Raef and others, The Leading Edge preprint

4D Seismic to Image a Thin Carbonate Reservoir During a Miscible CO2 Flood: Hall-Gurney Field, Kansas, USA

by Abdelmoneam E. Raef, Richard D. Miller, Evan K. Franseen, Alan P. Byrnes, W. Lynn Watney, and William E. Harrison, Kansas Geological Survey

published in The Leading Edge, v. 24, no. 5, p. 521-526.

Introduction

Time lapse (TL) seismic monitoring has proven a valuable tool for reservoir management. If available in a timely, cost-effective manner, it can be incorporated into a dynamic reservoir management approach, specifically in terms of assessing sweep efficiency through constraining reservoir simulations, quantifying compartmentalization, and enhancing the placement of infill-wells. Thus cost-effectiveness, rapid turnaround time, and sensitivity of the seismic response to subtle changes in reservoir properties evident in production data are critical assets of a robust and economic TL-seismic application. Successful applications of TL seismic monitoring has been predominantly offshore--Gulf of Mexico and North Sea--in thick clastic reservoirs characterized as low technical risk. Changes in TL seismic images as a direct result of enhanced oil recovery operations in low compressibility, "stiff," carbonate reservoirs are so small that distinguishing reservoir fluid changes from background noise on seismic attribute maps is challenging at best and many times impossible. Reservoirs predicted to produce weak TL anomalies associated with enhanced oil recovery (EOR) operations are classified as high-risk cases and languish in what has been designated the "stretch portfolio" of time-lapse application in reservoir management.

Significant reservoir heterogeneity and high rock frame stiffness complicate the use of 4D-seismic monitoring of fluid changes in carbonate rocks during EOR or carbon dioxide (CO2) sequestration activities. As well, background noise and residual differences in time, phase, amplitude and frequencies after cross-equalization represent obstacles to all 4D-seismic monitoring projects. For this application, the key to successfully monitoring the movement of miscible CO2 injected into a shallow (900 m) thin (3.6-6 m) carbonate reservoir was a highresolution approach. This approach concentrated on repeatability during acquisition and processing, and use of amplitude envelope 4D-horizon attributes. Amplitude envelope values were extracted from the baseline and three monitor surveys acquired at approximately twomonth intervals during the first eight months of a DOE-sponsored EOR pilot study in an Upper Pennsylvanian shelf carbonate reservoir in central Kansas. Production data and reservoir simulations compared to seismic images provided a measure of the effectiveness using TLseismic to detect weak anomalies associated with changes in fluid concentration.

Revealing TL-anomalies using a nonlinear color bar was instrumental in increasing interpretation confidence in areas with change. Assignment of a single color to all values above and below a progressively decreasing range of values while searching the data for spatially significant textural differences between the baseline and monitor datasets highlights areas that otherwise would not be distinguishable from noise due to the coarseness of the scale. These textural differences are relatively stable over a range (two-three color steps) of scanned levels, further advancing the suggestion of production and/or EOR effects. Production simulation, production data, and seismic consistency in multiple surveys solidified the relationship between these textural changes and reservoir response to CO2 injection. Our approach, called parallel progressive blanking (PPB), proved sensitive to weak time-lapse signatures that otherwise would have been concealed by noise and remnants of balancing/cross-equalization techniques.

The 4D-seismic interpretation approach applied here allows better focusing of color scales within the critical textural-difference range on time-lapse attribute maps, thereby enhancing apparent resolution within the critical range. Below the temporal resolution in thin heterogeneous stiff reservoirs, signatures of reservoir change are encoded in and interfered with main events. Small changes in fluid saturation will manifest themselves as subtle spatial changes in texture rather than monotonically increasing or decreasing horizon attributes. The following is a case study demonstrating successful use of the PPB approach to track the CO2 flood bank in a thin carbonate formation in the Hall-Gurney field, Kansas, USA. Production data from wells and field simulations provide evidence of the viability of the PPB approach when searching for weak TL anomalies. As will be shown later in this paper, this approach is also aiding our efforts analyzing these high-resolution data to distinguish subtle seismic characteristics and associated trends related to depositional lithofacies and geometries and structural elements of this carbonate reservoir that impact fluid character and EOR efforts.

Full Article

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