Geophysical Monitoring for Geologic Carbon Storage. Группа авторов

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СКАЧАТЬ pressure effects resulting from supercritical CO₂ (scCO₂) injection in a sandstone reservoir using 4D reflection seismic data and well logs. They use a 3D prestack seismic data set, two 3D seismic poststack volumes, and a wireline log from the sandstone reservoir, to perform forward modeling and predict reservoir seismic response of fluid substituted CO₂ as a component of the pore‐filling fluid mix in a reservoir unit. They use time‐lapse 3D seismic data acquired before and after CO₂ injection and postinjection common midpoint stacks to determine regions of possible fluid density and pore pressures signatures. The lateral changes in amplitude versus offset (AVO) can be an indicator of pore‐filling fluid changes. They implement the Aki‐Richards method to model reflected wave amplitudes at increasing offset angles on time‐lapse prestack seismic data from the Cranfield Field, Mississippi, USA. They present results on the effects of seismic‐wave reflectivity caused by the injection of CO₂ in a sandstone reservoir at Cranfield, and predict the fluid effects of CO₂ presence in the reservoir by applying fluid substitutions to produce reservoir property models and compare these effects with an actual 4D seismic survey.

1.4. SUBSURFACE NONSEISMIC MONITORING

      Migration of CO₂ at a geologic carbon storage site causes subsurface mass redistribution. Lower density CO₂ displaces higher density brine, which results in reduction of the bulk formation density. Time‐lapse gravity monitoring is sensitive to the bulk density changes. Gravity sensors can be deployed on the ground surface or in a borehole or on the seafloor. When CO₂ is present at a subsurface depth of greater than 700 m, CO₂ exists in a supercritical state and its density ranges from 0.25 to 0.70 g/cm³. The density contrast between brine and CO₂ in a CO₂ storage site is often less than 0.5 g/cm³.

      In Chapter 14, Appriou and Bonneville introduce the gravity method and discuss its modeling and application to various geologic carbon storage sites. Detection of gravity variations accompanying mass redistributions in the subsurface caused by fluid movements in reservoirs provides a unique means to monitor the dynamics of a carbon sequestration site. With the recent advancement in instrument capabilities, the gravity method provides a promising monitoring technique.

      Because a CO₂ storage reservoir is often located at a large depth and spatial resolution of gravity surveys decreases with the depth, there are often limited applications that land surface gravity surveys can provide. To improve the plume resolution, a gravimeter must be located closer to a CO₂ plume, either in a borehole or on the seafloor for an offshore geologic carbon storage site. There have been only two gravity monitoring applications at commercial‐scale CO₂ storage sites. A seafloor gravity monitoring survey was performed at the Sleipner CO₂ storage site in Norway. The high repeatability of 1.1–4.3 μGal in gravity measurements was achieved in a dynamic seafloor environment. Hydrocarbon production is another source of reservoir density change and complicates the gravity interpretation. The gravity anomaly reached tens of μGal after injection of 18 MMT CO₂. Gravity monitoring estimated that the mass of CO₂ in the storage reservoir agrees with the injected CO₂ mass without leakage.

      Compared with seismic monitoring, gravity monitoring is more cost‐effective and may be deployed between repeated seismic surveys if the model predicts a measurable gravity response. Downhole and seafloor gravity monitoring of deep CO₂ storage is more effective than surface gravity surveys, which may be better suited for monitoring shallow CO₂ leakage.

      The injection of carbon dioxide results in increased resistivity, which may be detected by electrical and EM imaging techniques, such as electrical resistivity tomography, complex resistivity method, magnetotelluric method, controlled source EM, and other EM methods. In Chapter 15, Gasperikova and Morrison present the electrical and electromagnetic (EM) techniques to map the electrical resistivity of the subsurface for monitoring CO₂ injection and migration. Both surface and borehole resistivity methods are relatively insensitive to horizontal resistive layers of CO₂ plumes. Monitoring with electromagnetic sources at depth shows great promise for detecting and monitoring the emplacement of tabular zones or bodies of resistive CO₂. Surface‐based EM techniques with an induction coil source are not suitable for detection of CO₂ plumes deeper than 1,000 m.

      Electrical and EM methods can be used to monitor CO₂ leakage into the shallow geologic formations. Gas‐phase CO₂ increases the bulk electrical resistivity. On the other hand, brine leakage and dissolved CO₂ in groundwater decrease the electrical resistivity. Electrical and EM monitoring should account for both effects of fluid salinity and CO₂ saturation.

      Electrical resistivity tomography (ERT) is an alternative high‐resolution technique for the shallow aquifer and deep reservoir monitoring. In Chapter 16, Yang and Carrigan describe the ERT method for tracking migration of a supercritical CO₂ plume in a deep storage reservoir and for detecting CO₂ leakage in a shallow aquifer. With downhole electrodes close to the target of interest, ERT can characterize the temporal and spatial resistivity changes effectively. A resistive supercritical CO₂ plume displaces conductive brine and increases the bulk resistivity of storage formation. However, brine leakage and CO₂ dissolution in a shallow aquifer lower the bulk resistivity of the formation. They present two case histories to demonstrate the capability of ERT that provides daily tomographic images of CO₂ distribution, which complements temporally sparse wireline logs and cross‐well seismic data. The surface ERT method is effective for monitoring shallower than 100 m in depth. The primary challenge of cross‐well ERT is the high cost of using two or more closely spaced monitoring wells to deploy vertical electrode arrays. Single‐well ERT may be used when closely spaced monitoring wells are not available.

The controlled source electromagnetic (CSEM) method is mostly an offshore monitoring technique using electromagnetic remote‐sensing technology to map the electric resistivity distribution of the subsurface. In Chapter 17, Tveit and Mannseth introduce a Bayesian sequential inversion methodology for combining geophysical data types of different resolutions. They first use CSEM or gravimetric data to invert for a low‐resolution result, which is used as prior information for inversion of seismic amplitude‐versus‐offset (AVO) data to improve resolution. The unknown geophysical parameters, for example, electric conductivity, density, and seismic velocity, are represented using a carefully designed parameterization based on the level‐set framework, which allows for representation of region boundaries defined by the large‐scale CO₂ plume and slowly varying geophysical properties inside and outside the CO2 plume. They show that seismic AVO inversion results are improved with the sequential inversion methodology using prior information from either CSEM or gravimetric inversion.

      Fluid flow through a porous media can generate electrical potential gradient or streaming potential along the flow path. The self‐potential (SP) technique is a passive electrical monitoring method that measures spontaneous or natural electrical potential from the subsurface. In Chapter 18, Nishi and Ishido introduce the SP monitoring technique. Two SP mechanisms are studied with numerical simulations. The electrokinetic coupling mechanism produces SP caused by changing reservoir conditions such as pressure disturbance from results of reservoir simulation. The geobattery mechanism creates a galvanic cell caused by subsurface electrochemical conditions near a metallic well casing. CO₂ injection alters deeper reducing environment and redox potential, which results in an SP anomaly. If a substantial amount of CO₂ migrates upward into a shallow fresh water aquifer through a vertical fault and creates substantial pressure disturbances, observable self‐potential measurements caused by electrokinetic coupling would provide a cost‐effective monitoring method СКАЧАТЬ