dual lateral wells. An opposed dual lateral well can have a total lateral section of more than 9,000 ft, compared to the approximate 6,000-ft practical limit for a single lateral.
As the Austin chalk play matures, the trend is toward drilling hot (280 F.), deep horizontal wells, as most of the easier (shallow and low temperature) wells have already been drilled.
WELL PLAN
The drilling plan called for a 17 1/2-in. surface hole to 3,200 ft MD. A string of 13 3/8-in. casing would be run and cemented to surface.
The intermediate hole was planned to extend 75 ft into the Austin chalk formation to a depth of 13,850 ft. A 12 1/4-in. hole was planned through the Wilcox formation and into the Midway shale. The 12 1/8-in. hole size was included in the well plan to provide for a contingency string of casing to be set through the Wilcox formation and still allow 7 5/8-in. casing to be set into the Austin chalk if adverse hole conditions developed.
If no hole problems were encountered, the hole size could be reduced to 9 7/8-in. at the top of the Midway shale, with drilling continued to 13,880 ft. A 7 5/8-in. casing string would be set into the chalk.
A native mud with gel sweeps for hole cleaning was planned for the majority of the intermediate hole. The Midway shale would require moderate water loss control. The mud pH would have to remain less than 9.5 upon reaching the Midway shale at 9,764 ft MD.
After the logging run, a string of 7 5/8-in. casing would be run and cemented.
A 6 1/2-in. pilot hole was planned to drill through the Austin chalk, Eagleford shale, and Buda formations to 14,344 ft. The pilot hole would be plugged with cement after logging and the well sidetracked 50 ft below the 7 7/8 in. casing shoe.
An angle-building motor assembly was scheduled to sidetrack the pilot hole and drill a 6 3/4-in. curved section with an 18.8/100 ft build rate. The B zone target was to be intersected with 87 inclination at 14,162 ft MD/14,005 ft TVD.
A steerable motor assembly was planned to drill a 6 1/2-in. hole in the remaining horizontal hole section. The plan called for approximately 3,100 ft of lateral with a total depth of 17,342 ft MD at a TVD of 14,172 ft.
At TD, a packer was to be set at 13,700 ft MD.
DRILLING RESULTS
The Navasota Unit No. 1 well, located about 1 mile east of Navasota, Tex., was spudded Aug. 6, 1994, with Nabors Rig No. 10. A 17 1/2-in. hole was drilled to 3,250 ft MD in 29.5 drilling hr. A string of 13 3/8-in., J-55 STC casing (54.5 lb/ft and 68 lb/ft) was run and cemented with 1,712 sacks of class A cement. A 13 5/8-in. casing head was installed, and a rotating head and blowout preventers (BOPS) were nippled up. Table 1 (16644 bytes) lists the bottom hole assembly (BHA) for the intermediate hole.
A 12 1/4-in. tricone bit drilled out the 13 3/8-in. casing shoe and formation to 6,079 ft MD in 46 drilling hr. The bit was tripped to pick up an 8-in. positive displacement mud motor (PDM) and tungsten carbide insert bit. This assembly drilled to 9,272 ft MD in 70.5 drilling hr, and the bit was then tripped. Drilling continued to 9,936 ft MD with the motor and a 12 1/4 in. milled-tooth bit. At 9,936 ft MD, the hole size was reduced, and a 9 7/8-in. polycrystalline diamond compact (PDC) bit and an 8-in. PDM were picked up. Table 2 (16069 bytes) lists the BHA components in the 9 7/8-in. intermediate hole.
The 9 7/8-in. PDC bit drilled only 96 ft before being pulled because of low penetration rate. A stray sand was probably encountered and broke cutters on the bit. Two additional mill tooth bits drilled until dull before another PDC bit was placed in the hole at 10,690 ft MD. Drilling with PDM motors and PDC bits continued to 13,724 ft MD.
At 13,724 ft MD, a "triple-combo" wire line log was run to confirm formation correlation. Drilling continued to 13,877 ft, where a neutron log was run through drill pipe to again confirm mud log correlation. Drilling continued to 14,317 ft, where a triple-combo wire line log was again run to verify the mud log pick of the top of the Austin chalk. The top of the Austin chalk was found at 14,250 ft MD on this logging run.
A string of 7 5/8-in. casing was run and cemented with the casing shoe set at 14,295 ft. A high-pressure rotating head and a 10,000-psi BOP were installed. A 4 3/4-in. PDM and a 6 1/2 in. PDC bit drilled out the casing shoe and formation from 14,317 ft MD to 14,944 ft MD in 44.5 drilling hr. A triple-combo wire line log was run from 13,600 ft to 14,944 ft MD.
Table 3 (11227 bytes) lists the BHA components for the pilot hole. The 6 1/2-in. open hole was plugged with cement. A 6 3/4-in. milled tooth bit cleaned out the 7 5/8-in. casing and drilled out the cement to 14,360 ft MD.
LATERAL SECTION
An angle-building motor assembly with a 6 3/4-in. rock bit sidetracked the well at 14,360 ft MD/14,357 ft TVD. Fig. 1 (86325 bytes) shows the motors used for the curved section and lateral reach. Table 4 (16534 bytes) lists the BHA components for the curved section of the well.
A measurement-while-drilling (MWD) tool failed after 14 drilling hr, and the BHA was tripped to replace the failed component.
The curve was built at a 21/100 ft build rate to 14,658 ft MD. There was sufficient angle in the well at this point to allow a steerable motor to finish the curve. A 2-bent-housing, tandem-power-section, steerable PDM was run with a 6 1/2 in. PDC bit. This assembly built angle while sliding at 10.4/100 ft from 14,658 ft to 15,012 ft MD. At 15,012 ft MD with approximately 86 of inclination, the curved portion of the well was completed and the lateral reach portion was begun. At 15,074 ft MD, a 10-ft slide was drilled to enable the well to traverse the entire target interval. Fig. 2 (34677 bytes) (shows the actual-vs.-planned curve portion of the well.
Soon after completion of the curve, the well kicked. While the kick was circulated out, pressure fluctuations caused the operator to believe that a washout occurred in the drillstring. The mud weight was raised from 10.7 ppg to 12.0 ppg to kill the well, and the BHA was pulled out of the hole to look for a drillstring washout. A leaking float in the BHA caused the pressure changes.
A new 2-bent-housing, tandem-power-section, steerable motor was picked up and run in the hole. The initial static downhole temperature was 349 F. after the trip. This assembly drilled from 15,151 ft to 17,899 ft MD in 146 rotating hr, with only one required slide from 15,265 ft to 15,279 ft for course correction. An MWD failure occurred after 126 rotating hr. Table 5 (18045 bytes) lists the BHA components for the lateral reach section of the well.
From 17,772 ft to TD, hole problems became severe because of excessive downhole torque and drag. Multiple lubricants were tried but did not appear to help. Drilling operations were halted at 17,899 ft MD/14,993 ft TVD. Fig. 3 (57474 bytes) shows the planned and actual curved and lateral reach sections. The 2-bent-housing, tandem-power-section motor was easily steered and required very little sliding time to maintain angle or course direction. The driller could easily control hole angle with this assembly by simply adjusting the rotary speed in the 30-60 rpm range, which caused the assembly to either drop or build angle. The PDC bit wall was severely damaged after TD was reached.
A packer was set in the 7 5/8-in. casing at 14,190 ft, and the well was completed open hole.
