New-generation hole-enlargement tools improve drilling efficiency

The advantages of reaming while drilling are clear: the pilot hole offers good directional control while the enlarged hole creates the right conditions for running casing or completion equipment. The end result can lead to significant savings in time and cost. Over the last several years, however, experience with this operation has highlighted issues that can erode savings and contribute heavily to nonproductive time (NPT).

The chief problem has been complex bottomhole assembly (BHA) dynamics that result from managing two hole diameters simultaneously. Often, the bit is drilling in a different formation type than the reamer, which is positioned in the BHA at quite a distance from the bit. Therefore, the issue of two hole sizes becomes even more complicated – the vibration tendencies induced by one type of rock are compounded by the characteristics of the higher rock.

Vibration, which is defined as unwanted axial, lateral, and torsional movement, not only dissipates energy that should be available at the bit face, but can lead to multiple costly failures related to fatigue, including:

• Drillstring twist-off;
• Tool damage/malfunction;
• Reduced mean time between failure for MWD/LWD and directional drilling components; and,
• Compromised performance in rotary steerable system (RSS) applications. One study estimates the cost of vibration-related failures to be in excess of US $300 million annually (Jacobsen-Plutt et al. 2009).

Analysis comparing the level of vibration before the activation of the reamer to the level after activation shows that vibration intensifies when the reamer is engaged. In short, the hole-enlargement tool is often the "noisiest" component in the BHA (Servaes 2009).

This graph shows the XR Reamer’s enhanced capabilities including opening ratio, temperature stability, and flow rate. (Images courtesy of Halliburton)

Other aspects of the operation can be impaired by the use of conventional reamers. The acquisition of important LWD data can be disrupted by high vibration levels and a lack of hole concentricity. Cutter design can severely limit the ROP and shorten the run time, resulting in multiple trips and tool replacements. Inability to produce and sustain a gauge hole will create issues for running and cementing casing. Factoring in today's challenging well trajectories, even moderate loss of hole quality can exacerbate all these potential problems.

But, even if a reamer can deliver acceptable performance in meeting these challenges, design limitations can significantly reduce its operational effectiveness. The selection of a hole-enlargement tool must address several capabilities including the determination of how much enlargement can be achieved (opening ratio); maximum flow rate capacity; and whether or not the arms and cutters offer prolonged life and fast drilling.

A new generation of hole-enlargement tools has been able to establish an excellent track record in all of the above performance criteria. In addition, it is the only tool that can be deactivated after enlarging for drilling ahead with the original pilot hole size and allow full-flow circulation while tripping out. The XR Reamer features multiple refinements that help minimize or eliminate the well-known issues typically encountered with hole-enlargement operations. Extreme downhole environments are also part of the repertoire: the XR Reamer is designed for temperatures up to 230°C (450°F).

The XR Reamer is engineered to take out the vibration-inducing aspects. Pictured is a reliability assessment based on an X-Y vibration data field test.

Hole size, flow rate

If hole enlargement is the goal, then a reamer that maximizes that value will help cut costs and improve the efficiency of subsequent casing-running operations. The XR Reamer features a wide range of diameters capable of enlargement to 50% over the pilot hole or drift diameter.

The high-powered rigs and drillships deployed around the world have sophisticated, high-output pumps that function at high efficiency. While the design of MWD/LWD tools has generally kept pace with the high flow rates generated by these pumps, many reamers require restricted flow rates. The XR Reamer has proven to operate reliably at flow rates up to 1,400 gal/min and at mud densities up to 16.0 lb/gal.

In the Norwegian North Sea, an operator recently drilled a 3,876-m (12,716-ft) section in one run to total depth, with a pump rate of 1,189 gal/min. The arms exhibited very little wear.

Reliabilty, run length

The XR Reamers are the result of several years of continued evolution. As an example, they have been deployed in deepwater Gulf of Mexico (GoM) operations for a major operator to successfully enlarge approx-

imately 19,050 m (62,500 ft) of well bore. In a total of 38 runs using a variety of RSS configurations, the high-performance reamers have accumulated 2,285 hours with 97% reliability.

On a 2010 field trial in the North Sea, the XR Reamer was used to successfully drill and ream a 3,101-m (10,173-ft) 8 ½ -in. x 9 ½ -in. section, the longest run achieved in the Brage field and a world record at that time. The average ROP was 61 ft/hr. One year later, a modified and improved tool was used in the same field to drill 2,931 m (9,616 ft), with an average ROP of 105 ft/hr.

In Egypt, performance records compiled for 18 wells drilled by one operator showed that the XR Reamer had zero impact on drilling performance and produced no detectible vibration in the BHA. The operator was able to sustain a high ROP and control equivalent circulating density (ECD) values within the programmed range. Caliper logs on these wells showed a gauge hole in every reamed section.

The runs ranged in length from 30 m to 577 m (98 ft to 1,893 ft). The opening diameters ranged from enlarging from 8 ½ in. to 12 ? in. up to enlarging from 17 ½ in. to 22 in.

In another Egypt-based operation, the 8 ½ -in. hole was enlarged while drilling with an RSS and holding at a 73° inclination to accommodate running a gravel pack application. The operator was able to drill and open the 53-m (174-ft) section to 9 ½ -in. in one run of 8.8 on-bottom hr, with an average ROP of 19.75 ft/hr. In this case, the XR Reamer was run and made up below the MWD/LWD tools. The LWD data were obtained without disruption.

The cutter quality and arrangement on the arms is integral to longevity and reaming effectiveness.

Vibration

The opening-size data for the one operator’s GoM wells is shown below.

When a hole-enlargement tool can reduce or eliminate vibration, it naturally optimizes the entire drilling operation – from BHA performance to ROP to log-data acquisition. The XR Reamers are carefully engineered to take out the vibration-inducing aspects. The reamer body helps minimizes BHA vibration and micro-doglegs at formation transition interfaces. Lateral vibration is reduced owing to the tight tolerance body-to-hole design. Resistance to bending and tilting helps reduce torsional vibration. Overall, the XR Reamer has outperformed others in key areas that impact cost and time.

A robust accumulation of field performance data confirms the ability of the XR Reamer to consistently meet or exceed hole enlargement requirements, even in difficult downhole environments. The technology is mature enough so that a wide range of tool diameters is available. The tool design is flexible enough to allow for the effective positioning in the BHA, optimal cutter strength and placement, and adaptation of function to wellbore conditions.