Well Design Evolution in a Semi-Conventional Reservoir: Example from the Nuiqsut Formation of North Slope, Alaska

Abstract

The Jurassic lower shoreface Nuiqsut formation is Pioneer’s primary target at the Oooguruk field on Alaska’s North Slope. The 9,000 acre field is developed as a line drive waterflood with ~6,000’ lateral wells and produces 19o-26o API oil. Well productivity is quite variable, with a 30-fold range in daily rate after 2 months of production. The reservoir has low horizontal permeability (1-10 mD) with extremely low vertical permeability at both the core plug scale due to clay laminae and at the wellbore scale due to horizontal baffles of shale and siderite. Several wellbore trajectory and completion strategies have been employed to develop the reservoir. Development commenced with undulating laterals with minimal fracture stimulation and culminated in flat wellbores with multi-stage, mechanically diverted fracture treatments like those commonly used in unconventional plays. The broad range in well performance is driven by reservoir quality, oil viscosity, and completion style. Of these drivers, completion style appears to be the most significant. Results from Oooguruk highlight the promise of employing unconventionalstyle completion techniques on low-quality conventional reservoirs. Introduction The Oooguruk field is located on Alaska’s North Slope ~5 miles offshore on a 6-acre manmade island. Figure 1 is a time-normalized production plot for all wells producing from the Upper Jurassic Nuiqsut Formation. Daily oil rate from this population of 14 wells exhibits a broad range, with a 30-fold difference observed at 60 days. Explaining this heterogeneity requires an integrated approach that considers variations in reservoir quality, oil quality, and completion efficiency. Placing these well results in proper context is critical to optimizing strategies for future development. Figure 2 shows the location of Alaska’s North Slope and indicates that the majority of fields sit along the Barrow Arch, a migration focus for kitchens to the north and south each of which contains up to 3 thermally mature, organic-rich source rocks. Historically, the majority of production has been from the high-quality sandstones of the Cretaceous Kuparuk (shoreface) and Triassic Ivishak (fluvio-deltaic) Formations. More recently, production from sands at the Alpine field (late 1990’s onward) has confirmed the presence of an Upper Jurassic shoreface and incised-valley fill play fairway, of which the Oooguruk field is a part (Figure 3a). The Nuiqsut Formation is a lower shoreface sandstone which represents one cycle within a prograding highstand system, with occasional incursions of offshore deposition that serve to stratigraphically separate the sand body into 3 flow units. Figure 3b is a depositional dip section through the field, showing its location immediately proximal to the transition from sand to offshore muds. The opportunity to exploit more proximal facies is limited by removal of the reservoir by the regional Lower Cretaceous Unconformity. This depositional setting in the lower shoreface, with occasional incursion of deeper water settings, has led to the creation of multiple impediments to vertical fluid flow within the reservoir. Figure 4 shows a core description from the Oooguruk disposal well and identifies the various depositional features that affect the vertical permeability of the reservoir at nearly every scale. Millimeter-scale clay laminae are a typical feature of lower shoreface sands and are present in the sands of zones 1, 3, and 5 (the flow units) throughout the reservoir. Siderite is an iron carbonate present throughout the Nuiqsut, but is most abundant in shalier URTeC 2013 Page 1341