Deepwater reservoirs hold a wealth of resources for a hydrocarbon-thirsty world, but they are complex, risky, and expensive to produce. Specialized drilling operations that are necessary to enhance production in this growing sector also can impact the economics and efficiency of the well construction process.

In deepwater Gulf of Mexico (GoM) wells operators frequently use hole enlargement while drilling (HEWD) to use larger casing sizes and increase the diameter of the production string, minimize hole-opening trips, enable better equivalent circulating density (ECD) management, and improve borehole quality. These are typically achieved through the use of a concentric expandable underreamer in a conventional bottom-hole assembly (BHA) design. The underreamer is positioned above the long, complex LWD string so that the enlarged borehole will not degrade the accuracy of formation evaluation measurements and to account for stabilization of the MWD/LWD tools, real-time communication of the MWD/LWD tools, and the ball-drop requirements of reamers. The rathole is a result of the underreamer being placed high in the BHA and is the unenlarged portion of the hole from the reamer to the bit. To open the long rathole to the larger borehole size to accommodate the liner string, the drilling BHA must be tripped back to surface to allow for a dedicated cleanout – or hole-opening – run, which adds time and cost to the entire operation. Ratholes can create various issues for operators such as causing difficulties in selecting a casing point, having negative effects on liner hanger setting depth, increasing ECD concerns while cementing if the casing is set in the rathole, and causing flat time between redrilling the interval between the shoe depth and the total depth.

A new integrated dual-reamer drilling system for HEWD operations enlarges the rathole at total depth (TD) and carries out the cleanout during the drilling run, eliminating the need for a separate rathole cleanout run. The system, which has been field-tested in deepwater fields globally, has been shown to deliver considerable reductions in well construction costs and rig time.

The new technology also gives operators greater control over the underreamer while downhole than in conventional HEWD operations, where the concentric expandable underreamer consists of a single cutter block and a simple expansion mechanism. In the conventional scenario a hydraulically activated piston forces the cutter blocks to traverse on a spline mechanism, resulting in the radial expansion of the blocks. The activation mechanism is typically deployed by dropping a ball down the drillstring. The ball shears an activation sleeve, allowing the remainder of the mechanism to deploy. A locking mechanism limits the cutter blocks to a predetermined radial position.

The conventional BHA also includes a rotary steerable system (RSS) and MWD/LWD tools, which are critical in deepwater operations.

On-demand reaming

To address the limitations of this conventional under-reamer design and meet operators’ ongoing need for greater efficiency, Schlumberger developed the Rhino RHE dual-reamer rathole elimination system for deep-water HEWD operations. The tandem design features a near-bit Rhino XC on-demand hydraulically actuated reamer working in concert with a conventional hydraulically actuated reamer for HEWD and an RSS paired with a customized polycrystalline diamond cutter bit.

Positioned between the RSS and MWD/LWD components, the ream-on-demand (RoD) tool features a sophisticated hydraulic system that can perform multiple activations or deactivations of the cutting blocks. The technology eliminates the need to drop an activation ball into the drillstring, which in turn allows a higher degree of flexibility in placing the underreamer in the BHA. The activation/deactivation mechanism includes a valve piston that selectively allows or prevents fluid flow in the underreamer drive position.

The position of the valve piston is controlled by a cam mechanism that can be manipulated with mud pump flow rates. By using specific indexing flow rates in a predetermined sequence, the modes can be switched between closed and open. Switching is confirmed by observing several surface and downhole parameters such as standpipe pressure, surface torque, hook load, and downhole data channels indicating flow-rate changes through the MWD/LWD string.

The on-demand reamer, which remains in passive mode during the primary drilling interval, has standard cutter blocks or optional cement cleanout cutter blocks with a minimum number of cutters on the gauge surface. Upon reaching TD, the BHA is tripped back to the depth where the on-demand reamer is placed above the pilot hole that was previously enlarged by the HEWD reamer. The on-demand passive reamer blocks are then activated to enlarge the rathole while the drilling process starts.

The RoD technology also allows two underreamers to be placed in the same drillstring and activated as needed during the drilling operation. This feature stabilizes the MWD/LWD tools and ensures real-time connections between those tools and accuracy of formation evaluation measurements while the rathole is eliminated. Another advantage is that the functionality of the RoD tool can be verified at surface.

Avoiding a cleanout run

The Rhino RHE system was successfully deployed in a 12¼-in. by 14½-in. exploration well in the GoM’s Mississippi Canyon in approximately 2,134 m (7,000 ft) of water. In this case the operator wanted to significantly reduce the length of the rathole during HEWD while using MWD/LWD services. The plan was to place a conventional ball-activated underreamer above the MWD/LWD tools in the BHA and place the RoD tool between the MWD/LWD components and the RSS. This would enable the operator to perform HEWD for the entire interval using the conventional reamer. The operator also could enlarge the remaining rathole by activating the RoD tool at the top of the rathole and reaming until the bit was back to bottom, eliminating the need for a separate cleanout run.

Before commencing the operation, an engineered drilling system was deployed that uses predictive modeling to minimize risk and determine optimal reamer placement, ensuring it did not interfere with RSS directional capabilities, establishing surface operating parameters, and customizing individual components. For example, the RoD was modified to act as a stabilizer in the deactivated state and as a reamer in the activated state, replacing the stabilizer typically included above the RSS tool in a conventional RSS BHA. Hydraulics analysis also was performed to verify that all of the tools in the BHA could operate within the planned flow rates and mud weights during the drilling phase.

The BHA was assembled normally at the rig site. After drilling out the casing shoe, the conventional underreamer in the BHA was deployed below the 135/8-in. casing shoe using an activation ball released from surface. The ball-activated underreamer and drillbit began enlarging the hole while drilling until TD was reached. The interval was drilled at an average ROP of 25.8 m/hr (84.5 ft/hr), with 372 m (1,221 ft) of 12¼-in. pilot drilled and simultaneously enlarged to 14½ in.

During this phase, the hydraulic RoD underreamer remained in the deactivated mode and was monitored from the rig floor until reaching TD. At TD the BHA was pulled back, and the RoD underreamer’s cutting mechanism, positioned at the top of the rathole, was activated and confirmed open by a 225-psi decrease in standpipe pressure and an increase in weight-on-bit torque. The system was able to enlarge 46.6 m (153 ft) of 12¼-in. rathole to 14½ in. in about three hours.

The success of the first running of this specific BHA configuration confirmed the reliability of the integrated dual-reamer drilling system in deepwater drilling operations. By eliminating the need for a dedicated rathole cleanout run, the procedure saved the operator an estimated 16 hours in nonproductive time and reduced operational costs by approximately US $625,000.