Mike Cure
Grace Drilling Co.
Dallas
Pete Fontana
FlowDril Corp.
Kent, Wash.
Recent changes in a high-pressure, jet-assisted drilling system have been successfully tested in several East Texas wells.
These encouraging results will help move the system from the test stage into commercial application.
The higher the formation angle or the harder the rock, the better the test for the high-pressure, jet-assisted (Fig. 1) drilling system. Grace/FlowDril, a joint venture between Grace Drilling of Dallas and FlowDril Corp. of Kent, Wash., is developing the new system.
Rates of penetration (ROP) with the ultrahigh-pressure (up to 35,000 psi) jet-assisted system have been from 1.5 to 3.0 times that of conventional drilling systems. The higher ROP can be achieved with less weight on bits designed for softer formations.
Jet-assisted drilling reduces wear rate on bits. This means that a driller can stay in the hole with fewer trips to change bits.
FlowDril began working on the system in 1985. Grace began working with FlowDril in July 1988 for further development of the jet-assisted drilling system. The current joint venture was formed in April 1990.
Since then, considerable strides have been made in putting together four operating prototype systems by July 1991.
The surface equipment (Fig. 2) includes drilling fluid-condition equipment, high-pressure pumps, an isolator, and a piping system to deliver clean drilling fluid to the drillstring (Fig. 3).
The drillstring uses the same equipment found on conventional rigs: swivel, kelly, kelly valves, drill pipe, collars, stabilizers, subs, and drill bits. However, these have been modified so that the high-pressure fluid is delivered to the hole bottom through an inner conduit in a stream separate from the conventional drilling mud (Fig. 4).
A high-pressure swivel was built into the gooseneck of the conventional swivel. High-pressure fluid is pumped through beryllium-copper tubing that runs down the center of the drillstring. The fluid stream impacts the rock face, fracturing the rock ahead of the bit.
The two fluid streams combine after being discharged from the nozzles at the drill bit and are circulated out of the hole with the cuttings.
In the system's most recent round of improvements, several modifications were made.
Previous drilling projects exposed weakness in the stab seal design. Changes in material and sealing sectional area have reduced the loss of downhole pressure integrity, reduced inner conduit pin wear, and reduced stabbing damage to sealing elements (Fig. 5). The new swivel has a proprietary design. The latest version of the swivel worked at 20,000-30,000 psi for over 160 hr, a major improvement over earlier designs.
Changes in the centralizer design within the bottom hole assembly (BHA) resulted in better stabilization of the inner conduit and lowered the friction pressure drop of the drilling mud flow in the annulus between the high-pressure tubing and the drill pipe.
In addition, the bit sub high-pressure check valve was improved, and the bit's jet nozzle was also redesigned, considerably reducing bit modification lead time.
ENGINEERING CONSIDERATIONS
A considerable amount of engineering has gone into the new equipment designs. In every design modification, cost, ease of operation, and safety have been driving forces.
SEAL LIFE
The proprietary seal in the high-pressure tubing consists of a slip-fit connection that is made up while the drill pipe is being threaded.
In the latest generation of equipment, the drillstring was upgraded with an improved stab seal design. Clearances were refined and optimal surface coatings for seal integrity were selected.
Changes in the material and sealing section area reduced the downhole friction pressure drop, inner-conduit pin wear, and stabbing damage to sealing elements.
PUMP DESIGN
The current mechanical-crankshaft, ultrahigh-pressure pump designs are more efficient than the hydraulic intensifiers used before.
The new pumps require less horsepower (600-800 bhp) to move 25-30 gal of clarified mud or water at 30,000 psi.
The older systems required as much as 2,400 bhp for pressure in the 12,00015,000 psi range.
CHECK VALVE
The bit sub high-pressure check valve was also improved. The one-way check valve closes when the pumps shut down, keeping the heavier drilling mud and cuttings from entering the inner conduit through the nozzles.
This provides well control and keeps the nozzles from plugging.
NOZZLES
A new generation of bit nozzles replaces the original welded design, where bit break down for welding could require as much as a 3-week lead time.
The new replacement nozzles can be installed on any bit, regardless of manufacturer, in less than 2 hr at any machine shop. They can also be reused on several different bits (Fig. 5).
The nozzles can be adapted to any off-the-shelf bit with minimal machining of the drill bit shank and throat. Bit selection is thus improved while rig inventory is reduced.
BIT LIFE
The development of the system thus far has been with 7 7/8 and 8 3/4-in. bits. Laboratory modifications are now being made to accommodate the testing of bits up to 12 1/4 in.
Bit selection depends on formation type, depth, and mud programs. Just like conventional bit programs, the parameters change from field to field.
Currently, soft to medium-formation bits (IADC bit classification 5-3-7 and 6-2-7) are able to drill hard and abrasive tight sands that usually require 7-3-7.
In drilling the Travis Peak and Cotton Valley sand formations in East Texas, a medium-formation bit (5-4-7) drilled 897 ft with the jet-assisted system. Using a conventional drilling system, a hard-formation bit in an offset well drilled only 500 ft.
Even the medium-formation bits show less wear.
Cost savings due to lower bit wear rate can be considerable. Generally, a bit costs around $5,000. The trip to change a bit can last as long as 12 hr and cost $5,000-$10,000 in lost drilling time.
TUBING LIFE
All tubular goods have dual-conduit design. The swivel, kelly, and drillstring are modified to include a small diameter (1 5/8-in. OD), concentric tube in the center.
The beryllium-copper tube comes in sections and is installed flush with the end of the tool joints. The API drill pipe is purchased in matched length. Make up is standard.
The original design 15-5 stainless steel tubing was sensitive to chloride-stress corrosion cracking. In addition, its greater wall thickness and 10-month lead time from the pipe mills led to a change in alloys.
Beryllium-copper tubing, although more expensive, is impervious to chloride-stress corrosion cracking and has a lead time from the mills of only 8-12 weeks.
The life of the components is more a function of the number of pressure cycles than wear. The fatigue life of the beryllium-copper tubing, indicated by laboratory fatigue pressure tests, goes beyond the life of the drill pipe.
Currently, the system is working with 4 1/2-in. drill pipe, which limits drilling to approximately 12,500 ft. A larger 5-in. drillstring is being developed now. This will extend the drilling capability to 15,000 ft.
A different style centralizer design will stabilize the inner conduit in the 5-in. drill pipe. This will further reduce the friction pressure drop of the drilling fluid inside the drill pipe-tubing annulus.
MUD HANDLING
Clear water or brine drilling fluids may be pumped directly by the ultrahigh-pressure pumps. Because of the high pressures, any suspended solids must be removed from those fluids traveling through the pumps.
Therefore, an "isolator" is being developed to isolate the ultrahigh-pressure pumps from the drilling fluid. The isolator works like an hydraulic intensifier with a one-to-one hydraulic ratio.
The ultrahigh-pressure pumps power clear water as hydraulic fluid to pump conditioned drilling mud with the "isolator." The "isolator" is being tested at this time.
FIELD EXPERIENCE
The jet-assisted system has recently been used in several wells in East Texas. ROP increased from 1.4 to 2.8 times that of conventional drilling in offset wells. Here is a breakdown of the higher ROPs:
- Test well No. 1 -Glen Rose, 1.7; Rodessa, 2.4; Pettit, 1.4; and Travis Peak, 1.9.
- Test well No. 2-Glen Rose, 1.9; Rodessa, 1.8; Pettit, 2.5; and Travis Peak, 2.4.
- Test well No. 3-(E.G. Powers No. 6) Glen Rose, 1.4; Rodessa, 2.0; Pettit, 2.4; Travis Peak, 2.8; and Cotton Valley, 2.3.
In Test Well No. 3, the jet-assisted system used one-third less weight on bit (WOB) to straighten existing deviation from conventional operations. Portions drilled with normal WOB exhibited an excess of 3.0 times greater ROP than conventional drilling.
The drilling, as shown, has been in the Glen Rose, Rodessa, Pettit, Travis Peak, and Cotton Valley sand formations, using soft to medium-formation bits.
On the E.C. Powers No. 6 (Test Well No. 3), the jet-assisted system drilled from 3,278 to 9,960 ft TD. Portions of that formation drilled with normal WOB had instantaneous ROP in excess of 3.0 times that of conventional drilling (Fig. 6).
Due to strict deviation limitations (2 1/2), approximately 2,000 ft of the well were drilled with reduced weight on bit from its original 30,000 lb, to 8,000-12,000 lb. Deviations were measured using a special slim-hole deviation tool acquired by the joint venture.
A soft-formation bit (4-4-7) was used for this interval. The bit was reusable even after drilling 2,696 ft for 83.2 hr, at an average ROP of 32.4 ft/hr through several sand sections in the Rodessa and upper Travis Peak formations.
This bit life compares favorably to conventional drilling of these formations where bits may last an average of 80 hr, but only average approximately 1,500-1,800 ft with an average ROP of 12-14 ft/hr.
Using the softer bit and one-third normal WOB, the jet-assisted system drilled a straight hole through the Glen Rose 1.4 times faster than its conventionally drilled offset. With normal WOB and jet assist, the well drilled 3.0 times faster than without jet assist.
Operated by Texaco, the E.C. Powers No. 6 well is located in Panola County, Tex.
The well was drilled to a TD of 9,960 ft with the top of the Travis Peak at 6,400 ft and the top of the Cotton Valley at 8,200 ft (Fig. 7).
After using the jet-assisted system on two wells in the hard-rock Travis Peak formation, the operator said the system is "workable."
By drilling a hard-rock formation in fewer days, rig costs can be reduced and the well brought on-line quicker. The operator said his company would most likely put the jet-assist system on the hole at the second or third bit in the well.
Several problems are still to be solved. The isolator must be improved, and the dual-pipe drillstring could possibly affect fishing operations.
When the isolator is redone, the operator sees potentially significant increases, as much as three times ROP, in hard-rock country.
The system has also been tested in West Texas in dolomite, limestone, and anhydrite.
FURTHER TESTING
For the next drilling project, planned for early 1991, Grace/FlowDril is looking for harder formations and more difficult strata for further testing of the system.
The jet-assisted drilling system could be very useful in overthrust regions, such as the Rocky Mountains and onshore Europe, where there are steep-dip formations. Because straight holes can be drilled with less WOB at higher ROP, it could be ideal for these areas.
The joint venture is also looking for a prospect in southern Oklahoma to test jet-assisted drilling in just such a formation.
As more wells are drilled, experience can be developed for bit selection and nozzle location for optimum drilling in various formations.
Currently, a nozzle system for 12 1/4-in. bits is being tested. Two, three, and even four-nozzle designs have been tested. In investigations of additional nozzles on the bits, tests have shown that ROP increases only about 20%/nozzle. However, added nozzles appear to increase bit protection and could lengthen bit life.
Bit optimization will be ongoing. The goal is to maximize both bit life and ROP. Targets for future development are 3.0 times conventional systems for overall ROP and 4.0 times for instantaneous ROP.
Several bit manufacturers have been working with Grace/FlowDril. At present, for example, a jet-assisted polycrystalline diamond bit is being developed.
Jet-assisted drilling has great potential for application to directional drilling, as shown by the successful test of the measurement-while-drilling (MWD) tool on well E.G. Powers No. 6.
Using an orient-jet and rotate operation, much like the slide and rotate steerable system used with downhole motors, the MWD tool orients the ultrahigh-pressure nozzle, directing the jetting action toward the target. Additional research and testing will proceed in this area.
Testing the ability to pump oil-based muds is also planned.
In addition, air drilling applications, where air/mist is currently being used, is being investigated for jet-assist application.
The joint venture has developed jars, fishing and wire line tools, and fishing procedures.
Second-generation equipment is already in the field. The equipment is smaller and requires less space on the well site than the previous system.
Plans call for making the equipment more modular, with skid mounts. Costs will be reduced further and maintenance made easier.
The jet-assisted drilling system can be moved from rig to rig easily, which means it can be used efficiently. Within 8 hr of arrival on location, the system can be rigged up and ready to pull up drill pipe.
Thus, the same drilling system could be used on multiple rigs in the same geographical area.
The evolution of the ultrahigh-pressure, jet-assisted drilling system has been rapid.
Once the concept of using a dual drillstring was developed, significant changes in materials allowed higher pressures in safer systems.
The company is learning to drill "all over again," and is eager to expand its drilling data base. The main goal continues to be increasing drilling performance while reducing costs.
BIBLIOGRAPHY
- Butler, T., Fontana, P., and Otta, R., "A Method for Combined Jet and Mechanical Drilling," Paper No. 20460, SPE 65th Annual Technical Conference and Exhibition, New Orleans, Sept. 23-26, 1990.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.