Joint cross‐well and single‐well seismic studies of CO₂injection in an oil reservoir

Abstract

A B S T R A C TA series of time-lapse seismic cross-well and single-well experiments were conducted in a diatomite reservoir to monitor the injection of CO 2 into a hydrofracture zone, based on P-and S-wave data. A high-frequency piezo-electric P-wave source and an orbital-vibrator S-wave source were used to generate waves that were recorded by hydrophones as well as 3-component geophones. During the first phase the set of seismic experiments was conducted after the injection of water into the hydrofractured zone. The set of seismic experiments was repeated after a time period of seven months during which CO 2 was injected into the hydrofractured zone. The questions to be answered ranged from the detectability of the geological structure in the diatomic reservoir to the detectability of CO 2 within the hydrofracture. Furthermore, it was intended to determine which experiment (cross-well or single-well) is best suited to resolve these features.During the pre-injection experiment, the P-wave velocities exhibited relatively low values between 1700 and 1900 m/s, which decreased to 1600-1800 m/s during the post-injection phase (−5%). The analysis of the pre-injection S-wave data revealed slow S-wave velocities between 600 and 800 m/s, while the post-injection data revealed velocities between 500 and 700 m/s (−6%). These velocity estimates produced high Poisson's ratios between 0.36 and 0.46 for this highly porous (∼50%) material. Differencing post-and pre-injection data revealed an increase in Poisson's ratio of up to 5%. Both velocity and Poisson's ratio estimates indicate the dissolution of CO 2 in the liquid phase of the reservoir accompanied by an increase in pore pressure.The single-well data supported the findings of the cross-well experiments. P-and S-wave velocities as well as Poisson's ratios were comparable to the estimates of the cross-well data.The cross-well experiment did not detect the presence of the hydrofracture but appeared to be sensitive to overall changes in the reservoir and possibly the presence of a fault. In contrast, the single-well reflection data revealed an arrival that could indicate the presence of the hydrofracture between the source and receiver wells, while it did not detect the presence of the fault, possibly due to out-of-plane reflections.

Time-lapse crosswell seismic and VSP monitoring of injected CO2 in a brine aquifer

“…Saline aquifers are generally considered a prime candidate for large scale storage. Initial studies have shown that time-lapse borehole and surface seismic surveys can be used to estimated the location of injected Cog in brine aquifers as well as in oil and gas reservoirs (Arts et al 2002;Hoversten et al 2003;Gritto et al 2004;Xue et al 2005). Monitoring of injected C02 will likely be a necessary component of any long term storage program.…”

“…Harris et al1995;Lazaratos and Marion 1997;Gritto et al 2004) and more recently as part of a sequestration pilot test (Xue et al 2005;Spetzler et al 2006). These studies were successful in detecting changes in seismic velocity caused by Cop injection into reservoirs.…”

Continuous active-source seismic monitoring of C O2 injection in a brine aquifer

“…Time-lapse seismic imaging has been shown to be an effective technique for subsurface CO 2 monitoring in both enhanced oil recovery (Lazaratos and Marion, 1997;Wang et al, 1998;Gritto et al, 2004;Majer et al, 2006) and sequestration (Skov et al, 2002;Arts et al, 2004).…”

Development of a 400 Level 3C Clamped Downhole Seismic Receiver Array for 3D Borehole Seismic Imaging of Gas Reservoirs

“…Decomposition of these two sweeps provides two equivalent sources with orthogonal horizontal oscillations (Daley and Cox, 2001). Component rotation using P-wave particle motion rotates these two sources into in-line and cross-line equivalents, with in-line being horizontal and in the plane of the two boreholes (Gritto, et al, 2004). Figure 1 shows a 6-component receiver gather (all source depths for one receiver depth) with in-line and cross-line sources for the vertical and two horizontal receiver components.…”