In 1991, Union Pacific Resources Company (UPRC) stimulated four horizontal wells in the Niobrara Formation in the Silo field, Laramie County, Wyoming. Stimulation treatments were required due to the rapid production declines seen early in the life of these wells, which were completed in 1990 and 1991. Horizontal and vertical wells in the Austin Chalk formation have been stimulated by injecting large volumes of water (10,000–30,000 bbls) at high rates (50-220 bbl/min) to improve productivity. This stimulation technique was applied in the Silo field because the Austin Chalk and Niobrara reservoirs have similar features; the most important similarity is that productivity requires connection to an adequate natural fracture system. These Niobrara stimulation treatments consisted of 30,000 bbls of water pumped at rates up to 150 bbl/min with wax beads as diverting material.
This paper describes the job designs, bottom-hole treating pressure (BHTP) trends observed during the job, and post-treatment well production. Early BHTP responses and post-treatment production are related to the extent of natural fracturing exposed within the horizontal (lateral) sections. The lack of sustained productivity after the treatments indicates that Niobrara wells respond differently than Austin Chalk wells after large-volume, high-rate treatments. The most likely explanation for this difference is that fractures opened during Niobrara large-volume, high-rate treatments either close during production or do not provide sustainable connectivity to high-conductivity fractures. Future treatment strategies are discussed concerning fluid selection and the use of diverting materials and proppants.