Drilling is a cost-driven business. As operators strive to find new ways to cut costs and minimize downtime, many specialists are developing advanced tools that maximize drilling efficiency.

Hart's E&P recently was invited to examine some of the latest downhole drilling tools on the market at Sperry-Sun's Nisku facility in Alberta.
Sperry-Sun has carried out research to produce a system that maintains the attributes of the steerable motor, such as directional control, while providing the hole-quality benefits of extended-gauge bit use.
Together with Japan's state-owned oil company, the Japan National Oil Co. (JNOC), Sperry-Sun has designed a bit-tilting steering technology that enables users to steer the wellbore while simultaneously rotating the drillpipe.
A new angle
Sperry-Sun claims Geo-Pilot, integrated with the group's built-in logging-while-drilling (LWD) sensors, enables operators to drill far more complex directional and extended-reach wells than ever before (Table 1).
The Geo-Pilot's self-correcting technology enables users to maintain the desired toolface and percent bit lift while maximizing rate of penetration (ROP).
Sperry Sun suggests this is because Geo-Pilot does not rely on the axial forces of sliding to steer the bit. Steering by sliding is normally accompanied by significant drag, which can limit the ability to transfer weight to the bit. A lack of weight on bit (WOB) can dramatically affect ROP.
"This downhole drilling tool is steered by tilting, but in a direction you want to go, while rotating the drillpipe. When you don't wish to steer, you instruct the tool to turn off the bit tilt and point the bit straight," Tom Gaynor, Sperry-Sun's drilling tool product line manager, said. Since no sliding is involved with Geo-Pilot, Sperry-Sun claims the tool can reduce stick slip, differential sliding, whirling and drag.
Cleaning of highly deviated holes can be troublesome; as hole inclination increases, axial particle slip shifts to radial particle slip, causing cuttings to fall to the low side of the borehole. Cuttings transport becomes more difficult and often results in the formation of a cuttings bed. Sperry-Sun found that because Geo-Pilot reduces the traditional downhole problems of torque and drag, hole cleaning is greatly improved, and continuous rotation enables better cuttings removal.
Unlike positive-displacement mud motors, Geo-Pilot's motors are designed to have no traditional elastomer mud motor stator, a component often exposed to wear and environmental dependency. This removes the requirement for a power section with a Geo-Pilot system. Torque is coupled directly through the drillpipe from the surface to the bit, resulting in a potentially longer bit run.
This "aviation-type quality" technology uses a shaft mounted with bearings at each end. The outer housing is free to rotate, but the speed of rotation is restricted to about two rotations per hour by an anti-rotation device. The inner driveshaft is deflected midway between the bearings by a pair of eccentric rings through which the shaft passes. By rotating the rings, the shaft can be deflected to any desired toolface setting with varying degrees of offset from center.
The computer-controlled bias unit automatically tracks any slippage of the outer housing and compensates for this by correcting the ring positions in the opposite direction.
The deflection of the shaft in the center results in the deflection of the assembly, pushing the bit in the opposite direction. By pointing the bit in the desired direction, the hole condition is greatly improved over those drilled with conventional bent-sub systems.
Field results promising
One recent commercial run was completed for Spirit 76 (a division of Unocal) in the Gulf of Mexico. With the Geo-Pilot system, 5,627ft (1,715m) were drilled on two runs of 4,507ft (1,373m) and 1,120ft (341m). The technically difficult well plan required a shallow kickoff, building up to 33° inclination at a rate of 2.5°/100ft, holding angle for 1,200ft (365m), dropping to near vertical and kicking off, building up to 55° and turning 182° to complete the section.
The same operator also achieved encouraging results in Louisiana, drilling 7,825ft (2,385m) with the system, reaching 4,505ft (1,373m) in a single run. Spirit 76 said drillstring torque and drag was reduced to much lower, more acceptable levels.
Halliburton Energy Services President Edgar Ortiz said: "We see the Geo-Pilot rotary steerable system as having a profound impact on drilling, even more than other rotary steerable systems. This is due to the fact that the point-the-bit concept lends itself to the use of long-gauge bits. These hole quality issues - straighter, smoother wellbores - will dramatically affect a host of problems that have plagued our industry for years."
With the Geo-Pilot system, two modes of rotary steering become possible. In the manual steering mode, the tool is pointed and controlled by a directional driller. In the automated mode, planned for introduction during the third quarter of 2000, a closed-loop system will drill along a preprogrammed path.
Better hole quality
To tackle the downhole problem of spiraling, Sperry-Sun has developed the SlickBore steerable drilling system. SlickBore is designed to improve hole quality and directional control (Table 2).
It integrates a pin-down positive displacement motor with a box-up, extended-gauge polycrystalline diamond compact (PDC) bit and has reduced bend angles and downhole vibrations. SlickBore uses a PDC bit design based on a nonaggressive cutting structure. Small, densely positioned cutters with high back-rake angles improve steerability. The bit's extended gauge design acts as a long-gauge stabilizer, providing a smoother wellbore by reducing spiraling and ledging.
So why is spiraling so important, and how does it affect the condition of the wellbore?
Spiraling is often caused by the side cutting action of short-gauge bits bouncing about the drillhole. It results in a deterioration in hole quality, an increase in torque, a decrease in ROP and a reduction in bit life.
Sperry-Sun's downhole engineers found that by running long-gauge bits they could prevent the bits from bouncing around the hole. They spent 3 years working to make steerable long-gauge bits. By improving all drilling conditions, they could run 20% less bend and get more dogleg.
Field applications reveal this technology can be used to turn, build, drop and hold angle. Sperry-Sun also claims the tool has improved hole-cleaning ability, reduced drag and improved measurement-while-drilling (MWD) reliability.
The group had mixed results with the downhole tool's early pilot runs. During operational trials in the North Sea, Sperry-Sun drilled one 12¼in. section with two-and-a-half bits and drilled two further sections with one bit. The group also drilled one section at its onshore test facility in eastern Canada with 10 bits. However, in the German sector of the North Sea, their persistence paid off when three "very hard" sections were drilled with the same bit. The group compiled data from new steerable system runs in the North Sea, Norway, eastern Canada, Southeast Asia and South Africa.
Favorable comparison
Operational trials on the Dunlin field in the North Sea resulted in a reduction of planned drilling days by 60%. During these early pilot studies, a comparison was made between eight conventionally drilled sections and eight matched-drilling system sections. The wells, chosen for comparable properties such as mud type, mud weight, flow rate and hydraulic horsepower, were drilled in the same block. Performance improvements in terms of circulating time were observed in rotated and oriented sliding modes. Trial runs that compared conventional mud motors against the Slickbore system repeatedly achieved greater penetration rates and reduced circulation rates.
These early operational trials were compared against results achieved with conventional drilling systems. Three MWD tool failures and two mud motor failures were recorded while the new steerable system suffered none.
At-the-bit measurements
The Nisku team also has developed its gamma-at-bit (GAB) MWD tool. The gamma ray is a formation-evaluation and depth-correlation tool.
So why do operators need a near-bit gamma tool?
In normal MWD equipment, the gamma tool is placed around 40 to 60ft (12 to 18m) above the drillbit. The sensor's depth lag requires the operator to drill more than 60ft (18m) of additional hole before the target zone can be analyzed. Often the time delay can cause further problems for the operator, including lost circulation zones (mud disappearing into natural fractures faster than it can be replaced), water-sensitive intervals (interstitial clays that swell and choke off pore throats when contaminated with water) and thin or inclined horizontal sections that require drilling close to the cap rock. That extra 60ft of drilling can dramatically increase drilling costs.
The initial GAB concept is a 4¾in. motor housing that incorporates a scintillation detector and lithium batteries mounted as inserts directly above the bearing pack. Circuit boards for instrument electronics, power regulation and short-hop telemetry are housed in a second set of inserts.
Sperry-Sun claims the gamma processor can measure up to 255 gamma counts per second at 2,000 rpm and tool temperature to within 2°. The tool's power supplies provide 500 hours of continuous operation from a pack of lithium batteries. Telemetry is improved with the Canadian group's new short-hop FM telemetry system, which can transmit data at rates around 10,000 times faster then EM telemetry.
During trials, the first-generation tool had a bit-to-bend of 90in., allowing it up to 15° of curve build per 100ft (30m). Its outside diameter is only 5.626in., allowing it access to most of the holes drilled by standard 4.750in. Sperry Drill motors.
Sperry-Sun plans to launch modified tools that include at-bit-inclination and azimuthal gamma capabilities.