Fireflood Microseismic Monitoring: Rock Mechanics Implications

Dusseault, Maurice B. (Faculties of Engineering and Science, Departments of Mineral Engineering and Physics, University of Alberta) | Nyland, Edo (Faculties of Engineering and Science, Departments of Mineral Engineering and Physics, University of Alberta)

OnePetro 

ABSTRACT

Numerous consistent seismic signals are being generated in a pilot fireflood in a 750 m deep high permeability unconsolidated channel sand in Eastern Alberta. The pilot has a central air injection well, eight production wells on artificial lift, and two internally placed observation wells. Surface seismometers and a 12-hydrophone downhole string were deployed to detect seismics signals. Date were collected for 22 hours employing signal-triggered event detectors recording on cassette tapes. Monitoring during twelve hours of steady injection at 625m 3/s yielded a clear event frequency of about 50 events/h. The events were mostly clear compressional wave arrivals originating in the vicinity of the firefront. Stopping air injection caused a gradual drop in event frequency, and the rate was reestablished when air injection was reinitiated. Spectral frequency analysis and knowledge of material properties indicate that the events are shears of a stick-slip nature associated with the strain-weakening behavior of the over-compacted reservoir sands. The event amplitude indicates small-scale shearing, and different amplitudes suggest that planes re-shear with lower energy release levels. The triggering mechanism for shear rupture is high shear stress created by thermal expansion in the area of highest thermal gradient. Other enhanced recovery techniques may also produce useful signals for process control. In particular, steam flooding, hydraulic fracturing, and high-pressure injection operations likely generate mappable microseismic events which can be applied directly to production/ injection strategy.



RESERVOIR CHARACTERISTICS

The reservoir is a 750 m deep channel sand of Cretaceous age averaging 35 m in thickness with a width of 1.5 km and a length of over 15 km. Alberta's heavy oil deposits, and most other heavy oils, are found in cohesionless sands. The oil contains dissolved gases and there are serious problems of sample disturbance because the fabric of the sands is destroyed as the gas evolves during core recovery (Dusseault, 1980; Dusseault and Van Domselaar, 1982). Core samples of the test reservoir show expansions of about 6% of total volume, or 20% of the pore volume. Porosities of 35% to 37% are calculated in the laboratory, whereas values of about 30% are determined from geophysical bulk density logs and specific gravity data on the rock components (Collins, 1977). The in situ stress state is not explicitly known for this region, but data from other areas (Dusseault, 1980; Gough and Bell, 1981) and analysis of geophysical density logs suggest that the vertical total stress is 16.5 MPa at a depth of 750 m, and that this stress is the principal stress. The intermediate and minor principal stresses are horizontal, and the minor stress is probably 70% to 80% of overburden and is oriented at Az 160º. The initial pore pressures in the reservoir are less than hydrostatic, but the present pore pressures are a function of the geometry of the fireflood as the air is being injected at a pressure below the hydraulic fracture pressure, and the production wells are produced with bottom-hole pumps which create a condition of low pressure to maximize the flow gradients.