Amit Padhi

CO2 saturation, distribution and seismic response in two-dimensional permeability model.

Carbon dioxide capture and storage (CCS) has been actively researched as a strategy to mitigate CO(2) emissions into the atmosphere. The three components in CCS are monitoring, verification, and accounting (MVA). Seismic monitoring technologies can meet the requirements of MVA, but they require a quantitative relationships between multiphase saturation distributions and wave propagation elastic properties. One of the main obstacles for quantitative MVA activities arises from the nature of the saturation distribution, typically classified anywhere from homogeneous to patchy. The emerging saturation distribution, in turn, regulates the relationship between compressional velocity and saturation. In this work, we carry out multiphase flow simulations in a 2-D aquifer model with a log-normal absolute permeability distribution and a capillary pressure function parametrized by permeability. The heterogeneity level is tuned by assigning the value of the Dykstra-Parson (DP) coefficient, which sets the variance of the log-normal horizontal permeability distribution in the entire domain. Vertical permeability is a 10th of the horizontal value in each gridcell. We show that despite apparent differences in saturation distribution among different realizations, CO(2) trapping and the V(p)-S(w) Rock Physics relationship are mostly functions of the DP coefficient. When the results are compared with the well accepted limits, Gassmann-Wood (homogeneous) (A Text Book of Sound; G. Bell and Suns LTD: London, 1941) and Gassmann-Hill (patchy) models, the V(p)-S(w) relationship never reaches the upper bound, that is, patchy model curve, even at the highest heterogeneity level in the model.

Time-lapse Monitoring Carbon Sequestrated Brine Aquifers- a Feasibility Study

Substantial research efforts are now underway on injecting (sequestrating) carbon dioxide (CO2) into deep saline aquifers. These sequestration efforts require remote monitoring using available geophysical tools to ensure that the sequestrated CO2 is in place and does not disturb the geological integrity of the surrounding rocks. Since seismic method is the only accepted geophysical tool that can potentially image detailed subsurface information to large depths, here we develop a monitoring strategy using seismic data alone. Fluid substitution at different saturations of CO2 in a brine filled aquifer and comparing its elastic properties with the original indicates that the formation density will play the key role in successful monitoring of carbon-sequestrated aquifers. As multicomponent seismic data are more sensitive to subsurface density variations than vertical (P-wave) component data, we believe that multicomponent seismic data are necessary for obtaining an accurate subsurface presequestration model. Multicomponent data are however more expensive than conventional (P-wave) data. Therefore, acquiring multicomponent data both for baseline and for successive monitoring surveys is not cost-effective. Since above-normal pore pressure due to sequestration may fracture the overlying formations, we investigate if microseismic events generated from these fractures could be used for monitoring. Inducing microseismic events with different fault-plane source mechanisms and computing passive seismic responses from them, we find that these computed responses are sensitive to the fracture fault plane geometry, and passive seismic data could be a potential monitoring tool. We conclude that if multicomponent seismic data are acquired prior to sequestration as a baseline survey and inverted for an accurate presequestration elastic earth model, we can then use passive seismic data for subsequent monitoring. This strategy, in turn, may provide a cost-effective way to monitor carbon sequestrated deep saline aquifers.

Prestack Waveform Inversion- the Present State And the Road Ahead

Prestack waveform inversion (PWI) is one of the emerging technologies for reservoir characterization. inversion uses the full wavefield information in modeling engine allowing the models to be sub-wavelength resolutions. Consequently, PWI much more accurate inversion result compared to the conventional amplitude-variation-with-offset (AVO) based inversions. PWI have been successfully applied in the recent past in a variety of applications such as obtaining an accurate velocity of the water-column for predicting ocean temperatures (Padhi et al, 2010), characterization of hydrocarbon reservoirs (Mallick, 1999; Sen among others), and predicting drilling hazards (Mallick and Dutta, 2002). Prestack waveform inversion (P-wave) and multicomponent seismic data are available (Mallick, 2000).