This paper describes the application of particle size distribution principles for determining materials to be added to the mud system during casing while drilling operations. Casing while drilling (CwD) has been demonstrated to stop or significantly reduce lost circulation and improve wellbore strength. The mechanism by which this improvement occurs is not understood, however the results from this work significantly advance what is needed to get repeatable results. If wellbore strengthening can be systematically achieved, then wells can be drilled in known loss areas without contingency strings of casing. In addition, wells drilled in mature fields, where producing horizons have altered pressures either from depletion or pressure maintenance, can be drilled with fewer casing strings. Sidetracks become economical because hole size can be preserved for an effective completion and well costs are lowered by not using additional liners to reach the objective. By adding particles to the mud to fill in the particle size distribution, losses to natural fractures were stopped while directional wells were drilled with casing in the Piceance Basin of Colorado. Applying what was learned in a field trial of casing while drilling in the Alaskan Tarn Field, the open hole leak off resistance was improved by 3.0 pound per gallon (ppg) drilling with 7.0 inch casing in a 67 degree angled well. With this success, a four well casing while drilling campaign was executed with two wells drilled each in the Kuparuk and Tarn fields on Alaska's North Slope. Results were positive for 7.0 inch and 7.625 inch casings but wellbore strengthening did not occur sufficiently in the 5.50 inch casing trials. Annular clearance appears to be a critical component to success and is not yet fully understood. The results demonstrate that a significant improvement in fracture gradient can be achieved with the right clearance between the hole and the casing and the proper sized particles added to the mud system. In addition, the amount of material added has been demonstrated to be as low as two pounds per barrel. With confidence that strengthening can be achieved to the levels of improvement demonstrated, wells can be evaluated with significant cost savings by eliminating casing strings and preserving hole size for completions or further drilling.

The complex wells currently being constructed to develop the West Sak Field on the North Slope of Alaska (Figure 1) are approaching the limits of extended reach drilling technology (Figure 2). Due to the shallow vertical depth and long horizontal departure of these wells, the ability to effectively transfer weight while drilling and running tubulars can be very challenging. This paper discusses the reasons those challenges exist, and the tools, techniques and learnings employed over the past seven years to successfully solve weight transfer challenges in this remote, environmentally sensitive area, including steerable motor assemblies, rotary steerable drilling assemblies, collaborative planning, on site drilling engineers, torque and drag reduction devices, fluid additives, vibrating down hole tools, and liner running with multiple trips. These learnings enabled the recent drilling and completion of a West Sak well with a total measured depth of 19,750 feet at a vertical depth of 3,055 feet. This well, with a horizontal departure of 18,472 feet and two laterals with lengths in excess of 7,500 feet, has a departure to depth ratio in excess of six. Background The West Sak field is within the Kuparuk River Unit on the North Slope of Alaska (Figure 1). The West Sak heavy oil sands contain highly viscous oil due to the low gravity of the crude (10 to 22 degrees API) and the low reservoir temperatures (caused by both the extreme northern latitude and the shallow (3,000 to 4,000 vertical feet) burial depth below 1,800 feet of permafrost in the overburden.) The West Sak reservoir has three primary sandstone targets: the " A2??, the " B?? and the " D?? intervals. The West Sak sands are " very-fine?? to " fine?? grained, single and amalgamated sandstone/siltstone beds. Geologic challenges while drilling include numerous fault crossings along the wellbore (Figure 3) as well as random encounters with calcite-cemented spheroids, also known as concretions (Figure 4). The concretions are much harder than the reservoir sand, having compressive strength of 25,000 pounds per square inch (psi) versus 500 psi for the reservoir sand. These concretions drill much more slowly than the adjacent sand which accelerates wear on the bit, the bottom hole assembly, and the drill string. This difference in hardness can also cause the drill bit to deflect off of the concretions resulting in unwanted severe doglegs. Such doglegs make it difficult to maintain directional control, increase torque and drag, and can cause down hole tool damage.

A multitude of extended reach technologies have been utilized in the West Sak field on the North Slope of Alaska to reduce surface impact in a remote and environmentally sensitive area. Significant to the shallow heavy oil West Sak development is the use of multilateral horizontal wells with a junction providing mechanical support and both through-tubing lateral isolation and re-entry capabilities. However, as extended reach drilling capabilities evolved to routinely reach departure to true vertical depth ratios in excess of five to one, multilateral junction technology did not evolve at the same pace. A new multilateral junction was designed to match current extended reach drilling capabilities and replace existing multilateral equipment which was utilized beyond its intended limit incurring both installation and production risk. The newly designed junction allows lateral liners to overcome drag limitations by rotating the liner and junction to setting depth in one trip and includes several positive indicators to ensure a successful installation. This paper discusses the evolution of multilateral wells in the West Sak development, the limitations of multilateral junctions when utilized in extended reach wells, the development and testing of a new multilateral junction, and several successful field installations. Operation highlights during the completion phase of a multilateral well with a lateral departure to true vertical depth ratio in excess of six to one are included. Existing tools such as oil based mud lubricants and thorough torque and drag prediction were combined with the new junction for a successful completion which progressed the application for multilateral junctions in extended reach wells Multilateral History in the West Sak Development The West Sak field is a heavy oil accumulation within the Kuparuk River Unit on the North Slope of Alaska (Figure 1). It is a Cretaceous, shallow marine sandstone at vertical depths from 2,400 feet on the western edge of the Unit to 3,800 feet on the eastern edge. The field contains 7–9 billion barrels of oil in place with an oil gravity that ranges from 10–22 degrees API. Initial oil production began in the late 1990's from 29 conventionally deviated wells (18 producers and 11 injectors) on a 40-acre water flood pattern with typical production rates of 150–250 barrels oil per day (BOPD). These rates did not support the high cost of the wells and alternative well designs were considered (Targac, et al, 2005). Consequently, operators on the North Slope began experimenting with horizontal and multilateral horizontal production wells which had become an attractive alternative to vertical wells in the multi-layered West Sak reservoir. With multilateral technology, two or more of the West Sak pay sands could be accessed from a single well. In the year 2000, three dual lateral horizontal wells were drilled and completed in the Kuparuk River Unit targeting the upper two West Sak sand intervals. The first of the these wells had an upper lateral length of 3,024 with an ERD ratio of 1.37 while the most difficult of these three wells had an upper lateral length of 3,580 feet with an ERD ratio of 2.04. Note: ERD ratio in this paper is calculated as the (unwrapped surface departure) ÷ (true vertical depth from RKB). The wells were completed with sand exclusion screens in the laterals, TAML (Technical Advancement of Multi-Laterals) Level 4 junctions with both the main bore and lateral cased and cemented at the junction, and artificially lifted with electric submersible pumps (ESP). Upon the economic success of the three first multilateral horizontal West Sak wells in 2000, it became readily apparent that the development would be even more profitable by optimizing well construction. Drilling and well completion costs were by far the largest portion of the capital cost and any reduction in these costs would decrease the cost per barrel produced. Additionally, increased reservoir exposure realized by drilling and completing longer laterals could increase production per well and would further decrease the cost per barrel produced. The West Sak drillsite would evolve to reach subsurface targets exceeding a 15,000 feet radius at depths approaching 3,000 feet TVD (Figure 2).