Seismic characterization of Niobrara fluid and rock properties: a 4D study and multicomponent (3C) analysis

Abstract

Unconventional plays in tight porosity, low permeability formations have found large success by stimulating hydraulic fractures to increase hydrocarbon production. Strategies to improve and optimize recovery can be aided by the use of multi-component, time-lapse seismic data for well placement, geologic understanding, monitoring production and identifying zones which have been under-stimulated, or by-passed entirely. Within the Wattenberg Field in the Denver-Julesburg (DJ) Basin, in a collaborative research project between the Reservoir Characterization Project (RCP) and field sponsor Anadarko Petroleum Corporation (APC), three time-lapse (4D) multicomponent (9C) seismic surveys were acquired within the Wishbone Section to analyze performance of the horizontal wells (seven horizontal wells were drilled in the Niobrara and four in the Codell): Baseline seismic survey, acquired just after the wells were drilled; Monitor 1, acquired after stimulating induced fractures; Monitor 2, acquired after two years of production. In order to determine the value added by incorporating multicomponent data in the inversion process, two inversions were performed and examined: rst, a single vertical component pre-stack PP inversion and second, a joint pre-stack PP-PS inversion, using three- component (3C) converted shear reflection (PS) seismic. Inverted volumes of P-impedance (ZP), S-impedance (ZS) and density () derived from joint pre-stack PP-PS simultaneous inversion were compared to those of pre-stack PP simultaneous inversion. The PS seismic incorporated into the joint inversion improved correlation of ZP, ZS and of inverted volumes with well log derived values by 8%, 19% and 45%, respectively. Improved correlation of fluid and rock properties of incompressibility (lambda-rho; ) and rigidity (mu-rho; ), components of analysis, resulted from the increased accuracy of inverted volumes (ZP, ZS and ). Incompressibility served as a fluid indicator which differentiated between oil and gas, while rigidity served as lithology indicator and differentiated between chalks and marls in the Niobrara. By incorporating PS seismic, the joint inversion increased correlation of and volumes with well log values by 22% and 24%, respectively. Including PS seismic data in the joint inversion created a more precise image of the subsurface which could allow for better well placement by more precisely landing wells within the targeted lithology. While 4D cross equalization of PP seismic showed excellent repeatability, poor repeatability of PS seismic showed an inability to use 4D PS seismic data. This led to 4D analysis using only cross equalized PP seismic surveys, though inclusion of PS seismic would have undoubtedly shown improved results. Visualized in cross section, and showed presence of gas ( < 0) and reservoir compaction ( > 0) in zones immediately surrounding producing wells. Crossplots of and cumulative gas production showed correlation of 0.84 with Niobrara wells and 0.71 with Codell wells, indicating strong correlation of with gas production. 4D analysis aided in identication of gas producing zones ( < 0), absence of gas and zones of understimulation ( 0), and areas which had been effectively drained and compacted ( > 0). Providing knowledge of productive zones and the ability to diferentiate between areas of effective stimulation and understimulation, time-lapse analysis could establish potential locations for inll drilling or re-stimulation.