New Automatic Vertical Drilling System Meets High Temperature, Harsh Environment and Performance Drilling Challenges

Even though most wells drilled today are intended to be vertical, different lithology and bottom hole assembly (BHA) imperfections cause the majority of the 100,000 wells drilled each year to deviate at least slightly from a true vertical trajectory. This deviation can have serious consequences if the surface location of the well is located near a property line or problematic formation and the well trajectory drifts across the line. Even wells that remain essentially beneath their surface location can experience borehole spiraling or tortuosity. Weaker formations interbedded with hard stringers can lead to abrupt changes in well trajectory angle (high local dogleg severity) at any inclination. The degree of borehole deviation and tortuosity might not be recognized until attempts at logging or casing-running operations fail to reach total depth because of excessive friction.

By far, the most common cause of unintentional–and sometimes quite significant–deviation is the presence of dipping formations. Without special drilling methods or technology, the well will follow the path of least resistance–the dip direction, rather than maintaining its verticality. The desire to drill fast, which usually leads to the use of maximum weight-on-bit (WOB), can exacerbate the issue. On the other hand, reducing WOB to manage deviation tendencies can result in reduced rate-of-penetration (ROP). Even when efforts are made to minimize deviation, a trip for a correction run is often required to bring the well back to vertical.

Typically, this results in the use of a steerable mud motor coupled with either a tricone or polycrystalline diamond compact (PDC) bit. Although PDC bit technology has seen impressive advances recently, steering with a PDC bit can be slow and problematic. Often, light bit weight must be used to avoid stalling the steerable motor and time is lost due to the need to realign the toolface with the desired correction direction. Both issues drastically reduce performance, and penetration rates may decrease by more than 50%.

Vertical Drilling Systems
Automated vertical drilling systems can markedly improve economics by automatically correcting for deviation problems and producing a straight hole in significantly less time. In addition, these automated systems can enable "lean" casing profiles--profiles which have significantly smaller top hole and intermediate casing diameters as compared to a standard casing profile, yet have an equivalent final production casing size. A smooth, full drift vertical (or directional) hole facilitates running casing with minimal clearance, making a lean casing profile achievable. A smaller hole is usually faster to drill and cuttings disposal, tubular, and cement costs are reduced by up to 30%.

A new device designed for high temperature, harsh environments and performance vertical drilling is the V-Pilot™ Vertical Drilling System (VDS). The system is based on a purely hydro-mechanical concept that automatically initiates corrective measures at a fraction of a degree deviation from vertical.

When used in performance drilling applications, the V-Pilot system allows operators to drill with higher WOB for improved ROP. Also, the system is less susceptible to vibration-related failures that arise when drilling in harsh environments because no electronics are on board. Typically it is a system's electronics that pose most of the reliability problems in high vibration and/or high temperature environments.

(Click on image to view VDS Configuration.)

System Description
The system consists of a vertical control subsystem and a positive displacement mud motor. Four vertical pads, evenly spaced around the tool circumference, are mounted close to the drill bit to apply smooth, corrective forces to the wellbore. Gravity-activated valves direct hydraulic fluid to one or two of the four pads, generating the corrective force. Although the system was designed specifically to take advantage of the new generation of high output even-wall motors for high performance applications, it can also be run with traditional performance mud motors to better match the hydraulic capability of smaller drilling rigs.

The V-Pilot system incorporates extended gauge helical near-bit stabilizers to achieve excellent hole quality. This eliminates or significantly reduces the propensity for ledging and spiraling, even when using very high bit weight, thereby improving overall drilling efficiency. Experience with extended gauge bits on this, and other advanced drilling systems, has proven that less time is spent back-reaming and bit life is greatly improved. This typically reduces NPT and total drilling days, compared to conventional drilling methods.

Incorporated in the system is a mechanical device that is sensitive to departures of less than 0.2 degrees from vertical. Harsh drilling conditions, common with hard rock formations, cause severe vibration in the drilling assembly. The mechanical system was designed to minimize friction and maximize damping, enabling the V-Pilot to respond to very small inclination angles, yet not over-react. Since the system is completely mechanical, temperature does not impede the performance. Also, it is critically damped, so vibration does not adversely affect inclination sensing.

The activation system opens and closes valves directing hydraulic pressure to one or two of the externally, equally spaced pads. The hydraulic pressure is "shop-configurable" to accommodate a wide range of drilling applications. The hydraulic system can produce extremely high side forces to overcome the highest natural deviation forces. The pads remain retracted until correction is required, which increases the pad life and reduces hole drag.

The vertical control subsystem is connected to a positive displacement motor. To maintain reliability, the V-Pilot system's oil chambers are mechanically isolated from the rotating components. The only seals in the tool are static seals with a temperature rating of 200°C. All oil chambers are pressure balanced, allowing for high hydrostatic pressures, and compensated for oil's thermal expansion.

High Performance
To overcome the extremely hard formations that are typically found near mountain ranges, the system has been designed to allow the use of high output even-wall motor technology. This provides the highest possible ROP with the highest WOB and torque demanded. The motors are selected to best suit the capabilities of the drilling rig and the geological conditions. The V-Pilot system is a two-piece modular design that allows the motor to be changed on the rig site thereby enabling the driller to accommodate formation or bit changes on site.

The V-Pilot correction system is automatic, thus no driller intervention is required. The hydro-mechanical system negates the need for computers, wireline, or highly skilled operators. Surveys can be acquired either with a measurement-while-drilling (MWD) system or with a periodic check-shot with a retrievable survey instrument.

The tool suspends auto-correction mode when string rotation exceeds 25 RPM, the point at which the pads are automatically retracted. This allows the tool to act as a conventional slick motor assembly and enables back-reaming. Keeping the pads retracted while rotating the string reduces the possibility of damaging a pad and reduces hole drag. The placement of a top stabilizer and the system's extremely high mass result in a strong pendulum effect, even when the pads are inactive, producing a natural tendency to drill straight down. The extreme rigidity of the system makes it very resistant to any flexing that might result from side forces generated by the bit's interaction with the dipping formations.

Field Testing
Two field tests have been completed using the system in Alberta, Canada in the foothills of the Canadian Rocky Mountains where vertical control is a major challenge.

Test No. 1
The first test consisted of two runs, with one run of each of two prototype tools. The objective of the test was to drill 610 m (2,001 ft) of surface hole, holding the angle as low as possible, preferably under 2 degrees. ROP in the area ranged from 2 to 7 m/hr.

The V-Pilot was equipped with a 9-5/8 in. 6:7 lobe GeoForce™ even-wall power section. The first tool went into the hole at surface and drilled 261 m (856 ft) at an average ROP of 7.7 m/hr (25 ft/hr). Very high levels of axial vibration were experienced throughout the run, but the tool continued to function. The V-Pilot maintained inclination at less than 1 degree until surveys indicated a slowly increasing inclination trend (0.2 degrees/30 m) and an inclination of 0.9 degrees. This was consistent with a known issue in the prototype tools that had been eliminated from the design, but not corrected in the prototype tools in time for the field test. At this point, the first tool was tripped, the second tool was picked up, and a shock sub was added to the BHA in an effort to reduce the extremely high vibration levels.

The second tool drilled 263 m (863 ft) at an average ROP of 4 m/hr. The system smoothly brought the inclination back to 0.2 degrees over 100 m (328 ft). Vibration levels were significantly lower, indicating the shock tool was performing well. The inclination then began to increase as with the first run, because of the same issue in the design, and reached 1.3 degrees before the run was terminated.

The remaining 87 meters of the section was completed with a conventional motor with a 1.15 degree bend setting. The sliding vs. rotating ratio required to maintain vertical with the steerable system indicated that the V-Pilot was able to overcome a constant 3 degrees/30 m natural build tendency of the formation. The same bit was used for all three runs for the interval.

Test Results
Normally three or four bits are required for this interval in the rest of the field. This is the result of the use of a near-bit centralizing sub, which acts to limit the displacement of the bit from the true centerline of the borehole. This geometry has been in use with both steerable and rotary steerable systems for seven years with extremely consistent results. This led the engineering team to incorporate this geometry into the V-Pilot design.

A testimony to the significant hole quality produced on this project is the fact that only 1.75 hours were required for reaming out of the hole, and on the runs into the hole, no significant drag was experienced, which was virtually unheard of for the field. The interval was completed ahead of the AFE target time. Upon teardown, evidence indicated that the known issue in both tools was the cause of the loss of corrective ability. However, despite the very high levels of vibration experienced on the first run, no significant vibration-related damage was observed in the tools.

Test No. 2
The second field test used V-Pilot tools with the design changes implemented. The V-Pilot began drilling at 95 m, where inclination was 0.79 degrees, and brought the inclination down to 0.4 degrees over the next 27 m (89 ft). At 128 m (420 ft), the rig lost power and the assembly became stuck. After jarring on the string, the assembly became free and drilling resumed, with the V-Pilot continuing to perform perfectly.

Test Results
The system successfully completed 621 m (2042 ft) of drilling, keeping the inclination to below 0.5 degrees for the entire interval with the exception of only one survey, when an inclination of 0.7 degrees was measured. ROP varied from 5 m/hr (16 ft/hr) to as high as 25 m/hr (82 ft/hr) due in part to the use of very high weights on bit (24,000 daN or 54,000 lbf) which the system is designed to accommodate. Typically, ROP ranges from 7 to 10 m/hr over similar intervals.

Inclination plot vs. Depth for Field Test #1

Inclination plot vs. Depth for Field Test #2

New Tool for New Challenges
A new vertical drilling system has been introduced to the oil and gas industry that has been designed specifically to address the requirements of high temperature, harsh environment and performance drilling applications.

It is a novel system in that it maintains vertical control without the use of electronics and with no intervention from surface. Also, the system is inherently rigid, has a pendulum behavior even when not activated, and can accommodate very high levels of weight on bit, all designed to optimize its performance.

Field testing has proven that the concept is effective at limiting inclination to below 1.0 degree and is capable of limiting inclination to below 0.5 degrees in areas known for high natural build rate tendencies.