Year 2 Progress

Technical Progress Report, Year 2, and Plan for Year 3: 4-D High-Resolution Seismic Reflection Monitoring of Miscible CO₂ Injected into a Carbonate Reservoir

by Richard D. Miller, Abdelmoneam E. Raef, Alan P. Byrnes, and William E. Harrison

KGS Open File Report 2005-32
Sept. 2005

Abstract

The objective of this research project is to acquire, process, and interpret multiple high-resolution 3-D compressional wave and 2-D, 2-C shear wave seismic data to observe changes in fluid characteristics in an oil field before, during, and after the miscible carbon dioxide (CO2) flood that began around December 1, 2003, as part of the DOE-sponsored Class Revisit Project (DOE #DE-AC26-00BC15124). Unique and key to this imaging activity is the high-resolution nature of the seismic data, minimal deployment design, and the temporal sampling throughout the flood. The 900-m-deep test reservoir is located in central Kansas oomoldic limestones of the Lansing-Kansas City Group, deposited on a shallow marine shelf in Pennsylvanian time. After 18 months of seismic monitoring, one baseline and six monitor surveys clearly imaged changes that appear consistent with movement of CO2 as modeled with fluid simulators.

Executive Summary

Efficiency of enhanced oil recovery (EOR) programs relies heavily on accurate reservoir models. Movement of miscible carbon dioxide (CO2) injected into a thin (~5 m), shallowshelf, oomoldic carbonate reservoir around 900 m deep in Russell County, Kansas, was successfully monitored using high-resolution 4-D/time-lapse seismic techniques. Highresolution seismic methods show great potential for incorporation into CO2-flood management, highlighting the necessity of frequently updated reservoir-simulation models, especially for carbonates. Use of an unconventional approach to acquisition and interpretation of the high-resolution time-lapse/4-D seismic data was key to the success of this monitoring project.

Interpretations of geologic features from seismic data have provided location-specific reservoir properties that appear to strongly influence fluid movement in this production interval. Lineaments identified on seismic sections likely (based on time-lapse monitoring and production data) play a role in sealing and/or diverting flow through the reservoir. Distribution and geometries associated with similarity seismic facies and seismic lineament patterns are suggestive of a complex ooid shoal depositional environment. By incorporating these features, using properties consistent with core data, a more realistic reservoir simulator results, honoring the production and core properties. Flow models after simulator updating (sealing lineaments and preferential permeability manifested by faster progression of the CO2 bank) show improvement in detail and provide correlation with the material balance.

Amplitude envelope attribute data possess changes in texture generally consistent with expectations and CO2 volumetrics. Arguably, a multitude of different boundaries could be drawn to define the shape of the CO2 plume, but the shapes suggested match the physical restraints, based on engineering data and the estimated amplitude response. Focusing on the injection well area and continuity of the characteristics defining the anomalous area, it is not difficult to identify a notable change in data character and texture likely associated with the displacement of reservoir fluids with CO2.

Advancement of the CO2 from the injector seems to honor both the lineaments identified on baseline data and changes in containment pressures. Overlaying the amplitude envelope attribute map with the lineament attribute map provides an enhanced view, and therefore perspective, of the overwhelming variability in the reservoir rocks and the associated consistency and control these features or irregularities have on fluid movement.

Increased northerly movement of the CO2, as interpreted on seismic data and inferred from production data, after several months of CO2 injection and oil production, stimulated an increase in injection rates at the water flood wells. After several months of increased water injection, the CO2 advancement to the northwest was halted and some regression was observed on seismic data.

Shortness of turnaround time of time-lapse seismic monitoring in the Hall-Gurney field provided timely support for reservoir-simulation adjustments and flood-management of the pilot study. Initial reservoir flow simulations utilized models based on pre-CO2 oil production history, measured rock properties from core, water injectivity testing, and interwell testing. These data did not completely constrain the possible permeability architecture in the reservoir and CO2-flood performance did not match pre-CO2 injection predicted performance. 4-D seismic data, obtained and interpreted while the CO2 flood was ongoing, was interpreted independent of simulations updated with the most current production data; therefore, to a limited extent, the interpretations of CO2 movement based on seismic data were performed without field production input. Seismic predictions of CO2 breakthrough at well 12 and the delay at well 13 were based on seismic data alone after the second monitor survey. Following initial seismic prediction, seismic and flood performance data were integrated to both validate the 4-D interpretation and confirm it was not inconsistent with flood performance, and to provide seismic input of flood progress to the flood management process. In general, seismically predicted changes in the CO2 plume and measurements at production wells have been consistent throughout the flood.

Interpretations of time-lapse seismic data are consistent with and have assisted understanding of field response for the pilot. In a similar fashion, 4-D seismic have provided input to reservoir simulations investigating full-field EOR-CO2 floods. Key observations from seismic data include

• accurate indication of solvent "CO2" breakthrough in well 12,
• predicted delayed response in well 13,
• interpretation of a permeability barrier between wells 13 and CO2I#1, and
• consistency with reservoir simulation prediction of CO2 movement and volume estimated to have moved north, outside the pattern.

Time-lapse seismic monitoring of EOR-CO2 can reveal weak anomalies in thin carbonates below temporal resolution and can be successful with moderate cross-equalization and attention to consistency in acquisition and processing details. Most of all, methods applied here avoid the complications associated with inversion-based attributes and extensive cross-equalization techniques. Spatial textural, rather than spatially sustainable magnitude, time-lapse anomalies were observed and should be expected for thin, shallow carbonate reservoirs. Non-inversion, direct seismic attributes proved both accurate and robust for monitoring the development of this EOR-CO2 flood.

Weak-anomaly enhancement of selected non-inversion, 4-D seismic attribute data represented a significant interpretation development and proved key to seismic monitoring of CO2 movement. Also noteworthy was the improved definition of heterogeneities affecting the expanding flood bank. Among other findings, this time-lapse seismic feasibility study demonstrated that miscible CO2 injected into a shallow, thin carbonate reservoir could be monitored, even below the classic temporal seismic resolution limits.

Complete Report

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