Summary A nine–well subsea development project has been completed using casedhole frac packs (CHFPs) for sand control and multizone intelligent–well systems (IWSs) to improve recovery from a series of shallow, low–pressure gas reservoirs. In these wells, CHFPs have been installed to provide reliable sand control over the long, low–net–to–gross–ratio sand/shale target sequence: typically, three to six frac packs per well. This outer CHFP completion is then augmented with a multizone IWS, consisting of isolation seals, surface–controlled zonal–isolation valves, and downhole–pressure/temperature (DHP/T) gauges. The IWS string is run as a separate inner string to provide flow–monitoring capability and allow shutoff of zones producing high water volumes. This critical water–shutoff capability eliminates the risk of one or more high–water–production zones loading up and killing adjacent low–pressure gas zones, with the associated loss of reserves. To date, a total of nine wells have been completed and are being produced from three subsea gas fields. To maximize recovery from the fields’ numerous but relatively thin gas reservoirs, production wells are completed over three to six separate intervals. These frac–packed intervals are then grouped to allow flow control and pressure/temperature monitoring to occur through up to six surface–operated interval control valves (ICVs) and associated downhole gauges. This combination of sand control and intelligent–well control has provided an ability to perform multirate tests (MRTs) and pressure–buildup (PBU) tests on each reservoir interval to detect the start of water production or identify other impending production issues. After approximately 6 years of production service to the October 2018 date of this paper, 16 of the 34 zones completed in the nine–well project have been shut in to eliminate high water production. These water–shutoff actions performed using the surface–controlled ICVs are estimated to have improved gas–recovery factors from 50 to 60% without requiring rig intervention. This paper describes the reservoir challenges addressed and the completion–design and –operating practices used in this successful program.
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).
A nine well subsea development project has been completed using cased hole frac-packs for sand control and an innovative multi-zone intelligent well system to improve recovery from a series of shallow, low pressure, gas reservoirs. In these wells, cased hole frac-packs have been installed to provide reliable sand control over the long, low net-to-gross, sand/shale target sequence: typically, three to six frac-packs per well. This outer cased hole frac-pack completion is then augmented with a multi-zone intelligent well system, consisting of isolation seals, surface controlled zonal isolation valves and downhole pressure/temperature gauges. The intelligent well system is run as a separate inner string to provide flow monitoring capability and allow shut-off of zones producing high water volumes. This critical water shut-off capability eliminates the risk of one or more high water production zones loading-up and killing adjacent low-pressure gas zones with associated loss of reserves. To date, a total of nine wells have been completed and are being successfully produced from three subsea gas fields. To maximize recovery from the fields' numerous but relatively thin gas reservoirs, production wells are completed over three to six separate intervals. These frac-packed intervals are then grouped to allow flow control and pressure/temperature monitoring to occur through up to six surface operated Interval Control Valves and associated downhole gauges. This combination of sand control and intelligent well control has provided an ability to perform multi-rate flow tests and pressure build-up tests on each reservoir interval to detect the start of water production or identify other impending production issues. After roughly five years of production service to June 2017, four of the 34 zones completed in the nine well project have had to be shut-in to eliminate high water production. These water shut-off actions performed via the surface controlled interval control valves are estimated to have improved gas recovery factors from 50% to 60% without requiring rig intervention. This paper will describe the reservoir challenges, completion design and operating practices employed in this successful program.
TX 75083-3836 U.S.A., fax 1.972.952.9435. AbstractThe West Sak viscous oilfield on the North Slope of Alaska is currently being developed with extended reach multilateral wells, with departure to depth ratios up to 5 to 1, in which horizontal slotted liners are utilized in conjunction with a TAML level 3 multilateral junction system. Centralizers are considered necessary on the slotted liners to avoid slot plugging, reduce drag, and limit differential sticking. Selection of proper centralizers to run through a casing exit, without a whipstock in place, has been key to ensuring a successful multilateral installation.Several failures of centralizers run on liners through casing exits have resulted in significant drilling lost time associated with fishing and milling pieces of centralizers in order to place the wells into proper service. After three such failures, the requirement to study the passage of a centralized liner through a casing exit became essential. A surface test fixture was utilized to simulate liners run through a casing exit to test several potential centralizer candidates using the loads estimated from modeling.Torque and drag modeling provided the side force estimates exerted on the liner and centralizer as they passed through a casing exit. This paper will the discuss the liner centralizer installation problems prior to the testing program, detail the modeling used to determine the loads exerted on the centralizer at the casing exit, show the results of the yard tests conducted on several commonly utilized industry centralizers, and make recommendations for proper liner centralization in multilaterals.
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