Browsing by Subject "Time-lapse"
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Item Temporal changes in gas hydrate mound topography and ecology: deep-sea time-lapse camera observations(Texas A&M University, 2004-09-30) Vardaro, Michael FredricA deep-sea time-lapse camera and several temperature probes were deployed on the Gulf of Mexico continental shelf at a biological community associated with a gas hydrate outcropping to study topographic and hydrologic changes over time. The deployment site, Bush Hill (GC 185), is located at 27?47.5' N and 91?15.0' W at depths of ~540m. The digital camera recorded one still image every six hours for three months in 2001, every two hours for the month of June 2002 and every six hours for the month of July 2002. Temperature probes were in place at the site for the entire experimental period. The data recovered provide a record of processes that occur at gas hydrate mounds. Biological activity was documented by identifying the fauna observed in the time-lapse record and recording the number of individuals and species in each image. 1,381 individual organisms representing 16 species were observed. Sediment resuspension and redistribution were regular occurrences during the deployment periods. By digitally analyzing the luminosity of the water column above the mound and plotting the results over time, the turbidity at the site was quantified. A significant diurnal pattern can be seen in both luminosity and temperature records, indicating a possible tidal or inertial component to deep-sea currents in this area. Contrary to expectations, there was no major change in shape or size of the gas hydrate outcrop at this site on the time frame of this study. This indicates that this particular mound was more stable than suggested by laboratory studies and prior in situ observations. The stable topography of the gas hydrate mound combined with high bacterial activity and sediment turnover appears to focus benthic predatory activity in the mound area. The frequency and recurrence of sediment resuspension indicates that short-term change in the depth and distribution of surface sediments is a feature of the benthos at the site. Because the sediment interface is a critical environment for hydrocarbon oxidation and chemosynthesis, short-term variability and heterogeneity may be important characteristics of these settings.Item Time-lapse seismic monitoring for enhanced oil recovery and carbon capture and storage field site at Cranfield field, Mississippi(2013-12) Ditkof, Julie Nicole; Bangs, Nathan Lawrence Bailey; Meckel, Timothy AshworthThe Cranfield field, located in southwest Mississippi, is an enhanced oil recovery and carbon sequestration project that has been under a continuous supercritical CO₂ injection by Denbury Onshore LLC since 2008. Two 3D seismic surveys were collected in 2007, pre-CO₂ injection, and in 2010 after > 2 million tons of CO₂ was injected into the subsurface. The goal of this study is to characterize a time-lapse response between two seismic surveys to understand where injected CO₂ is migrating and to map the injected CO₂ plume edge. In order to characterize a time-lapse response, the seismic surveys were cross equalized using a trace-by-trace time shift. A normalized root-mean-square (NRMS) difference value was then calculated to determine the repeatability of the data. The data were considered to have “good repeatability,” so a difference volume was calculated and showed a coherent seismic amplitude anomaly located through the area of interest. A coherent seismic amplitude anomaly was also present below the area of interest, so a time delay analysis was performed and calculated a significant added velocity change. A Gassmann-Wood fluid substitution workflow was then performed at two well locations to predict a saturation profile and observe post-injection expected changes in compressional velocity values at variable CO₂ saturations. Finally, acoustic impedance inversions were performed on the two seismic surveys and an acoustic impedance difference volume was calculated to compare with the fluid substitution results. The Gassmann-Wood fluid substitution results predicted smaller changes in acoustic impedance than those observed from acoustic impedance inversions. At the Cranfield field, time-lapse seismic analysis was successful in mapping and quantifying the acoustic impedance change for some seismic amplitude anomalies associated with injected CO₂. Additional well log data and refinement of the fluid substitution workflow and the model-based inversion performed is necessary to obtain more accurate impedance changes throughout the field instead of at a single well location.