Challenges of geophysical processes and interpretation of an unconventional play: a case study from Wattenberg field, Denver-Julesburg Basin, Colorado, The
Carmichael, James Brady
Carmichael, James Brady
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2024
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Abstract
Unconventional production formations are frequently complex to characterize, due to what could be interpreted as sudden lateral stratigraphic changes, potential stress variations, and the inability to use Gassmann's Fluid Substitution theory directly. While general trends of limited permeability, over-pressurized tight-porosity, and highly-fractured mediums tend to hold, those fractures are likely cemented, stress regimes within unconventional targets can change over distances, and long-term production from offset legacy wells may alter localized pressure. Basins comprised of unconventional, hydrocarbon-bearing zones have thus challenged the exploration industry, to push technological advances, and led to systematic improvements to assess and diagnose the success of a developing field.
The Postle project provided various datasets to investigate, including seismic, borehole logs, distributed fiber optic sensing data, microseismic, and production. Relevant research findings were compared and if applicable were linked to other project data from larger regional analyses to enhance the characterization of the Niobrara-Codell petroleum system. The targeted units of this system are still under great scrutiny to determine the best method(s) to acquire meaningful seismic data and maximize production proficiency while reducing completion costs, lowering lateral interaction and interference, and minimizing the environmental footprint.
This study attempted to find correlations between seismic attribute interpretations, pre-stack inversion products of pre-stimulated seismic data, and passively observed microseismic activity trends from hydraulic treatments. Identification of these correlations would improve the petroleum system characterization of the western flank of the basin’s syncline. However, the unforeseen presence of amplitude artifacts from acquisition footprint coverage compounded with a lack of any robust microseismic data cloud trends hindered the larger integration of geophysical seismic interpretation with log analysis and production data for the multidisciplinary assessment of an area’s reservoir. The observed amplitude versus angle (AVA) trends from 1) an overlying reflection event (Lower Pierre), 2) a reflection event within the target zone (Niobrara A), and 3) from an underlying reflection event (Muddy J) all generally increased in amplitude with increasing angle. These AVA trends are contrary to the expected amplitude decrease with angle or offset, evidenced from both Zoeppritz and Elastic forward modeling based on rock properties obtained from a well located in the northwestern part of the seismic survey and supported by recent previous work within the basin.
Post-stack inversion was used to assess relative differences across the survey, due to the inability to use pre-stack inverted products and the lack of microseismic trends. A stratal flattening technique on an angle-gather stack also offered an alternative method to investigate the space between discrete reflection events. Combining horizontal slices from the stratal flattened volume, the interpreted structures based on seismic attributes, with year-one hydrocarbon production values, provided a visual connection between the influence of structural trends and production. Upon reviewing water-to-oil ratios (WOR), a link was identified between well production and the linear trends intersected along the trajectory. A direct relationship between hydrocarbon production to variations in geology was not possible, due to the number of completion design variations.
Production decline curves indicated wellbore interaction between child and parent wells. This shows that despite the expected in-situ low connectivity, a long duration after parent lateral hydraulic treatments and production, child treatments likely re-stimulated depleted areas and created communication pathways back to parent laterals.
The processing and analysis of 1) post-stack structural interpretation, 2) the post-stack inversion product, 3) both Zoeppritz and Elastic AVO modeling, 4) pre-stack inversion of an Elastic AVO synthetic gather, 5) stratal flattening, and 6) integration of various data sets, supports the value of full-scale project planning if there is an interest in answering other geological and geophysical questions, such as A) determining the degree of lateral lithological consistency and B) identification of anomalies.
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