CO2 storage in depleted gas reservoirs: A study on the effect of residual gas saturation

Depleted gas reservoirs are recognized as the most promising candidate for carbon dioxide storage. Primary gas production followed by injection of carbon dioxide after depletion is the strategy adopted for secondary gas recovery and storage practices. This strategy, however, depends on the injection strategy, reservoir characteristics and operational parameters. There have been many studies to-date discussing critical factors influencing the storage performance in depleted gas reservoirs while little attention was given to the effect of residual gas. In this paper, an attempt was made to highlight the importance of residual gas on the capacity, injectivity, reservoir pressurization, and trapping mechanisms of storage sites through the use of numerical simulation. The results obtained indicated that the storage performance is proportionally linked to the amount of residual gas in the medium and reservoirs with low residual fluids are a better choice for storage purposes. Therefore, it would be wise to perform the secondary recovery before storage in order to have the least amount of residual gas in the medium. Although the results of this study are useful to screen depleted gas reservoirs for the storage purpose, more studies are required to confirm the finding presented in this paper

Characterization of Trapped Gas Saturation and Heterogeneity in Core Samples Using Miscible-Displacement Experiments

Trapped gas saturation and permeability heterogeneity were evaluated in Berea cores at reservoir conditions, using standard miscible displacement experiments, with and without surfactants. Pressure and production history were influenced by core heterogeneity and foam lamellae formation when aqueous surfactant was present in the core. A simple dispersion model and a three-coefficient dispersion-capacitance model (Coates-Smith) were fit to the experimental data. The dispersion-capacitance model successfully matched the experiments in which foam lamella formed, while the simple dispersion model was used only for determining initial core flow heterogeneity. The objective of the dispersion-capacitance model was to estimate trapped gas saturations; however longitudinal dispersion and mass transfer also were examined. The results show that the dispersion-capacitance model accurately fits trapped gas saturation controlled by rock heterogeneities and foam lamellae for lamella generating mechanisms that allow a continuous gas phase (leave-behind lamellae). The practical applications resulting from this study can aid in core sample selection and scaling short laboratory corefloods to field dimensions for applications to foam stimulation and underground storage of natural gas

Velocity measurements in reservoir rock samples from the SACROC unit using various pore fluids, and integration into a seismic survey taken before and after a CO<inf>2</inf> sequestration flood

The SACROC field, located in west Texas in the Permian Basin, is t he oldes t CO2 enhanced oil recovery site in the United States, with over 93 milli on tons of CO2 injected as of 2007. Recently, t he Na tional Energy Technology Labora tory (NETL) of the United Sta tes Department of Energy has begun to support an enhanced oil recovery project at the SACROC field in north central Texas, working in close collaboration with the Bureau of Economic Geology in Austin, Texas. The project requires the injection of CO2 at a depth of approximately 2040 meters into a reef structure. This project involves both repeat reflection seismic surveys and rock physics based analysis of core material. In the SACROC field, hydrocarbons were flooded out using water, and will be flooded w ith liquid CO2 during the summer of 2008. An experiment is planned whereby we first image fluid floods through reservoir rock using a CT scanner at NETL in order to see what the residual saturations of the previous fluids are. We th en plan to conduct velocity measurements in this order, so that we can obtain a good estimate of conditions in different parts of the reservoir. W e will then measure the P and S wave velo cities, porosity, and permeability at varying pressures and temperatures that simulate reservoir conditions after each successive flood. In addition to velocity ulus, Poisson's ratio, and stress and strain measuremen ts, we will measure porosity, permeability, Young's Mod measuremen t under simulate d in situ reservoir conditions. Preliminary P and S wave velocity measurements were made in dry Berea sandstone samples in order to have a base r eference material to compare our results to. Our Berea measurements were 2402 m/s for Vp, 1688 m/s for Vs, and 0.703 for Vs / Vp for an unstressed sample, which agrees well with the literature. The measured ve locities, the P and S waveforms, the velocity vs. confining pressure will be used to calibrate our acoustical measurements both in dry and saturated samples of the carbonate SACROC reservoir rock using oil, gas, water, and CO2. We then plan on doing the first of several seismic surveys in the SACROC field in west Texas. The first survey will be comple ted during the summer of 2008, and presented at this meeting. Using our velocity measurement s, we hope to be able to discern the difference between areas of the reservo ir rock that are saturated with different pore - filling phases. With repeated surveys over the next few years, we hope to be able to observe the area of extent of various floods of CO2 that are scheduled to be injected into several wells in the area using the laboratory me asurements we collected. © 2009 Elsevier Ltd. All rights reserved

Suitability of depleted gas reservoirs for geological CO2 storage: A simulation study

Hydrocarbon reservoirs, particularly depleted gas formations, are promising geological sites for CO2storage. Although there have been many studies on the storage aspects of gas reservoirs, the suitability of these formations in terms of fluid types such as dry, wet, and condensate gas has not been properly addressed at the reservoir level. In this study, an attempt was made to evaluate different gas reservoirs in order to provide an insight into their storage capabilities. A dynamic numerical simulation was carried out to simulate CO2injection in a synthetic but realistic model of a geologic formation having dry, wet, or condensate gas. The results obtained under particular conditions revealed that the condensate gas medium offers a good storage potential, favorable injectivity, and reasonable pressure buildup over a long period of time, whereas dry gas formations were found to be the least favorable sites for storage among gas reservoirs. A sensitivity analysis was done to evaluate the injection rate and the permeability variation of different media during and after the storage. It indicated that the storage behavior of gas reservoirs is sensitive to the injection rate, and selection of an optimum injection rate might help to achieve a good storage capacity in condensate gas systems. The results also highlighted that CO2immobilization in gas reservoirs after injection is enhanced due to the reduction of permeability, whereas no heterogeneity effect was observed under different permeability realizations