Structural and Stratigraphic Compartments in a Horizontal Well Drilled in the Eolian Tensleep Sandstone, Byron Field, Wyoming
N. F. Hurley, A. A. Aviantara, D. R. Kerr, 2003. "Structural and Stratigraphic Compartments in a Horizontal Well Drilled in the Eolian Tensleep Sandstone, Byron Field, Wyoming", Horizontal Wells: Focus on the Reservoir, Timothy R. Carr, Erik P. Mason, Charles T. Feazel
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The Tensleep Sandstone is a major oil and gas producer in the Bighorn Basin, Wyoming. Cores, borehole images, and outcrop descriptions have been used to characterize the formation. Lithofacies include: (1) porous and permeable eolian cross-stratified units; (2) lower permeability, finely laminated interdune units; and (3) relatively impermeable marine carbonate and clastic units. Individual eolian dunes range in thickness from 2 to 10 m (6 to 30 ft), with first-, second-, and third-order bounding surfaces identified in outcrops, cores, and logs.
Byron field is a northwest-southeast-trending asymmetric anticline. In 1992, a medium-radius lateral hole was drilled from the southwest flank toward the crest of the fold. The horizontal well was cased from the surface to the top of the Tensleep Sandstone. The re-mainder of the well, which was drilled for approximately 150 m (500 ft) on an uphill slant, stayed in the uppermost Tensleep Sandstone in a 6-m- (20-ft-) thick stratigraphic interval. This part of the borehole was left uncased. The general shape of the borehole is that of a fishhook, where the structural high is at total depth.
Borehole-image log interpretation in the horizontal well showed two sets of roughly orthogonal fractures. Average spacing, corrected for borehole geometry, is 2.3 m (7.5 ft) for Set 1 fractures, which lie approximately perpendicular to the borehole path. Set 2 fractures, which lie roughly parallel to the borehole path, have a spacing corrected for borehole geometry of 0.9 m (3.0 ft).
Using prior knowledge of the paleowind direction, the horizontal well was drilled approximately perpendicular to the trend of the eolian dunes. Interpretations of bed boundaries suggest that the borehole crossed at least five eolian-facies architectural elements in 96 m (315 ft) of lateral distance. This suggests an average spacing of19 m (63 ft) for dune-related compartments. Outcrop and core studies show that flow barriers or baffles commonly exist at bounding surfaces between such compartments.
The last 46 m (150 ft) of the borehole was full of oil. The oil-water contact in the borehole, which was stable during several days of logging, may represent the height of the oil-water contact in the fractures. Note that this level is hundreds of meters (or feet) above the original oil-water contact in the intergranular porosity. This well suggests a novel but untested way to complete a horizontal well: (1) run tubing to total depth, and (2) use the borehole as a downhole oil-water separator. This completion technique could reduce water cuts and prolong the life of Byron field and other fractured reservoirs with strong water drives or active waterfloods.
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Horizontal Wells: Focus on the Reservoir
This book provides an overview of the new technical approaches required for best use of horizontal and extended-reach technology in different reservoir situations. The volume is a selection from more than 50 papers presented at an AAPG/SPWLA Hedberg Research Symposium, “International Horizontal and Extended Reach Well Symposium: Focus on the Reservoir,” held in The Woodlands, Texas, on October 10 14, 1999. The 16 chapters describe horizontal and extended-reach wells and drilling programs in a variety of geologic settings all over the world.