In-situ borehole seismic monitoring of injected CO2 at the Frio Site

PROCEEDINGS, CO2SC Symposium Lawrence Berkeley National Laboratory, Berkeley, California, March 20-22, 2006 IN-SITU BOREHOLE SEISMIC MONITORING OF INJECTED CO2 AT THE FRIO SITE Thomas M. Daley and Valeri A.Korneev Lawrence Berkeley National Laboratory, Earth Science Division 1 Cyclotron Rd. Berkeley, CA, 94720 tmdaley@lbl.gov, vakorneev@lbl.gov INTRODUCTION The U.S. Dept. of Energy funded Frio Brine Pilot provided an opportunity to test borehole seismic monitoring techniques in a saline formation in south- east Texas. A relatively small amount of CO 2 was injected (about 1600 tons) into a thin injection inter- val (about 6 m thick at 1500 m depth). Designed tests included time-lapse vertical seismic profile (VSP) and crosswell surveys which investigated the detect- ability of CO 2 with surface-to-borehole and borehole- to-borehole measurement . DATA ACQUISITION Two time-lapse borehole geophysical surveys were acquired at the Frio site, a vertical seismic profile (VSP) and a crosswell seismic survey. A VSP uses surface sources with borehole sensors, while a crosswell survey uses borehole sources and sensors. Both surveys had baseline, pre-injection, data acqui- sition in July of 2004 and post injection data acquisi- tion in late Nov., 2004 (approximately 1.5 months post injection). The VSP was intended to detect changes in seismic properties on the scale of 10's - 100's of meters around the injection well, while the crosswell survey was designed to detect changes be- tween the injection and monitoring wells on the scale of 2 - 10 meters. Both surveys used an 80-level, 3-component borehole geophone string with 7.6 m (25 ft.) spacing. The crosswell survey had 5 moves of the sensor string to acquire data at 1.5 m (5 ft.) spacing for both sensor and source. The crosswell source was an orbital vi- brator (Daley and Cox, 2001) which generates both P- and S-waves, allowing imaging of compressional and shear properties, respectively. The VSP source was explosive shots at surface sites reoccupied after injection. Initial processing and analysis of the VSP data has provided measurement of seismic reflection amplitude changes on 3 azimuths, northwest, north and northeast of the injection well. VSP AND CROSSWELL RESULTS A large change in VSP reflection amplitude (about 70%) from the Frio zone was observed and the results were compared favorably with flow modeling predictions of saturation (Figure 1). The difference in VSP response for each azimuth of acquisition indicates subsurface heterogeneity affecting saturation. The crosswell data provided very good P- and S- wave tomographic images. The time-lapse tomographic image mapped large changes in P-wave velocity (up to 20%), with well defined boundaries, due to the CO 2 plume (Figure 2). The S-wave tomogram shows no measurable change except at the injection well near the perforations. The results of both VSP and crosswell demonstrate that CO2 injected in brine reservoirs can be spatially located with borehole seismic techniques. Estimates of CO2 saturation from the seismic response are difficult due to complex rock physics which require site dependent models usually obtained from laboratory core studies. Figure 1. Change in VSP reflection amplitude (sym- bols), as a function of offset from injection well, for each of three azimuths, along with modeled gas saturation (lines)along the same azimuths. Gas saturation model- ing courtesy of C. Doughty.