BENT-HOUSING TURBODRILLS IMPROVE HARD-FORMATION DIRECTIONAL DRILLING

Leo Koot, Koos KooleShell U.K. Exploration & Production Lowestoft, England

Tom GaynorNeyrfor-Weir Ltd. Aberdeen, Scotland

Improvements in the design of turbine-powered downhole motors allowed steerable drilling in a hard formation at a high rate of penetration (ROP).

Drilling in this dolomite formation with the rotary or with positive-displacement motors (PDMs) was slow during steering operations.

In the southern North Sea, Shell U.K. Exploration & Production has drilled numerous horizontal gas wells into the Rotliegendes sandstone at around 8,000 ft TVD. Optimum reservoir development required an even spread of directional wells across the reservoir. However, directional control and drilling were a problem during drilling of the Zechstein section (composed of halites, dolomites, and anhydrites).

The typical wells drilled to produce nearer the platforms had an L profile (Fig. 1). Most of the buildup was drilled in the 8 1/2-in. hole with the final approach and the lateral in the 6-in. hole.

The typical wells drilled to produce farther from the platforms had a J profile (Fig. 2). These more distant laterals had the main buildup in the 17 1/2-in. hole, and the 12 1/4-in. and 8 1/2-in. sections were drilled as tangent sections. The final approach and horizontal section were in the 6-in. hole.

Unfortunately, the ideal L-profile well was difficult to drill with steerable motors because of the hard Zechstein section. Standard turbodrills had high ROPs but poor directional control. Steerable PDMs had reasonable directional control but poor ROPs.

Table 1 shows the performance of the different drilling methods used in a series of wells Shell drilled in the Barque and Clipper fields over several months. In each well the total length of the section ranged from 2,000 ft to 3,000 ft.

Shell's solution to the steering and penetration rate problems was to change the well plans if suitable directional drilling tools weren't available. Where possible, the wells were designed with the Zechstein interval drilled as a tangent section with non-steerable turbodrills. However, a better solution was the use of a steerable turbodrill-a tool unavailable on the market at that time.

MOTOR DEVELOPMENT

In 1987, Neyrfor Weir Ltd. began a project to improve turbine-powered downhole motors and develop an efficient steerable turbodrill. The goals of the 4-year project included the following:

Reduce bearing loads (improve bearing life and efficiency) by using thrust balancing drums.
Develop efficient mud-lubricated bearings.
Eliminate elastomers to remove temperature limitations.
Reduce the power section length without a sacrifice of power.
Improve torque.
Make the tool easier to run for the drillers.
Design a high-speed, high-strength flexible coupling to allow building of a bent-housing turbodrill.

The use of thrust balancing drums, the use of efficient mud-lubricated bearings, and the elimination of elastomers allowed a reduction in the bearing section length. This reduction in length, along with the reduced bearing loads, made one-piece turbodrills practicable.

The coupling and bent housing offered a possible solution to the performance-vs.-steering problem for Shell's southern North Sea wells.

A trial run was made with a 9 1/2-in. turbodrill in a Shell well in the North Sea in February 1992. The flexible coupling was installed in a conventional turbodrill with thrust balancing drum and elastomer bearings.

Drilling performance was normal for the formation. Teething problems stopped the run early, but steering and sliding performance exceeded expectations. The success of this tool led to the design of a 6 5/8-in.-7 1/4-in. version of the tool to tackle the difficult buildups in the 8 1/2-in. hole in the Zechstein section.

FIELD TEST

In April 1992, the 6 5/8-in. SBS turbodrill was field tested in the 8 1/2-in. section of a horizontal well for Shell. The directional requirement was to build angle from 40 to 50 while the azimuth was held over an 1,800-ft section.

The tool went through builds, turns, and drops to establish its deviation performance envelope. Two-thirds of the run were rotated and one-third steered. The test run was successful:

The average ROP, including steering, was 40 ft/hr.
The best previously recorded ROPs for PDM and rotary drilling in the area were 22 ft/hr and 26 ft/hr, respectively.
No steering had been possible in those runs.
In the harder formations there was no difference between ROP during steering and rotation.
There was no hanging-up.
The motor drilled where it was steered.
Tool face was easily obtained and easily maintained.
Reactive torque was steady and predictable.
The maximum dog leg severity was 6/100 ft.
The 7-in. liner went straight to bottom with no need for clean up and reaming trips.

The primary benefit for the southern North Sea gas wells was the flexibility in reservoir development. The well profiles did not have to be compromised geometrically because of poor ROP in hard formations.

Figs. 3 and 4 are ROP and deviation plots for a well drilled with the SBS turbodrill. Note that there is no automatic drop in ROP during oriented drilling.

MOTOR DESCRIPTION

The 6 5/8-in. SBS turbodrill is a one-piece motor without elastomer bearings (Fig. 5). This motor can produce more than 300 hp at 500 gpm (turbodrill horsepower varies with the flow rate cubed). The torque is comparable to that for a 6 5/8-in. multilobe PDM.

This turbodrill slides better than offset-stabilizer turbodrills and bent-housing motors.

The motor length has been reduced from 68 to 34 ft because of several key features:

Thrust balancing drums reduce axial loads on the bearing pack by reducing hydraulic thrust by up to 70%. The balancing drums equalize pressure on top of the turbodrill drive shaft with the pressure in the annulus (Fig. 6). This balance is achieved by allowing drilling mud to leak through a microannulus between the rotor shaft and the well annulus.
Fewer bearings are required, hence the bearing packs are shorter. The primary benefit is increased motor efficiency.
New metallic bearings can carry high loads at high efficiency for a long time. This bearing design also allows shorter bearing packs.
Changes to the blade profiles give more power per stage, reducing the number of stages necessary and therefore the length. The design has power, torque, and speed characteristics particularly suited to steerable motor work.
The stabilizer on the bit box stabilizes like a long gauge bit but without a length or deviation performance penalty.

FLEXIBLE COUPLING

The operational life of the flexible coupling is determined by the balance between misalignment and power transmitted. The 6 5/8-in. design has operated for more than 100 hr transmitting 300 hp through a 3/4 misalignment (the longest field test to date).

A bend of 3/4 will change well direction at up to 8/100 ft.

ELIMINATION OF ELASTOMERS

Elastomers swell and harden when heated, impairing their sealing abilities. Moineau-powered motors (PDMs) rely on sealing between the metal rotor and the elastomer stator for power and are adversely affected by high temperature.

Turbodrills do not rely on elastomers for power, but elastomers are used as friction bearings. Turbodrills are therefore relatively easy to configure for high temperature applications by allowing for the swelling in advance. However, high temperature reduces elastomer life, and elastomer failure can be rapid once it begins. This uncertainty was a good reason for eliminating elastomers in downhole motors. Furthermore, elastomer bearings are not efficient at carrying large thrust loads and are not equally efficient in all muds.

Weight and complexity can be reduced through use of product-lubricated bearings. The bearings are designed to be run in specific fluids (hydrocarbons, water, mud, etc.).

The bent-housing turbodrill uses mud-lubricated thrust bearings. These nonelastomer bearings have endured 145 C. for more than 300 hr and 210 C. for more than 150 hr. The endurance is not affected by temperature.

OPERATION

Generally, turbodrills deliver high power at high speed. A relatively small depth of cut will give a high ROP. Although the blade designs in the bent-housing turbodrills deliver more torque at lower speed, there is no need for aggressive bits to deliver a big cut. The torque demand can be relatively, low. Thus, reactive torque and torque fluctuation, which directly affect tool face and ease of operation, are also low.

Conventional turbodrills have a flat pressure profile. The pressure drop across the motor is virtually unchanged whether the motor is on or off bottom, drilling or stalled.

In the past, this characteristic has made steerable turbodrills difficult to manage. For example, a driller may have difficulty determining whether the turbodrill was hung up off bottom, on bottom stalled, or on bottom drilling slowly.

The SBS turbodrills use recently developed pressure-profile blades; the pressure drop varies with speed and torque like that for a Moineau motor, but in an opposite relationship. The SBS turbodrills drop pressure as torque increases and speed decreases. The driller and directional driller can use this information to monitor turbodrill speed. If the SBS turbodrill stalls, the pressure drops abruptly by several hundred psi.

When a drillstring is pressured up, it lengthens; when pressure drops, it shortens.

If drillstring pressure drops as the turbodrill begins to stall, the motor will tend to resist stalling. The depth of cut decreases, and torque approaches maximum. Drillers report that several 9 1/2-in. turbodrills with the new profile blades have stalled then restarted without intervention.

The power available to resist stalling is at least three times greater than that for a 6 3/4-in. Moineau-powered motor.

HOLE QUALITY

Recent work with antiwhirl bits has pointed to the similarity (hole spiralling) between cutting metal and drilling with polycrystalline diamond compact (PDC) bits.

The spiralling phenomenon is complex. The following simplified explanation, easily reproduced with a domestic electric drill, is analogous to downhole drilling.

If a hole is drilled in metal with a twist drill, the tip of the drill will wobble until the hole reaches the drill diameter and the spiral body enters the metal. The tendency to wobble increases with flatter pointed drills. A drill with two cutting edges will wobble in a roughly triangular path.

If the wobbling has produced an overgauge hole, subsequent spiralling will be reduced but not eliminated. Longer, and thus less stiff, drills produce heavier cuts (torque demand), worsening the problem.

If a well bore is drilled with a short-gauge PDC bit without stabilization close behind the bit, a spiralled hole is likely to occur. Bits with a long, tapered nose spiral least; flat-bottom bits have the worst spiralling tendencies. Fig. 7 is reproduced from sonic logs taken in a 17 1/2-in. hole drilled with a steerable motor. The spiral is generated by a slightly different mechanism. The well bore is similar to a threaded hole and has a distinctive wave profile.

This spiralling can make running logs and casing more difficult. During sliding and steering, the stabilizers on a stabilized motor can stick in the troughs of the wave profile, preventing the motor from sliding properly. The only motor which can be slid and steered is a slick motor with a short-gauge bit; however, this setup can continue the spiralling problem.

The SBS turbodrill overcomes this problem with a bit box stabilizer that does not interfere with steering during sliding. The proximity of the body stabilizers to the bit increases support and helps reduce spiralling.