Expandable reamers were developed to improve efficiencies and reduce risk while drilling through problematic formations in deep water and other offshore wells, where wellbore stability is a major challenge.
Lean-profile well designs have been implemented to enable operators to drill faster and more efficiently, with better well control and less material, within a narrow pressure window between formation fracture and shale collapse.
Using this concept, the well is drilled using the smallest possible hole sections and minimum clearances between casing strings. This allows access to the reservoir with a larger-diameter hole to increase production flow rate or access to additional reserves that otherwise would be unreachable because of their extreme depth.
Lean-profile wells require rotary steerable systems to create smooth wellbores, and expandable reamers to manage equivalent circulating densities (ECDs) and maintain wellbore stability while enlarging the hole under the casing sufficiently to allow reliable cementing.
The first concentric expandable reamers were ball-activated. Once expanded, they could not be closed without stopping circulation. The second iteration was modified so that the reamer could be deactivated to allow circulation after reaming for better hole cleaning.
The advantages of the concentric expandable reamer over its mechanical-arm predecessors included the attainment of a larger borehole size that could accommodate larger bottomhole assembly (BHA) components, flexibility in pilot bit selection, higher flow rates, and the ability to ream a previously drilled hole or to back ream a hole after drilling.
The advances in underreaming technology expanded its application to the point at which it is now used on a high percentage of development wells. The most common application is shoe-to-shoe underreaming to enlarge the clearance for easier tripping and casing running.
However, there are drawbacks to traditional expandable underreaming systems. Activation cannot be confirmed. Additionally, the placement of the reamer at 100 to 300 ft above the bit means that the equivalent portion of the hole, known as a rathole, cannot be enlarged without a dedicated trip that typically requires 1 to 2 days of rig time at a daily cost ranging from USD 1 million to USD 2 million.
On-Command Digital Reamer
To address the limitations of existing underreamers, Baker Hughes and Statoil jointly developed the industry’s first on-command digital reamer as a single size prototype in 2007. The prototype was exclusively used in the Norwegian and UK sectors of the North Sea. The new remotely controlled hydraulic-electric reamer was developed using hydraulic blade activation, which was controllable from the surface.
In 2012, Baker Hughes began to develop several sizes of the GaugePro Echo on-command digital reamer (Fig. 1 above), an improved, downlink-activated reamer for the global market. The new product was commercially tested in the US Gulf of Mexico, Norway, and Malaysia, and is being globally commercialized in the current quarter. The on-command reamer brings reliability, flexibility, and real-time operational insight to high-spread-cost drilling applications and eliminates the need for a separate rathole reaming run.
No longer confined by mechanical activation restrictions, the downlink reamer has the ability to undergo as many activation cycles as needed and provide real-time confirmation of blade status and position, an objective that has long been sought. Additionally, it is able to send back information on oil pressure, oil temperature, and vibration in real time for diagnostic and optimization purposes.
The wired, modular reamer can be placed anywhere in the BHA, an especially effective feature for rathole reaming. When the reamer is placed near the bit, the traditional second rathole reaming run is eliminated, wellbore conditions are improved, and casing is run significantly more quickly and safely.
The tool is powered by an electrical current from the measurement-while-drilling (MWD) unit, which is generated by a mud-driven turbine, or through wired pipe. The blades are retracted by means of a hydraulically driven piston that is independent of the flow.
The reamer is operated with three main commands: activate, deactivate, and updrill. To activate, the power from the MWD unit drives an electrical motor, which runs a pump driving a piston. The piston connects to a yoke, which drives the cutter blocks up the ramp. Once the blades reach the stop blocks, the pressure on the piston is reduced and kept at a level sufficient to maintain a stable positioning of the blocks.
In deactivate mode, the pressure is applied on the other side of the piston, pulling the yoke back into the inactive position. In updrill mode, a maximum pressure is applied to the cutter blocks to keep them in place while reaming up, or back reaming.
The tool is the only reamer that can digitally confirm activation and deactivation. The reamer operates independently of drilling parameters such as fluid pressure, flow rate, revolutions per minute (rev/min), or weight on bit (WOB).
The on-command reamer is available in S12 and S14 models, enlarging 12¼-in. holes up to 14¾ in. and 14½-in. holes up to 17½ in., respectively.
Matching Bit and Reamer
For best performance, reamers and bits should be matched for cutter size and aggressiveness (depth of cut) to balance loads more closely while drilling through homogeneous formations. For example, if the drill bit out-drills the reamer, some inner formation stresses may have been released by the time the reamer starts drilling a new formation. In this situation, there is no WOB and the weight is transferred to the reamer, which can damage the cutters.
Additionally, with no WOB, lateral and whirl vibrations can damage the lower BHA and quickly compromise wellbore stability. Matching the bit and reamer for aggressiveness can alleviate or eliminate these risks.
Case Studies
Extremely long Norway run. Using the GaugePro Echo on-command digital reamer in the Oseberg South field in the North Sea enabled an operator to drill an extremely long section of 8,195 ft (2498 m) by 2.7 days faster than scheduled. The operational challenges included high calculated ECD in the 12¼-in. hole section, low rate of penetration (ROP) because of hard formations, and an unstable formation near the end of the section.
The reamer was placed directly at the top of the BHA, reducing the reamer-to-bit distance by approximately 164 ft (50 m). After drilling the float equipment and cement, the reamer was activated by a downlink at 8,343 ft (2543 m) in measured depth (MD). The reamer opened the hole from 12¼ in. to 13½ in. over a distance of 8,031 ft (2448 m) in MD. Throughout the underreaming operation, tool performance, including the real-time confirmation of the opening diameter, was continuously monitored using mud-pulse telemetry.
The section was drilled with a controlled ROP of 66 ft/hr (20 m/h) until approximately 9,842 ft (3000 m) in MD, when drilling reached the Shetland formation, which is interbedded with hard limestone stringer intervals. When drilling this interval, the ROP slowed to 33 ft/hr (10 m/h) and occasionally down to 7 ft/hr (2 m/h), and WOB was adjusted to control torque increases and spikes.
Once free from stringers, WOB could be optimized again and ROP returned to the controlled 66 ft/hr. The horizontal interval and the drop to 55° inclination were drilled without any drilling dysfunctions. After the long section was completed, the tool was used to back ream from a total depth (TD) to 15,912 ft (4850 m). Low vibrations and stick-slip were observed throughout the run, and the reamer was graded with no wear when coming out of the run.
Gulf of Mexico rig-time savings. In a challenging deepwater well in the US Gulf of Mexico, positioning a GaugePro Echo digital reamer near the bit enabled an enlargement of the wellbore through several types of hard, abrasive formations without a dedicated rathole reaming trip, saving the operator 36 hours of rig time at a rate of USD 1.4 million per day.
At 21,290 ft (6489 m), the cement plug was drilled to 22,073 ft (6728 m), and drilling continued to a TD of 25,760 ft (7852 m). The BHA was then backed out to 25,584 ft (7798 m), where the digital reamer was engaged within 5 minutes. When the blade status was confirmed on the surface by digital communication from the reamer, it began enlarging the rathole from 12¼ in. to 14½ in.
Using the gamma ray log, the drilling engineer identified limestone stringers and shale sections and recommended rev/min and WOB levels for the best reaming performance. The ROP ranged from 50 ft/hr (15.2 m/h) to 70 ft/hr (21.3 m/h). The reamer finished opening the rathole at 25,739 ft (7845 m), reducing it from 176 ft (53.7 m) to 21 ft (6.4 m), and was pulled out of the hole in excellent condition.
The subsequent casing run and cement operation were successful, with higher-than-expected formation integrity test results.