As the industry continues to push the envelope of existing technologies, a number of technological synergies are beginning to emerge. The integration of extended-reach drilling with single-diameter, solid expandables is one of the most promising of these synergies.
The coupling of these two leading-edge technologies has received considerable attention for its potential to substantially enhance field development optimization. Using these two technologies as companion pieces in a drilling design can positively impact development scenarios and reduce capital costs, resulting in economically superior development programs.

Single-diameter technology
Single-diameter solid expandable technology, a natural derivative of the conventional solid expandable tubular system, provides the foundation for a significant increase in the lateral reach of many extended-reach well bores. This technology fundamentally installs similar-sized liners sequentially in a well without a decrease in the inside diameter (ID). This process can be repeated to maintain consistent wellbore ID to total depth (TD), virtually eliminating the telescoping effect that occurs when running conventional casing.
This extended lateral reach results in the increased ability to drill into previously deemed "out-of-reach reservoirs" because of distance or economics, and greatly increase reservoir penetration. In turn, this increased lateral reach and reservoir contact can ultimately reduce the required field development well counts and infrastructure, resulting in improved field development economics. Using single-diameter solid expandable technology with significant friction reduction could extend the current extended reach drilling (ERD) envelope to over 50,000 ft (15,243 m) at depths of 5,000 ft to 10,000 ft (1,524 m to 3,048 m) total vertical depth.
Several technical challenges of using single-diameter solid expandable technology in a 50,000-ft ERD well must be overcome. These include:
• Mechanical (torque and drag) limits;
• Tortuosity;
• Drag risk;
• Hydraulics (pump pressure, hole cleaning);
• wellbore stability;
• Wellbore profile - catenary shape;
• Casing wear;
• Casing integrity (tubular design);
• Well control implications; and
• Operational considerations, including the need for frequent trips to install the liners.

Scenarios for engineering studies
Engineering studies by the authors have specifically investigated the use of ERD with expandables for an offshore platform development project (drilling from a fixed platform) and a subsea gas development project (drilling from a floater). These two studies were used to assess and quantify the lateral reach improvements possible with single-diameter solid expandable technology in ERD wells. Optimal well paths were designed using the constrained optimization methods proposed by Suryanarayana, et al. Mechanical (torque, drag, stress, fatigue and tortuosity) analysis was conducted using an in-house program. The analysis incorporated actual operator-provided drilling and completion data, lithology, mud parameters, torque and drag and well designs, in conjunction with existing literature on ERD performance. Where data was not available, assumptions were based on reasonable engineering judgment and experience in ERD. The historical operator data provided a baseline for ERD in the subject fields and a context for estimation of trouble times and expected time to drill extended-reach wells beyond the conventional technology field reach record of 27,000 ft (8,231 m), up to 50,000 ft in lateral length (over 55,000 ft or 16,768 m in measured depth). Literature and actual field historical data were used as the basis for the selection of base case friction factor profiles in the mechanical analysis.
Hydraulics were investigated using an industry-standard hydraulics program. Wear analysis was based on Archard's adhesive wear mode, the basis of all industry wear-analysis programs. Casing integrity was evaluated using the working stress design approach (deterministic design) and supplemented by probabilistic design where appropriate. Drag risk was evaluated using the methods proposed by Schamp, et al. All implications of single-diameter solid expandable liners were considered in the context of ERD beyond conventional technology limits, including the unique force transmission during the expandable technology operations.
The well design models assessed the achievable lateral reach and reduction in drilling trouble time with respect to the following:
• Conventional drilling technology;
• Conventional technology with improved drilling practices;
• Installation of up to two solid expandable tubular liners to preserve the desired completion size; and
• Installation of multiple, successive single-diameter solid expandable drilling liners.

Results
In the case of the fixed platform field development, initial modeling work indicated that the practical drilling limit with conventional technology was 28,000 ft (8,536 m) of lateral reach with significant drilling risk. This modeling was validated by the operator's recent lateral reach drilling record established with significant drilling trouble. To mitigate the primary drilling trouble and optimize the lateral reach, two conventional solid expandable tubular liners were engineered into the well design model. With the two liners, lateral reach can be increased to a maximum of 41,000 ft (12,500 m). To further the lateral reach to the desired 50,000 ft while preserving desired completion size, the model incorporated various single-diameter solid expandable liner lengths. The model indicated that a 50,000 ft lateral reach was achievable with nine to eleven successive single-diameter liners varying in length from 3,000 ft (914 m) in the early liners to 1,000 ft (305 m) near TD.
One of the main conclusions of the work was that achieving reach beyond 41,000 ft and preserving completion size requirements requires the use of single-diameter solid expandable liners. Tapered (65¼8 in. x 51¼2 in.) S-grade drillstring and high-torque tool joints are also required for greater reach to be mechanically feasible. Hydraulic analysis suggests that increased pump capacity beyond the currently available 5,500 psi will be required to efficiently drill and complete the longer reach wells and may require capacity beyond conventional triplex pumps. Wear analysis indicates that the maximum expected casing wear in the shallowest single-diameter liner (which would experience the greatest wear), is an acceptable 5% to 6%. Post-wear integrity analysis of the casing design and the single-diameter solid expandables for expected load conditions indicates adequate integrity. To address drag risk during set down, it was recommended that appropriate friction reduction techniques be designed, tested and deployed in the well. Monitoring the drag risk during operations is crucial.
In the case of the subsea field development, achieving 50,000 ft lateral reach is technically more challenging, largely due to the shallower (below mudline) kickoff point necessitated by the significantly deeper water depth. A combination of single-diameter solid expandable liners, rotary drilling, friction reducers, drillpipe "shuffling" and careful operational monitoring can make 50,000 ft reach achievable in these wells. However, a maximum practical reach target of 44,000 ft (13,414 m) was recommended. Several mechanical risks highlighted by the analysis prompted this recommendation. The reach at which the minimum required weight-on-bit (10,000 lbf) is available with a tortuous well path is limited to 44,000 ft even with single-diameter solid expandable liners. Further optimization of liner lengths can increase this only to 45,000 ft (13,719 m). Set down weight reduces to zero at 47,000 ft (14,329 m) reach, which implies that the drillpipe cannot be tripped to depths beyond this reach without rotation. Minimum required overpull of 100,000 lbf is no longer available beyond 41,000 ft reach. At 44,000 ft reach, the overpull available reduces to 94,500 lbf. Tortuosity further reduces the overpull margin by 20,000 lbf. This implies that careful stuck-pipe prevention planning will be required to reduce risk, even if the target reach is limited to 44,000 ft.
Hydraulic analysis indicates minimal problems with hole cleaning for circulating rates in excess of 500 gpm during drilling. Annular pressure profiles are within the fracture gradient limits of openhole sections. However, pump pressure in excess of 7,250 psi will be required at a minimum.
Casing wear analysis indicates that a maximum 5% is a reasonable expectation of wear. Casing integrity analysis indicates that limiting burst loads can be withstood. Although the factor of safety for the limiting loads is lower than 1.1, probabilistic design indicates that casing reliability is acceptable. Casing integrity analysis indicates that the single-diameter liner can withstand (within acceptable limits) collapse loads because of drillpipe wear if the well bore is not evacuated beyond 30% at any time during drilling and completion operations.
Reach can be further improved by rotation. If it is feasible to rotary-steer the drillpipe beyond 44,000 ft, it is theoretically possible to drill to TD with 50,000 ft reach. However, the risk of not being able to slide beyond 47,000 ft still remains and exceeding this limit is unadvisable. Shuffling drillpipe improves reach to some extent. However, rotation will still be required to go beyond a reach of 44,000 ft.
In summary, the integration of single-diameter solid expandables and ERD wells shows the promise and potential to greatly extend lateral reach to reduce the required number of fixed platforms and satellite subsea installation requirements, reduce well counts, drill into out-of-reach reservoirs, increase reservoir contact and improve capital efficiency for field development economics. These results in turn may serve to optimize new field development plans, revitalize sub-economic or marginal fields, and provide efficient access to unreachable or uneconomic fields with reduced infrastructure requirements.

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