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Time-lapse PP seismic to characterize stimulation and production behavior in the Niobrara and Codell reservoirs Wattenberg field, Colorado, US
Utley, Jacob W.
Utley, Jacob W.
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2017
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Abstract
The primary source of present-day, on-shore oil and natural gas production in the United States is within unconventional, tight reservoirs. Recovery factors in unconventional reservoirs are typically 5-15\%, and substantially less than in conventional reservoirs. Current hydraulic fracturing and completion efforts are directed to improving recovery, where even a 1\% increase could substantially impact the economics of the field. The Wattenberg Field is among the most successful unconventional fields in the United States, targeting the Niobrara and Codell formations. The Reservoir Characterization Project in conjunction with the field operator, Anadarko Petroleum Corporation, acquired three multi-component, time-lapse seismic surveys over a four-square mile area to study the stimulation and production effects associated with 11 horizontal wells. The baseline survey was acquired prior to drilling the wells. The first monitor was acquired during completion of these wells, and the second monitor acquired after two years of production to observe dynamic changes to the reservoir. Time-lapse, pre-stack seismic inversion of the compressional wave data was performed to analyze the changes to the reservoir at these two pivotal instances. Observing changes from monitor one (during completion) to the baseline has provided an understanding of the effectiveness of the hydraulic stimulation, and the opening of existing and new fracture networks. There is no in-situ free gas in the reservoir; however, after 90 days of production, near-wellbore reservoir pressures drop below bubble point and gas is released from solution. During the two-year interval between the acquisition of the first and second time-lapse seismic data, the 11 wells experienced an increased gas production and increasing gas-oil-ratios over time. The inversion result from the second monitor with reference to the baseline survey provides information about the influence of gas on the system, thereby indicating the effectiveness of the completion and contributing reservoir volume. Time-lapse seismic inversion has allowed for a self-consistent interpretation of the oil and gas production, directly correlating to the stimulation and production, in addition to in-situ stress and structure as it relates to a change in compressional velocity. The results of this study will assist in understanding these variables to help guide future field operations and completion strategies.
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