Seismic monitoring of a CO₂ flood in a carbonate reservoir: A rock physics study

Abstract

A carbon dioxide (CO2) injection pilot project is underway in Section 205 of the McElroy field, West Texas. High-resolution crosswell seismic imaging surveys were conducted before and after CO2 flooding to monitor the CO2 flood process and map the flooded zones. The velocity changes observed by these time-lapse surveys are typically on the order of -6%, with maximum values on the order of -10% in the vicinity of the injection well. These values generally agree with laboratory measurements if the effects of changing pore pressure are included.The observed dramatic compressional (VP) and shear (VS ) velocity changes are considerably greater than we had initially predicted using the Gassmann (1951) fluid substitution analysis (Nolen-Hoeksema et al., 1995) because we had assumed reservoir pressure would not change from survey to survey. However, the post-CO2 reservoir pore fluid pressure was substantially higher than the original pore pressure. In addition, our original petrophysical data for dry and brine-saturated reservoir rocks did not cover the range of pressures actually seen in the field. Therefore, we undertook a rock physics study of CO2 flooding in the laboratory, under the simulated McElroy pressures and temperature.Our results show that the combined effects of pore pressure buildup and fluid substitution caused by CO2

Pressure and fluid saturation prediction in a multicomponent reservoir using combined seismic and electromagnetic imaging

“…The majority of effort, as measured by the topics of published and presented work, has concentrated on developing and improving algorithms for estimating the geophysical parameters themselves (Newman, 1995;Lazaratos et al, 1995;Wilt et al, 1995;Nemeth et al, 1997;Goudswaard et al 1998 to list but a few). In most applications where nongeophysical parameters, such as temperature during a steam flood (Lee et al, 1995) or CO 2 saturations during CO 2 flood Wang et al, 1998) are the object of the crosswell survey, correlations between the geophysical parameters, e.g., velocity or electrical conductivity, and the desired reservoir parameter are derived and used to infer the distribution of reservoir parameters from the distribution of the geophysical parameters. The output from the survey is still most commonly a cross section of velocity, electrical conductivity or the time-lapse change of these parameters, which is then interpreted in terms of its implications for the distribution and/or change of the parameter of interest (temperature, CO 2 saturation, etc.…”

Applying compactness constraints to differential traveltime tomography

“…Preliminary rock-physics analysis using poroelastic fluid substitution models similar to those used in Nolen-Hoeksema et al [1995] indicate that such effects are insufficient to produce this change. Likewise, Wang et al [1998] observe significantly smaller reductions in V p during core-scale laboratory measurements of CO 2 injection. The absence of significant changes in V s (not shown) suggests that an increase in pore pressure can also be ruled out as a secondary factor.…”

Time-lapse walkaway vertical seismic profile monitoring for CO2 injection at the SACROC enhanced oil recovery field: A case study

“…With the assumption that there is no compaction within the reservoir and the pore pressure stays approximately the same, we can give a rough estimation of the P-wave velocity change due to a simple fluid substitution using the Gassmann equation (Gassmann, 1951;Wang et al, 1998):…”