4D SEISMIC MONITORING OF STEAM INJECTION AT COALINGA FIELD, CALIFORNIA

The Coalinga field is located in the Central San Joaquin Valley approximately 100 miles north of Bakersfield, California. Heavy oil is produced from moderately consolidated sand-shale reservoirs at an approximate depth of 1,500 feet (0.5 seconds). Steam injection in the Chevron-operated study portion of the Coalinga field began in 1995, with the objective of increasing production in heavy-oil-bearing reservoirs.

A time-lapse 4D seismic project was initiated to monitor the lateral distribution of injected steam in the producing oil reservoirs. Repeat 3D seismic surveys were acquired in 1996, 1997 and more recently again in 2000. The seismic data were carefully acquired with the objective of maintaining a high degree of repeatability in survey direction, source and receiver specifications, bin spacing, and offset and azimuth distributions. The 3D seismic data sets were carefully processed to enhance time-lapse repeatability, and subsequently cross-equalized to refine final steam-related time-lapse seismic anomalies.

The seismic data sets were calibrated with log and production data, and transforms were created using rock physics relations and forward seismic modeling to relate changes in reservoir temperature and steam thickness to changes in seismic amplitude attributes. Reservoir-rock P-wave velocities range from 2375-2525 m/s at ambient (non-steamed) reservoir conditions. Rock physics measurements on oil-saturated Coalinga core samples show an approximate 1% decrease in P-wave velocity (Vp) for every 19 degrees Fahrenheit increase in reservoir temperature, and an additional 8% decrease in Vp if steam is present in the free gas phase, resulting in total Vp changes as high as 25% in steamed zones. The log, production and rock physics information is combined in the transforms to generate separate maps of steam thickness and temperature estimated from the 4D seismic data. These maps can be very useful in assessing the lateral distribution and conformance of injected steam and heat in producing heavy oil reservoirs.