One of the many issues that set unconventional plays apart from their conventional cousins is their proximity to populated areas. This compounds the entire upstream cycle.
The Wattenberg Field in the Denver-Julesburg Basin in Colorado is directly overlain by the town of Greeley, home to the University of Northern Colorado as well as some 95,000 residents. A paper titled “The Greeley 3-D Seismic Survey: One of the Nation’s Largest Urban Surveys Leads to Niobrara and Codell Horizontal Activity” by Jack Wiener of Halliburton, Tagir Galikeev of Unified Geosystems LLC and Collin Richardson of MRI was recently presented at the Unconventional Resources Technology Conference in Denver. The authors describe the Wattenberg Field as one of the largest overpressured basin-centered oil and gas fields in the Rockies in terms of total proved reserves, areal extent and number of wells. While horizontal drilling has been active in the outskirts of Greeley, very little activity has taken place within the city limits. A private company decided to undertake one of the largest urban 3-D programs in the country, 59 sq km (23 sq miles).
The survey required several months to plan due to the densely populated nature of the area. A wireless recording system was chosen both for aesthetic purposes and to avoid safety issues posed by cables. Underground utility locations were surveyed ahead of time to avoid damage during vibroseis acquisition, and traffic control during operational hours was put in place.
During acquisition, particle wave ground motion recorders were used at every vibration point to monitor vibrator sweeps and their effects on nearby structures. Operation hours were restricted to the 12 hours from 7 a.m. to 7 p.m. Residents were alerted well in advance of the survey and were treated to an open house to answer their questions. “Public sensitivity and total transparency were key to successful and smooth operations during the survey,” the authors noted.
The layout was irregular because of easement limitations, and sweep frequencies were nonlinear to help avoid ground roll damage. These types of urban limitations can result in a low signal-to-noise (S/N) ratio, but the authors noted that the structural geology is characterized by subhorizontal layering, which lends itself to the common depth point method for increasing S/N.
Processing methods also helped maximize the quality and frequency content of the final image.
Interpretive products included accurate time-to-depth conversion of the horizons and volumes, detailed structural analysis and framework modeling, and neural network inversion for rock properties.
So was it worth the hassle? According to the authors, the final products are providing accurate information for well planning as far as landing the laterals and drilling and completing the horizontal portions of the wells. “Results from the drilling program have been excellent in terms of accurate prediction of the subsurface geology and exceeding production expectations by 25% of published Niobrara EURs,” the authors wrote.
It’s nice to see the industry combining its ingenuity with its common sense to carry out a survey of this magnitude. This kind of grassroots PR may be just what we need.
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