Horizontal development of unconventional shale reservoirs has given revival to the use of 3-D seismic data. Seismic data is a critical tool to maximize time in the pay zone, avoid drilling hazards or plan for faulting in the reservoir. However, attempts to use this powerful technology for more than just steering in these shale plays is still in its infancy. Much of the quantitative seismic work completed to date has been on characterizing static reservoir parameters such as “sweet spots” of more brittle, frackable rock.

Anadarko has undertaken a first-of-its-kind study in collaboration with the Reservoir Characterization Project (RCP) at the Colorado School of Mines (CSM) to characterize how seismic can help oil companies understand the dynamic response of unconventional shale reservoirs to hydraulic fracturing and production.

This study focuses on a 2.58-sq-km (1-sq-mile) section within the Wattenberg Field with 11 horizontal wells drilled in two reservoir zones. This section was designated a test area to optimize well spacing, hydraulic fracturing parameters and other engineering-driven questions. Therefore, this is an excellent area to study the usefulness of dynamic seismic reservoir characterization.

An initial multicomponent (9-C) 3-D seismic survey was acquired over this focus area after the wells were drilled but prior to hydraulic fracturing and production. This survey serves as the baseline for future monitor seismic surveys. It also is being used for static parameter analyses including brittleness, natural fracture characterization, geologic structure, stratigraphy and stress state.

A second survey was acquired immediately after the wells were hydraulically fractured to study the differences in the earth response to stimulation. Finally, a third survey was to be acquired during the winter of 2015 to study the response of the reservoir to multiple years of production. This integrated study also includes data provided by Anadarko, including core, well logs, microseismic, chemical tracers and the production data.

At least 24 students in RCP at the CSM use these data for their thesis research in geophysics, geology and reservoir engineering. Early theses already have been published that are changing the way Anadarko thinks about the reservoir and its response to hydraulic stimulation. Anadarko is using these results to optimize well spacing and hydraulic fracturing plans for future drilling campaigns.

Static characterization, development and frack design
The Wattenberg Field is dominated by many normal faults that cut through the two main geologic reservoirs, the Smoky Hill Member of the Niobrara Formation and the Codell Sandstone of the Carlile Formation. Understanding and accurately predicting the faulting patterns ahead of the bit is critical for well planning and steering, both to optimize the time in zone and to avoid swelling clays that are drilling hazards. Additionally, faults need to be taken into account when planning for the hydraulic stimulation.

The compressional- (P)-wave portion of the data acquired during this study has helped Anadarko and CSM do an even better job characterizing these faults due to the high-quality nature of the data. Understanding the direction and magnitude of natural fractures and faults helps optimize the stimulation design. By carefully analyzing the multicomponent dataset, students have found that the shear (S) components of the 9-C data better characterize the natural fracture patterns in the rock than P-wave only data.

Work is currently underway to see if there are any ways to get this same information out of converted wave data due to the difficulty and expense in acquiring pure S seismic.

Time-lapse seismic data help to determine the effects of completions on the reservoir, overburden and underburden intervals. Hydraulic fracturing occurs along planes of weakness and therefore may open preexisting fractures. Time-lapse inversions were performed on the P (normal) and S seismic datasets. Post- and prestack inversions delineate small faults and observe geologic responses of varying completion techniques, well spacing and geology. The time-lapse post-stack S inversions characterize the stimulated reservoir volume through S inversion differences.

Time-lapse anomalies
Figure 1 represents the base map of the study area and the locations of the horizontal wells drilled in the section. The map represents the percent difference in P-impedance values between the baseline and the first monitor survey, where the warmer colors are higher percent differences. These percent differences displayed with the incoherence attribute show the major faults in the survey.

The time-lapse anomalies show a larger percent difference in P-impedance values in the western portion of the survey. The anomalies also can mark boundaries of the faults, represented by sharp changes in color. Therefore, these anomalies can be used to refine fault interpretations in the study area. By using the inversion results from the prestack seismic rather than the post-stack, the resolution is improved. Faults and fractures are further refined with the use of the S-wave seismic.

Figure 2 depicts the S inversion of the percent difference between the monitor and baseline surveys within the upper reservoir interval. Generally, lower percent differences indicate faulted areas, and higher percent differences indicate stimulated areas. In some cases, the inversion is able to detect faults that the incoherence attribute does not show.

Fault compartmentalization and communication has an effect upon the overall effectiveness of completions. Some of these features are highlighted by S inversion results and microseismic patterns. The S difference slice of the upper reservoir interval illustrates faults that compartmentalize individual stages, which are not visible on the incoherence map (Figure 2).

FIGURE 2. This base map shows the percent difference in S impedance derived from S inversion. Microseismic events from Well 6N diverge around the area of low percent difference, representing a faulted area partially picked up by the incoherence attribute of top Niobrara (pictured in dark gray). (Source: Anadarko Petroleum Corp.)

The frack barriers were interpreted in the prestack P inversion and post-stack S inversions based on microseismic patterns correlating with inversion anomalies. Figure 3 shows all of the interpreted frack barriers, which correspond mostly to north-south trending faults. Further studies will incorporate the converted-wave seismic to determine the value of acquiring pure S data.

Hydraulic fracturing
Microseismic and time-lapse seismic acquired pre- and post-stimulation provide unique characterization of stimulated rock volume (SRV). Both techniques are limited but can be used in conjunction with one another for better results. By comparing the two different SRV predictions, Anadarko is gaining better insights into the effectiveness of the hydraulic fracturing.

Microseismic shows how the frack progresses out from the wellbore in real time, but the magnitude of these events are very small and therefore do not represent the big picture. Time-lapse seismic shows the gross response of the reservoir to the stimulation—in particular open, propped fractures and stress changes due to the pressure of the fluid injection—but cannot resolve vertical and temporal detail that the microseismic can provide. Therefore, the results are not expected to be the same, and they need to be used together for an optimal understanding of the hydraulic fracturing. The early results look promising.

A map of the SRV was created (Figure 4a) by taking percentage differences of S inversions in the upper reservoir interval (40 ms window below top reservoir horizon) and then subtracting the baseline from the monitor survey. The higher percentages in Figure 4a indicate areas that a) were not initially fractured and were stimulated or b) were initially fractured and encountered changes due to stimulation. Lower percentages correlate with major faults.

Overall, the stimulation is not uniform along the wells, and faults have a significant effect upon the SRV as indicated by the S differences. The microseismic density is shown in greater concentrations on the western portion of the study area (Figure 4b).

Figure 4a shows a high anomaly occurring in the northwest corner of the map. This anomaly probably occurs because the S datasets did not go through a residual rotation during processing. However, this area might also be affected by the vertical well shown in that corner, along with vertical wells in adjacent sections.

Figure 4a

Figure 4b

FIGURE 4. The stimulated reservoir volume is shown as the time-lapse percent difference between S inversions (a) and the microseismic moment (b). (Source: Anadarko Petroleum Corp.)

Production
After the reservoir has been produced, the propped fractures have closed and the stress state of the reservoir has changed due to depletion. The post-production time-lapse should have the ability to detect those changes. Prior results show depletion in the reservoir due to production from previous wellbores, and it is expected that the post-production seismic time-lapse will provide a greater understanding of produced rock volume. Anadarko is looking forward to these results, coming up in the next year or so.

Seismic already has proven to be very useful in the development of unconventional shale reservoirs, particularly for steering. The multicomponent time-lapse seismic studies being done now are allowing Anadarko and CSM to quantitatively characterize the static and dynamic states of the reservoir.

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